Cassini–Huygens
Setting off for Saturn
S
NASA/ESA
until, at about half the planetary radius, a
aturn and its rings make a glorious object
Andrew Coates sets the scene for
metallic hydrogen mantle forms. Currents
in the night sky. At 9–10 AU from the Sun
an epic journey of exploration to
flowing here power Saturn’s magnetic field.
it is rather remote. Up to now only three
Saturn, its rings and moons, which
Inside this, spacecraft flyby trajectories indiNASA flyby missions have visited the planet –
cate the presence of a small rocky core about a
Pioneer 11 and Voyagers 1 and 2 in 1979–81.
begins with the launch of Cassini
quarter of the planetary radius. One of the
These missions revolutionized our knowledge
and its probe, Huygens.
tasks for Cassini is to probe the internal strucgained from remote observation, but many
ture and refine these models.
questions remain. Now, we are going back
Saturn’s large orbit means that it takes almost
with NASA’s Cassini Saturn orbiter and ESA’s
origin, but the clouds are not as brightly
30 Earth years to go around the Sun, and the
Huygens Titan probe, to learn more about the
coloured as Jupiter’s or as blue as Neptune’s.
temperature at the cloud tops is 140 K. The
planet and its companions. The results will
The detailed composition of the atmosphere is
chemical composition is closely related to the
help us to understand conditions at the beginone of Cassini’s targets.
solar nebula from which the planet condensed.
ning of the Solar System.
Moving below the clouds the gas pressure
The light elements were kept because the evolvCassini–Huygens may well be the last broadincreases under the huge gravitational field
ing planet was heavy
ly instrumented mission to
enough and cold enough.
visit the outer Solar System. The Cassini Spacecraft
The average density of SatThe international planetary
low-gain antenna
4 m high-gain
urn is less than that of
community is poised to reap
(1 of 2)
antenna
water – the lowest of all the
the scientific rewards to be
planets with the possible
gained from this unique
11 m magnetometer
boom
exception of Pluto. Comventure, following on from
radar bay
bined with the rapid spin
Galileo at Jupiter. If Cassini
period of 10 hours, this
were proposed to NASA
means that the radius of the
now it would not be selectfields and
planet is 10% lower at the
ed: it is far from the “smallparticles pallet
radio/plasma
poles than at the equator.
er, faster, cheaper” and
wave subsystem
One of the interesting
“focused science” philosoantenna (1 of 3)
topics
for Cassini will be
phies in vogue today. Other
Huygens Titan probe
the wind structures on the
agencies would be unlikely
remote sensing pallet
surface. The wind speed
to do it on their own. With
reaches 1800 kilometres
Huygens, however, ESA has
radioisotope
per hour at the equator,
secured a central role in the
thermoelectric
moving towards the east.
Cassini mission and its scigenerator (1 of 3)
445 N engine (1 of 2)
This is two thirds of the
ence. The UK is particularly
speed of sound locally and
well represented.
n 6 October a 6.8 metre tall, 5.6
much higher, and less structured, than Jupiter’s
The probe is named after Christiaan Huygens
tonne spacecraft will leave
wind patterns. There are also features such as
who found that the rings were separate from
the long-lived ovals and brown spots, and
Saturn itself and, in 1655, discovered Titan.
Earth on its journey towards Saturn.
shorter lived features such as the white spots
Jean-Dominique Cassini, whose name the
On its arrival in July 2004, the
which appear in the Saturn summer. These
orbiter bears, discovered the icy satellites IapeCassini–Huygens mission will begin
were last seen in 1990–1994 in Saturn’s northtus, Rhea, Tethys and Dione and in 1675 disthe most broad ranging and detailed
ern hemisphere. Cassini’s tour will be during
covered a division in the rings which now also
the southern hemisphere summer and autumn,
bears his name. Cassini also postulated that the
exploration of the Saturn system
so a close-up study of white spot formation
rings were composed of individual satellites
that mankind is likely to undertake
and dynamics may be possible.
rather than being a solid object. This idea had
in the foreseeable future. The fourSaturn is a source of heat, throwing out 1.8
to wait until 1857 to be proved theoretically by
times as much energy as it receives from the
James Clerk Maxwell, and a further 30 years
year mission will centre on Saturn
Sun. This is unlikely to be left over from the
for experimental proof.
and its rings, the large and rocky
heat of formation. A possible explanation
moon Titan, the icy satellites and the
Planet Saturn
involves the precipitation of helium which is
magnetosphere. Here I review what
not soluble in the elevated pressure and densiThe second largest gas giant in our Solar Systy in the metallic hydrogen mantle. As the pretem, Saturn is composed mainly of hydrogen
is known about this fascinating
cipitation occurs, gravitational energy is
and helium. The visible cloud structures, at
object, describe the mission and
released, acting as a heat source and depleting
almost 10 Earth radii from the centre of the
focus on the broad range of UK
the upper layers of helium. A helium depletion
planet, are caused by ammonia crystals susinvolvement in the science.
was seen by Voyager, but Cassini will study the
pended in the lighter gases. There are light and
mechanism in much more detail.
dark bands in the clouds of unknown chemical
O
October/November 1997 Vol 38 Issue 5
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Cassini–Huygens
Ring system
Saturn’s rings form the only planetary ring system visible from Earth. The ring system is
huge; if Saturn were at Earth’s position, the
main rings would reach almost to our moon.
Of Saturn’s main ring systems, labelled A–G in
order of their discovery, only the five brightest
were known prior to the flyby missions. The
flybys discovered further faint features including an eighth system which is formed from
structure within the Cassini division itself.
The rings are composed of billions of individual pieces of ice and dust, with sizes which
vary from microns to tens of metres. The rings
are less than a kilometre thick and as thin as
tens of metres in places. As well as the additional rings, the flybys found that the structure
in each ring is extremely complex; the results
opened new avenues of Solar System dynamics.
The ring structures include warps, grooves,
braids, clumps, spokes, kinks, splits, resonances and gaps. Some well known causes
include gravitational interactions between the
ring particles themselves and with nearby
“shepherd” moons. Less well known is the
interaction with the plasma and magnetic field.
It is likely that electrostatic charging produces
the spoke structures in the B-ring seen by Voyager, but Cassini will study this aspect as well
as many other questions concerning the ring
structure. The dust–plasma–radiation interactions are reminiscent of those occurring in the
forming solar nebula and in much more distant
objects.
The composition of the rings will also be
studied using Cassini’s sensitive mass spectrometers and dust instrumentation. The
results will help to show where the rings came
from in the first place. At present, it is unclear
whether a forming satellite was torn apart by
gravitational forces, as the major part of the
rings is within the Roche limit, or whether
smaller bodies were bombarded by asteroids
from outside Saturn’s system. Composition
studies will also be important in providing
information about the formation of the early
Saturn from the solar nebula.
Titan
The Huygens probe will make a detailed study
of Titan during its two and a half hour descent
through the thick atmosphere. At the surface
the atmospheric pressure is larger than that at
Earth. The flyby missions found the face of
Titan to be shrouded by cloud and haze. The
principal atmospheric gas was nitrogen with
the bulk of the remainder being methane. The
chemical processes at work depend critically
on height, the amount of light penetrating the
clouds, and at high altitudes on the influence of
Saturn’s magnetosphere.
Complex hydrocarbons formed by photochemical reactions are probably present in the
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atmosphere and may even precipitate onto the
surface of the moon. The frigid surface could
be solid rock or ice, or could be covered by
lakes of liquid methane and/or ethane. The
temperature at the surface is close to the triple
point of methane, 90.7 K, but until Huygens
reaches the surface its physical nature will
remain unclear.
The probe’s in-situ atmospheric and surface
measurements will be complemented by measurements from the orbiter during over 40 flybys of Titan. Remote sensing instruments, such
as the Cassini radar, and in-situ measurements
such as the ionosphere and plasma environment, will make major contributions to our
understanding of Titan’s nature and environment. Titan flybys are also used as a gravitational “slingshot” to “crank” the orbital petals
of Cassini’s four year tour.
Icy satellites
The diversity of Saturn’s satellites is remarkable. As well as Titan, similar to a terrestrial
planet, there are at least 20 icy moons which
are quite different from each other. Voyager
scientists discovered six new moons and a further three were added by HST observations
during Earth’s recent ring-plane crossing. Some
of these are in the same orbit at different phases, another is inside Encke’s division in ring A,
and yet another is on that ring’s outside edge.
Iapetus and Enceladus, two of the more major
moons, illustrate the diversity well. Enceladus
orbits at four Saturn radii and has a relatively
smooth face illustrating recent geological activity. The heat driving this activity is from an
unknown source. Ice volcanoes may still be
active, belching material into Saturn’s E-ring.
Iapetus, on the other hand, orbits at almost 60
Saturn radii on an elliptical path. It has both a
light and a dark face. The origin of this difference could be due to geological activity or due
to material swept up along the orbit. Cassini
will unshroud some of the mysteries of Iapetus.
Many of the moons show evidence of internal activity, and Mimas spectacularly shows
the result of outside influences. It has a crater
covering a third of its diameter, as well as other
marks that show that it was fortunate to survive the enormous collision that left this scar.
Cassini will have close flybys of Mimas, Enceladus, Dione, Rhea and Iapetus, with more distant flybys of other moons. The geological and
thermal histories, as well as the detailed composition, will be determined.
Saturn’s magnetosphere
Saturn has a strong magnetic field; at the cloud
tops its magnitude is close to that at the Earth’s
surface. This means that the planet has a large
magnetosphere where the planetary magnetic
field dominates that of the solar wind. As at
Jupiter, a large portion of the magnetosphere
co-rotates with the planet. The magnetic field
strength, plasma density and temperature in
the solar wind are less than those at the Earth
due to the larger distance from the Sun, but the
speed is about the same. The solar wind sonic
and Alfven Mach numbers are higher than at
Earth, modifying the bow shock. Unlike other
planets, the magnetic field axis is closely
aligned with the spin axis.
As well as different solar wind conditions
and larger size, there are other major differences to Earth’s case which make Saturn’s magnetosphere uniquely complex. First, the dust in
Saturn’s system (including the rings) soaks up
energetic particles and supports dust–plasma
interactions and modified wave modes. Secondly, material from Titan’s atmosphere forms
a neutral torus which is a source of nitrogen
and hydrogen ions for the magnetosphere.
Thirdly, sputtered material from the icy satellites add water and other ions into the magnetosphere. Direct electron penetration into Saturn’s magnetic cusp regions probably forms the
Saturn kilometric radiation which makes the
planet a radio source. Modulations of this
revealed the spin rate of the planet.
Cassini will determine the sources and sinks
of particles in the magnetosphere. It will also
discover the importance of distance and
timescales compared to Earth’s magnetosphere
which will test theories of both. Composition
of sputtered products from the satellites will
tell us more about their composition. The presence of magnetic fields or cometary interactions of the satellites will be revealed. As the
solar wind changes, Saturn’s magnetopause
spends some time inside Titan’s orbit and the
difference this causes to the interaction and
perhaps the atmospheric chemistry will be
looked at. Studies of this complex magnetosphere will be challenging, but the processes at
work are relevant in other Solar System contexts as well as in astrophysics.
The spacecraft and the mission
Cassini and Huygens will remain attached to
each other until after the crucial Saturn orbit
insertion burn some seven years into the mission. The orbiter design is driven by this
requirement, as well as by the usual design
requirements from communications, attitude
sensing, propulsion, power, thermal and instrument accommodation. The high gain antenna,
provided by the Italian space agency and used
for science data transmission, is 4 m across.
The transmitter power is only 19 W and the
data rate to Earth varies between 5 and
249 kilobits per second. The antenna also collects data from Huygens during its descent
towards Titan and acts as the transmitter for
the Cassini radar. From Saturn, radio signals
take over an hour to reach the Earth and significant autonomy is needed. Due to the large
October/November 1997 Vol 38 Issue 5
Cassini–Huygens
1
2
3
1 This Voyager image shows spokes in the rings of
Saturn, features that Cassini will investigate.
(NASA/JPL)
2 The Electron Spectrometer in the calibration system
at the Mullard Space Science Laboratory. (MSSL-UCL)
3 An artist’s impression of Cassini flying close to the
rings and firing engines to begin its orbits of Saturn.
(Dave Seal/ NASA)
distance from the Sun it is impractical to use
solar power, so three radioisotope thermoelectric generators provide the necessary 650 W
for the spacecraft and instruments.
The Titan 4 launch, although powerful, does
not give Cassini/Huygens enough energy to
reach Saturn directly. The spacecraft will be
directed to fly past Venus (twice), Earth and
Jupiter for gravity assists which will ultimately
take the craft to its destination almost seven
years into the mission. During this time
preparatory events such as cover releases and
calibration sequences will be performed by the
on-board instruments. The Earth and Jupiter
flybys offer attractive additional science
returns from these tests.
Operations will start in a relatively continuous manner just past Jupiter, two years before
the Saturn orbit insertion. For the plasma payload this gives another important target – measurements in the distant solar wind. Although
the Pioneers and Voyagers made some measurements here, they were not sufficient to
measure, for example, the electron heat flux.
This is important for theories of the solar wind
expansion and Cassini measurements are
expected to provide the answers. Dust measurements during cruise will also be important
for determining dust stream dynamics near and
beyond Jupiter, following on from Galileo
results. A flyby of distant moon Phoebe, which
seems to have some characteristics of an asteroid, will be made nearly three weeks before
Saturn orbit insertion, and the magnetic effect
October/November 1997 Vol 38 Issue 5
of Saturn will be felt for the first time as the
spacecraft crosses the bow shock a couple of
days before this important manoeuvre.
On 1 July 2004, the spacecraft will finally
reach its target, flying close above Saturn’s
rings. During this time the electron, magnetic
field and imaging experiments may find the
answer to how the ring spoke structure occurs.
Near apoapsis, the main engine on board will
fire for over an hour to slow the spacecraft so
that it is captured into Saturn orbit. The first
orbit around Saturn will be huge, taking about
five months.
On the way towards the first Titan flyby, the
Huygens probe will be released. The batterydriven probe has thermal protection tiles on its
front surface to protect it against the 12 000 K
temperatures expected on entry into the
atmosphere. Three separate parachutes are
used one after the other to slow the probe
through the Titan atmosphere. Five of the six
probe instruments will measure the atmospheric winds, composition and chemistry, and one
will search for lightning, before a 5.5 metre per
second impact on the surface. If the probe survives impact the nature of the surface will be
determined. The probe and orbiter will reach
Titan on 27 November 2004. During the probe
mission the orbiter is used as a data relay.
The orbiter will then be well into its four year
tour of Saturn and its satellites. The scientific
priorities for the various parts of the tour,
including flybys, local time coverage, and inclination requirements have already been iterated
by the Project Science Group and the various
discipline working groups, but much of the
detailed planning is postponed until after
launch. This will allow time to determine onboard scientific performance of the payload
and to fold this into the plan. The four year
tour of Saturn and its moons will be an exciting and hectic time for planetary scientists.
UK involvement
The UK has contributed to six of the twelve
orbiter instruments and two of the six on the
probe. One UK Principal Investigator is on
each spacecraft. This is a substantial involvement costing almost £7.5 million.
UK scientists are involved in all areas of science that Cassini–Huygens will study. Imaging
of Saturn, its moons and rings from visible to
infrared, particles and field measurements with
both plasma and planetological relevance, the
Saturn and Titan atmospheres and the nature
of the surface of Titan will all be studied. It is
the UK’s excellence in space science and instrumentation, and hard work over the seven years
since selection, that has led to this unrivalled
scientific opportunity. In another seven years
we should be reaping the benefits. ●
Dr Andrew Coates is a Royal Society University
Research Fellow and Reader in Physics at University
College London’s Mullard Space Science Laboratory.
He is a co-investigator on the Cassini Plasma Spectrometer and leads the team providing the Electron
Spectrometer.
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