Nuclear Power Applications in Space
NUCLEAR POWER ALREADY IN USE
Why Nuclear For Space Exploration?
Radioisotope Thermoelectric Generators (RTGs)
• Nuclear fuels are a million times more energy dense than chemical fuels
• Chemical fuels have reached their practical limits
• Nuclear reactors give more thrust allowing missions to be completed faster,
meaning less exposure time for astronauts to hostile space environment
• Radioactive isotopes are able to provide heat and electricity for several decades
• Only nuclear reactors are a practical source of electricity as we move farther
and farther away from the Sun
RTGs have been used to produce power on space probes
and other missions for the past 25 years. They use the natural
decay of Plutonium-238 to create about 230 W of electricity.
Ideal for interplanetary missions, they are compact weighing
only 120 lbs, 45 inches in height, 18 inches in diameter and
operate unattended for several decades.
The Cassini Mission is powered
by RTGs and the systems kept
warm by pellets of Plutonium.
Plutonium Heat Generators
Energy is Derived from Nuclear Reactions
What About Radiation From Space Reactors?
Nuclear Fission
Space is essentially an ocean of radiation. The Sun
gives off far more radiation from its fusion than we
could ever become close to matching. The Earth’s
magnetic field protects us from this harmful
radiation. However, astronauts are exposed to this,
and spending too much time in space can lead to
health effects. It is important that space crafts be
shielded from the hostile radiation environment of
space.
• Fission occurs when a free neutron strikes a heavy atom such as
Uranium or Plutonium. This collision causes the atom to break apart
or fission.
• The atom splits apart into two highly energetic fragments which
deposit their energy making heat
• Also 2-3 additional neutrons result which can strike other atoms
causing them to fission resulting in a chain reaction
• The reaction rate can be controlled in a nuclear reactor allowing
production of electricity from the heat generated
Nuclear Fusion
• Fusion occurs when two light atoms smash into each other and
combine
• The products are lighter than the reactants meaning some of the
mass gets converted to energy
• Fusion is more energy dense than fission
• The most common reaction involves two hydrogen isotopes
(Deuterium and Tritium) fusing to make Helium
• Nuclear fusion is the process powering the stars
• As of yet, fusion as an electricity source has not yet been achieved
and is currently being researched
Although nuclear reactors give off radiation, the
crew can be protected by distance and shielding.
Note the reactor is located on the end of the boom
in the picture of the ship on the right, a safe
distance from the crew. Nuclear reactors allow
ships to reach their destination faster actually
lowering their total radiation exposure. Since
reactors are well contained, it can withstand any
reentry disasters and pose little to no risk to the
general public should such a scenario occur.
Fission Reactors In Space
Nuclear Fission Propulsion works by having a reactor generate
heat. Liquid Hydrogen or Ammonia propellant is pumped into a
vessel by the reactor. The propellant is heated up, vaporizes,
and is ejected out of a nozzle propelling a spacecraft forward.
NERVA Rocket Prototype
Designs for early Nuclear Fission Reactor
Propulsion systems in 1960s and 1970s.
Artist’s conception of the Jupiter Icy Moons
Orbiter approaching Europa. The fission reactor
is located at the end of the boom near the top of
the picture.
The first fission propulsion systems were
investigated in the 1960s and 1970s. The
capstone design from this program was
called NERVA (Nuclear Engine for Rocket
Vehicle Application). The program was
cancelled in 1972 as the finishing touches
of the propulsion system were being applied.
Fission propulsion is a tested and feasible
technology. Current research is in
engineering nozzles and propellant
circulation systems.
Small amounts of Plutonium-238 are often placed on space
probes and vehicles. Because the natural decay produces heat,
they are optimal for providing warmth for computers and
other systems needing room temperature operation.
Fission Fragment Interstellar Probes
The fragments from a fission reaction are extremely energetic and
could be used for propulsion. The fuel is located on thin disks that
rotate in and out of the reactor (see figure to left). Because the
disks are thin, many of the fragments can escape and be accelerated
by a magnetic field. These fragments are ejected out of the probe
and the ship is propelled forward at extremely fast velocities. It is
also possible to attach a sail to the probe allowing the fragments to
push the probe even more when far away from the Sun. The high
speeds this craft can reach make it ideal for probing nearby stars in
the future.
Design and conception of the Fission
Fragment Interstellar Probe.
Radioisotope Thermoelectric
Generator (RTG)
Fusion & Future Propulsion
Magnetic Confined Fusion (MCF) Propulsion
This concept is based on the Magnetic Fusion concept. It confines
Deuterium and Tritium (D-T) ions with a magnetic field. The D-T ions
are heated to a temperature of 100 million degrees C. All matter at this
state becomes a plasma or ionized gas and must be confined with a
magnetic field. These ions are moving so fast that they fuse when they
smash into each other. The reaction creates highly energetic
byproducts which are accelerated out the back of the engine propelling
the craft forward.
Inertial Confined Fusion (ICF) Propulsion
This engine works on the Inertial Fusion concept. A small D-T pellet is
injected into the reactor chamber. Several lasers or heavy ion beams
fire simultaneously at the target pellet causing the pellet to collapse
and inducing a small thermonuclear explosion similar to a hydrogen
bomb. The force of the explosion propels the craft forward. Main
technical difficulties are in the laser driver systems being very heavy
and requiring a great deal of power.
Inertial Electrostatic Confinement (IEC) Fusion Propulsion
Electrostatic Fields are used to accelerate fusion fuels (either D, T, or
3He) toward the center of the grid. The grid is mostly transparent and
the particles are accelerated toward the center at which point they
strike each other and fuse. The fusion fragments are accelerated out of
the reactor and are used to propel the craft forward.
Jupiter Icy Moons Orbiter
NASA has recently proposed to start work on the Jupiter Icy
Moons Orbiter (JIMO) to be completed by around 2011. JIMO is
designed to orbit three of Jupiter’s moons: Europa, Ganymede,
and Callisto.
JIMO’s mission is to find evidence of life on the moons such as
the existence of oceans. It will collect data that will hopefully tell
us about their surfaces and perhaps some clues as to their origins.
Additionally, JIMO will measure the radiation levels near the
moons. JIMO is to be powered by a nuclear fission reactor
projected to have a power output of around 250,000 Watts.
Compare this to Cassini which runs on a mere 100 Watts of
electricity. JIMO will illustrate the power of nuclear fission
reactors on space probes.
Antiproton Catalyzed Micro-fission/Fusion Propulsion
This propulsion scheme uses pellets mixed of Uranium and D-T fuels.
Lasers or heavy ion beams compress the pellet. At maximum
compression, a small number of antiprotons (10^9) are fired at the
pellet to catalyze the Uranium fission process. The fission heat causes
a fusion burn and the expanding plasma pushes the craft forward.
This system gets around typical restrictions of antimatter propulsion
because it uses a relatively small amount of expensive antimatter. This
craft would be capable of reaching Pluto in 3 years with a 100 million
ton payload.
“Without nuclear-powered spacecraft, we'll never get anywhere”
-- Dr. Robert Zubrin
Images courtesy of NASA, JK Rawlings, and JPL
Poster by Brian C Kiedrowski