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 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