Nuclear power has applications for space exploration due to its high energy density and ability to provide power for decades. Nuclear fission and fusion reactions can be used to provide electricity, heat, and propulsion. Current technologies using radioisotope thermoelectric generators and heat generators have powered missions for over 25 years. Future missions could use more advanced fission and fusion reactors to power longer missions like the proposed Jupiter Icy Moons Orbiter, and innovative designs may allow for interstellar travel using fission fragments or various fusion propulsion methods. Nuclear power provides benefits for space exploration by allowing longer missions with less radiation exposure compared to chemical fuels.

1. Nuclear Power Applications in Space
American Nuclear Society
Why Nuclear For Space Exploration?
• 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
“Without nuclear-powered spacecraft, we'll never get anywhere”
-- Dr. Robert Zubrin
Energy is Derived from Nuclear Reactions
Nuclear Fission
• 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
NUCLEAR POWER ALREADY IN USE
Radioisotope Thermoelectric Generators (RTGs)
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.
Plutonium Heat Generators
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.
The Cassini Mission is powered
by RTGs and the systems kept
warm by pellets of Plutonium.
Radioisotope Thermoelectric
Generator (RTG)
What About Radiation From Space Reactors?
Fission Reactors In Space
Fusion & Future Propulsion
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.
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.
NERVA Rocket Prototype
Designs for early Nuclear Fission Reactor
Propulsion systems in 1960s and 1970s.
Jupiter Icy Moons Orbiter
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.
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.
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.
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.
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.
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.
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.
Images courtesy of NASA, JK Rawlings, and JPL
Poster by Brian C Kiedrowski