Nuclear Applications in Space

Why Nuclear For Space Exploration? ALREADY IN USE

• Nuclear are a million times more dense than chemical fuels Radioisotope Thermoelectric Generators (RTGs) • Chemical fuels have reached their practical limits • Nuclear reactors give more thrust allowing missions to be completed faster, RTGs have been used to produce power on space probes meaning less exposure time for astronauts to hostile space environment and other missions for the past 25 years. They use the natural • Radioactive are able to provide and for several decades decay of -238 to create about 230 W of electricity. The Cassini Mission is powered • Only nuclear reactors are a practical source of electricity as we move farther by RTGs and the systems kept Ideal for interplanetary missions, they are compact weighing and farther away from the warm by pellets of Plutonium. 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 Energy is Derived from Nuclear Reactions What About From Space Reactors? probes and vehicles. Because the natural decay produces heat, they are optimal for providing warmth for computers and other systems needing room temperature operation. Radioisotope Thermoelectric Space is essentially an ocean of radiation. The Sun Generator (RTG) • Fission occurs when a free strikes a heavy such as gives off far more radiation from its fusion than we or Plutonium. This collision causes the atom to break apart or fission. could ever become close to matching. The Earth’s • The atom splits apart into two highly energetic fragments which magnetic field protects us from this harmful deposit their energy making heat • Also 2-3 additional result which can strike other radiation. However, astronauts are exposed to this, causing them to fission resulting in a chain reaction and spending too much time in space can lead to • The reaction rate can be controlled in a allowing production of electricity from the heat generated health effects. It is important that space crafts be shielded from the hostile radiation environment of space.

Nuclear Fusion Fusion & Future Propulsion Although nuclear reactors give off radiation, the • Fusion occurs when two atoms smash into each other and crew can be protected by distance and shielding. combine Magnetic Confined Fusion (MCF) Propulsion • The products are lighter than the reactants meaning some of the Note the reactor is located on the end of the boom gets converted to energy in the picture of the ship on the right, a safe • Fusion is more energy dense than fission • The most common reaction involves two isotopes distance from the crew. Nuclear reactors allow This concept is based on the Magnetic Fusion concept. It confines ( and ) fusing to make Deuterium and Tritium (D-T) with a magnetic field. The D-T ions • is the process powering the ships to reach their destination faster actually • As of yet, fusion as an electricity source has not yet been achieved lowering their total . Since are heated to a temperature of 100 million degrees C. All at this and is currently being researched reactors are well contained, it can withstand any state becomes a or ionized gas and must be confined with a reentry disasters and pose little to no risk to the magnetic field. These ions are moving so fast that they fuse when they smash into each other. The reaction creates highly energetic general public should such a scenario occur. byproducts which are accelerated out the back of the engine propelling the craft forward.

Inertial Confined Fusion (ICF) Propulsion

Fission Reactors In Space This engine works on the Inertial Fusion concept. A small D-T pellet is injected into the reactor chamber. Several or heavy beams Nuclear Fission Propulsion works by having a reactor generate Fission Fragment Interstellar Probes fire simultaneously at the target pellet causing the pellet to collapse heat. Liquid Hydrogen or Ammonia propellant is pumped into a and inducing a small thermonuclear explosion similar to a hydrogen

vessel by the reactor. The propellant is heated up, vaporizes, . The force of the explosion propels the craft forward. Main The fragments from a fission reaction are extremely energetic and and is ejected out of a nozzle propelling a spacecraft forward. technical difficulties are in the driver systems being very heavy could be used for propulsion. The is located on thin disks that and requiring a great deal of power. rotate in and out of the reactor (see figure to left). Because the The first fission propulsion systems were investigated in the 1960s and 1970s. The disks are thin, many of the fragments can escape and be accelerated

capstone design from this program was by a magnetic field. These fragments are ejected out of the probe and the ship is propelled forward at extremely fast velocities. It is Inertial Electrostatic Confinement (IEC) Fusion Propulsion called NERVA (Nuclear Engine for Rocket Vehicle Application). The program was also possible to attach a sail to the probe allowing the fragments to

cancelled in 1972 as the finishing touches push the probe even more when far away from the Sun. The high Electrostatic Fields are used to accelerate fusion fuels (either D, T, or speeds this craft can reach make it ideal for probing nearby stars in 3He) toward the center of the grid. The grid is mostly transparent and of the propulsion system were being applied. Fission propulsion is a tested and feasible the future. the particles are accelerated toward the center at which point they technology. Current research is in strike each other and fuse. The fusion fragments are accelerated out of NERVA Rocket Prototype engineering nozzles and propellant Design and conception of the Fission the reactor and are used to propel the craft forward. circulation systems. Fragment Interstellar Probe. Designs for early Nuclear Fission Reactor Propulsion systems in 1960s and 1970s. Icy Moons Orbiter

Antiproton Catalyzed Micro-fission/Fusion Propulsion NASA has recently proposed to start on the Jupiter Icy

Moons Orbiter (JIMO) to be completed by around 2011. JIMO is This propulsion scheme uses pellets mixed of Uranium and D-T fuels. designed to orbit three of Jupiter’s moons: Europa, Ganymede, and Callisto. Lasers or heavy ion beams compress the pellet. At maximum compression, a small number of antiprotons (10^9) are fired at the 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 pellet to catalyze the Uranium fission process. The fission heat causes us about their surfaces and perhaps some clues as to their origins. a fusion burn and the expanding plasma pushes the craft forward. Additionally, JIMO will measure the radiation levels near the This system gets around typical restrictions of propulsion moons. JIMO is to be powered by a nuclear fission reactor 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 Artist’s conception of the Jupiter Icy Moons projected to have a power output of around 250,000 Watts. Orbiter approaching Europa. The fission reactor Compare this to Cassini which runs on a mere 100 Watts of ton payload. is located at the end of the boom near the top of the picture. electricity. JIMO will illustrate the power of nuclear fission

reactors on space probes.

“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