Student Astronaut Challenge
2017 Student Textbook
Table of Contents
1 - A History of Rocketry............................................................................................. 2
2 - Basic Concepts of Orbital Spaceflight................................................................. 11
3 - Launch Phase......................................................................................................... 30
4 - Cruise Phase........................................................................................................... 37
5 - Encounter Phase.................................................................................................... 42
6 - Extended Operations Phase.................................................................................. 50
7 - Space Shuttle Operations...................................................................................... 53
8 - Emergency Operation Management..................................................................... 67
9 - The Future of Space Propulsion........................................................................... 71
Glossary of Essential Terms...................................................................................... 76
Reference Sources...................................................................................................... 92
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Chapter 1
A History of Rocketry
Early History
The earliest solid rocket fuel was a form of gunpowder, and the earliest recorded
mention of gunpowder comes from China late in the third century before Christ. Bamboo
tubes filled with saltpeter, sulphur and charcoal were tossed into ceremonial fires during
religious festivals in hopes the noise of the explosion would frighten evil spirits.
It's probable that more than a few of these bamboo tubes were imperfectly sealed and,
instead of bursting with an explosion, simply went skittering out of the fire, propelled by
the rapidly burning gunpowder. Some clever observer whose name is lost to history
may have then begun experiments to deliberately produce the same effect as the
bamboo tubes which leaked fire.
Certainly by the year 1045 A.D. -- 21 years before William the Conqueror would land on
the shores of England -- the use of gunpowder and rockets formed an integral aspect of
Chinese military tactics.
A point of confusion arises tracing the history of rocketry back before 1045. Chinese
documents record the use of "fire arrows," a term which can mean either rockets or an
arrow carrying a flammable substance. By the beginning of the 13th Century, the
Chinese Sung Dynasty, under pressure from growing Mongolian hordes, found itself
forced to rely more and more on technology to counter the threat. Chinese ordnance
experts introduced and perfected many types of projectiles, including explosive
grenades and cannon.
Rocket fire-arrows were certainly used to repel Mongol invaders at the battle of Kaifung-fu in 1232 A.D. The rockets were huge and apparently quite powerful, and
according to a report, "when the rocket was lit, it made a noise that resembled thunder
that could be heard for five leagues -- about 15 miles. When it fell to Earth, the point of
impact was devastated for 2,000 feet in all directions." Apparently these large military
rockets carried incendiary material and iron shrapnel. These rockets may have included
the first combustion chamber, for sources describe the design as incorporating an "iron
pot" to contain and direct the thrust of the gunpowder propellant.
The rocket seems to have arrived in Europe around 1241 A.D. Contemporary accounts
describe rocket-like weapons being used by the Mongols against Magyar forces at the
battle of Sejo which preceded their capture of Buda (now known as Budapest) Dec. 25,
1241. Accounts also describe Mongol's use of a noxious smoke screen -- possibly the
first instance of chemical warfare.
Rockets appear in Arab literature in 1258 A.D., describing Mongol invaders' use of them
on February 15 to capture the city of Baghdad. Quick to learn, the Arabs adopted the
rocket into their own arms inventory and, during the Seventh Crusade, used them
against the French Army of King Louis IX in 1268.
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It is certain that, not later than the year 1300, rockets had found their way into European
arsenals, reaching Italy by the year1500, Germany shortly afterwards, and later,
England. A 1647 study of the "Art of Gunnery" published in London contains a 43-page
segment on rockets. The Italians are credited, by the way, with adopting military rockets
for use as fireworks -- completing the circle, so to speak, of the bursting bamboo used
at the Chinese festivals 1,700 years earlier.
The French Army traditionally has been among the largest, if not THE largest, army in
Europe and was quick to adopt rockets to military operations. Records from 1429 show
rockets in use at the siege of Orleans during the Hundred Years War against the
English. Dutch military rockets appear by 1650 and the Germans' first military rocket
experiments began in 1668. By 1730, a German field artillery colonel Christoph Fredrich
von Geissler, was manufacturing rockets weighing 55 to 120 pounds.
Rocket Warfare
As the 18th Century dawned, European military experts began to take a serious
interest in rockets -- if only because they, like the Magyars 500 years earlier, found
themselves on the receiving end of rocket warfare. Both the French and the British,
during the Eighteenth Century, began wrestling for control of the riches of India. In
addition to fighting one another, they also found themselves frequently engaged against
the Mogol forces of Tippoo Sultan of Mysore. During the two battles of Seringapatam in
1792 and 1799, rockets were used against the British. One of Tippoo Sultan's rockets is
now displayed in the Royal Ordnance Museum at Woolwich Arsenal, near London.
Tippoo Sultan's father, Hyder Ally, had incorporated a 1,200 man contingent of
rocketeers into his army in the year 1788. Tippoo Sultan increased this force to about
5,000 men, about a seventh of his total Army’s strength.
Profiting from their Indian experience, the British, led by Sir William Congrieve (KONgreeve), began development of a series of barrage rockets ranging in weight from 300
to 18pounds. Congrieve-design rockets were used against Napoleon. It is surprising
that Napoleon seems to have made no use of rockets in the French Army but it must be
remembered Napoleon was an artillery officer and may have simply been too hidebound a traditionalist to favor new-fangled rockets over more familiar cannons.
The scope of the British use of the Congrieve rocket can be ascertained from the 1807
attack on Copenhagen. The Danes were subjected to a barrage of 25,000 rockets which
burnt many houses and warehouses. An official rocket brigade was created in the
British Army in 1818. Rockets came to the New World during the War of 1812.
During the Battle of Bladensburg, August 24, 1814, the British85th Light Infantry used
rockets against an American rifle battalion commanded by U.S. Attorney General
William Pickney. British Lieutenant George Gleig witnessed the Americans' response to
the new threat-- "Never did men with arms in their hands make better use of their legs,"
he wrote. On December 4, 1846, a brigade of rocketeers was authorized to accompany
Maj. Gen. Winfield Scott's expedition against Mexico. The Army's first battalion of
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rocketeers -- consisting of about 150 men and armed with about 50 rockets -- was
placed under the command of First Lieutenant George H. Talcott.
The rocket battery was used March 24, 1847 against Mexican forces at the siege of
Veracruz. On April 8 the rocketeers moved inland, being placed in their firing position
by Captain Robert E. Lee (later to command the Confederate Army of Northern Virginia
in the War Between the States). About 30 rockets were fired during the battle for
Telegraph Hill. Later, the rockets were used in the capture of the fortress of
Chapultepec, which forced the surrender of Mexico City.
With typical foresight, as soon as the fighting in Mexico was over, the rocketeer
battalion was disbanded and the remaining rockets were placed in storage. They
remained in mothballs for about 13 years-- until 1861 when they were hauled out for use
in the Civil War. The rockets were found to have deteriorated; however, so new ones
were made.
The first recorded use of rockets in the Civil War came on July 3,1862, when Maj. Gen.
J.E.B. Stuart's Confederate cavalry fired rockets at Maj. Gen. George B. McClellan's
Union troops at Harrison's Landing, VA. No record exists of the Northerners' opinion of
this premature “Fourth of July" fireworks demonstration.
Later in 1862, an attempt was made by the Union Army's New York Rocket Battalion -160 men under the command of British-born Major Thomas W. Lion -- to use rockets
against Confederates defending Richmond and Yorktown, Virginia. It wasn't an
overwhelming success. When ignited, the rockets skittered wildly across the ground,
passing between the legs of a number of mules. One detonated harmlessly under a
mule, lifting the animal several feet off the ground and precipitating its immediate
desertion to the Confederate Army.
The only other documented use of rockets is at Charleston, S.C., in 1864. Union troops
under Maj. Gen. Alexander Schimmelfennig found rockets "especially practical in driving
off Confederate picket boats, especially at night."
As an interesting sidelight, the author Burke Davis, in his book “Our Incredible Civil
War," tells a tale of a Confederate attempt to fire a ballistic missile at Washington, D.C.,
from a point outside Richmond, Va. According to the author, Jefferson Davis witnessed
the event at which a 12-foot-long, solid-fueled rocket, carrying a10-pound gunpowder
warhead in a brass case engraved with the letters C.S.A., was ignited and seen to roar
rapidly up and out of sight. Noone ever saw the rocket land. It's interesting to speculate
whether, almost 100 years before Sputnik, a satellite marked with the initials of the
Confederate States of America might have been launched into orbit.
The military appears to have remained underwhelmed with the potential of rockets.
They were employed in fits and starts in many of the brushfire wars which punctuated
the otherwise calm closing days of the late Victorian Era. If the military was luke warm
to rockets, another profession welcomed them with open arms.
The international whaling industry developed rocket-powered, explosive-tipped
harpoons which were most effective against the ocean-going leviathans.
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Rockets and World War
During the First World War, rockets were first fired from aircraft attempting to shoot
down enemy hydrogen gas-filled observation balloons. Successes were rare and pilots
resisted being asked to fire rockets from the highly flammable, cloth and varnish
covered wings of their biplanes. The French were the principal users of aerial rockets,
using a model developed by a naval lieutenant, Y.P.G. LePrieur.
The principal drawback to rockets throughout this period of development was the type of
fuel. Both here and abroad, experiments were under way to develop a more powerful,
liquid-propelled rocket. Two young men stand out in this effort -- one an American,
Robert H. Goddard -- the other a German, Werner von Braun.
Radio commentator Paul Harvey tells a story of how young von Braun's interest in
rocketry almost got him labeled as a juvenile delinquent. At the age of 13, von Braun
exhibited an interest in explosives and fireworks. His father could not understand his
son’s consuming interest in so dangerous a hobby. He feared his son would become
safecracker. One day the young teenager obtained six skyrockets, strapped them to a
toy red wagon and set them off. Streaming flames and a long trail of smoke, the wagon
roared five blocks into the center of the von Braun family's home town, where they
finally exploded.
As the smoke cleared, the toy wagon emerged as a charred wreck. Young von Braun
emerged in the firm grasp of policeman. Despite being severely reprimanded by his
father, the youngster’s interest would not be denied. By the age of 22 he had earned his
doctorate in physics. Two years later he was directing Germany’s military rocket
development program.
Von Braun and his colleagues produced a number of experimental designs, the most
famous of which was the A-4 rocket, which has gained distinction in history under
another name -- the vengeance weapon number two -- V-2 for short. The V-2 was the
first successful, long-range ballistic missile, and von Braun is credited as its principal
developer. As World War II drew to a close, von Braun led his contingent of several
hundred rocket scientists and engineers –all marked for death by the Nazis to prevent
their capture by the Allies-- into American lines.
In 1946, von Braun and his team arrived at White Sands, N.M., where, for the first time,
von Braun learned of work done by the American rocket pioneer Robert Goddard.
Goddard's interest in rockets began in 1898 when, as a 16-year-old, he read the latest
publication of that early science fiction writer, English novelist H.G. Wells. The book
which so excited Goddard was later made into a 1938 radio program that nearly
panicked our entire nation when it was broadcast. Orson Well’s too realistic rendition of
the "War of the Worlds" still causes many to shudder.
As the 20th Century began -- Wilbur and Orville Wright were preparing to become the
first men to fly. Goddard, however, was already designing rockets to probe the upper
atmosphere and delve into space. Half a world away -- and unknown to Goddard -- a
Russian schoolteacher, Konstantin Tsiolkovsky was thinking along much the same
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lines. Both came to the conclusion independently that, if a rocket was going to do the
things they dreamed of, it would have to be powered by liquid fuels. Solid fuels of the
time simply didn't have sufficient power. Tsiolkovski lacked Goddard's practicality. While
Tsiolkovski worked out many principles of astronautics and designed suitable rockets,
he never built any. By contrast, Goddard was a technical man. He could and did build
rockets. By the time he died in 1945, Goddard held 214 patents in rocketry -- patents
which still produce royalties for his estate.
Goddard began his experiments in rocketry while studying for his doctorate at Clark
University in Worcester, Mass. He first attracted attention in 1919 when he published a
paper titled, "A Method of Reaching Extreme Altitudes." In his paper he outlined his
ideas on rocketry and suggested none too seriously, that a demonstration rocket should
be flown to the Moon. The general public ignored the scientific merit of the paper -latching instead onto Goddard's Moon rocket proposal. At the time, such an endeavor
was absurd and most dismissed Goddard as a "crank."
The experience taught Goddard a hard lesson -- one which caused him to shy away
from future opportunities to publicize his work. Publicity was far from Goddard's mind on
the morning of March16, 1926. On that day, barely a year after Werner von Braun's
rocket wagon fiasco, Goddard launched a liquid-powered rocket he had designed and
built from a snow-covered field at his Aunt Effie Goddard's farming Auburn, Mass. The
rocket flew -- 152 feet -- about the same distance as the Wright Brothers' first manned
flight -- but it did fly! It was the first flight of a liquid-fueled rocket in history.
When Goddard was later approached by the American Interplanetary Society in 1930 to
publicize his work, Goddard refused. The society, rebuffed and learning that no one in
the United States aside from Goddard was working with rockets, turned its attention to
rocket research under way in Europe, where rocketry was beginning to develop
following.
In the spring of 1931, two founder-members of the American society, husband and wife
Edward and Lee Pendray, travelled on vacation to Germany where they made contact
with the German Rocket Society, which had been formed in 1927. The visiting
Americans were given a preview of the future when a member of the German Rocket
Society -- Prof. Willy Ley -- took the pair to the Germans' rocket flying test ground in the
suburbs of Berlin.
Returning home, the Pendrays filed an enthusiastic report of their visit, prompting the
American society to build its first rocket. The attempted test flight in November 1932
ended with the American design firmly on the ground. It's unfortunate the Pendrays
didn't meet another future rocketry Hall-of-Famer who also was a member of the
German society. Rumanian-born Hermann Oberth wrote, in 1923, a highly prophetic
book: "The Rocket into Interplanetary Space." The book enthralled many with dreams of
space flight, including that precocious German teenager, Werner von Braun who read
the book in 1925. Five years later, von Braun had joined Oberth and was assisting with
rocket experiments.
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By 1932, the German Army was beginning to show an interest in the German Rocket
Society's efforts, and in July of that year, a "Mirak" rocket was launched as a
demonstration for the head of the newly created German Army rocket research group,
Captain (later Major General) Walter Dornberger. Mirak didn't impress Dornberger but
von Braun did.
Three months after the demonstration flight, von Braun was engaged to work on liquid
propelled rockets for the Army. Most of the German Rocket Society followed von Braun
into national service and the society disbanded. By December 1934, von Braun scored
his first successes with an A2 rocket powered by ethanol and liquid oxygen. Two years
later, as plans for the follow-on A3 rocket were being finalized, initial planning began for
the A4 rocket -- a rocket that was to be, in Dornberger's words, a practical weapon, not
a research tool. As noted earlier, most know the A4 by another name -- the V-2.
The rocket researchers quickly outgrew their facilities at Kummersdorf on the outskirts
of Berlin and, in 1936, operations were transferred to a remote island on Germany's
Baltic coast --Peenemunde. Between 1937 and 1941, von Braun's group launched
some70 A3 and A5 rockets, each testing components for use in the proposedA4 rocket.
The first A4 rocket flew in March 1942. The rocket barely cleared some low clouds
before crashing into the sea a half mile from the launch site. The second launch in
August 1942 saw the A4 rise to an altitude of 7 miles before exploding. The third try
was the charm. On October 3, 1942, another A4 roared aloft from Peenemunde,
followed its programmed trajectory perfectly, and landed on target 120 miles away. This
launch can fairly be said to mark the beginning of the space age. The A4, the first
successful ballistic rocket, is the ancestor of practically every rocket flown in the world
today.
Production of the A4 began in 1943 and the first A4s, now renamedV2s, were launched
against London in September 1944. The V-2offensive came too late to affect the
course of the war. By April1945, the German Army was in full retreat everywhere and
Hitler had committed suicide in his bunker in Berlin.
At an inn near Oberjoch, the HausIngeburg, von Braun and over 100of his rocket
experts waited for the end. The entire team had been ordered executed by Hitler to
prevent their capture. Werner von Braun's brother, Magnus, however, managed to
contact nearby American forces before Hitler's SS henchmen could reach the rocket
team. On May2, the same day Berlin fell to the Soviet Army, von Braun and his rocket
team entered American lines and safety.
With the fighting over, von Braun and his team were heavily interrogated and jealously
protected from Russian agents. V2s and V2components were assembled. German
rocket technicians were rounded up. In June, General Eisenhower sanctioned the final
series of V2launches in Europe. Watching each of the three V2s which rose from launch
site at Cuxhaven was a Russian Army colonel, Sergei Korolev. Ten years later, Korolev
would be hailed as the Soviet Union’s chief designer of spacecraft and the individual
responsible for building the Vostok, Voshkod and Soyuz spacecraft which, since1961,
have carried all Soviet cosmonauts into orbit.
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Few members of von Braun's team participated in the Cuxhaven launches. Most had
already begun setting up shop at Fort Bliss, near El Paso, Texas. Piled up in the desert
near Las Cruces, New Mexico, were enough parts to build 100 V2s. Von Braun and his
team soon moved to nearby White Sands Proving Ground where work began
assembling and launching V2s. By February 1946, von Braun's entire Peenemunde
team had been reunited at White Sands and, on April 16, the first V2 was launched in
the United States. The U.S. space program was under way!
The Space Race
Up to 1952, 64 V2s were launched at White Sands. Instruments, not explosives, packed
the missiles' nosecones. A V2 variant saw the missile become the first stage of a two
stage rocket named Bumper. The top half was a WAC Corporal rocket. The need for
more room to fire the rockets quickly became evident and, in 1949, the Joint Long
Range Proving Ground was established at remote, deserted Cape Canaveral, Fla. On
July 24, 1950, a two-stage Bumper rocket became the first of hundreds to be launched
from "the Cape."
The transfer of launch operations to the Cape coincided with the transfer of the Army's
missile program from White Sands to a post just outside a north Alabama cotton town
called Huntsville. Von Braun and his team arrived in April 1950 -- it was to remain his
home for the next 20 years -- 20 years in which the city's population increased tenfold.
The von Braun team worked to develop what was essentially asuper-V2 rocket, named
for the U.S. Army arsenal where it was being designed -- the Redstone. In 1956, the
Army Ballistic Missile Agency was established at Redstone Arsenal under von Braun's
leadership to develop the Jupiter intermediate range ballistic missile. A version of the
Redstone rocket, known as the Jupiter C, on January 31, 1958, was used to launch
America's first satellite, Explorer I. Three years later, Mercury Redstones launched Alan
Sheppard and Gus Grissom on suborbital space flights, paving the way for John Glenn's
first orbital flight.
In 1958, NASA was established, and, two years later, von Braun, his team, and the
entire Army Ballistic Missile Agency were transferred to NASA to become the nucleus of
the agency's space program. The Army Missile Command, which owns Redstone
Arsenal, continued its vital national defense mission after the transfer of ABMA to
NASA, chalking up a remarkable number of successful programs to augment America's
land power. MICOM's successes include the Pershing II, the NIKE weapons systems,
the HAWK system, Improved HAWK, Corporal, Sergeant, Lance and Chaparral, to
name a few.
Pursuing a separate course, that of developing rockets for space exploration, the
Marshall Space Flight Center's past quarter century has been a time of superlatives. In
1961, almost as Alan Sheppard was drying off from his landing in the Atlantic following
his riding a Marshall-designed Redstone rocket on a sub-orbital flight which made him
the first American in space, President Kennedy committed this nation to being first on
the Moon. NASA's Marshall Center was charged with developing the family of giant
rockets which would take us there.
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The Saturn rockets developed at Marshall to support the Apollo program and to honor
President Kennedy's pledge were, at the time, the most powerful space launch vehicles
yet to have been invented. Engineers, scientists, contractors, and other support
personnel built well. On July 20, 1969, a transmission from the Moon's Sea of
Tranquility reported "the Eagle has landed."
Marshall's Saturn rockets first took us around the Moon, then to its cratered surface.
Marshall-developed lunar excursion vehicles --the ungainly Moon Buggies -- carried
astronauts on far-ranging excursions in pursuit of samples of lunar soil and rock.
Closer to home, the team at Marshall developed America's first space station -- Skylab.
Built to replace the upper stage of a Saturn V moon rocket, the Skylab module was
successfully placed in orbit early on May 14, 1973. Placing Skylab in orbit marked a
major transition in the story of rocketry. Up until Skylab, the rocket had been the star -the featured attraction. The focus had been on the up and down -- launch and recovery.
Skylab, in essence stole the show. For the first time, space became a place in which to
live and work. Flying aboard a rocket was about the Earth side equivalent of driving the
family car to work. Just as having to drive to work is only incidental to work itself -- flying
aboard a rocket became secondary to the work done once Skylab had been reached.
The rocket, simply stated, became a means to an end -- the end in this case being the
opportunity to learn to live and work in space.
A rash of malfunctions plagued Skylab's early days –problems which tested the
resourcefulness of the entire NASA team. The problems were overcome, however, and
Skylab went on to become one of Marshall’s proudest achievements.
A Marshall-developed Saturn I-B also carried aloft America's half of the first joint U.S.Soviet space endeavors, the Apollo-Soyuz project. After Apollo, the team at Marshall
tackled designing revolutionary national space transportation system, which came to be
known simply as "The Space Shuttle."
The space shuttle main engines were among the most powerful, most sophisticated
devices ever invented. They represented a quantum leap in technology advancement
over the engines which powered the Saturn V. Each of the three main engines in tail of
the shuttle provided almost a half-million pounds of thrust, a thrust equal to that
produced by all eight of the Saturn I's first stage engines. Unlike most previous rocket
engines, which were designed to be used only once-- and then for only a few minutes -the space shuttle's main engines were designed to be used again and again, for up to
7.5 hours. The thrust to weight ratio for these engines were the best in the world --each
engine weighs less than 7,000 pounds but put out the power equivalent of seven
Hoover Dams!
Twenty-four successful flights of the space shuttle lulled America into a sense of
complacency. Shuttle launches became routine – a ho-hum event which had to
scramble for an inch or two on page 2 until the Challenger disaster. The time since the
loss of Challenger had been the busiest in the history of Marshall Spaceflight Center.
Teams of experts have been organized to find and fix the problems which led to the
accident. Investigation quickly focused on a defective joint in the space shuttle's solid
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rocket motors. Rocket propulsion experts devised a number of modifications tithe solid
rocket motor design to remedy the fault.
A vigorous test program was undertaken to show the problems had been solved. The
disaster-enforced hiatus in shuttle operations has given Marshall -- and other NASA
installations -- an opportunity to address other shuttle-related concerns. Major steps
were made at enhancing the reliability and safety of the turbine blades and turbopumps
in the shuttle's main engines. An escape system was implemented for the shuttle crew
during leveled flight and Improvements were made to the orbiter's landing gear and
brakes. On September 29, 1988, the shuttle program returned to flight with the launch
of STS-26R and continued until its retirement in 2011.
NASA continues to use more expendable launch vehicles on missions that do not
require the shuttle's unique capabilities, and continues to look into development of a
new generation of heavy lift launch vehicles. These will become the next chapter in the
story of rocketry -- a story whose first chapters were written more than 2,400 years ago.
No one can say where our path will lead or when the last chapter in this history will be
written.
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Chapter 2
Basic Concepts of Orbital Spaceflight
Introduction
The common conception of orbital flight is that of an object flying through space,
following a circular or elliptical path around another object. That description is accurate,
but incomplete. It leaves out the most important element of orbital flight: gravity.
An orbit is the path of an object in space as it moves around another object due to the
force of gravity. Orbital flight is not powered flight, it is the result of a careful balance
between gravity and momentum. A rocket is used to carry a spacecraft into space, but
once the spacecraft has achieved sufficient altitude and velocity, the rocket engine is
turned off, and often discarded. At this point, the spacecraft will be held in orbit by
gravity - the same force that holds the moon in orbit and makes the planets revolve
around the sun.
Newton's Cannon
In 1687, in a book titled Principia Mathematica, the physicist Isaac Newton presented
the first explanation of orbital motion. It is still one of the simplest and clearest
explanations available.
Newton imagined a canon on top of a very high mountain. A cannonball is shot out,
which travels for a distance, but eventually gravity pulls it down and it strikes the
ground. On a second shot, more gunpowder is used, and the cannonball travels further
before striking the ground. In each case, the cannonball follows a curved path to the
ground.
The surface of the earth is also curved. If enough gunpowder were used, Newton
suggested, the curvature of the cannonball's trajectory would be the same as the
curvature of the earth. The cannonball would be falling, but it would never reach the
ground. The cannonball would "fall" all the way around the world and strike the
cannoneer in the back of the head.
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Basic Mechanics and Newton
In order to understand the discoveries of Newton, we must have an understanding of
three basic quantities: (1) velocity, (2) acceleration, and (3) force. Vectors are quantities
that require not only a magnitude, but a direction to specify them completely. Let us
illustrate by first citing some examples of quantities that are not vectors. The number of
gallons of gasoline in the fuel tank of your car is an example of a quantity that can be
specified by a single number---it makes no sense to talk about a "direction" associated
with the amount of gasoline in a tank. Such quantities, which can be specified by giving
a single number (in appropriate units), are called scalars. Other examples of scalar
quantities include the temperature, your weight, or the population of a country; these are
scalars because they are completely defined by a single number (with appropriate
units).
However, consider a velocity. If we say that a car is going 70 km/hour, we have not
completely specified its motion, because we have not specified the direction that it is
going. Thus, velocity is an example of a vector quantity. A vector generally requires
more than one number to specify it; in this example we could give the magnitude of the
velocity (70km/hour), a compass heading to specify the direction (say 30 degrees from
North), and an number giving the vertical angle with respect to the Earth's surface (zero
degrees).
Velocity and Acceleration
Let us now give a precise definition of velocity and acceleration. They are vectors, so
we must give a magnitude and a direction for them. The velocity v and the acceleration
a are defined in the following illustration,
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This illustration also demonstrates graphically that velocity (and therefore acceleration)
is a vector: the direction of the rock's velocity is certainly of critical interest to the person
standing under the rock in the two illustrations!
Uniform Circular Motion is Accelerated Motion
Notice that velocity, which is a vector, is changed if either its magnitude or its direction
is changed. Thus, acceleration occurs when either the magnitude or direction of the
velocity (or both) are altered.
Circular motion (even at uniform angular velocity) implies a continual acceleration,
because the direction of the velocity is continuously changing, even if its magnitude is
constant. This point, that motion on a curved path is accelerated motion, will prove
crucial to our subsequent understanding of motion in gravitational fields.
Newton's Principles of Mechanics
Sir Isaac Newton realized that the force that makes apples fall to the ground is the same
force that makes the planets "fall" around the sun. Newton had been asked to address
the question of why planets move as they do. He established that a force of attraction
toward the sun becomes weaker in proportion to the square of the distance from the
sun.
Newton postulated that the shape of an orbit should be an ellipse. Circular orbits are
merely a special case of an ellipse where the foci are coincident. Newton described his
work in the Mathematical Principles of Natural Philosophy (often called simply the
Principia), which he published in 1685. Newton gave his laws of motion as follows:
1. Every body continues in a state of rest, or of uniform motion in a straight line,
unless it is compelled to change that state by forces impressed upon it.
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2. The change of motion (linear momentum) is proportional to the force impressed
and is made in the direction of the straight line in which that force is impressed.
3. To every action there is always an equal and opposite reaction; or, the mutual
actions of two bodies upon each other are always equal, and act in opposite
directions.
(Notice that Newton's laws describe the behavior of inertia, they do not explain what the
nature of inertia is. This is still a valid question in physics, as is the nature of mass.)
There are three ways to modify the momentum of a body. The mass can be changed;
the velocity can be changed (acceleration), or both.
Acceleration
Force (F) equals change in velocity (acceleration, A) times mass (M):
F = MA
Acceleration may be produced by applying a force to a mass (such as a spacecraft). If
applied in the same direction as an object's velocity, the object's velocity increases in
relation to an un-accelerated observer. If acceleration is produced by applying a force in
the opposite direction from the object's original velocity, it will slow down relative to an
un-accelerated observer. If the acceleration is produced by a force at some other angle
to the velocity, the object will be deflected. These cases are illustrated below.
The world standard of mass is the kilogram, whose definition is based on the mass of a
metal cylinder kept in France. Previously, the standard was based upon the mass of
one cubic centimeter of water being one gram, which is approximately correct. The
standard unit of force is the Newton, which is the force required to accelerate a 1-kg
mass 1 m/sec2 (one meter per second per second). A Newton is equal to the force from
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the weight of about 100 g of water in Earth's gravity. That's about half a cup. A dyne is
the force required to accelerate a 1-g mass 1 cm/s2.
Acceleration in Orbit
Newton's first law describes how, once in motion, planets remain in motion. What it
does not do is explain how the planets are observed to move in nearly circular orbits
rather than straight lines. Enter the second law. To move in a curved path, a planet
must have acceleration toward the center of the circle. This is called centripetal
acceleration and is supplied by the mutual gravitational attraction between the sun and
the planet.
Momentum
Momentum is the tendency of a moving object to stay in motion. When you are in a
moving vehicle that stops suddenly, it is your own momentum that throws you forward.
Momentum is related to velocity. The greater velocity an object has the more
momentum it has.
This is the definition of an orbit. The momentum of the cannonball and the pull of gravity
are balanced. The cannonball is in a continuous state of free fall, and will remain so until
affected by another force (Newton's theoretical example assumes the cannon is high
enough that the resistance of earth's atmosphere can be ignored).
Orbital Velocity
The velocity needed to maintain a given orbit is called orbital velocity. The velocity
required depends on the altitude of the orbit. The closer an orbiting spacecraft is to
earth, the higher the velocity required to remain in that orbit. To give you an idea of the
speeds required, maintaining a circular orbit at an altitude of 100 miles requires a
velocity of 17,478 miles per hour.
Getting Into Orbit
Rockets are used to give a spacecraft sufficient altitude and velocity to achieve orbit. A
rocket is launched vertically, but as it ascends it adjusts its trajectory to become more
horizontal. This puts the spacecraft in the proper position for orbit.
When the desired altitude and velocity have been reached, the rocket engine is cut off.
If altitude and velocity are correct, the spacecraft will be in orbit. The path of the orbit is
determined by the spacecraft's velocity.
Once in the desired orbit, the spacecraft's velocity must be carefully monitored.
Changes in velocity will change the path of the orbit. In fact, this is how a spacecraft in
orbit returns to earth. Small rockets, known as retrorockets, fire briefly against the
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direction of flight, changing the velocity of the spacecraft. This changes the path of the
spacecraft's orbit to one that will bring the spacecraft back into the atmosphere.
Weightlessness
Weightlessness is an interesting side effect of being in orbit. A common misconception
is that objects in space float because there is no gravity in space. The term
"weightlessness" reinforces this idea, but the word is a misnomer. In fact, the pull of
gravity on objects in orbit is diminished only by a small amount. For example, an
astronaut that weighs 160 pounds on earth will weigh about 140 pounds while in orbit.
The astronaut feels weightless because he is in a constant state of free fall. Few of us
have ever been in free fall long enough to notice, but a person in free fall doesn't feel his
own weight. Only when something is opposing the pull of gravity do we experience the
feeling of weight. On earth, gravity pulls us into something - the ground or the floor, for
example - and this opposition to gravity is why we feel weight.
Everything in orbit with the astronaut floats around him because it all is continuously
falling at the same rate as he is. Although travelling at thousands of miles per hour,
these objects are not moving at all relative to each other, and they appear to be floating
in space, or "weightless".
Apogee and Perigee
Not all orbits are circular. Many take the shape of an ellipse. The point of highest
altitude in an elliptical orbit is called the apogee, and the lowest point is the perigee. A
circular orbit is just a special case of an elliptical orbit where the apogee and perigee
are the same.
In a circular orbit, the velocity of an object is constant, while velocity varies throughout
an elliptical orbit. In such an orbit, velocity is greatest at the perigee, and lowest at
apogee.
Atmospheric drag
Earth's atmosphere doesn't end suddenly at the edge of space. It gradually thins out. A
spacecraft in low-earth orbit will experience drag, or resistance, from the thin
atmosphere it encounters. This drag will cause an orbit to deteriorate, or decay, over
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time. All current manned space activity takes place in low-earth orbit, and must take
drag into account. The International Space Station, for example, must periodically be
boosted to its proper altitude and velocity to compensate for the effects of atmospheric
drag.
What is an Orbit?
An orbit is the movement of a (small body) object around a usually much larger body
(e.g. planet or moon) in space in the shape of an elliptical. The elliptical shape is
determined by the velocity of the small body and its distance to the center of gravity of
the larger body. The location of the closest and farthest point of an orbiting object
around a body is fixed irrespective of the larger bodies’ rotation. This brings up the
difference between a revolution and an orbit. A revolution is defined relative to a
position on the body that you are orbiting.
For example since the Earth turns around its axis a revolution around the Earth will be
more than an orbit if you orbit in the direction of the rotation. So the time to complete an
orbit may not be the same as the revolution time. An object in orbit is continuously
falling towards the body that it is orbiting but because of its velocity never reaches the
surface. When an object moves in an elliptical orbit there will be a point where it is
closest (periapsis) to the body it orbits and a point where it is farthest (apoapsis).
When in apoapsis the object has its slowest speed in the orbit and will start falling back
to the body and exchange potential energy for kinetic energy. If the object travels to
slow at a certain distance then its path will intersect with the object it was orbiting or if it
travels to fast at a certain distance then it will never return (a.k.a escape velocity).
Depending on the body that is being orbited the names of the closest and farthest
approach change their suffix (e.g. apogee and perigee when orbiting the Earth, aphelion
and perihelion when orbiting the Sun and apolune and perilune when orbiting the moon,
Gravitation and Mechanics
Gravitation is the mutual attraction of all masses in the universe. While its effect
decreases in proportion to distance squared, it nonetheless applies, to some extent,
regardless of the sizes of the masses or their distance apart.
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The concepts associated with planetary motions developed by Johannes Kepler (15711630) describe the positions and motions of objects in our solar system. Isaac Newton
(1643-1727) later explained why Kepler's laws worked, by showing they depend on
gravitation. Albert Einstein (1879-1955) posed an explanation of how gravitation works
in his general theory of relativity.
In any solar system, planetary motions are orbits gravitationally bound to a star. Since
orbits are ellipses, a review of ellipses follows.
Ellipses
An ellipse is a closed plane curve generated in such a way that the sum of its distances
from two fixed points (called the foci) is constant. In the illustration below, the sum of
Distance A + Distance B is constant for any point on the curve.
Ellipse Foci
An ellipse results from the intersection of a circular cone and a plane cutting completely
though the cone. The maximum diameter is called the major axis. It determines the size
of an ellipse. Half the maximum diameter, the distance from the center of the ellipse to
one end, is called the semi-major axis.
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The shape of an ellipse is determined by how close together the foci are in relation to
the major axis. Eccentricity is the distance between the foci divided by the major axis. If
the foci coincide, the ellipse is a circle. Therefore, a circle is an ellipse with an
eccentricity of zero.
Motion in a Circular Orbit
Kepler's Laws
Johannes Kepler's laws of planetary motion are:
1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals
of time.
3. The square of the orbital period of a planet is directly proportional to the cube of
the semi-major axis of its orbit.
The major application of Kepler's first law is to precisely describe the geometric shape
of an orbit: an ellipse, unless perturbed by other objects. Kepler's first law also informs
us that if a comet, or other object, is observed to have a hyperbolic path, it will visit the
sun only once, unless its encounter with a planet alters its trajectory again.
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Kepler's second law addresses the velocity of an object in orbit. Conforming to this law,
a comet with a highly elliptical orbit has a velocity at closest approach to the sun that is
many times its velocity when farthest from the sun. Even so, the area of the orbital
plane swept is still constant for any given period of time.
Kepler's third law describes the relationship between the masses of two objects
mutually revolving around each other and the determination of orbital parameters.
Consider a small star in orbit about a more massive one. Both stars actually revolve
about a common center of mass, which is called the barycenter. This is true no matter
what the size or mass of each of the objects involved. Measuring a star's motion about
its barycenter with a massive planet is one method that has been used to discover
planetary systems associated with distant stars.
Obviously, these statements apply to a two-dimensional picture of planetary motion,
which is all that is needed for describing orbits. A three-dimensional picture of motion
would include the path of the sun through space.
Gravity Gradients & Tidal Forces
Gravity's strength is inversely proportional to the square of the objects' distance from
each other. For an object in orbit about a planet, the parts of the object closer to the
planet feel a slightly stronger gravitational attraction than do parts on the other side of
the object. This is known as gravity gradient. It causes a slight torque to be applied to
any orbiting mass which has asymmetric mass distribution (for example, is not
spherical), until it assumes a stable attitude with the more massive parts pointing toward
the planet. An object whose mass is distributed like a bowling pin would end up in an
attitude with its more massive end pointing toward the planet, if all other forces were
equal.
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Consider the case of a fairly massive body such as our Moon in Earth orbit. The gravity
gradient effect has caused the Moon, whose mass is unevenly distributed, to assume a
stable rotational rate which keeps one face towards Earth at all times, like the bowling
pin described above.
The Moon's gravitation acts upon the Earth's oceans and atmosphere, causing two
bulges to form. The bulge on the side of Earth that faces the moon is caused by the
proximity of the moon and its relatively stronger gravitational pull on that side. The bulge
on the opposite side of Earth results from that side being attracted toward the moon less
strongly than is the central part of Earth. Earth's atmosphere and crust are also affected
to a smaller degree. Other factors, including Earth's rotation and surface roughness,
complicate the tidal effect. On planets or satellites without oceans, the same forces
apply, causing slight deformations in the body. This mechanical stress can translate into
heat, as in the case of Jupiter's volcanic moon Io.
Reference Systems in Navigation
Spatial coordinates and timing conventions are adopted in order to consistently identify
locations and motions of an observer, of natural objects in the solar system, and of
spacecraft traversing interplanetary space or orbiting planets or other bodies. Without
these conventions it would be impossible to navigate the solar system.
Terrestrial Coordinates
A great circle is an imaginary circle on the surface of a sphere whose
center is the center of the sphere. Great circles that pass through both the
north and south poles are called meridians, or lines of longitude. For any
point on the surface of Earth a meridian can be defined.
The prime meridian, the starting point measuring the east-west locations of other
meridians, marks the site of the old Royal Observatory in Greenwich, England.
Longitude is expressed in degrees, minutes, and seconds of arc from 0 to 180 degrees
eastward or westward from the prime meridian. For example, downtown Pasadena,
California, is located at 118 degrees, 8 minutes, 41 seconds of arc west of the prime
meridian: 118° 8' 41" W.
The starting point for measuring north-south locations on Earth is the
equator, a great circle which is everywhere equidistant from the poles.
Circles in planes parallel to the equator define north-south measurements
called parallels, or lines of latitude. Latitude is expressed as an arc
subtended between the equator and the parallel, as seen from the center
of the Earth. Downtown Pasadena is located at 34 degrees, 08 minutes,
44 seconds latitude north of the equator: 34° 08' 44" N.
One degree of latitude equals approximately 111 km on the Earth's surface, and by
definition exactly 60 nautical miles. Because meridians converge at the poles, the length
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of a degree of longitude varies from 111 km at the equator to 0 at the poles where
longitude becomes a point.
Terrestrial Coordinates Grid
Rotation and Revolution
"Rotation" refers to an object's spinning motion about its own axis. "Revolution" refers
the object's orbital motion around another object. For example, Earth rotates on its own
axis, producing the 24-hour day. Earth revolves about the Sun, producing the 365-day
year. A satellite revolves around a planet.
Earth's Rotation
The Earth rotates on its axis relative to the sun every 24.0 hours mean solar time, with
an inclination of 23.45 degrees from the plane of its orbit around the sun. Mean solar
time represents an average of the variations caused by Earth's non-circular orbit. Its
rotation relative to "fixed" stars (sidereal time) is 3 minutes 56.55 seconds shorter than
the mean solar day, the equivalent of one solar day per year.
Precession of Earth's Axis
Forces associated with the rotation of Earth cause the planet to be slightly oblate,
displaying a bulge at the equator. The moon's gravity primarily, and to a lesser degree
the sun's gravity, act on Earth's oblateness to move the axis perpendicular to the plane
of Earth's orbit. However, due to gyroscopic action, Earth's poles do not "right
themselves" to a position perpendicular to the orbital plane. Instead, they precess at 90
degrees to the force applied. This precession causes the axis of Earth to describe a
circle having a 23.4 degree radius relative to a fixed point in space over about 26,000
years, a slow wobble reminiscent of the axis of a spinning top swinging around before it
falls over.
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Precession of Earth's Axis Over 26,000 Years
Because of the precession of the poles over 26,000 years, all the stars, and other
celestial objects, appear to shift west to east at the rate of .014 degree each year (360
degrees in 26,000 years). This apparent motion is the main reason for astronomers as
well as spacecraft operators to refer to a common epoch such as J2000.0.
At the present time in Earth's 26,000 year precession cycle, a bright star happens to be
very close, less than a degree, from the north celestial pole. This star is called Polaris,
or the North Star. Stars do have their own real motion, called proper motion. In our
vicinity of the galaxy, only a few bright stars exhibit a large enough proper motion to
measure over the course of a human lifetime, so their motion does not generally enter
into spacecraft navigation. Because of their immense distance, stars can be treated as
though they are references fixed in space. (Some stars at the center of our galaxy,
though, display tremendous proper motion speeds as they orbit close to the massive
black hole located there.)
Nutation
Superimposed on the 26,000-year precession is a small nodding motion with a period of
18.6 years and an amplitude of 9.2 arc seconds. This nutation can trace its cause to the
5 degree difference between the plane of the Moon's orbit, the plane of the Earth's orbit,
and the gravitational tug on one other.
Revolution of Earth
Earth revolves in orbit around the sun in 365 days, 6 hours, 9 minutes with reference to
the stars, at a speed ranging from 29.29 to 30.29 km/s. The 6 hours, 9 minutes adds up
to about an extra day every fourth year, which is designated a leap year, with the extra
day added as February 29th. Earth's orbit is elliptical and reaches its closest approach
to the sun, a perihelion of 147,090,000 km, on about January fourth of each year.
Aphelion comes six months later at 152,100,000 km.
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Shorter-term Polar Motion
Aside from the long-term motions, the Earth's rotational axis and poles have two shorter
periodic motions. One, called the Chandler wobble, is a free nutation with a period of
about 435 days. There is also a yearly circular motion, and a steady drift toward the
west caused by fluid motions in the Earth's mantle and on the surface. These motions
are tracked by the International Earth Rotation and Reference Systems Service, IERS.
Movement of Earth's rotational poles 2001 to 2006, and the mean pole
location from the year 1900 to 2000. Units are milliarcseconds .
Epochs
Because we make observations from Earth, knowledge of Earth's natural motions is
essential. As described above, our planet rotates on its axis daily and revolves around
the sun annually. Its axis precesses and nutates. Even the "fixed" stars move about on
their own. Considering all these motions, a useful coordinate system for locating stars,
planets, and spacecraft must be pinned to a single snapshot in time. This snapshot is
called an epoch.
By convention, the epoch in use today is called J2000.0, which refers to the mean
equator and equinox of year 2000, nominally January 1st 12:00 hours Universal Time
(UT). The "J" means Julian year, which is 365.25 days long. Only the 26,000-year
precession part of the whole precession/nutation effect is considered, defining the mean
equator and equinox for the epoch.
The last epoch in use previously was B1950.0 - the mean equator and equinox of 1949
December 31st 22:09 UT, the "B" meaning Besselian year, the fictitious solar year
introduced by F. W. Bessell in the nineteenth century. Equations are published for
interpreting data based on past and present epochs.
Making Sense
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Given an understanding of the Earth's suite of motions -- rotation on axis, precession,
nutation, short-term polar motions, and revolution around the sun -- and given
knowledge of an observer's location in latitude and longitude, meaningful observations
can be made. For example, to measure the precise speed of a spacecraft flying to
Saturn, you have to know exactly where you are on the Earth's surface as you make the
measurement, and then subtract out the Earth's motions from that measurement to
obtain the spacecraft's speed. The same applies if you are trying to measure the proper
motion of a distant star -- or a star's subtle wobble, to reveal a family of planets.
Spacecraft Navigation
Spacecraft navigation comprises two aspects: (1) knowledge and prediction of
spacecraft position and velocity, which is orbit determination, and (2) firing the rocket
motor to alter the spacecraft's velocity, which is flight path control.
A spacecraft on its way to a distant planet is actually in orbit about the sun, and the
portion of its solar orbit between launch and destination is called the spacecraft's
trajectory. Orbit determination involves finding the spacecraft's orbital elements and
accounting for perturbations to its natural orbit. Flight path control involves commanding
the spacecraft's propulsion system to alter the vehicle's velocity. Comparing the
accurately determined spacecraft's trajectory with knowledge of the destination object's
orbit is the bases for determining what velocity changes are needed.
Since the Earth's own orbital parameters and inherent motions are well known, the
measurements we make of the spacecraft's motion as seen from Earth can be
converted into the sun-centered or heliocentric orbital parameters needed to describe
the spacecraft's trajectory. The meaningful measurements we can make from Earth of
the spacecraft's motion are:



Its distance or range from Earth,
The component of its velocity that is directly toward or away from Earth, and
To the extent discussed below, its position in Earth's sky.
Some spacecraft can generate a fourth type of navigational data,

Optical navigation, wherein the spacecraft uses its imaging instrument to view a
target planet or body against the background stars.
By repeatedly acquiring these three or four types of data, a mathematical model may be
constructed and maintained describing the history of a spacecraft's location in threedimensional space over time. The navigation history of a spacecraft is incorporated not
only in planning its future maneuvers, but also in reconstructing its observations of a
planet or body it encounters. This is essential to constructing SAR (synthetic aperture
radar) images, tracking the spacecraft's passage through planetary magnetospheres or
rings, and interpreting imaging results.
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Another use of navigation data is the creation of predicts, which are data sets predicting
locations in the sky and radio frequencies for the Deep Space Network, DSN, to use in
acquiring and tracking the spacecraft.
Navigation Data Acquisition
The basic factors involved in acquiring the types of navigation data mentioned above
are described below.
Spacecraft Velocity Measurement
Measurements of the Doppler shift of a coherent downlink carrier provide the radial
component of a spacecraft's Earth-relative velocity. Doppler is a form of the tracking
data type, TRK, provided by the DSN.
Spacecraft Distance Measurement
A uniquely coded ranging pulse can be added to the uplink to a spacecraft and its
transmission time recorded. When the spacecraft receives the ranging pulse, it returns
the pulse on its downlink. The time it takes the spacecraft to turn the pulse around
within its electronics is known from pre-launch testing. For example, Cassini takes 420
nanoseconds, give or take 9 ns. There are many other calibrated delays in the system,
including the several microseconds needed to go from the computers to the antenna
within DSN, which is calibrated prior to each use. When the pulse is received at the
DSN, its true elapsed time at light-speed is determined, corrections are applied for
known atmospheric effects, and the spacecraft's distance is then computed. Ranging is
also a type of TRK data provided by the DSN. Distance may also be determined using
angular measurement.
Spacecraft Angular Measurement
The spacecraft's position in the sky is expressed in the angular quantities Right
Ascension and Declination. While the angles at which the DSN antennas point are
monitored with an accuracy of thousandths of a degree, they are not accurate enough
to be used in determining a distant interplanetary spacecraft's position in the sky for
navigation. DSN tracking antenna angles are useful only for pointing the antenna to the
predicts given for acquiring the spacecraft's signal.
Fairly accurate determination of Right Ascension is a direct byproduct of measuring
Doppler shift during a DSN pass of several hours. Declination can also be measured by
the set of Doppler-shift data during a DSN pass, but to a lesser accuracy, especially
when the Declination value is near zero, i.e., near the celestial equator. Better accuracy
in measuring a distant spacecraft's angular position can be obtained by:
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
VLBI
Extremely accurate angular measurements can be provided by a process
independent from Doppler and range, VLBI, Very Long Baseline Interferometry.
A VLBI observation of a spacecraft begins when two DSN stations on different
continents (separated by a VLB) track a single spacecraft simultaneously. Highrate recordings are made of the downlink's wave fronts by each station, together
with precise timing data. After a few minutes,
both DSN antennas slew directly to the position
of a quasar, which is an extragalactic object
whose position on the plane of the sky is known
to a high precision. Recordings are made of the
quasar's radio-noise wave fronts.
Correlation and analysis of the recorded wave fronts yields a very precise
triangulation from which the angular position may be determined by direct
comparison to the position of a quasar whose RA and DEC are well known. VLBI
is considered a distinct DSN data type, different from TRK and TLM. This VLBI
observation of a spacecraft is called a "delta DOR," DOR meaning differenced
one-way ranging, and the "delta" meaning the difference between spacecraft and
quasar positions.

Precision Ranging
Precision ranging refers to a set of procedures
to ensure that range measurements are
accurate to about 1 meter. Knowledge of the
spacecraft's Declination can be improved with
Range measurements from two stations that
have a large north-south displacement, for example between Spain and
Australia, via triangulation.

Differenced Doppler
Differenced Doppler can provide a measure of a spacecraft's changing threedimensional position. To visualize this, consider a spacecraft orbiting a distant
planet. If the orbit is in a vertical plane exactly edge on to you at position A, you
would observe the downlink to take a higher frequency as it travels towards you.
As it recedes away from you to go behind the planet, you observe a lower
frequency.
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Now, imagine a second observer way across the Earth, at position B.
Since the orbit plane is not exactly edge-on as that observer sees it,
that person will record a slightly different Doppler signature. If you
and the other observer were to compare notes and difference your
data sets, you would have enough information to determine both the
spacecraft's changing velocity and position in three-dimensional
space.
Two DSSs separated by a large baseline can do basically this. One
DSS provides an uplink to the spacecraft so it can generate a
coherent downlink, and then it receives two-way. The other DSS
receives a three-way coherent downlink. The differenced data sets
are frequently called "two-way minus three-way."
These techniques, combined with high-precision knowledge of DSN Station positions, a
precise characterization of atmospheric refraction, and extremely stable frequency and
timing references (F&T, which is another one of the DSN data types), makes it possible
for DSN to measure spacecraft velocities accurate to within hundredths of a millimeter
per second, and angular position on the sky to within 10 nano-radians.
Optical Navigation
Spacecraft that are equipped with imaging instruments can use them to observe the
spacecraft's destination planet or other body, such as a satellite, against a known
background star field. These images are called opnav images. The observations are
carefully planned and uplinked far in advance as part of the command sequence
development process. The primary body often appears overexposed in an opnav, so
that the background stars will be clearly visible. When the opnav images are downlinked
in telemetry (TLM) they are immediately processed by the navigation team.
Interpretation of opnavs provides a very precise data set useful for refining knowledge
of a spacecraft's trajectory as it approaches a target. Note that this form of navigation
data resides in the TLM data type.
Orbit Determination
The process of spacecraft orbit determination solves for a description of a spacecraft's
orbit in terms of a state vector (position and velocity) at an epoch, based upon the types
of observations and measurements described above. If the spacecraft is en-route to a
planet, the orbit is heliocentric; if it is in orbit about a planet, the orbit determination is
made with respect to that planet. Orbit determination is an iterative process, building
upon the results of previous solutions. Many different data inputs are selected as
appropriate for input to computer software, which uses the laws of Newton. The inputs
include the various types of navigation data described above, as well as data such as
the mass of the sun and planets, their ephemeris and barycentric movement, the effects
of the solar wind and other non-gravitational effects, a detailed planetary gravity field
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model (for planetary orbits), attitude management thruster firings, atmospheric friction,
and other factors.
Flight Path Control
Trajectory Correction Maneuvers: Once a spacecraft's solar or planetary orbital
parameters are known, they may be compared to those desired by the project. To
correct any discrepancy, a Trajectory Correction Maneuver (TCM) may be planned and
executed. This adjustment involves computing the direction and magnitude of the vector
required to correct to the desired trajectory. An opportune time is determined for making
the change.
For example, a smaller magnitude of change would be required immediately following a
planetary flyby, than would be required after the spacecraft had flown an undesirable
trajectory for many weeks or months. The spacecraft is commanded to rotate to the
attitude in three-dimensional space computed for implementing the change, and its
thrusters are fired for a determined amount of time. TCMs generally involve a velocity
change (delta-V) on the order of meters, or sometimes tens of meters, per second. The
velocity magnitude is necessarily small due to the limited amount of propellant typically
carried.
Orbit Trim Maneuvers: Small changes in a spacecraft's orbit around a planet may be
desired for the purpose of adjusting an instrument's field-of-view footprint, improving
sensitivity of a gravity field survey, or preventing too much orbital decay. Orbit Trim
Maneuvers (OTMs) are carried out generally in the same manner as TCMs. To make a
change increasing the altitude of periapsis, an OTM would be designed to increase the
spacecraft's velocity when it is at apoapsis. To decrease the apoapsis altitude, an OTM
would be executed at periapsis, reducing the spacecraft's velocity. Slight changes in the
orbital plane's orientation may also be made with OTMs. Again, the magnitude is
necessarily small due to the limited amount of propellant spacecraft typically carry.
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Chapter 3
Launch Phase
Launch Vehicles
The launch of a spacecraft comprises a period of powered flight during which the
vehicle rises above Earth's atmosphere and accelerates at least to orbital velocity.
Powered flight ends when the rocket's last stage burns out, and the spacecraft
separates and continues in freefall. If the spacecraft has achieved escape from Earth's
gravitation, rather than entering Earth orbit, its flight path will then be purely a solar orbit
of some description (since the launch pad was also in solar orbit).
To date, the only practical way to produce the propulsive energy needed for launching
spacecraft from Earth has been by combustion of chemical propellants. Mass drivers,
however, may be useful in the future for launching material from the Moon or other
small airless bodies. Ion-engine propulsion, whether powered by photovoltaics or
nuclear reactors, is useful not for launch phase, but for gently but steadily accelerating
spacecraft that are already in Earth orbit or beyond.
There are two groups of propellants for chemical-combustion rockets, liquids and solids.
Many spacecraft launches involve the use of both types of rockets, for example the solid
rocket boosters attached to liquid-propelled rockets. Hybrid rockets, which use a
combination of solid and liquid, are also being developed. Solid rockets are generally
simpler than liquid, but they cannot be shut down once ignited. Liquid and hybrid
engines may be shut down after ignition and, in some designs, can be re-ignited as
needed.
Expendable launch vehicles, ELV, are used once. The U.S. Space Transportation
System, STS, or Space Shuttle, was designed as a reusable system to reach Low Earth
Orbit. Most of its components are refurbished and reused multiple times. Upper Stage
Rockets can be selected for placement atop the launch vehicle's lower stages (or within
the Shuttle's cargo bay) to provide the performance needed for a particular payload.
The Centaur high-energy upper stage has been a choice for robotic missions to the
Moon and planets for many years. One useful measure of performance for comparison
among launch vehicles is the amount of mass it can lift to Geosynchronous Transfer
Orbit, GTO.
Delta
Delta is a family of two- or three-stage liquid-propelled ELVs that use multiple strap-on
solid rocket boosters in several configurations. Originally made by McDonnell Douglas,
it is now produced and launched by the Boeing and Lockheed Martin joint venture,
United Launch Alliance (ULA).
The Delta II, whose liquid-propellant engines burn kerosene and liquid oxygen (LOX),
can be configured as two- or three-stage vehicles and with three, four or nine strap-on
solid rocket graphite epoxy motors. Delta II payload delivery options range from about
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891 to 2,142 kg to geosynchronous transfer orbit (GTO) and 2.7 to 6.0 metric tons to
low-Earth orbit (LEO). Two-stage Delta II rockets typically fly LEO missions, while threestage Delta II vehicles generally deliver payloads to GTO, or are used for deep-space
explorations such as NASA's missions to Mars, a comet or near-Earth asteroids.
The Delta IV family of launch vehicles is capable of carrying payloads ranging from
4,210 kg to 13,130 kg to GTO. The three Delta IV Medium-Plus vehicles use a single
common booster core that employs the RS-68 liquid hydrogen/liquid oxygen engine,
which produces 2,949 kN (663,000 lb) of thrust. They are augmented by either two or
four 1.5-meter diameter solid rocket strap-on graphite epoxy motors.
Delta's launch record (350 flights as of January 2013) includes Earth orbiters and
interplanetary missions dating back to 1960. Space Launch Complex 37, a historic
Saturn-1 launch pad at the Kennedy Space Center (KSC), has become the site for the
Delta-IV launch facility.
Titan
Titan was a family of U.S. expendable rockets used between 1959 and 2005. A total of
368 rockets of this family were launched. Titan IV, produced and launched for the U.S.
Air Force by Lockheed Martin, was the nation's most powerful ELV until it was retired in
2005. Titan IV was capable of placing 18,000 kg into LEO, over 14,000 kg into polar
orbit, or 4,500 kg into a geostationary transfer orbit (GTO). A Titan III launched the
Viking spacecraft to Mars in 1975. A Titan IV, equipped with two upgraded solid rocket
boosters and a Centaur high-energy upper stage, launched the Cassini spacecraft on its
gravity-assist trajectory to Saturn in 1997. Titan III vehicles launched JPL's Voyager 1
and 2 in 1977, and the Mars Observer spacecraft from the Kennedy Space Center
(KSC), Cape Canaveral in 1992.
A Titan IV consisted of two solid-propellant stage-zero motors, a liquid propellant 2stage core and a 16.7-ft diameter payload fairing. Upgraded 3-segment solid rocket
motors increased the vehicle's payload capability by approximately 25%. The Titan IV
configurations included a cryogenic Centaur upper stage, a solid-propellant Inertial
Upper Stage (IUS), or no upper stage. Titan IV rockets were launched from Vandenberg
Air Force Base, California, or Cape Canaveral Air Station, Florida.
Atlas
Atlas, The Atlas V rocket is an expendable launch vehicle formerly built by Lockheed
Martin. It is now built by the Lockheed Martin-Boeing joint venture United Launch
Alliance. Aerojet develops and manufactures the Atlas V boosters. The rocket, built in
Decatur, Alabama, consists of a first stage powered by kerosene and liquid oxygen,
which uses a Russian made RD-180 engine, and a liquid hydrogen liquid oxygen
powered Centaur upper stage. Some configurations also use strap-on booster rockets.
Together these components are referred to as the Atlas launch vehicle.
The Atlas-IIAS can put 3,833 kg in GTO. The Atlas-IIA was retired in December 2002,
and the Atlas II was retired in March 1998. Re-engineering the Atlas-II booster resulted
in an Atlas-III with at least a 5% improvement in performance. Atlas-V
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The Atlas-V uses a "structurally stable" design; previous Atlas models depended on a
pressurized propellant load for structural rigidity. With provisions for adding between
one and five strap-on solid rocket boosters, the Atlas-V offers the capability of placing
up to 8,670 kg in GTO.
Ariane
Today, Arianespace serves more than half the world's market for spacecraft launches to
geostationary transfer orbit (GTO). Created as the first commercial space transportation
company in 1980, Arianespace is responsible for the production, operation and
marketing of the Ariane 5 launchers.
Ariane 5, launched from the Kourou Space Center in French Guiana, entered service
with a 6.5 metric ton payload capability to geostationary orbit. In 1996, the maiden flight
of the Ariane 5 launcher ended in a failure. In 1997 Ariane 5's second test flight
succeeded, and it is now in service.
Proton
The Proton is a liquid-propellant ELV, capable of placing 20,000 kg into LEO, originally
developed by the Soviet CIS Interkosmos. It is launched from the Baykonur
Kosmodrome in Kasakhstan, with launch services marketed by International Launch
Services (a company formed in 1995 by Lockheed Martin, Khrunichev Enterprises and
NPO Energia). With an outstanding reliability record and over 200 launches, the Proton
is the largest Russian launch vehicle in operational service. It is used as a three-stage
vehicle primarily to launch large space station type payloads into low earth orbit, and in
a four-stage configuration to launch spacecraft into GTO and interplanetary trajectories.
Soyuz
The proven Soyuz launch vehicle is one of the world's most reliable and frequently used
launch vehicles. As of January 2013, almost 1,800 missions have been performed by
Soyuz launchers to orbit satellites for telecommunications, Earth observation, weather
and scientific missions, as well as for piloted flights.
Soyuz is being evolved to meet commercial market needs, offering payload lift capability
of 4,100 kg. to 5,500 kg. into a 450-km. circular orbit. Soyuz is marketed commercially
by Starsem, a French-registered company. The Russian government is directing
development of a new launcher under the Prospective Piloted Transport System (PPTS)
project.
Space Transportation System
America's Space Shuttle, as the Space Transportation System (STS) is commonly
known, was a reusable launching system whose main engines burned liquid hydrogen
and LOX. After each flight, its main components, except the external propellant tank,
could be refurbished to be used on future flights. The STS could put payloads of up to
30,000 kg in LEO. With the appropriate upper stage, spacecraft could be boosted to a
geosynchronous orbit or injected into an interplanetary trajectory. Galileo, Magellan, and
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Ulysses were launched by the STS, using an Inertial Upper Stage (IUS), which is a twostage solid-propellant vehicle. The STS could be operated to transport spacecraft to
orbit, perform satellite rescue, assemble and service the International Space Station,
and carry out a wide variety of scientific missions ranging from the use of orbiting
laboratories to small self-contained experiments. Thirty years after its first flight, the STS
was retired in 2011. Its planned successor, the Space Launch System (SLS), was
defined as part of The NASA Authorization Act of 2010 and announced in 2011, and will
carry on the duties of the Space Shuttle.
Smaller Launch Vehicles
Many NASA experiments, as well as commercial and military payloads, are becoming
smaller and lower in mass, as the art of miniaturization advances. The range of payload
mass, broadly from 100 to 1300 kg, is becoming increasingly significant as smaller
spacecraft are designed to have more operational capability. The market for launch
vehicles with capacities in this range is growing.
1. Pegasus is a small, winged solid-propellant ELV built and flown by Orbital
Sciences Corporation. It resembles a cruise missile, and is launched from under
the fuselage of an aircraft while in flight at high altitude, currently Orbital
Sciences' L-1011. Pegasus can lift 400 kg into LEO.
2. Taurus is the ground-based variant of Orbital Sciences Corporation's airlaunched Pegasus rocket. This four-stage, transportable, inertially guided, all
solid propellant vehicle is capable of putting 1,350 kg into LEO, or 350 kg in
GTO.
3. The Saturn V launch vehicle served admirably during the Apollo Program that
landed a dozen humans on the Moon. Although this vehicle was never used
again, honorable mention seems appropriate. This most powerful rocket ever
launched was developed at NASA's Marshall Space Flight Center under the
direction of Werner von Braun. Fifteen were built. Its fueled first stage alone
weighed more than an entire Space Shuttle.
Launch Sites
If a spacecraft is launched from a site near Earth's equator, it can take optimum
advantage of the Earth's substantial rotational speed. Sitting on the launch pad near the
equator, it is already moving at a speed of over 1650 km per hour relative to Earth's
center. This can be applied to the speed required to orbit the Earth (approximately
28,000 km per hour). Compared to a launch far from the equator, the equator-launched
vehicle would need less propellant, or a given vehicle can launch a more massive
spacecraft.
A spacecraft intended for a high-inclination Earth orbit has no such free ride, though.
The launch vehicle must provide a much larger part, or all, of the energy for the
spacecraft's orbital speed, depending on the inclination.
For interplanetary launches, the vehicle will have to take advantage of Earth's orbital
motion as well, to accommodate the limited energy available from today's launch
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vehicles. In the diagram below, the launch vehicle is accelerating generally in the
direction of the Earth's orbital motion (in addition to using Earth's rotational speed),
which has an average velocity of approximately 100,000 km per hour along its orbital
path.
In the case of a spacecraft embarking on a Hohmann interplanetary transfer orbit, recall
the Earth's orbital speed represents the speed at aphelion or perihelion of the transfer
orbit, and the spacecraft's velocity merely needs to be increased or decreased in the
tangential direction to achieve the desired transfer orbit.
The launch site must also have a clear pathway downrange so the launch vehicle will
not fly over populated areas, in case of accidents. The STS has the additional constraint
of requiring a landing strip with acceptable wind, weather, and lighting conditions near
the launch site as well as at landing sites across the Atlantic Ocean, in case an
emergency landing must be attempted.
Launches from the east coast of the United States (the Kennedy Space Center at Cape
Canaveral, Florida) are suitable only for low inclination orbits because major population
centers underlie the trajectory required for high-inclination launches. High-inclination
launches are accomplished from Vandenberg Air Force Base on the west coast, in
California, where the trajectory for high-inclination orbits avoids population centers. An
equatorial site is not preferred for high-inclination orbital launches. They can depart from
any latitude.
Complex ground facilities are required for heavy launch vehicles, but smaller vehicles
such as the Taurus can use simpler, transportable facilities. The Pegasus requires none
once its parent airplane is in flight.
Launch Windows
A launch window is the span of time during which a launch may take place while
satisfying the constraints imposed by safety and mission objectives. For an
interplanetary launch, the window is constrained typically within a number of weeks by
the location of Earth in its orbit around the sun, in order to permit the vehicle to use
Earth's orbital motion for its trajectory, while timing it to arrive at its destination when the
target planet is in position. The launch window may also be constrained to a number of
hours each day, in order to take best advantage of Earth's rotational motion. In the
illustration above, the vehicle is launching from a site near the Earth's terminator which
is going into night as the Earth's rotation takes it around away from the sun. If the
example in the illustration were to launch in the early morning hours on the other side of
the depicted Earth, it would be launching in a direction opposite Earth's orbital motion.
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The daily launch window may be further constrained by other factors, for example, the
STS's emergency landing site constraints. Of course, a launch which is to rendezvous
with another vehicle in Earth orbit must time its launch with the orbital motion of that
object. This has been the case with the Hubble Space Telescope repair missions
executed in December 1993, February 1997, and December 1999.
ATLO stands for Assembly, Test, and Launch Operations. This period is usually
scheduled very tightly. Spacecraft engineering components and instruments are all
delivered according to plan where the spacecraft first takes shape in a large clean room.
They are integrated and tested using computer programs for command and telemetry
very much like those that will be used in flight. Communications are maintained with the
growing spacecraft nearly continuously throughout ATLO. The spacecraft is transported
to an environmental test lab where it is installed on a shaker table and subjected to
launch-like vibrations. It is installed in a thermal-vacuum chamber to test its thermal
properties, all the while communicating with engineers. Adjustments are made as
needed in thermal blanketing, and thermal-vacuum tests may be repeated.
Then the spacecraft is transported to the launch site. The spacecraft is sealed inside an
environmentally controlled carrier for the trip, and internal conditions are carefully
monitored throughout the journey whether it is by truck or airplane.
Once at the launch site, additional testing takes place. Propellants are loaded aboard.
Any pyrotechnic devices are armed. Then the spacecraft is mated to its upper stage,
and the stack is hoisted and mated atop the launch vehicle. Clean-room conditions are
maintained atop the launch vehicle while the payload shroud (fairing) is put in place.
Pre-launch and launch operations of a spacecraft are typically carried out by personnel
at the launch site while in direct communication with persons at the Space Flight
Operations Facility at JPL. Additional controllers and engineers at a different location
may be involved with the particular upper stage vehicle, for example the Lockheed
personnel at Sunnyvale, California, monitoring performance of the inertial upper stage
(IUS) or the Centaur upper stage. The spacecraft's telecommunications link is
maintained through ground facilities close to the launch pad prior to launch and during
launch, linking the spacecraft's telemetry to controllers and engineers at JPL. Command
35
sequences must be loaded aboard the spacecraft, verified, and initiated at the proper
time prior to launch. Spacecraft health must be monitored, and the launch process
interrupted if any critical tolerances are exceeded.
As soon as the spacecraft is launched, the DSN begins tracking, acquiring the task from
the launch-site tracking station, and the mission's cruise phase is set to begin.
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Chapter 4
Cruise Phase
The cruise phase is bounded by launch phase at the beginning and encounter phase at
the end. It may be as short as a few months, or it may span years with the spacecraft
looping the sun to perform gravity-assist planetary flybys. It is a time during which
ground system upgrades and tests may be conducted, and spacecraft flight software
modifications are implemented and tested. Cruise operations for missions are typically
carried out from the Space Flight Operations Facility.
On August 5, 2011, the Juno spacecraft was launched on a mission to Jupiter. Currently
in cruise phase, it is scheduled to conduct a gravity-assist flyby of Earth on October 9,
2013 before completing its five-year journey on July 5, 2016, when it will enter orbit
around Jupiter. During its six-year primary mission, Juno will search for clues about
Jupiter's composition and study its gravity and magnetic fields.
Spacecraft Checkout and Characterization
After launch, the spacecraft is commanded to configure for cruise. Appendages that
might have been stowed in order to fit within the launch vehicle are deployed either fully
or to intermediate cruise positions. Telemetry is analyzed to determine the health of the
spacecraft, indicating how well it survived its launch. Any components that appear
questionable might be put through tests specially designed and commanded in or near
real time, and their actual state determined as closely as possible by subsequent
telemetry analysis.
During the cruise period, additional command sequences are uplinked and loaded
aboard for execution, to replace the command sequence exercised during launch.
These take the spacecraft through its routine cruise operations, such as tracking Earth
with its HGA and monitoring celestial references for attitude control. Flight team
members begin to get the feel of their spacecraft in flight.
Commonly, unforeseen problems arise, and the onboard fault protection algorithms
receive their inevitable tests; the spacecraft will, more likely than not, go into safing or
contingency modes, and nominal cruise operations must be painstakingly recovered.
TCMs (Trajectory Correction Maneuvers), are executed to fine-tune the trajectory. As
the spacecraft nears its target, or earlier during designated checkout periods, the
science instruments are powered on, exercised and calibrated.
Real-time Commanding
Frequently, command sequences stored on the spacecraft during cruise or other
phases must be augmented by commands sent and executed in or near real time, as
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new activities become desirable, or, less frequently, as mistakes are discovered in the
on-board command sequence.
There are risks inherent in real-time commanding. It is possible to select the wrong
command file for uplink, especially if an extensive set of files exists containing some
similar or anagrammatic filenames. Also, it is possible that a command file built in haste
may contain an error. Planned sequences of commands (generally just called
"sequences") are typically less risky than real-time commands because they benefit
from a long process of extensive debate and selection, testing and checking and
simulation prior to uplink.
Some flight projects permit real-time commanding of a sort wherein command
mnemonics are entered at the command system keyboard in real time and uplinked
directly. Other projects do not permit this inherently high-risk operation because even a
typographical error passing undetected might introduce a problem with the spacecraft or
the mission.
These factors may limit the desirability of undertaking many activities by real-time
commands, but the necessity, as well as the convenience, of at least some form of realtime commanding frequently prevails.
Typical Daily Operations
Mission Control Usually, at least one person is on duty at the Flight Project's real-time
mission support area at JPL or other location, during periods when the DSN (Deep
Space Network), is tracking the spacecraft. This person, typically the mission controller,
or "Ace," watches real-time data from the spacecraft and ground system, and responds
to any anomalous indications via pre-planned responses. Anomaly response plans
usually include getting in touch with appropriate subsystem experts in real time, no
matter what time it is locally, to evaluate the situation and determine how best to
proceed.
The Ace is a person on the Mission Control Team or Real-time Operations Team who is
the single point of contact between the entire flight team, consisting of, for example, a
Spacecraft Team, a Navigation Team, Science Teams, and other teams on the one
hand, and teams external to the flight project such as DSN, Facilities Maintenance,
multi-mission Data Systems Operations Team (DSOT), Ground Communications
Facility (GCF), Advanced Multi-Mission Operations System (AMMOS), System
Administrators (SAs), Network Administrators, and others, on the other hand.
“Ace" is not an acronym, despite attempts to make it one. It simply refers to one single
point of contact for a project's real-time flight operations, and is not too inappropriately a
pun for an expert pilot. Most Aces have experience with many different missions. It is
possible for one Ace to be serving more than one flight project at a time. In 1993, one
particular Ace was serving Magellan, Voyager 1, Voyager 2, and Mars Observer, all at
the same time during regular work shifts.
The Ace executes commanding, manages the ground systems, insures the capture and
delivery of telemetry, monitor, and tracking data, watches for alarm limit violations in
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telemetry, manages the alarm limits, evaluates data quality, performs limited real-time
analyses to determine such things as maneuver effectiveness and spacecraft health to
first order, and coordinates the activities of the DSN and other external teams in support
of the flight project(s). Typically, a large portion of the Ace's interactions are with the
Spacecraft Team, the DSN, and the DSOT. On the infrequent occasions when the Ace
detects an anomaly and rules out any false indications, he or she proceeds to invoke
and follow the anomaly response plan that has been approved by the flight project. That
plan is then followed by appropriate flight team members until the anomaly is resolved,
and nominal operations are restored.
Monitoring Spacecraft and Ground Events
In order to tell whether everything is proceeding nominally, an Ace needs an accurate
list of expected events, to compare with spacecraft events as they are observed in real
time. For example, the spacecraft's downlink signal may change or disappear. Was it
planned, or is this an anomaly?
Such a list is also required for the purpose of directing DSN station activity, and for
planning command uplinks and other real-time operations. For example, if we uplink a
command at 0200 UTC, will the spacecraft actually receive it one-way light-time later, or
will the spacecraft be off Earthpoint or behind a planet at that time?
That list is called the sequence of events, SOE. It contains a list of spacecraft events
being commanded from the onboard sequence, and DSN ground events such as the
beginning and end of tracking periods, transmitter operations, and one-way light times. .
Compiling an SOE and related products begins with a list of the commands that will be
uplinked to the spacecraft's sequencing memory, and that will execute over a period of
typically a week or a month or more into the future. The list of commands, sorted into
time-order, comes from engineers responsible for spacecraft subsystems, scientists
responsible for their instruments' operations, and from others. Times for the events'
execution are included with the commands. The team responsible for generating the
command sequence then creates a spacecraft event file (SEF). This file goes on as an
input to the remainder of the sequence generation process for eventual uplink to the
spacecraft.
A copy of the SEF also goes to the sequence of events generator software, SEGS,
where commands are adjusted for light time, and are merged with DSN station schedule
information and events. Station view period files and light time files are typically
provided by the navigation team. One of SEGS output products is the DSN keyword file
(DKF). This file is provided to the DSN, who then combines it with similar listings from
other projects to create an SOE for each particular station. SEGS outputs the SOE in
tabular form, and also arranges most of the same information into a high-level graphics
product, the space flight operations schedule, SFOS. Users can view each of these
products, or create hardcopy, using SEGS viewing and editing software.
Tracking the Spacecraft in Flight
39
DSN tracking requirements and schedules have been negotiated months or even years
in advance of launch. Now the spacecraft is in flight. Near the time when the spacecraft
will be rising in the sky due to Earth's rotation, its assigned DSN tracking activity begins.
Precal: During the period allotted for "precal" activities, the Network Monitor and Control
(NMC) operator sits down at her or his console in the Signal Processing Center (SPC)
of one the DSN's three Deep Space Communications Complexes (DSCC). The operator
will be controlling and monitoring the assigned antenna, called a Deep Space Station
(DSS), as well as an assigned set of computers that control its pointing, tracking,
commanding, receiving, telemetry processing, ground communications, and other
functions.
This string of equipment from the antenna to the project people at JPL is called a
"connection", referring to the two-way communications link between the spacecraft and
the project. The NMC operator in this role is called the "Connection Controller." Prior to
the Connection Controller's arrival, the Complex Manager (a higher-level NMC function)
operator will have assigned, via directives sent out to the station components over a
local area network (LAN), applicable equipment to become part of the connection.
At about this time, a JPL Communications technician establishes a voice link between
the Connection Controller and the Ace. The Ace offers a pre-pass briefing to the
Connection Controller, highlighting the important activities for the upcoming pass, and
any changes in the latest documentation. Now the DSS14 Connection Controller begins
sending more directives over the SPC LAN to configure each of the link components
specifically for the upcoming support. Predict sets containing uplink and downlink
frequencies, Doppler bias ramp rates, pointing angles and bit rates, command
modulation levels, and hundreds of other parameters, are all sent to the link
components. Any problems are identified and corrected as the Connection Controller
and the Ace communicate as needed during the "precal."
Near the end of the precal period, the Connection Controller checks the DSS area via
closed circuit TV, makes a warning announcement over its outdoor loudspeakers, and
the DSS antenna swings to point precisely to the spacecraft's apparent location above
the eastern horizon.
BOT: Beginning of Track (BOT) is declared. At the prearranged time, typically ten
minutes after BOT, the DSS's transmitter comes on, and red rotating beacons on the
antenna illuminate as a warning of the microwave power present. Upon locking the
receivers, telemetry, and tracking equipment to the spacecraft's signal, the link is
established. This marks the Beginning of Track (BOT) and Acquisition of Signal (AOS).
The Ace interacts with the Connection Controller as needed during the course of the
track to be sure the flight project's objectives are met. Depending on the nature of the
spacecraft's activities, there may be Loss of Signal (LOS) temporarily when the
spacecraft turns away to maneuver, or if it goes into occultation behind a planet. This
LOS would presumably be followed by another AOS when the maneuver or occultation
is complete. During the day, the DSS antenna moves slowly to follow, or track, the
spacecraft as the Earth rotates.
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EOT: Near the end of the Connection Controller's shift, the DSS is pointing lower on
the western horizon. At the same time, if continuous tracking is scheduled, another
Connection Controller inside the SPC of another DSCC a third of the way around the
world, is in touch with the Ace, conducting a precal as the same spacecraft is rising in
the east from the new station's point of view. To accomplish an uplink transfer, the
setting DSS's transmitter is turned off precisely two seconds after the rising DSS's
transmitter comes on. Scheduled End of Track (EOT) arrives, and the Connection
Controller at the setting DSS begins postcal activities, idling the link components and
returning control of them to the Complex Manager operator. After a de-briefing with the
Ace, the Ace releases the station, so it can begin preparing for its next scheduled
activity.
Preparation for Encounter
Command loads uplinked to the spacecraft are valid for varying lengths of time. Socalled quiescent periods such as the lengthy cruises between planets require relatively
few activities, and a command load may be valid for several weeks or even months. By
comparison, during the closest-approach part of a flyby encounter of a prime target
body, a very long and complex load may execute in only a matter of hours.
Prior to the Voyagers' encounters, the spacecraft was generally sent a command
sequence that took it through activities simulating the activities of encounter. Nowadays,
this kind of rehearsal is largely accomplished with ground-based simulation, using
simulated data broadcast to users. Changes in data rate and format, and spacecraft
maneuvers, are designed to put the flight team and ground systems through their paces
during a realistic simulation, in order to provide some practice for readiness, to shake
down the systems and procedures, and to try to uncover flaws or areas for
improvement. In a few critical cases, the actual encounter command load, slightly
modified so that some activities (such as rocket firings) are skipped, may be actually
executed aboard the spacecraft as a final test. This was the case with Cassini's Saturn
Orbit Insertion critical sequence, and also with its Huygens Mission Relay critical
sequence.
Instrument calibrations are undertaken prior to encounter, and again afterwards, to be
sure that experiments are being carried out in a scientifically controlled fashion.
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Chapter 5
Encounter Phase
The term "encounter" is used in this chapter to indicate the high-priority data-gathering
period of operations for which the mission was intended. It may last a few months or
weeks or less as in the case of a flyby encounter or atmospheric probe entry, or it may
last a number of years as in the case of an orbiter. For missions, brief critical portions of
encounter operations are typically carried out from a special room within the Space
Flight Operations Facility that is equipped for television cameras and VIP visits.
Flyby Operations
All the interplanetary navigation and course corrections accomplished during cruise
result in placement of the spacecraft at precisely the correct point, and at the correct
time to carry out its encounter observations. A flyby spacecraft has a limited opportunity
to gather data. Once it has flown by its target, it cannot return to recover any lost data.
Its operations are planned years in advance of the encounter, and the plans are refined
and practiced in the months prior to the encounter date. Sequences of commands are
prepared, tested, and uplinked by the flight team to carry out operations in various
phases of the flyby, depending on the spacecraft's distance from its target.
During each of the six Voyager encounters, the phases were called Observatory phase,
Far Encounter phase, Near Encounter phase, and Post Encounter phase.
These phases may be given other names on different missions, but many of the
functions most likely will be similar. Many of these also apply in some degree to
missions other than flyby missions as well. In fact, for a spacecraft orbiting Jupiter or
Saturn, for example, flyby encounters occur repeatedly, as the spacecraft approaches
each targeted satellite, on orbit after orbit.
In a flyby operation, Observatory phase (OB) begins when the target can be better
resolved in the spacecraft's optical instruments than it can from Earth-based
instruments. This phase generally begins a few months prior to the date of flyby. OB is
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marked by the spacecraft being for the first time completely involved in making
observations of its target (rather than cruise activities), and ground resources are
completely operational in support of the encounter. The start of this phase marks the
end of the interplanetary flyby cruise phase. Ground system upgrades and tests have
been completed, spacecraft flight software modifications have been implemented and
tested, and the encounter command sequences have been placed on board.
Far Encounter phase (FE) begins when the full disc of a planet can no longer fit within
the field of view of the instruments. Observations are designed to accommodate parts of
the planet rather than the whole disc, and to take best advantage of the higher
resolution available.
Near Encounter phase (NE) includes the period of closest approach to the target. It is
marked by intensely active observations with all of the spacecraft's science
experiments, including onboard instruments and radio science investigations. It includes
the opportunity to obtain the highest resolution data about the target. During NE, radio
science observations may include ring plane measurements during which ring structure
and particle sizes can be determined. Celestial mechanics observations can determine
the planet's or satellites' mass, and atmospheric occultations can determine
atmospheric structures and compositions.
During the end of FE or the beginning of NE, a bow shock crossing may be identified
through data from the magnetometer, the plasma instrument and plasma wave
instrument as the spacecraft flies into a planet's magnetosphere and leaves the solar
wind. When the solar wind is in a state of flux, these crossings may occur again and
again as the magnetosphere and the solar wind push back and forth over millions of
kilometers.
Post encounter phase (PE) begins when NE completes, and the spacecraft is receding
from the planet. It is characterized by day after day of observations of a diminishing, thin
crescent of the planet just encountered. This is the opportunity to make extensive
observations of the night side of the planet. After PE is over, the spacecraft stops
observing its target planet, and returns to the activities of cruise phase. DSN resources
are relieved of their continuous support of the encounter, and they are generally
scheduled to provide less frequent coverage to the mission during PE.
After encounter, instrument calibrations are repeated to be sure that any changes in the
instruments' states are accounted for.
Planetary Orbit Insertion Operations
The same type of highly precise interplanetary navigation and course correction used
for flyby missions also applies during cruise for an orbiter spacecraft. This process
places the spacecraft at precisely the correct location at the correct time to enter into
planetary orbit. Orbit insertion requires not only precise position and timing, but also
controlled deceleration.
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As the spacecraft's trajectory is bent by the planet's gravity, the command sequence
aboard the spacecraft places the spacecraft in the correct attitude, and fires its
engine(s) at the proper moment and for the proper duration. Once the retro-burn has
completed, the spacecraft has been captured into orbit by its target planet. If the retroburn were to fail, the spacecraft would continue to fly on past the planet as though it
were a flyby mission. It is common for the retro-burn to occur on the far side of a planet
as viewed from Earth, with little or no data available until well after the burn has
completed and the spacecraft has emerged from behind the planet, successfully in orbit.
Once inserted into a highly elliptical orbit, Mars Global Surveyor continued to adjust its
orbit via aerobraking near periapsis to decelerate the spacecraft further, causing a
reduction in the apoapsis altitude, and establishing a close circular orbit at Mars. Mars
Odyssey used the same technique. Galileo used a gravity assist from a close flyby of
Jupiter's moon Io to decelerate, augmenting the deceleration provided by its 400 N
rocket engine. Thereafter, additional Orbit Trim Maneuvers (OTMs) over a span of
several years were used to vary the orbit slightly and choreograph multiple encounters
with the Galilean satellites and the magnetosphere. Cassini is currently doing the same
in orbit at Saturn.
System Exploration or Planetary Mapping
At least two broad categories of orbital science-gathering operations may be identified:
system exploration and planetary mapping. Exploring a planetary system includes
making observations of the planet, its atmosphere, its satellites, its rings, and its
magnetosphere during a tour typically a few years or more in duration, using the
spacecraft's complement of remote-sensing and direct-sensing instruments. On the
other hand, mapping a planet means concentrating observations on the planet itself,
using the spacecraft's instruments to obtain data mainly from the planet's surface and
atmosphere.
Galileo explored the entire Jovian system, including its satellites, rings, magnetosphere,
the planet, its atmosphere, and its radiation environment. At Saturn, Cassini is engaged
in a similar exploratory mission, examining the planet's atmosphere, rings,
magnetosphere, icy satellites, and the large satellite Titan, which has its own
atmosphere. Magellan, a planetary mapper, covered more than 99% the surface of
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Venus in great detail using Synthetic Aperture Radar (SAR) imaging, altimetry,
radiometry, gravity field, and mass distribution. A few special experiments were also
carried out, including bistatic radar, aerobraking, windmilling, and destructive
atmospheric entry. Mars Global Surveyor and Mars Odyssey are mapping the surface of
their planet using imaging, altimetry, spectroscopy, and a gravity field survey.
An orbit of low inclination at the target planet (equatorial, for example) is well suited to a
system exploration mission, because it provides repeated exposure to satellites orbiting
within the equatorial plane, as well as adequate coverage of the planet and its
magnetosphere. An orbit of high inclination (polar, for example) is better suited for a
mapping mission, since the target planet or body will rotate fully below the spacecraft's
orbit, providing eventual exposure to every part of the planet's surface.
In either case, during system exploration or planetary mapping, the orbiting spacecraft
is involved in an extended encounter period, requiring continuous or dependably regular
support from the flight team members, the DSN, and other institutional teams.
Occultations
Occultations provide unique opportunities to conduct scientific experiments.
Occultations of interest include the earth, the sun, or another star disappearing behind a
planet, behind its rings, or behind its atmosphere, as viewed from the spacecraft. They
are of interest when involving either the main planet or its satellites. During the one-time
only opportunity for occultation by a planet during a flyby mission encounter, or
repeatedly during an orbital mission, onboard instruments may make unique
observations. Here are a few examples:
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An ultraviolet spectrometer watches the sun as it disappears behind a planet's
atmosphere (or a satellite's atmosphere), obtaining spectra deep into the
atmosphere, that can be studied to determine its composition and structure.
A photometer watches a bright distant star as it passes behind a ring system.
Results yield high-resolution data on the sizes and structures of the ring system
and its particles.
The telecommunications system is used by Radio Science to measure the
Doppler shift around closest approach to a satellite, to determine the object's
mass.
Radar Science commandeers the telecommunications system to perform a
bistatic radar experiment: The Deep Space Network (DSN) observes the radio
signal glancing off a satellite's surface as the spacecraft directs its downlink
signal at just the right spots on the satellite to achieve this specular reflection.
Results can yield information on the structure and composition of the surface.
(While this kind of experiment doesn't actually need an occultation, it might be
performed near occultation.)
Radio Science observes the spacecraft's radio signal on Earth as the spacecraft
passes behind a planet or a satellite, yielding data on the composition and
structure of its atmosphere.
45
Recall that the most precise Radio Science investigations require a two- or three-way
coherent mode, receiving an uplink from the DSN. However, with an atmospheric
occultation, or an occultation of an opaque ring section, this coherent link is generally
possible on ingress only; the spacecraft is likely to lose the uplink from DSN when it
passes close behind the planet, and therefore would be incapable of producing a
coherent downlink. For this reason, some spacecraft are equipped with an Ultra Stable
Oscillator (USO) in a temperature-controlled "oven" on the spacecraft which is capable
of providing a fairly stable downlink frequency for a short time when an uplink is not
available. Calibrations, of course would, be accomplished shortly before and after the
occultation.
Extrasolar Planetary Occultations
Occultation may be one method of identifying extrasolar planets (also known as
"exoplanets") by measuring the slightly reduced brightness of a distant star as one of its
planets happens to pass in front of it as viewed from Earth. A spacecraft is not
necessarily required for this kind of measurement; it can be undertaken to some extent
using earth-based telescopes.
Somewhat related to this subject, many Earth-based telescopes, and some proposed
orbiting instruments, are equipped with an occulting disk (coronagraph) that can block
out light from the central star. This facilitates examining the star's "planetary" disk of
material orbiting it. Scattered light in the instrument and/or in the Earth's atmosphere
limits its effectiveness, but studies exist for orbiting an external large occulting disk to
improve experiments' effectiveness from Earth-based or orbiting telescopes.
Gravity Field Surveying
Planets are not perfectly spherical. Terrestrial planets are rough-surfaced, and most
planets are at least slightly oblate. Thus they have variations in their mass
concentrations, sometimes associated with mountain ranges or other features visible on
the surface. A gravity field survey identifies local areas of a planet that exhibit slightly
more or slightly less gravitational attraction. These differences are due to the variation
of mass distribution on and beneath the surface.
There are two reasons for surveying the gravity field of a planet. First, highly accurate
navigation in orbit at a planet requires a good model of variations in the gravity field,
which can be obtained by such a survey. Second, gravity field measurements have the
unique advantage of offering scientists a "view" of mass distribution both at and below
the surface. They are extremely valuable in determining the nature and origin of
features identifiable in imaging data. Application of these techniques to Earth helps
geologists locate petroleum and mineral
To obtain gravity field data, a spacecraft is required to provide only a downlink carrier
signal coherent with a highly stable uplink from the DSN. It may be modulated with
telemetry, command and ranging data, or it may be unmodulated. After the removal of
known Doppler shifts induced by planetary motions and the spacecraft's primary orbit,
and other factors, the residual Doppler shifts are indicative of miniscule spacecraft
46
accelerations resulting from variations in mass distribution at, and below, the surface of
the planet. The gravity feature size that can be resolved is roughly equal to the
spacecraft's altitude; with a 250-km altitude, a spacecraft should resolve gravity features
down to roughly 250 km in diameter.
With an X-band (3.6 cm wavelength) uplink received at a spacecraft, and a coherent Xband downlink, spacecraft accelerations can be measured to tens of micrometers per
second squared. This translates to a sensitivity of milligals in a planetary gravity field.
(One gal represents a gravitational acceleration of 1 cm/sec2).
The most accurate and complete gravity field coverage is obtained from low circular
orbit. Mars Global Surveyor is conducting a gravity field survey from circular orbit as one
of its first-priority investigations. Magellan's orbit was elliptical during its primary mission,
and meaningful gravity data could be taken only for the portion of the orbit that was plus
and minus about 30 degrees true anomaly from periapsis, which occurred at about 10
degrees north latitude. After aerobraking to a low circular orbit, Magellan conducted a
high-resolution gravity field survey of the entire planet.
Atmospheric Entry and Aerobraking
Aerobraking is the process of decelerating by converting velocity mostly into heat via
supersonic compression in a planetary atmosphere. Galileo's atmospheric probe is a
typical example of an atmospheric entry and aerobraking mission. The probe was
designed with an aeroshell that sustained thousands of degrees of heat as it entered
the Jovian atmosphere. In fact its aeroshell reached a higher temperature than the sun's
photosphere. It decelerated at hundreds of Gs, until it reached a speed where its
parachute became effective. At that time, the spent aeroshell was discarded, and the
probe successfully carried out its experiments characterizing Jupiter's upper
atmosphere.
The Magellan spacecraft was not designed for atmospheric entry. However, the
periapsis altitude of Magellan's orbit was lowered, by the use of propulsive maneuvers,
into the upper reaches of Venus's atmosphere near 140 km above the surface. This is
still high above the cloud tops, which are at about 70 km. Flying at this altitude induced
deceleration via atmospheric friction during the portion of the spacecraft's orbit near
periapsis, thus reducing the height to which it could climb to apoapsis. The solar array,
consisting of two large square panels, was kept flat-on to the velocity vector during each
pass through the atmosphere, while the Magellan HGA trailed in the wind. The solar
array reached a maximum of 160° C, and the HGA a maximum of 180° C. After
approximately 70 earth days and one thousand orbits of encountering the free
molecular flow, and decelerating a total of about 1250 m/sec, the apoapsis altitude was
lowered to a desirable altitude. The periapsis altitude was then raised to achieve a
nearly circular orbit. The objectives of this aerobraking experiment were to demonstrate
the use of aerobraking for use on future missions, to characterize the upper atmosphere
of Venus, and to be in position to conduct a full-planet gravity field survey from a nearly
circular orbit.
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Descent and Landing
Magellan Landing on a planet is generally accomplished first by aerobraking while
entering the planet's atmosphere under the protection of an aeroshell. From there, the
lander might be designed to parachute to the surface, or to use a propulsion system to
soft-land, or both, as did the Viking landers on Mars. In addition to aeroshell, parachutes
and propulsion, the Mars Pathfinder spacecraft used airbags to cushion its impact. This
technique was repeated with the Mars Exploration Rovers, Spirit and Opportunity. The
Soviet Verena spacecraft parachuted to the surface of Venus by means of a small rigid
disk integral with the spacecraft’s' structure which helped slow their descent sufficiently
through the very dense atmosphere. A crushable foot pad absorbed the energy from
their final impact on the surface.
Even though Huygens enjoyed the immense fortune of surviving impact with the surface
of Titan, and returning data for over 90 minutes, the mission was classified as an
atmospheric probe, not a lander. Its prime mission was to collect data while descending
for more than two hours through the atmosphere.
Surveyor missions landed on the Moon via propulsive descent, coming to rest on
crushable foot pads at the lunar surface. For a possible future mission, some members
of the international science community desire to land a network of seismometerequipped spacecraft on the surface of Venus to measure seismic activity over a period
of months or years.
Balloon Tracking
Once deployed within a planet's atmosphere, having undergone atmospheric entry
operations as discussed above, a balloon may ride with the wind and depend on the
DSN to directly track its progress, or it can use an orbiting spacecraft to relay its data to
Earth. In 1986, DSN tracked the Venus balloons deployed by the Soviet Vega
spacecraft when it was on its way to encounter comet Halley. The process of tracking
the balloon across the disc of Venus yielded data on the circulation of the planet's
atmosphere.
The planned Mars Balloon is designed to descend to just above the surface. Carrying
an instrument package, including a camera, within a long, snake- or rope-like structure,
it will rise and float when heated by the daytime sunlight, and will sink and allow the
"rope" to rest on the surface at night. In this way it is hoped that the balloon package will
visit many different locations pseudo-randomly as the winds carry it. In doing so, it will
also yield information on atmospheric circulation patterns. The Mars balloon is designed
jointly by Russia, CNES, and The Planetary Society, a public non-profit space-interest
group in Pasadena. It will depend upon an orbiting spacecraft to relay its data home.
The Mars Global Surveyor spacecraft carries radio relay equipment designed to relay
information from landers, surface penetrators, balloons, or Mars aircraft. The Mars
Odyssey and Mars Express orbiters are similarly equipped. Future Mars orbiting
spacecraft will also have relay capability, as did two Mars-bound spacecraft that were
lost: Mars Observer in 1993 and Mars Climate Orbiter in 1999.
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Sampling
One of the major advantages of having a spacecraft land on the surface of a planet is
that it can take direct measurements of the soil. The several Soviet Venera landers
accomplished this on the 900° F surface of Venus, and the Viking landers accomplished
this on the surface of Mars. Samples are taken from the soil and transported into the
spacecraft's instruments where they are analyzed for chemical composition, and the
data are relayed back to Earth.
This is all good, but the scientific community really would like to directly examine
samples in the laboratory. A robotic sample return mission to Mars could be in the
future. NASA has conducted long-range studies and technology development and may
continue to do so for planning purposes. Several different scenarios are envisioned for
accomplishing this, some of which would include a rover to go around and gather up
rock and soil samples to deposit inside containers aboard the return vehicle. Interest in
returning samples from Mars has heightened recently in the wake of discoveries
including recently flowing springs, and ancient lake beds on Mars. Such samples would
be examined for fossil evidence of life forms. Stardust
Sampling of cosmic dust in the vicinity of the Earth has also become an endeavor of
great interest, since interplanetary dust particles can reveal some aspects of the history
of solar system formation. Space shuttle experiments have so far been successful at
capturing three 10 µm particles from Earth orbit, one intact.
Launched in 1999, Stardust collected samples of material from the coma of Comet Wild
2 in January 2004, for return to Earth January 15, 2006. The return capsule's entry,
descent, and landing on Earth were flawless, and scientific analysis of its thousands of
samples has yielded new insights about the composition and formation of comets.
Genesis succeeded in returning samples of the solar wind, despite its unexpectedly
hard landing on Earth in September 2004, when its parachute failed to deploy.
A spacecraft isn't always needed if you want to collect interplanetary material. Dust from
interplanetary space rains continuously into Earth's atmosphere, which slows it gently
because of the particles' low mass. In 1998 the NASA Dryden Flight Research Center
flew one of its ER-2 high-altitude research aircraft with an experiment for the Johnson
Space Center that collected high-altitude particulate matter -- or "cosmic dust" -- on two
collector instruments mounted on pods under the wings.
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Chapter 6
Extended Operations Phase
Completion of Primary Objectives
A mission's primary objectives are spelled out well in advance of the spacecraft's
launch. The efforts of all of the flight team members are concentrated during the life of
the mission toward achieving those objectives. A measure of a mission's success is
whether it has gathered enough data to complete or exceed its originally stated
objectives.
During the course of a mission, there may be inadvertent losses of data. In the case of
an orbiter mission, it might be possible to recover the losses by repeating observations
of areas where the loss was sustained. Such data recovery might require additional time
be added to the portion of a mission during which its primary objectives are being
achieved. However, major data losses and their recovery are usually planned for during
mission design. One predictable data loss occurs during superior conjunction, when the
sun interferes with spacecraft communications for a number of days.
Additional Science Data
Once a spacecraft has completed its primary objectives, it may still be in a healthy and
operable state. Since it has already undergone all the efforts involved in conception,
design and construction, launch, cruise and perhaps orbit insertion, it can be very
economical to redirect an existing spacecraft toward accomplishing new objectives and
to retrieve data over and above the initially planned objectives. This has been the case
with several spacecraft. It is common for a flight project to have goals in mind for
extended missions to take advantage of a still-viable spacecraft in a unique location
when the original funding expires.
Voyager was originally approved as a mission only to Jupiter and Saturn. But Voyager
2's original trajectory was selected with the hope that the spacecraft might be healthy
after a successful Saturn flyby, and that it could take advantage of that good fortune.
After Voyager 1 was successful in achieving its objective of reconnaissance of the
Saturnian system, including a tricky solar occultation of Titan and associated
observations, Voyager 2 was not required to be used solely as a backup spacecraft to
duplicate these experiments. Voyager 2's trajectory to Uranus and Neptune was
therefore preserved and successfully executed. Approval of additional funding enabled
making some necessary modifications, both in the ground data system and in the
spacecraft's onboard flight software, to continue on to encounter and observe the
Uranus and Neptune systems.
By the time Voyager 2 reached Uranus after a five-year cruise from Saturn, it had many
new capabilities, such as increased three-axis stability, extended imaging exposure
modes, image motion compensation, data compression, and new error-correction
coding.
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In 1993, after 15 years of flight, Voyagers 1 and 2 both observed the first direct
evidence of the long-sought-after heliopause. They identified a low frequency signature
of solar flare material interacting with the heliopause at an estimated distance of 40 to
70 AU ahead of Voyager 1's location, which was 52 AU from the sun at the time.
After fulfilling its goal of mapping at least 70% of the surface of Venus, the Magellan
mission went on with more than one mission extensions, eventually to accomplish
special stereo imaging tests, and interferometric observation tests. Mapping coverage
reached over 99% of the surface.
Rather than abandon the spacecraft in orbit, the Magellan Project applied funding which
had been saved up over the course of the primary mission to begin an adventurous
transition experiment, pioneering the use of aerobraking to attain a nearly circular
Venusian orbit, and a low-latitude gravity survey was completed. All of these
accomplishments far exceeded the mission's original objectives.
Orbiting Relay Operations
Some Mars orbiting spacecraft are equipped with radio relay capability intended to
receive uplink from surface or airborne craft. Typically such relay equipment operates at
UHF frequencies.
In order to serve as a relay, at least some of the orbiter's own science data gathering
activities have to be reduced or interrupted while its data handling and storage
subsystems process the relay data. This may or may not present an undesirable impact
to the mission's ability to meet its primary objectives. Relay service, then, is a good
candidate for extended mission operations. Since relay service entails neither keeping
optical instruments pointed, nor flying a precise ground track, the demand on the
attitude control and propulsion systems is minimal, and a little propellant can go a long
way. The demand on other subsystems, such as electrical supply, can also be reduced
in the absence of other science
End of Mission
Resources give out eventually. Due to the age of their RTGs in 2000, the Pioneer 10
and 11 spacecraft, plying the solar system's outer reaches, faced the need to turn off
electrical heaters for the propellant lines in order to conserve electrical power for
continued operation of science instruments. Doing so allowed the propellant to freeze,
making it impossible to re-thaw for use in additional spacecraft maneuvers. The
spacecraft were still downlinking science data while Earth eventually drifted away from
their view, and over the following months contact was lost forever.
Voyagers 1 and 2 continue to make extraordinary use of their extended mission. They
are expected to survive until the sunlight they observe is too weak to register on their
sun sensors, causing a loss of attitude reference. This is forecast to happen near the
year 2015, which may or may not be after they have crossed the heliopause. Electrical
energy from their RTGs may fall below a useable level about the same time or shortly
thereafter. The spacecraft's supply of hydrazine may become depleted sometime after
that, making continued three-axis stabilization impossible.
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Pioneer 12 ran out of hydrazine propellant in 1993, and was unable to further resist the
slow decay of its orbit about Venus, resulting from friction with the tenuous upper
atmosphere. It entered the atmosphere and burned up like a meteor after fourteen years
of service.
Components wear out and fail. The Hubble Space Telescope has been fitted with many
new components, including new attitude-reference gyroscopes, to replace failed and
failing units. Two of Magellan's attitude-reference gyroscopes had failed prior to the start
of the transition experiment, but of course no replacement was possible.
Once a mission has ended, the flight team personnel are disbanded, and the ground
hardware is returned to the loan pool or sent into long-term storage. Sometimes it is
possible to donate excess computers to schools. Oversubscribed DSN resources are
freed of contention from the terminated mission, and the additional tracking time
allocations can be made available to missions currently in their prime.
While layoffs are not uncommon, many personnel from a disbanded flight team are
assigned to new flight projects to take advantage of valuable experience gained. Interim
work is often available within the Section itself. Many Viking team members joined the
Voyager mission after Viking achieved its success at Mars in the late 1970s. Many of
the Voyager flight team members joined the Magellan project after Voyager's last
planetary encounter ended in October 1989. Other ex-Voyager people joined the
Galileo and Topex/ Poseidon missions. Some ex-Magellan people have worked on
Cassini, Mars Global Surveyor, Mars Pathfinder, SIRTF, and Mars Exploration Rover.
Mission's end also provides a convenient time for some employees to begin their
retirement, and for new employees to be hired and begin building careers in
interplanetary exploration.
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Chapter 7
Space Shuttle Operations
The space shuttle was the world's first reusable spacecraft, and the first spacecraft in
history that could carry large satellites both to and from orbit. The shuttle launches like a
rocket, maneuvers in Earth orbit like a spacecraft and lands like an airplane.
Because of these requirements the Shuttle was shaped to look like an aircraft but to
operate as a spacecraft. The structure of the Shuttle Orbiter comprises nine separate
sections, or elements: the forward fuselage, the forward reaction control system
module, the mid-fuselage, the payload bay doors, the aft fuselage, the vertical tail, the
two orbital maneuvering system/reaction control modules and the wing.
The demands are greater than is usually the case with a conventional aircraft because
the stresses imposed upon the structure are unique to the Shuttle. Because of this, the
design team at North American Aviation had no precedents on which to base their
prototype. It was the first of its kind, without the advantage of any previous learning
curve, and one of a kind without parallel.
Columbia was the first space shuttle orbiter to be delivered to NASA's Kennedy Space
Center, Fla., in March 1979. Columbia and the STS-107 crew were lost Feb. 1, 2003,
during re-entry. The orbiter Challenger was delivered to KSC in July 1982 and was
destroyed in an explosion during ascent in January 1986. Discovery was delivered in
November 1983. Atlantis was delivered in April 1985. Endeavour was built as a
replacement following the Challenger accident and was delivered to Florida in May
1991. An early space shuttle orbiter, the Enterprise, never flew in space but was used
for approach and landing tests at the Dryden Flight Research Center and several launch
pad studies in the late 1970s.
A typical shuttle mission lasts seven to eight days, but can extend to as much as 14
days depending upon the objectives of the mission.
Launching the Space Shuttle
To lift the 4.5 million pound (2.05 million kg) shuttle from the pad to orbit (115 to 400
miles/185 to 643 km) above the Earth, the shuttle uses the following components:
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two solid rocket boosters (SRB)
three main engines of the orbiter (SSME)
the external fuel tank (ET)
orbital maneuvering system (OMS) on the orbiter
Solid Rocket Boosters
The SRBs are solid rockets that provide most of the main force or thrust (71 percent)
needed to lift the space shuttle off the launch pad. In addition, the SRBs support the
entire weight of the space shuttle orbiter and fuel tank on the launch pad. Each SRB
has the following parts:
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solid rocket motor
o case
o propellant
o igniter
o nozzle
solid propellant fuel
o atomized aluminum (16 percent) oxidizers
o ammonium perchlorate (70 percent) catalyst
o iron oxide powder (0.2 percent) binder
o polybutadiene acrylic acid acrylonite (12 percent) curing agent
o epoxy resin (2 percent)
jointed structure
synthetic rubber O-rings between joints
flight instruments
recovery systems
o parachutes (drogue, main)
o floatation devices
o signaling devices
explosive charges for separating from the external tank
thrust control systems
self-destruct mechanism
Because the SRBs are solid rocket engines, once they are ignited, they cannot be shut
down. Therefore, they are the last component to light at launch.
Main Engines
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The orbiter has three main engines located in the aft (back) fuselage (body of the
spacecraft). Each engine is 14 feet (4.3 m) long, 7.5 feet (2. 3 m) in diameter at its
widest point (the nozzle) and weighs about 6,700 lb (3039 kg).
The main engines provide the remainder of the thrust (29 percent) to lift the shuttle off
the pad and into orbit. The main engines burn liquid hydrogen and liquid oxygen as fuel
which are stored in the external fuel tank (ET), at a ratio of 6:1. They draw liquid
hydrogen and oxygen from the ET at an amazing rate, equivalent to emptying a family
swimming pool every 10 seconds! The fuel is partially burned in a pre-chamber to
produce high pressure, hot gases that drive the turbo-pumps (fuel pumps). The fuel is
then fully burned in the main combustion chamber and the exhaust gases (water vapor)
leave the nozzle at approximately 6,000 mph (10,000 km/h). Each engine can generate
between 375,000 and 470,000 lb (1,668,083 to 2,090,664 N) of thrust; the rate of thrust
can be controlled from 65 percent to 109 percent maximum thrust. The engines are
mounted on gimbals (round bearings) that control the direction of the exhaust, which
controls the forward direction of the rocket
External Fuel Tank
As mentioned above, the fuel for the main engines is stored in the ET. The ET is 158 ft.
(48 m) long and has a diameter of 27.6 ft. (8.4 m). When empty, the ET weighs 78,000
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lb (35,455 kg). It holds about 1.6 million lb (719,000 kg) of propellant with a total volume
of about 526,000 gallons (2 million liters).
The ET is made of aluminum and aluminum composite materials. It has two separate
tanks inside, the forward tank for oxygen and the aft tank for hydrogen, separated by an
inter-tank region. Each tank has baffles to dampen the motion of fluid inside. Fluid flows
from each tank through a 17-inch (43 cm) diameter feed line out of the ET through an
umbilical line into the shuttle's main engines. Through these lines, oxygen can flow at a
maximum rate of 17,600 gallons/min (66,600 l/min) and hydrogen can flow at a
maximum rate of 47,400 gallons/min (179,000 l/min).
The ET is covered with a 1-inch (2.5 cm) thick layer of spray-on, polyisocyanurate foam
insulation. The insulation keeps the fuels cold, protects the fuel from heat that builds up
on the ET skin in flight, and minimizes ice formation. When Columbia launched in 2003,
pieces of the insulating foam broke off the ET and damaged the left wing of the orbiter,
which ultimately caused Columbia to break up upon re-entry.
Space Shuttle Liftoff
The two orbital maneuvering systems' (OMS) engines are located in pods on the aft
section of the orbiter, one on either side of the tail. These engines place the shuttle into
final orbit, change the shuttle's position from one orbit to another, and slow the shuttle
down for re-entry.
The OMS engines burn monomethyl hydrazine fuel (CH3NHNH2) and nitrogen tetroxide
oxidizer (N2O4). Interestingly, when these two substances come in contact, they ignite
and burn automatically (i.e., no spark required) in the absence of oxygen. The fuel and
oxidizer are kept in separate tanks, each pressurized by helium. The helium pushes the
fluids through the fuel lines (i.e., no mechanical pump required). In each fuel line, there
are two spring-loaded solenoid valves that close the lines. Pressurized nitrogen gas,
56
from a small tank located near the engine, opens the valves and allows the fuel and
oxidizer to flow into the combustion chamber of the engine. When the engines shut off,
the nitrogen goes from the valves into the fuel lines momentarily to flush the lines of any
remaining fuel and oxidizer; this purge of the line prevents any unwanted explosions.
During a single flight, there is enough nitrogen to open the valves and purge the lines 10
times!
Either one or both of the OMS engines can fire, depending upon the orbital maneuver.
Each OMS engine can produce 6,000 lb (26,400 N) of thrust. The OMS engines
together can accelerate the shuttle by 2 ft/s2(0.6 m/s2). This acceleration can change
the shuttle's velocity by as much as 1,000 ft/s (305 m/s). To place into orbit or to de-orbit
takes about 100-500 ft/s (31-153 m/s) change in velocity. Orbital adjustments take
about 2 ft/s (0.61 m/s) change in velocity. The engines can start and stop 1,000 times
and have a total of 15 hours of burn time.
Profile of shuttle launch and ascent into orbit
As the shuttle rests on the pad fully fueled, it weighs about 4.5 million pounds or 2
million kg. The shuttle rests on the SRBs as pre-launch and final launch preparations
are going on through T minus 31 seconds:
1. T minus 31 s - the on-board computers take over the launch sequence.
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2. T minus 6.6 s - the shuttle's main engines ignite one at a time (0.12 s apart).
The engines build up to more than 90 percent of their maximum thrust.
3. T minus 3 s - shuttle main engines are in lift-off position.
4. T minus 0 s -the SRBs are ignited and the shuttle lifts off the pad.
5. T plus 20 s - the shuttle rolls right (180 degree roll, 78 degree pitch).
6. T plus 60 s - shuttle engines are at maximum throttle.
7. T plus 2 min - SRBs separate from the orbiter and fuel tank at an altitude of 28
miles (45 km). Main engines continue firing. Parachutes deploy from the SRBs.
SRBs will land in the ocean about 140 miles (225 km) off the coast of Florida.
Ships will recover the SRBs and tow them back to Cape Canaveral for
processing and re-use.
8. T plus 7.7 min - main engines throttled down to keep acceleration below 3g's
so that the shuttle does not break apart.
9. T plus 8.5 min - main engines shutdown.
10. T plus 9 min - ET separates from the orbiter. The ET will burn up upon reentry.
11. T plus 10.5 min - OMS engines fire to place the shuttle in a low orbit.
12. T plus 45 min - OMS engines fire again to place the shuttle in a higher,
circular orbit (about 250 miles/400 km).
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Orbiter in Space
Once in space, the shuttle orbiter can be the home for astronauts for seven to 14 days.
The orbiter can be oriented so that the cargo bay doors face toward the Earth or away
from the Earth depending upon the mission objectives; in fact, the orientation can be
changed throughout the mission. One of the first things that the commander will do is to
open the cargo bay doors to cool the orbiter.
The orbiter consists of the following parts:
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crew compartment - where astronauts live and work
forward fuselage (upper, lower parts) - contains support equipment (fuel cells,
gas tanks) for crew compartment
forward reaction control system (RCS) module - contains forward rocket jets for
turning the orbiter in various directions
movable airlock - used for spacewalks and can be placed inside the crew
compartment or inside the cargo bay
mid-fuselage: contains essential parts (gas tanks, wiring, etc.) to connect the
crew compartment with the aft engines; forms the floor of the cargo bay
cargo bay doors - roof of the cargo bay and essential for cooling the orbiter
remote manipulator arm - located in the cargo bay: moves large pieces of
equipment in and out of the cargo bay; platform for spacewalking astronauts
aft fuselage - contains the main engines
OMS/RCS pods (2) - contain the orbital maneuvering engines and the aft RCS
module; turn the orbiter and change orbits
airplane parts of the orbiter - fly the shuttle upon landing (wings, tail, body flap)
The crew compartment is located in the forward fuselage. The crew compartment has
2,325 cu. ft. of space with the airlock inside or 2,625 cu. ft. with the airlock outside. The
crew compartment has three decks:
Flight deck - uppermost deck
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forward deck - contains all of the controls and warning systems for the space
shuttle (also known as the cockpit)
seats - commander, pilot, specialist seats (two)
aft deck - contains controls for orbital operations: maneuvering the orbiter while
in orbit (rendezvous, docking, deploying payload, and working the remote
manipulator arm
Mid-deck
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living quarters (galley, sleeping bunks, toilet)
stowage compartments (personal gear, mission-essential equipment,
experiments)
exercise equipment
airlock - on some flights
entry hatch
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Lower deck (equipment bay)
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contains life support equipment, electrical systems, etc.
Living Environment
The shuttle orbiter provides an environment where astronauts can live and work in
space. The shuttle provides the following:
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life support - atmosphere control, supply and recycling; water; temperature
control; light; food supply; waste removal; fire protection
ability to change position and change orbits
capability to talk with ground-based flight controllers (communications and
tracking)
stellar navigation to find its way around in orbit
make its own electrical power
coordinate and handle information (computers)
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base from which to launch/retrieve satellites; construction - such as building the
International Space Station and conduct experiments
The orbiter must provide astronauts with an environment similar to Earth. The shuttle
must have air, food, water, and a comfortable temperature. The orbiter must also take
away the wastes products produced by the astronauts (carbon dioxide, urine, feces)
and protect them from fire
Our atmosphere is a mixture of gases (78 percent nitrogen, 21 percent oxygen, 1
percent other gases) at a pressure of 14 lbs/in2 (1 atm) that we breathe in and out. The
space shuttle must provide a similar atmosphere. To do this, the orbiter carries liquid
oxygen and liquid nitrogen in two systems of pressurized tanks, which are located in the
mid-fuselage (each system has two tanks for a total of four tanks). The cabin
pressurization system combines the gases in the correct mixture at normal atmospheric
pressure. While in orbit, only one oxygen-nitrogen system is used to pressurize the
orbiter. During launch and landing, both systems of each gas are used.
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Five loops of fans circulate the atmosphere. The circulated air picks up carbon
dioxide, heat and moisture:
Chemical carbon dioxide canisters remove carbon dioxide by reacting it with
lithium hydroxide. These canisters are located in the lower deck of the crew
compartment and are changed every 11 hours.
Filters and charcoal canisters remove trace odors, dust and volatile chemicals
from leaks, spills and out gassing.
A cabin heat exchanger in the lower deck cools the air and condenses the
moisture, which collects in a slurper. Water from the slurper is moved with air to a
fan separator, which uses centrifugal force to separate water from air. The air is
re-circulated and the water goes to a wastewater tank.
Besides air, water is the most important quantity aboard the orbiter. Water is made from
liquid oxygen and hydrogen in the space shuttle's fuel cells (the fuel cells can make 25
lb (11 kg) of water per hour). The water passes through a hydrogen separator to
eliminate any trapped hydrogen gas (excess hydrogen gas is dumped overboard). The
water is then stored in four water storage tanks located in the lower deck. Each tank can
hold 165 lb (75 kg).
The water tanks are pressurized by nitrogen so that water can flow to the mid-deck for
use by the crew. Drinkable water is then filtered to remove microbes and can be
warmed or chilled through various heat exchangers depending upon the use (food
preparation, consumption, personal hygiene). Excess water produced by the fuel cells
gets routed to a wastewater tank and subsequently dumped overboard.
Outer space is an extremely cold environment and temperatures will vary drastically in
different parts of the orbiter. You might think that heating the orbiter would be a
problem. However, the electronic equipment generates more than enough heat for the
ship. The problem is getting rid of the excess heat. So the temperature control system
has to carry out two major functions:
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Distribute heat where it is needed on the orbiter (mid-fuselage and aft sections)
so that vital systems do not freeze in the cold of space.
Get rid of the excess heat.
To do this, the shuttle has two methods to handle temperature control:
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Passive methods - generally simple methods that handle small heat loads and
require little maintenance; insulating materials (blankets), surface coatings,
paints, all reduce heat loss through the walls of the various components just like
home insulation. Electrical heaters - use electrically-heated wires like a toaster to
heat various areas.
Active methods – generally more complex, these systems use fluid to handle
large heat loads and require maintenance. Cold plates are metal plates that
collect heat by direct contact with equipment or conduction. Heat exchangers are
used to collect heat from equipment using fluid. The equipment radiates heat to a
fluid (water, ammonia) which in turn passes heat on to Freon. Both fluids are
pumped and re-circulated to remove heat. Pumps, lines, valves - transport the
collected heat from one area to another. Radiators - located on the inside
surfaces of the cargo bay doors that radiate the collected heat to outer space
Flash evaporator/ammonia boilers - these devices are located in the aft fuselage
and transfer heat from Freon coolant loops overboard when cargo bay doors are
closed or when cargo bay radiators are overloaded. Flash evaporator Freon
coolant loops wrap around an inner core. The evaporator sprays water on the
heated core. The water evaporates removing heat. The water vapor is vented
overboard. Ammonia boiler Freon coolant loops pass through a tank of
pressurized ammonia. Heat released from the Freon causes the ammonia to boil.
Ammonia vapor is dumped overboard.
The cabin heat exchanger also controls the cabin temperature. It circulates cool water
to remove excess heat (cabin air is also used to cool electronic equipment) and
transfers this heat to a Freon exchanger. The Freon then transfers the heat to other
orbiter systems (e.g., cryogenic gas tanks, hydraulic systems) and radiates excess heat
to outer space.
The orbiter has internal fluorescent floodlights that illuminate the crew compartment.
The orbiter has external floodlights to illuminate the cargo bay. Finally, the control
panels are lighted internally for easy viewing.
Food is stored on the mid-deck of the crew compartment. Food comes in several forms
(dehydrated, low moisture, heat-stabilized, irradiated, natural and fresh). The orbiter has
a galley-style kitchen module along the wall next to the entry hatch, which is equipped
with the following:
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food storage compartments
food warmers
a food preparation area with warm and cold water outlets
metal trays so the food packages and utensils do not float away
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Like any home, the orbiter must be kept clean, especially in space when floating dirt and
debris could present a hazard. Wastes are made from cleaning, eating, work and
personal hygiene. To maintain general housecleaning, the astronauts use various wipes
(wet, dry, fabric, detergent and disinfectant), detergents, and wet/dry vacuum cleaners
are used to clean surfaces, filters and the astronauts themselves. Trash is separated
into wet trash bags and dry trash bags, and the wet trash is placed in an evaporator that
will remove the water. All trash bags are stowed in the lower deck to be returned to
Earth for disposal. Solid waste from the toilet is compacted, dried and stored in bags
where it is returned to Earth for disposal (burning). Liquid waste from the toilet goes to
the wastewater tank where it is dumped overboard.
Fire is one of the most dangerous hazards in space. The orbiter has a Fire Detection
and Suppression Subsystem that consists of the following:
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area smoke detectors on each deck
smoke detectors in each rack of electrical equipment
alarms and warning lights in each module
non-toxic portable fire extinguishers (carbon dioxide-based)
personal breathing apparatus - mask and oxygen bottle for each crew member
After a fire is extinguished, the atmosphere control system will filter the air to remove
particulates and toxic substances.
Work aboard the Shuttle
The shuttle was designed to deploy and retrieve satellites as well as deliver payloads to
Earth orbit. To do this, the shuttle uses the Remote Manipulator System (RMS). The
RMS was built by Canada and is a long arm with an elbow and wrist joint. The RMS can
be controlled from the aft flight deck. The RMS can grab payloads (satellites) from the
cargo bay and deploy them, or grab on to payloads and place them into the bay.
In the past, the shuttle was used for delivering satellites and conducting experiments in
space. Within the mid-deck, there are racks of experiments to be conducted during each
mission. When more space was needed, the mission used the Spacelab module, which
was built by the European Space Agency (ESA). It fit into the cargo bay and was
accessed by a tunnel from the mid-deck of the crew compartment. It provided a "shirtsleeve" environment in which you could work. The Spacelab was lost along with
Columbia in 2003. Now, most experiments are conducted aboard the International
Space Station.
The shuttle's major role was to build and re-supply the International Space Station. The
shuttle delivers components built on Earth. Astronauts use the RMS to remove
components from the cargo bay and to help attach them to existing modules in space
station.
Space Shuttle Positioning, Communication and Navigation
To change the direction that the orbiter is pointed (attitude), the reaction control system
(RCS) located on the nose and OMS pods of the aft fuselage is used.
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The RCS has 14 jets that can move the orbiter along each axis of rotation (pitch, roll,
and yaw). The RCS thruster’s burn monomethyl hydrazine fuel and nitrogen tetroxide
oxidizer just like the OMS engines described previously. Attitude changes are required
for deploying satellites or for pointing (mapping instruments, telescopes) at the Earth or
stars. To change orbits (e.g., rendezvous, docking maneuvers), you must fire the OMS
engines. As described above, these engines change the velocity of the orbiter to place it
in a higher or lower orbit.
Tracking and Communication
The astronauts talk with flight controllers on the ground daily for the routine operation of
the mission. In addition, they must be able to communicate with each other inside the
orbiter or its payload modules and when conducting spacewalks outside.
NASA's Mission Control in Houston will send signals to a 60 ft. radio antenna at White
Sands Test Facility in New Mexico. White Sands will relay the signals to a pair of
Tracking and Data Relay satellites in orbit 22,300 miles above the Earth. The satellites
will relay the signals to the space shuttle. The system works in reverse as well.
The orbiter has two systems for communicating with the ground:
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
S-band - voice, commands, telemetry and data files
Ku-band (high bandwidth) - video and transferring two-way data files
The orbiter has several intercom plug-in audio terminal units located throughout the
crew compartment. Each astronaut wears a personal communications control with a
headset. The communications control is battery-powered and can be switched from
intercom to transmit functions. They can either push to talk and release to listen or have
a continuously open communication line. To talk with spacewalkers, the system uses a
UHF frequency, which is picked up in the astronaut's space suit. The orbiter also has a
series of internal and external video cameras to see inside and outside.
Navigation, Power and Computers
The orbiter must be able to know precisely where it is in space, where other objects are
and how to change orbit. To know where it is and how fast it is moving, the orbiter uses
global positioning systems (GPS). To know which way it is pointing (attitude), the orbiter
has several gyroscopes. All of this information is fed into the flight computers for
rendezvous and docking maneuvers, which are controlled in the aft station of the flight
deck.
All of the on-board systems of the orbiter require electrical power. Three fuel cells make
electricity; they are located in the mid fuselage under the payload bay. These fuel cells
combine oxygen and hydrogen from pressurized tanks in the mid fuselage to make
electricity and water. Like a power grid on Earth, the orbiter has a distribution system to
supply electrical power to various instrument bays and areas of the ship. The water is
used by the crew and for cooling.
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The orbiter has five on-board computers that handle data processing and control critical
flight systems. The computers monitor equipment and talk to each other and vote to
settle arguments. Computers control critical adjustments especially during launch and
landing:
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operations of the orbiter (housekeeping functions, payload operations,
rendezvous/docking)
interface with the crew
caution and warning systems
data acquisition and processing from experiments
flight maneuvers
Pilots essentially fly the computers, which fly the shuttle. To make this easier, the
shuttles have a Multifunctional Electronic Display Subsystem (MEDS), which is a full
color, flat, 11-panel display system. The MEDS, also known as the "glass cockpit",
provides graphic portrayals of key light indicators (attitude, altitude, speed). The MEDS
panels are easy to read and make it easier for shuttle pilots to interact with the orbiter.
The Shuttle's Return to Earth
For a successful return to Earth and landing, dozens of things have to go just right.
First, the orbiter must be maneuvered into the proper position. This is crucial to a safe
landing. When a mission is finished and the shuttle is halfway around the world from the
landing site (Kennedy Space Center, Edwards Air Force Base), mission control gives
the command to come home, which prompts the crew to:
1. Close the cargo bay doors. In most cases, they have been flying nose-first and
upside down, so they then fire the RCS thrusters to turn the orbiter tail first.
2. Once the orbiter is tail first, the crew fires the OMS engines to slow the orbiter
down and fall back to Earth; it will take about 25 minutes before the shuttle
reaches the upper atmosphere.
3. During that time, the crew fires the RCS thrusters to pitch the orbiter over so
that the bottom of the orbiter faces the atmosphere (about 40 degrees) and they
are moving nose first again.
4. Finally, they burn leftover fuel from the forward RCS as a safety precaution
because this area encounters the highest heat of re-entry.
Because it is moving at about 17,000 mph (28,000 km/h), the orbiter hits air molecules
and builds up heat from friction (approximately 3000 degrees F, or 1650 degrees C).
The orbiter is covered with ceramic insulating materials designed to protect it from this
heat. The materials include:
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Reinforced carbon-carbon (RCC) on the wing surfaces and underside
High-temperature black surface insulation tiles on the upper forward fuselage
and around the windows
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White Nomex blankets on the upper payload bay doors, portions of the upper
wing and mid/aft fuselage
Low-temperature white surface tiles on the remaining areas
Maneuvering of the orbiter for re-entry
These materials are designed to absorb large quantities of heat without increasing their
temperature very much. In other words, they have a high heat capacity. During re-entry,
the aft steering jets help to keep the orbiter at its 40 degree attitude. The hot ionized
gases of the atmosphere that surround the orbiter prevent radio communication with the
ground for about 12 minutes (i.e., ionization blackout).
When re-entry is successful, the orbiter encounters the main air of the atmosphere and
is able to fly like an airplane. The orbiter is designed from a lifting body design with
swept back "delta" wings. With this design, the orbiter can generate lift with a small wing
area. At this point, flight computers fly the orbiter. The orbiter makes a series of Sshaped, banking turns to slow its descent speed as it begins its final approach to the
runway. The commander picks up a radio beacon from the runway (Tactical Air
Navigation System) when the orbiter is about 140 miles (225 km) away from the landing
site and 150,000 feet (45,700 m) high. At 25 miles (40 km) out, the shuttle's landing
computers give up control to the commander. The commander flies the shuttle around
an imaginary cylinder (18,000 feet or 5,500 m in diameter) to line the orbiter up with the
runway and drop the altitude. During the final approach, the commander steepens the
angle of descent to minus 20 degrees (almost seven times steeper than the descent of
a commercial airliner).
Shuttle flight path for landing
When the orbiter is 2,000 ft. (610 m) above the ground, the commander pulls up the
nose to slow the rate of descent. The pilot deploys the landing gear and the orbiter
touches down. As the commander applies the wheel brakes, the speed brake on the
vertical tail is opened to help slow down the shuttle. A parachute is deployed from the
back to help stop the orbiter. The parachute and the speed brake on the tail increase
the drag on the orbiter. The orbiter stops about midway to three-quarters of the way
down the runway.
After landing, the crew goes through the shutdown procedures to power down the
spacecraft. This process takes about 20 minutes. During this time, the orbiter is cooling
and noxious gases, which were made during the heat of re-entry, blow away. Once the
orbiter is powered down, the crew exits the vehicle. Ground crews are on-hand to begin
servicing the orbiter.
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Chapter 8
Emergency Operation Management
When an emergency occurs it is Mission Controls responsibility to evaluate the event,
triage the process, and evaluate the most important jobs that need to be accomplished.
The safety of the flight crew is the primary focus of an emergency as Mission Control
takes on a new series of responsibilities.
Understandably the flight crew gets very engaged after an anomaly. They want to help
ensure the mission is a success and failure is a big concern. They need to be provided
frequent updates on findings and progress and participate in the evaluation of the
emergency and its mitigation. This mission is very important to the flight crew but
reason and balance needed to prevail.
The first concern is to “stop the bleeding”, questions naturally begin to surface about
why the anomaly occurred. These queries, while important to understanding your
continuing risk, should not distract the team from focusing their attention on continuing
the mission and managing the problem.
Watch for Things Getting Complicated
After the anomaly, Mission Control needs to work through the data, consider responses,
and to solve the problem. Teams have a tendency to create complex, multilayer
solutions to mitigate the problem. Sometimes discussions work their way from one
incremental fix to another, arriving at complex fixes and patches that would move the
team far from its operations training and might not address the real problem. This
complexity growth actually grows risk that the system will become so sophisticated it will
be prone to operator error or create unforeseen interactions. In the heat of battle, there
needs to be someone who keeps an eye on the risk of the solution. This is the
responsibility of Mission Control, someone needs to ask, “Do we need to go that far, or
can we live with just the first corrective measure?” Sometimes you need to agree that
you can accept residual risk after addressing the principal problem. Missions have been
lost because smart people did well-intended things that made problems worse.
Meeting the Challenge of the Emergency
The triage process must be a mix of urgency and focus, which comes from many, many
operational rehearsals where the team trains for what is supposed to happen and even
what is not supposed to happen. You need to focus not just on the specifics of what
could go wrong, but on your behavior and process when something goes wrong.
Mission Control has many responsibilities when an emergency happens. You will have
to depend on individual and team capabilities, training, and roles in ways that are hard
to describe. You know that you must trust the team’s abilities and judgment, but also
watch for signs, both within the team and outside, of good intentions yielding
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problematic results. You must be reasonable and evenhanded, understanding that you
cannot eliminate risk. The emergency is a time when a mission team shows what it is
really made of.
Space Shuttle Abort Modes
The worst possible outcome of an Emergency is the “Mission Abort”. To meet this need
NASA developed multiple procedures to mitigate this specific possibility. A Space
Shuttle abort procedure is an emergency procedure that is needed due to equipment
failure on NASA's Space Shuttle, most commonly during ascent. A main engine failure
was a typical abort scenario. There were fewer abort options during reentry and
descent. For example, the Columbia disaster happened during reentry, and there were
no alternatives in that portion of flight.
Later in descent, certain failures were survivable, although not usually classified as an
abort. For example, a flight control system problem or multiple auxiliary power unit
failure would make reaching a landing site impossible, thus requiring the astronauts to
bail out.
There were five abort modes available during ascent, in addition to pad (RSLS) aborts.
These were divided into the categories of intact aborts and contingency aborts. The
choice of abort mode depended on how urgent the situation was, and what emergency
landing site could be reached. The abort modes covered a wide range of potential
problems, but the most commonly expected problem was Space Shuttle Main Engine
(SSME) failure, causing inability either to cross the Atlantic or to achieve orbit,
depending on timing and number of failed engines. Other possible non-engine failures
necessitating an abort included multiple auxiliary power unit (APU) failure, cabin leak,
and external tank leak (ullage leak).
Redundant Set Launch Sequencer (RSLS) Abort
The main engines were ignited roughly 6.6 seconds before liftoff. From that point to
ignition of the Solid Rocket Boosters at T - 0 seconds, the main engines could be shut
down. This was called a "Redundant Set Launch Sequencer Abort", and happened five
times, on STS-41-D, STS-51-F, STS-51, STS-55, and STS-68. It always happened
under computer (not human) control, caused by computers sensing a problem with the
main engines after starting but before the SRBs ignited. The SRBs could not be turned
off once ignited, and afterwards the shuttle was committed to take off. If an event such
as an SSME failure requiring an abort happened after SRB ignition, acting on the abort
would have to wait until SRB burnout 123 seconds after launch. No abort options
existed if that wait was not possible.
Intact abort modes
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There were four intact abort modes for the Space Shuttle. Intact aborts were designed
to provide a safe return of the orbiter to a planned landing site or to a lower orbit than
planned for the mission.
Return To Launch Site (RTLS)
In a Return To Launch Site (RTLS) abort, the Shuttle would have continued downrange
until the solid rocket boosters were jettisoned. It would then pitch around, so the SSMEs
fired retrograde. This maneuver would have occurred in a near-vacuum above the
appreciable atmosphere and was conceptually no different from the OMS engines firing
retrograde to de-orbit. The main engines continued burning until downrange velocity
was killed and the vehicle began heading back toward the launch site at sufficient
velocity to reach a runway. Afterwards the SSMEs were stopped, the external tank was
jettisoned, and the orbiter made a normal gliding landing on the runway at Kennedy
Space Center about 25 minutes after lift-off. The CAPCOM would call out the point in
the ascent at which an RTLS was no longer possible as "negative return",
approximately four minutes after lift-off.
Should all three SSMEs have failed, the shuttle would not have been able to make it
back to the runway at KSC, forcing the crew to bail out. While this would have resulted
in the loss of the Shuttle, the crew could escape safely and then be recovered by the
SRB recovery ships.
This abort mode was never needed in the history of the Shuttle program. Astronaut
Mike Mullane referred to the RTLS abort as an "unnatural act of physics," and many
pilot astronauts hoped that they would not have to perform such an abort due to its
difficulty.
Transoceanic Abort Landing (TAL)
A Transoceanic Abort Landing (TAL) involved landing at a predetermined location in
Africa or western Europe about 25 to 30 minutes after lift-off. It was used when velocity,
altitude, and distance downrange did not allow return to the launch point via RTLS. It
was also used when a less time-critical failure did not require the faster but possibly
more stressful RTLS abort.
A TAL abort would be declared between roughly T+2:30 minutes (2 minutes and 30
seconds after liftoff) and Main Engine Cutoff (MECO), about T+8:30 minutes. The
Shuttle would then land at a pre-designated friendly airstrip in Europe. The last four TAL
sites until the Shuttle's retirement were Istres Air Base in France, Zaragoza and Morón
air bases in Spain, and RAF Fairford in England. Prior to a Shuttle launch, two of them
were selected depending on the flight plan, and staffed with standby personnel in case
they were used. The list of TAL sites changed over time; most recently Ben Guerir Air
Base in Morocco (TAL site from July 1988–June 2002) was eliminated due to terrorist
attack concerns. Other previous TAL sites included Lajes Air Base, Terceira, Azores,
Mallam Aminu Kano International Airport, Kano, Nigeria; Mataveri International Airport,
Easter Island, Chile (for Vandenberg launches); Rota, Spain; Casablanca, Morocco;
Banjul, Gambia; and Dakar, Senegal.
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Preparations of TAL sites took 4 to 5 days and began a week before a launch with the
majority of personnel from NASA, the Department of Defense, and contractors arriving
48 hours before launch. Additionally, two C-130 aircraft from the Manned Space Flight
support office from the adjacent Patrick Air Force Base including eight crew members,
nine para-rescue men, two flight surgeons, a nurse and medical technician, along with
2,500 pounds of medical equipment were deployed to Zaragoza, Istres, or both. One or
more C-21 or a C-12 aircraft were also deployed to provide weather reconnaissance in
the event of an abort with a TALCOM, or astronaut flight controller aboard for
communications with the shuttle pilot and commander. This abort mode was never
needed during the entire history of the space shuttle program.
Abort Once Around (AOA)
An Abort Once Around (AOA) was available when the shuttle could not reach a stable
orbit but had sufficient velocity to circle the earth once and land, about 90 minutes after
lift-off. The time window for using the AOA abort was very short – just a few seconds
between the TAL and ATO abort opportunities. Therefore, taking this option was very
unlikely. This abort mode was never needed during the entire history of the space
shuttle program.
Abort to Orbit (ATO)
An Abort to Orbit (ATO) was available when the intended orbit could not be reached but
a lower stable orbit was possible. This occurred on mission STS-51-F, which continued
despite the abort to a lower orbit. The Mission Control Center in Houston (located at
Lyndon B. Johnson Space Center) observed an SSME failure and called "Challenger-Houston, Abort ATO. Abort ATO".
The moment at which an ATO became possible was referred to as the "press to ATO"
moment. In an ATO situation, the spacecraft commander rotated the cockpit abort mode
switch to the ATO position and depressed the abort push button. This initiated the flight
control software routines which handled the abort. In the event of lost communications,
the spacecraft commander could have made the abort decision and taken action
independently.
A hydrogen fuel leak in one of the SSMEs on STS-93 resulted in a slightly lower orbit
than anticipated, but was not an ATO; if the leak had been more severe, it might have
necessitated an ATO, RTLS, or TAL abort.
Emergency landing sites
Pre-determined emergency landing sites for the Orbiter were determined on a missionby-mission basis according to the mission profile, weather and regional political
situations.
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Chapter 9
The Future of Space Propulsion
The use of chemical based rockets to leave our planet and explore space may very well
be a dead end technology. It’s old, outdated and it’s extremely inefficient. Surely we’ve
discovered or improved upon newer, more efficient technology in these last 60 years,
right? The answer to that is yes, and we’re going to go over them in detail.
We will explore exotic technology that includes using solar wind to sail amongst the
stars, using nuclear bombs to approach light speed, and even dabbling with technology
that exploits loopholes in the laws of physics which NASA has recently been
experimenting with.
What’s Wrong with Chemical Rockets?
Chemical rockets may be a dead end because of their extreme inefficiency. Just to put
the space shuttle into earth orbit (to reach 17,500 MPH), the rockets need to carry 15
times its weight in fuel – and that’s considered extremely efficient among other
chemical-based rocket systems. To escape earth's gravitational pull and explore our
solar system (to reach 25,000 MPH), you would need significantly more fuel.
Occasionally, space agencies can mitigate some of the problems by using gravitational
assists from planets. They use a planet’s gravity well to slingshot a probe toward its
destination, significantly speeding it up.
The problem with this solution is one of availability. To take advantage of a planet’s
gravity well, the planet has to be in a specific place, at a specific time. This leaves a
small window which a probe would need to be launched. Some of these windows can
be incredibly rare. The Voyager space probes, which explored the planets in the outer
solar system, took advantage of a planet alignment that happens only once every 176
years.
Then there is the cost. The average cost to put the space shuttle into orbit is 450 million
USD per mission. That’s a huge price tag just to reach low earth orbit, and it’s also a big
part of the reason the shuttle program was scrapped. If we wanted to leave earth orbit
and explore our solar system with such an inefficient technology (without gravitational
assists), the problems become severely compounded. Because there aren’t any fuel
stations in space, a spaceship has to carry all its fuel with it, fuel which is not only
pricey, but heavy.
If we wanted to leave our solar system and travel to our closest neighboring star in a
reasonable time frame (say, 900 years) using standard chemical-based rockets, it would
require 10137 kilograms of fuel – that is more fuel than exists on our planet. Thus, we
need to look towards developing a better, more efficient method of propulsion.
Solar Sails
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Solar sails do exactly as the name suggests; they sail on the solar wind. There is no
actual wind in space because space is a vacuum, but there is something similar that a
spacecraft could use to propel itself. A craft equipped with a giant sail made out of ultrathin mirrors can harness a combination of light and high speed ejected gasses from the
sun to reach incredible speeds.
The pressure of the light and gasses is very small, but since there no friction in the
vacuum of space, it allows that small pressure to build up over time. Given enough time,
this pressure can propel a craft to a significant fraction of light speed. The time to reach
top speeds could be lessened by aiming extremely powerful lasers or masers at the
sails from a base on the moon or other satellite without an atmosphere.
However, a solar sail does have its drawbacks. Once far enough away from the sun
(and any laser boosting stations we have setup), the craft would no longer be
accelerating and instead rely on its own inertia to travel to its destination. The craft
would then have to direct its sails towards the destination star to decelerate and slow
down.
Solar sail spacecraft became a reality when, back in May 2010, the Japanese launched
the Ikaros probe. It successfully deployed its solar sails and is currently in a wide orbit
around the sun. It’s expected to reach Jupiter in a few years.
Ion Drive
An ion thruster (or ion drive) is a lot less exciting than how it is often portrayed in
science fiction books and movies. It operates on similar principle as the solar sail; using
very low thrust but over an extended period of time. It achieves this thrust by ejecting
charged ions, gas or plasma out of its electric engine which propels the spacecraft.
This method of acceleration allows a craft to achieve a very high specific impulse. Such
a craft would only work in the vacuum of space since the thrust is so low. However, the
fuel required by the engine is significantly less than is required by chemical rockets
which max out thanks to the Carnot limit (a limit on efficiency).
This technology is being heavily considered for future space missions and has already
proven its feasibility in space. In 1998, NASA launched the Deep Space 1 probe which
was powered by a xenon gas ion engine and was the first ion drive in space. In 2003,
Japan launched the Hayabusa probe which used 4 xenon ion engines. Its mission was
to rendezvous with an asteroid and collect samples. It completed its mission and
returned to earth in June of 2010.
Like the solar sail, ion drives also have their drawbacks. First, they would need to carry
their fuel with them. While the amount required to get the nearest star is technically
feasible, it wouldn’t be very practical. Travel time is another issue. While an ion drive is
significantly more efficient than rocket engines, and is great for jaunts around our solar
system, interstellar travel is another matter entirely. With a gravitational assist from our
sun, it still would take 19,000 years to reach Proxima Centauri with a ship using an ion
engine.
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We need more speed if we want to leave the confines of our solar cradle.
Nuclear Propulsion
If we wanted to get to our nearest neighboring star using the best technology available
to us right now, nuclear propulsion is our best option. It’s fast, proven and relatively
cheap. A ship equipped with nuclear pulse propulsion and could theoretically reach 12%
the speed of light. That is so fast, you could travel completely around the earth and end
up back at your starting point in just under 2 seconds. Or you could travel to the moon in
13 seconds – it took Apollo 11 four days to reach the moon by comparison.
While it would take 19,000 years to reach Proxima Centauri with an Ion Drive, it would
take a relatively manageable 35 years using nuclear pulse propulsion. A human would
be able to travel to our nearest neighboring star within his or her lifetime. And it could be
done with technology that already exists.
The way nuclear propulsion works sounds a bit crazy, but it is proven and it is relatively
simple. Small nuclear bombs are dropped out of the back of the spacecraft which
detonate. The resulting force from the explosion accelerates the craft. This is done
repeatedly until the desired speed has been attained. An incredibly large, reinforced
pusher plate would shield the craft from damages and radiation while dampeners would
be used to mitigate the effects of G force and provide smooth acceleration.
The US military began looking into nuclear pulse propulsion back in 1958 under the
project name “Orion”. The project was shelved in 1963 thanks to the Partial Test Ban
Treaty which prevents nuclear devices being detonated in space. The idea wasn’t
forgotten however. In 1973, the British Interplanetary Society developed a similar
concept, called Project Daedalus. Then in 1998, the nuclear engineering department at
PSU began developing two improved versions of the Daedalus design known as Project
Ican and Project Aimstar.
One of the obvious drawbacks to nuclear pulse propulsion is that you have to carry your
fuel with you. This means carrying hundreds or thousands of small nuclear bombs.
There is also the problem of ablation of the pusher plate. Repeated exposure to nuclear
blasts will cause erosion if not sprayed with special oil before each detonation. Yet
another problem is nuclear fallout. This could be averted if a craft is launched from a
polar region, or if a craft is launched into space using conventional rockets, then once
far enough away began using its nuclear propulsion.
The late Carl Sagan once suggested that nuclear pulse propulsion would be an
excellent use for our current stockpiles of nuclear weapons.
Nuclear Fusion
A spacecraft equipped with a nuclear fusion engine could explore our solar system
without the need to carry a large fuel supply thanks to its efficient, long-term
acceleration capability.
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There are two ways a fusion engine could work. The first is using the energy created by
a fusion reaction to generate electricity. This electricity could be used to superheat
plasma which then would be ejected out the back of the craft, providing thrust. The
second method would be more direct. It would use the plasma-based exhaust from the
fusion reaction to provide thrust.
The drawbacks of a fusion engine are very similar to that of the ion drive. While fusion is
a huge improvement over ion drives, it would be very hard to achieve the higher speeds
necessary when traveling between stars. Fusion technology is also still in the
experimental stage of development. The technology must overcome hurdles with
plasma confinement to become viable, then a reactor would need to be miniaturized to
a size manageable for a spacecraft. Currently, experimental laser-based ICF reactors
are as large as football stadiums and are struggling to break even with power output.
Antimatter
Antimatter is the most potent fuel source that we currently know of. It’s also the most
efficient. Antimatter is as the name implies, matter which has its charges reversed.
When antimatter comes into contact with normal matter, the two annihilate one another
in a ferocious blast of pure energy. A piece of antimatter the size of a small coin
contains enough energy to propel a fully loaded space shuttle into orbit. Once in orbit,
NASA claims that a trip to Mars would only require as little as 10 milligrams worth of
antimatter.
An engine using antimatter is pretty simple in its operation. A beam of anti-electrons is
released into an engine core where it annihilates the surface of a metal plate. This
creates a small explosion which propels the craft forward. Another proposed design
uses a sail, similar to the solar sail described above. A cloud of anti-particles is released
which then reacts explosively with surface of the sail. This reaction can propel the craft
to incredible speeds. According to NASA, an antimatter powered craft would be able to
reach speeds up to 70% the speed of light. That means we could reach Proxima
Centauri in just under 6 years.
The drawbacks of using antimatter are production and containment. Antimatter is a
byproduct of atom-smashing tests done at particle accelerators, these tests are very
expensive to operate. If we wanted to produce a single gram of antimatter, it would cost
over a trillion dollars. Containment is also another issue. Since antimatter violently
reacts when it comes into contact with normal matter, it would have to be stored in
vacuum containers at incredibly low temperatures, suspended by strong magnetic
fields. This becomes a challenge because anti-electrons (positrons) repel each other,
often explosively. Some solutions have been proposed, one suggests that by combining
positrons with electrons, researchers can create an element called positronium which
can theoretically store the anti-electrons indefinitely.
Faster Than Light
Faster than light travel is just the stuff of science fiction, right? After all, didn’t Einstein
say that the speed of light is the ultimate speed limit? Not necessarily, claim physicists.
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The devil is in the details. According to physics, there are ways around the universes
ultimate speed limit. These technical loopholes could theoretically and potentially allow
us to race a beam of light, and win.
NASA researchers know that nothing can accelerate faster than the speed of light, but
they also know there is no such restriction regarding space itself. Space-time has no
such limit on how fast it can move, and it is believed that space-time exceeded the
speed of light during the expansion of the big bang. Researchers at NASA’s advanced
propulsion division have been wondering if space-time can make a repeat performance.
A warp drive, normally the stuff of science fiction, would travel faster than light by riding
on a wave of space-time. It creates this wave by compressing the space-time in front of
the ship and expanding the space-time behind it. A ship then sits in the middle of this
wave, and is propelled through space. Since the ship itself isn’t moving, and only the
space-time around the ship is moving, no laws of physics are broken.
At NASA Eagleworks, researchers have begun to attempt to prove the concept of warp
drive with lab experiments. There, the researchers set up a mini warp drive called the
“White-Juday Warp Field Interferometer”. The experiment seeks to generate a very tiny
instance of a warp field. A warp field that is so small, it is only expected to perturb
space-time by one part in 10 million. While the results will be underwhelming if
successful, it will be existence for proof of concept. The location for the new project is
the facility that was built for the Apollo program, the very same one that put astronauts
on the moon.
The first scientific paper which took warp drives seriously was written in 1994 by
Mexican physicist Miguel Alcubierre. Alcubierre’s paper called for enormous energies to
power his theoretical warp drive (The mass-energy equivalent of Jupiter). Harnessing
that kind of energy is impractical and virtually impossible, so his paper went largely
ignored.
In October of 2012, at the 100 Year Starship Symposium, NASA researcher Harold
White gave a presentation where he announced that he discovered loopholes in the
mathematical equations. Loopholes which brought down the energy requirements to
levels much lower than previously thought. He calculated that by altering the design of
the warp engine and the ship itself, he could get the energy requirements down to just a
few thousand pounds of mass. This advancement and others like it edge warp drives
ever further out of the realm of science fiction and closer to reality.
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Glossary of Essential Terms
A
a, A -- Acceleration. a = Δ velocity / Δ time. Acceleration = Force / Mass
A -- Ampere, the SI base unit of electric current.
AC -- Alternating current
AC Bus Sensor - A system that distributes electrical power
Acceleration -- Change in velocity. Note that since velocity comprises both direction and
magnitude (speed), a change in either direction or speed constitutes
acceleration.
ALT -- Altitude.
ALT -- Altimetry data.
AO -- Announcement of Opportunity.
AOS -- Acquisition of Signal.
Aphelion -- Apoapsis in solar orbit.
Apoapsis -- The farthest point in an orbit from the body being orbited.
Apogee -- Apoapsis in Earth orbit.
APU –Auxiliary Power Unit provides hydraulic pressure. They can provide hydraulic
power for gimballing of engines and control surfaces. During landing, they power
the control surfaces and brakes.
Argument -- Angular distance.
Argument of periapsis -- The argument (angular distance) of periapsis from the
ascending node.
Ascending node -- The point at which an orbit crosses a reference plane (such as a
planet's equatorial plane or the ecliptic plane) going north.
Asteroids -- Small bodies composed of rock and metal in orbit about the sun.
AU -- Astronomical Unit, based on the mean Earth-to-sun distance, 149,597,870 km.
Refer to "Units of Measure" section for complete information.
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AZ -- Azimuth.
B
Boiler System – this water system cools the Auxiliary Power Unit (APU) lubrication oil
and hydraulic fluid. Three independent Water Spray Boilers each serve a
corresponding APU. The Water Spray Boiler System sprays water onto the
APU lubrication oil and hydraulic fluid lines, thus cooling the fluids within them.
BPS -- Bits per Second, same as Baud rate.
C
c -- The speed of light, 299,792 km per second.
C-band -- A range of microwave radio frequencies in the neighborhood of 4 to 8 GHz.
Carrier -- The main frequency of a radio signal generated by a transmitter prior to
application of any modulation.
Centrifugal force -- The outward-tending apparent force of a body revolving around
another body.
Centripetal acceleration -- The inward acceleration of a body revolving around another
body.
Chandler wobble -- A small motion in the Earth's rotation axis relative to the surface,
discovered by American astronomer Seth Carlo Chandler in 1891. Its amplitude
is about 0.7 arcseconds (about 15 meters on the surface) with a period of 433
days. It combines with another wobble with a period of one year, so the total
polar motion varies with a period of about 7 years. The Chandler wobble is an
example of free nutation for a spinning non-spherical object.
Channel -- In telemetry, one particular measurement to which changing values may be
assigned.
Clarke orbit -- Geostationary orbit.
Conjunction -- A configuration in which two celestial bodies have their least apparent
separation.
Coma -- The cloud of diffuse material surrounding the nucleus of a comet.
Comets -- Small bodies composed of ice and rock in various orbits about the sun.
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COMM – communication system
CRT -- Cathode ray tube video display device.
CST -- Central Standard Time.
D
DAP – The Digital Auto Pilot controls the RCS thrusters when in orbit.
DC -- Direct current.
DEC -- Declination.
Declination -- The measure of a celestial body's apparent height above or below the
celestial equator.
Density -- Mass per unit volume. For example, the density of water can be stated as 1
gram/cm3.
Descending node -- The point at which an orbit crosses a reference plane (such as a
planet's equatorial plane or the ecliptic plane) going south.
Doppler Effect -- The effect on frequency imposed by relative motion between
transmitter and receiver. See Chapters 2, 4 and 5.
Downlink -- Signal received from a spacecraft.
E
Eccentricity -- The distance between the foci of an ellipse divided by the major axis.
Ecliptic -- The plane in which Earth orbits the sun and in which solar and lunar eclipses
occur.
EDL -- (Atmospheric) Entry, Descent, and Landing.
EGT – APU Exhaust Gas Temperature.
Ellipse -- A closed plane curve generated in such a way that the sums of its distances
from the two fixed points (the foci) is constant.
ELV -- Expendable launch vehicle.
EM -- Electromagnetic.
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EMF -- Electromagnetic force (radiation).
EMR -- Electromagnetic radiation.
Equator -- An imaginary circle around a body which is everywhere equidistant from the
poles, defining the boundary between the northern and southern hemispheres.
Equinox -- The equinoxes are times at which the center of the Sun is directly above the
Earth's equator. The day and night would be of equal length at that time, if the
Sun were a point and not a disc, and if there were no atmospheric refraction.
Given the apparent disc of the Sun, and the Earth's atmospheric refraction, day
and night actually become equal at a point within a few days of each equinox.
The vernal equinox marks the beginning of spring in the northern hemisphere,
and the autumnal equinox marks the beginning of autumn in the northern
hemisphere.
ERT -- Earth-received time, UTC of an event at DSN receive-time, equal to SCET plus
OWLT.
EST -- Eastern Standard Time.
ET -- Ephemeris time, a measurement of time defined by orbital motions. Equates to
Mean Solar Time corrected for irregularities in Earth's motions. Obsolete,
replaced by TT, Terrestrial Time.
eV -- Electron volt, a measure of the energy of subatomic particles.
F
f, F -- Force. Two commonly used units of force are the Newton and the dyne. Force =
Mass X Acceleration.
FDS -- Flight Data Subsystem.
FE -- Far Encounter phase of mission operations.
Fluorescence -- The phenomenon of emitting light upon absorbing radiation of an
invisible wavelength.
FM -- Frequency modulation.
FTS -- DSN Frequency and Timing System. Also, frequency and timing data.
G
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G -- Universal Constant of Gravitation. Its tiny value (G = 6.6726 x 10-11 Nm2/kg2) is
unchanging throughout the universe.
g -- Acceleration due to a body's gravity. Constant at any given place, the value of g
varies from object to object (e.g. planets), and also with the distance from the
center of the object. The relationship between the two constants is: g = GM/r2
where r is the radius of separation between the masses' centers, and M is the
mass of the primary body (e.g. a planet). At Earth's surface, the value of g = 9.8
meters per second per second (9.8m/s2). See also weight.
Gamma rays -- Electromagnetic radiation in the neighborhood of 100 femtometers
wavelength.
GEO -- Geosynchronous Earth Orbit.
Geostationary -- A geosynchronous equatorial circular orbit. Also called Clarke orbit.
Geosynchronous -- A direct, circular, low inclination orbit about the Earth having a
period of 23 hours 56 minutes 4 seconds.
GMT -- Greenwich Mean Time. Obsolete. UT, Universal Time is preferred.
GPC – General Purpose Computer Control. When the toggle switch is in the straight
up or middle position (not on or off) it allows the valve to be controlled by the
flight software loaded in the general purpose computer.
Gravitation -- The mutual attraction of all masses in the universe. Newton's Law of
Universal Gravitation holds that every two bodies attract each other with a force
that is directly proportional to the product of their masses, and inversely
proportional to the square of the distance between them. This relation is given by
the formula at right, where F is the force of attraction between the two objects,
given G the Universal Constant of Gravitation, masses m1 and m2, and d
distance. Also stated as Fg = GMm/r2 where Fg is the force of gravitational
attraction, M the larger of the two masses, m the smaller mass, and r the radius
of separation of the centers of the masses. See also weight.
Gravitational waves -- Einsteinian distortions of the space-time medium predicted by
general relativity theory (not yet directly detected as of March 2010). (Not to be
confused with gravity waves, see below.)
Gravity assist -- Technique whereby a spacecraft takes angular momentum from a
planet's solar orbit (or a satellite's orbit) to accelerate the spacecraft, or the
reverse.
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Gravity waves -- Certain dynamical features in a planet's atmosphere (not to be
confused with gravitational waves, see above).
GTO -- Geostationary (or geosynchronous) Transfer Orbit.
H
H2 Main Propulsion System - Within the orbiter aft fuselage, liquid hydrogen and liquid
oxygen pass through the manifolds, distribution lines and valves of the propellant
management subsystem. During prelaunch activities, this subsystem is used to
control the loading of liquid oxygen and liquid hydrogen in the external tank.
During SSME thrusting periods, propellants from the external tank flow into this
subsystem and to the three SSMEs. The subsystem also provides a path that
allows gases tapped from the three SSMEs to flow back to the external tank
through two gas umbilical’s to maintain pressure in
the external tank's liquid
oxygen and liquid hydrogen tanks. After MECO, this subsystem controls MPS
dumps, vacuum inerting and MPS re-pressurization for entry.
HA -- Hour Angle.
Halo orbit -- A spacecraft's pattern of controlled drift about an unstable Lagrange point
(L1 or L2 for example) while in orbit about the primary body (e.g. the Sun).
Heliocentric -- Sun-centered.
Heliopause -- The boundary theorized to be roughly circular or teardrop-shaped,
marking the edge of the sun's influence, perhaps 100 AU from the sun.
Heliosphere -- The space within the boundary of the heliopause, containing the sun and
solar system.
Helium System - During prelaunch, the pneumatic helium supply provides pressure to
operate the liquid oxygen and hydrogen pre-valves and outboard and
inboard fill and drain valves. The three engine helium supply systems are used
to provide anti-icing purges.
HGA -- High-Gain Antenna onboard a spacecraft.
Hohmann Transfer Orbit -- Interplanetary trajectory using the least amount of propulsive
energy.
Horizon -- The line marking the apparent junction of Earth and sky.
h -- Hour, 60 minutes of time.
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Hour Angle -- The angular distance of a celestial object measured westward along the
celestial equator from the zenith crossing. In effect, HA represents the RA for a
particular location and time of day.
HSI – the horizontal situation indicator is used to follow both the glideslope and
localizer. When tuned to the proper frequency, the navigation radio, or NAV,
sends a signal to the HSI and two indicators will appear. The indicators are
oriented perpendicular to each other - one oriented horizontally and the other
vertically. The pilot maneuvers the aircraft so that the indicators form a "+" in the
center of the HSI. When this occurs, the pilot knows that the aircraft is both on
the proper glide path and is lined up with the runway.
HUD -- Head-Up Display or Heads-Up Display is any transparent display that
presents data without requiring users to look away from their usual viewpoints.
The origin of the name stems from a pilot being able to view information with the
head positioned "up" and looking forward, instead of angled down looking at
lower instruments.
Hydraulic System -- This system distributes the hydraulic pressure produced by the
Auxiliary Power Unit (APU) System. The Hydraulic System is made up of three
independent hydraulic systems, each of which is mated to a corresponding APU.
I
IF -- Intermediate Frequency. In a radio system, a selected processing frequency
between RF (Radio Frequency) and the end product (e.g. audio frequency).
IMU – The Inertial Measurement Units consist of an all-attitude, four-gimbal, inertially
stabilized platform. They provide inertial attitude and velocity data to the
navigation software. Guidance uses the attitude data, along with state vectors
from the navigation software, to develop steering commands for flight
control.
Inclination -- The angular distance of the orbital plane from the plane of the planet's
equator, stated in degrees.
Inferior planet -- Planet which orbits closer to the Sun than the Earth's orbit.
Inferior conjunction -- Alignment of Earth, sun, and an inferior planet on the same side
of the sun.
Ion -- A charged particle consisting of an atom stripped of one or more of its electrons.
IR -- Infrared, meaning "below red" radiation. Electromagnetic radiation in the
neighborhood of 100 micrometers wavelength.
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ISOE -- Integrated Sequence of Events.
Isolation valves - The propellant tank isolation valves are located between the
propellant tanks and the manifold isolation valves and are used to isolate the
propellant tanks from the remainder of the propellant distribution system
Isotropic -- Having uniform properties in all directions.
IUS -- Inertial Upper Stage.
K
K-band -- A range of microwave radio frequencies in the neighborhood of 12 to 40 GHz.
Keyhole -- An area in the sky where an antenna cannot track a spacecraft because the
required angular rates would be too high. Mechanical limitations may also
contribute to keyhole size.
Klystron -- A microwave travelling wave tube power amplifier used in transmitters.
Kuiper belt -- A disk-shaped region about 30 to 100 AU from the sun considered to be
the source of the short-period comets.
L
Lagrange points -- Five points with respect to an orbit which a body can stably occupy.
Designated L1 through L5.
LAN -- Local area network for inter-computer communications.
Laser -- Light Amplification by Stimulated Emission of Radiation. Compare with Maser.
Latitude -- Circles in parallel planes to that of the equator defining north-south
measurements, also called parallels.
L-band -- A range of microwave radio frequencies in the neighborhood of 1 to 2 GHz.
LCP -- Left-hand circular polarization.
LEO -- Low Equatorial Orbit.
LGA -- Low-Gain Antenna onboard a spacecraft.
Light -- Electromagnetic radiation in the neighborhood of 1 nanometer wavelength.
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Light speed -- 299,792 km per second, the constant c.
Light time -- The amount of time it takes light or radio signals to travel a certain distance
at light speed.
Light year -- A measure of distance, the distance light travels in one year, about 63,197
AU.
Local time -- Time adjusted for location around the Earth or other planets in time zones.
Longitude -- Great circles that pass through both the north and south poles, also called
meridians.
LOS -- Loss of Signal, used in DSN operations.
LOX -- Liquid oxygen.
M
m, M -- Mass. The kilogram is the standard unit of mass. Mass = Acceleration / Force.
Major axis -- The maximum diameter of an ellipse.
Maser -- A microwave travelling wave tube amplifier named for its process of Microwave
Amplification by Stimulated Emission of Radiation. Compare with Laser. In the
Deep Space Network, masers are used as low-noise amplifiers of downlink
signals, and also as frequency standards.
Mass -- A fundamental property of an object comprising a numerical measure of its
inertia; the amount of matter in the object. While an object's mass is constant
(ignoring Relativity for this purpose), its weight will vary depending on its location.
Mass can only be measured in conjunction with force and acceleration.
Mean solar time -- Time based on an average of the variations caused by Earth's noncircular orbit. The 24-hour day is based on mean solar time.
MECO - Main Engine Cut Off point is where the engines shutdown at about
8 minutes and 30 seconds into the flight.
Meridians -- Great circles that pass through both the north and south poles, also called
lines of longitude.
Meteor -- A meteoroid which is in the process of entering Earth's atmosphere. It is called
a meteorite after landing.
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Meteorite -- Rocky or metallic material which has fallen to Earth or to another planet.
Meteoroid -- Small bodies in orbit about the sun which are candidates for falling to Earth
or to another planet.
MFD -- Multi-function display is a small screen in an aircraft that can be used to
display information to the pilot in numerous configurable ways.
MGA -- Medium-Gain Antenna onboard a spacecraft.
MLI -- Multi-layer insulation (spacecraft blanketing).
Modulation -- The process of modifying a radio frequency by shifting its phase,
frequency, or amplitude to carry information.
MST -- Mountain Standard Time.
Multiplexing -- A scheme for delivering many different measurements in one data
stream.
N
N -- Newton, the SI unit of force equal to that required to accelerate a 1-kg mass 1 m
per second per second (1m/sec2).
Nadir -- The direction from a spacecraft directly down toward the center of a planet.
Opposite the zenith.
NE -- Near Encounter phase in flyby mission operations.
NiCad -- Nickel-cadmium rechargeable battery.
Nodes -- Points where an orbit crosses a reference plane.
Non-coherent -- Communications mode wherein a spacecraft generates its downlink
frequency independent of any uplink frequency.
Nucleus -- The central body of a comet.
Nutation -- A small nodding motion in a rotating body. Earth's nutation has a period of
18.6 years and an amplitude of 9.2 arc seconds.
O
OB -- Observatory phase in flyby mission operations encounter period.
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OMS - The Space Shuttle Orbital Maneuvering System, is a system of rocket engines
for use on the space shuttle orbiter for orbital injection and modifying its orbit
One-way -- Communications mode consisting only of downlink received from a
spacecraft.
Oort cloud -- A large number of comets theorized to orbit the sun in the neighborhood of
50,000 AU.
Opposition -- Configuration in which one celestial body is opposite another in the sky. A
planet is in opposition when it is 180 degrees away from the sun as viewed from
another planet (such as Earth). For example, Saturn is at opposition when it is
directly overhead at midnight on Earth.
OTM -- Orbit Trim Maneuver, spacecraft propulsive maneuver.
OWLT -- One-Way Light Time, elapsed time between Earth and spacecraft or solar
system body.
P
PAM -- Payload Assist Module upper stage.
Parallels -- Circles in parallel planes to that of the equator defining north-south
measurements, also called lines of latitude.
PDT -- Pacific Daylight Time.
PE -- Post Encounter phase in flyby mission operations.
Periapsis -- The point in an orbit closest to the body being orbited.
Perigee -- Periapsis in Earth orbit.
Phase -- The angular distance between peaks or troughs of two waveforms of similar
frequency.
Phase -- The particular appearance of a body's state of illumination, such as the full or
crescent phases of the Moon.
Phase -- Any one of several predefined periods in a mission or other activity.
Photovoltaic -- Materials that convert light into electric current.
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Plasma -- Electrically conductive fourth state of matter (other than solid, liquid, or gas),
consisting of ions and electrons.
PM -- Post meridiem (Latin: after midday), afternoon.
Prograde -- Orbit in which the spacecraft moves in the same direction as the planet
rotates. See retrograde.
PST -- Pacific Standard Time.
Q
Quasar -- Quasi-stellar object observed mainly in radio waves. Quasars are
extragalactic objects believed to be the very distant centers of active galaxies.
R
RA -- Right Ascension.
Radian -- Unit of angular measurement equal to the angle at the center of a circle
subtended by an arc equal in length to the radius. Equals about 57.296 degrees.
RAM -- Random Access Memory.
RCS – The reaction control system is a subsystem of a spacecraft whose purpose
is
attitude control and steering by the use of thrusters. An RCS system is capable
of providing small amounts of thrust in any desired direction or combination of
directions The RCS engines use a Hypergolic Fuel which lights up when its two
components (Fuel and Oxidizer) come into contact. This allows the system to be
almost fail-safe due to the simple nature of the system.
Reflection -- The deflection or bouncing of electromagnetic waves when they encounter
a surface.
Refraction -- The deflection or bending of electromagnetic waves when they pass from
one kind of transparent medium into another.
Retrograde -- Orbit in which the spacecraft moves in the opposite direction from the
planet's rotation. See prograde.
RF -- Radio Frequency.
RFI -- Radio Frequency Interference.
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RGA - The rate gyro assemblies are used by the flight control system during
ascent, entry and aborts as feedbacks to final rate
errors that are used to
augment stability and for display on the commander's and pilot's attitude director
indicator.
Right Ascension -- The angular distance of a celestial object measured in hours,
minutes, and seconds along the celestial equator eastward from the vernal
equinox.
Rise -- As in ascending above the horizon,
ROM -- Read Only Memory.
S
s -- Second, the SI base unit of time.
SA -- Solar Array, photovoltaic panels onboard a spacecraft.
SAR -- Synthetic Aperture Radar
Satellite -- A small body which orbits a larger one. A natural or an artificial moon. Earthorbiting spacecraft are called satellites. While deep-space vehicles are
technically satellites of the sun or of another planet, or of the galactic center, they
are generally called spacecraft instead of satellites.
S-band -- A range of microwave radio frequencies in the neighborhood of 2 to 4 GHz.
SCET -- Spacecraft Event Time, equal to ERT minus OWLT.
SCLK -- Spacecraft Clock Time, a counter onboard a spacecraft.
Sec -- Abbreviation for Second.
Second -- the SI base unit of time.
Semi-major axis -- Half the distance of an ellipse's maximum diameter, the distance
from the center of the ellipse to one end.
Set -- As in going below the horizon.
SI -- The International System of Units (metric system).
SI base unit -- One of seven SI units of measure from which all the other SI units are
derived.
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SI derived unit -- One of many SI units of measure expressed as relationships of the SI
base units. For example, the watt, W, is the SI derived unit of power. It is equal to
joules per second. W = m2 ⋅ kg ⋅ s–3 (Note: the joule, J, is the SI derived unit
for energy, work, or quantity of heat.)
Sidereal time -- Time relative to the stars other than the sun.
SSME - Space Shuttle Main Engines are reusable liquid-fuel rocket engines,
Space Shuttle ascent to orbit is propelled by three engines
each
SOE -- Sequence of Events.
Solar wind -- Flow of lightweight ions and electrons (which together comprise plasma)
thrown from the sun.
SNR -- Signal-to-Noise Ratio.
Specific Impulse -- A measurement of a rocket's relative performance. Expressed in
seconds, the number of which a rocket can produce one pound of thrust from
one pound of fuel. The higher the specific impulse, the less fuel required to
produce a given amount of thrust.
Spectrum -- A range of frequencies or wavelengths.
Star tracker - The star tracker system is part of the orbiter's navigation system which
works to help maintain the IMU during flight.
STS -- Space Transportation System (Space Shuttle).
Subcarrier -- Modulation applied to a carrier which is itself modulated with informationcarrying variations.
T
TCM -- Trajectory Correction Maneuver, spacecraft propulsive maneuver.
TCS - Thermal Conditioning system consists of an air revitalization system, water
coolant loop systems, atmosphere revitalization pressure control system, active
thermal control system, supply water and waste water system, waste collection
system and airlock support system. These systems interact to provide a
habitable environment for the flight crew in the crew compartment in addition to
cooling or heating various orbiter systems or components.
TOS -- Transfer Orbit Stage, upper stage.
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Transducer -- Device for changing one kind of energy into another, typically from heat,
microphone or speaker.
Transponder -- Electronic device which combines a transmitter and a receiver.
TRM -- Transmission Time, UTC Earth time of uplink.
True anomaly -- The angular distance of a point in an orbit past the point of periapsis,
measured in degrees.
U
UHF -- Ultra-high frequency (around 300MHz).
µm -- Micrometer (10-6 m).
Uplink -- Signal sent to a spacecraft.
UT -- Universal Time, also called Zulu (Z) time, previously Greenwich Mean Time. UT is
based on the imaginary "mean sun," which averages out the effects on the length
of the solar day caused by Earth's slightly non-circular orbit about the sun. UT is
not updated with leap seconds as is UTC.
UTC -- Coordinated Universal Time, the world-wide scientific standard of timekeeping. It
is based upon carefully maintained atomic clocks and is highly stable. Its rate
does not change by more than about 100 picoseconds per day. The addition or
subtraction of leap seconds, as necessary, at two opportunities every year
adjusts UTC for irregularities in Earth's rotation.
UV -- Ultraviolet (meaning "above violet") radiation. Electromagnetic radiation in the
neighborhood of 100 nanometers wavelength.
V
Velocity -- A vector quantity whose magnitude is a body's speed and whose direction is
the body's direction of motion.
W
W -- Watt, a measure of electrical power equal to potential in volts times current in
amps.
Walking orbit -- A spacecraft orbit that precesses, wherein the location of periapsis
changes with respect to the planet's surface in a useful way.
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Wavelength -- The distance that a wave from a single oscillation of electromagnetic
radiation will propagate during the time required for one oscillation.
Weight -- The gravitational force exerted on an object of a certain mass. The weight of
mass m is mg Newtons, where g is the local acceleration due to a body's gravity.
WWW -- World-Wide Web.
X
X-band -- A range of microwave radio frequencies in the neighborhood of 8 to 12 GHz.
X-ray -- Electromagnetic radiation in the neighborhood of 100 picometer wavelength.
Z
Z -- Zulu in phonetic alphabet, stands for UT, Universal Time.
Zenith -- The point on the celestial sphere directly above the observer. Opposite the
nadir.
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Reference Sources
“How Space Shuttles Works” by Craig Freudenrich, Ph.D.
http://science.howstuffworks.com/space-shuttle1.htm
“Space Shuttle Operating Systems” by National Aeronautics and Space Administration”
http://science.ksc.nasa.gov/shuttle
“The Basics of Flight” by Jet Propulsion Laboratory, California Institute of Technology
http://www2.jpl.nasa.gov/basics/index.php
This New Ocean, The History of Space Flight”, Prof. James Schombert
http://abyss.uoregon.edu/~js/space/
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