To The Galileo Spacecraft Contents PageNuclear-powered spacecraft have orbited the Earth and probed deep space for over twenty years. Nuclear power supplies a constant source of electricity over a long lifetime with high reliability, insensitivity to the chilling cold of the outer reaches, and virtual invulnerability to high radiation fields such as Earth's Van Allen belts and Jupiter's sizzling magnetosphere.
The Galileo mission will be the 24th U.S. space mission since 1961 to be powered partially or totally by nuclear power sources. These missions, for both the U.S. military and NASA, have included Earth-orbiting weather, communications, and navigational satellites, as well as the Apollo, Pioneer, Viking, and Voyager space programs. The Soviet Union uses nuclear-powered spacecraft as well. The Galileo orbiter will carry two 285-watt (electrical)* general purpose heat source (GPHS) radioisotope thermoelectric generators (RTGs) while approximately 112 one-watt (thermal) radioisotope heater units (RHUs) will warm scientific instruments onboard both the orbiter and the probe.
An RTG (see figure) basically consists of two parts: a source of heat and a system for converting the heat to electricity. The heat source contains a radioisotope, such as plutonium-238, that becomes physically hot from its own radioactivity decay. This heat is converted to electricity by a thermoelectric converter which uses the Seebeck effect, a basic principle of thermoelectricity discovered in 1822. An electromotive force, or voltage, is produced from the diffusion of electrons across the junction of two different materials (e.g., metals or semiconductors) that have been joined together to form a circuit when the junctions are at different temperatures. Junctions of different metal wires are used to measure temperatures and are called thermocouples.
Doping semiconductor materials such as silicon-germanium with small amounts of impurities such as boron or phosphorus produces an excess or deficiency of electrons, and therefore makes the semiconductor a more efficient power converter than metals. The joining of these thermoelectric materials with hot radioisotopes produces a reliable source of power with no moving parts. The temperature difference between the hot and cold junctions in these thermocouples is about 700 K (800 deg F).
Nuclear safety is a major factor in the design of these power sources. Plutonium-238 decays primarily by emitting alpha particles, which are completely absorbed in the heat source to produce heat; thus, no special radiation shielding is necessary to absorb these particles. (Moderate neutron and gamma-ray fields exist external to the RTG, requiring isolation of the RTGs from the rest of the spacecraft to prevent interference with the scientific measurements. Therefore, each RTG will be mounted at the end of a 5-meter (16-foot) boom.) The principal safety objective connected with the use of plutonium-238 is to keep it contained to prevent contamination of the surrounding environment. The half-life of 238Pu is about 87.8 years, and nuclear-powered Earth-orbiting satellites have been replaced in orbits where they will not reenter the Earth's atmosphere until the radioactive material has decayed to harmless levels. After the Soviet Cosmos 954 satellite (which carried a nuclear reactor) fell to Earth over Canada in 1978, the U.N. established a working group on the use of nuclear power sources in outer space which concluded that nuclear power sources "can be used safely in outer space provided that all necessary safety requirements are met."**
Each 55-kg (121-pound) GPHS RTG contains approximately 11 kg (24 pounds) of plutonium dioxide fuel, pressed into 72 solid ceramic-like cylindrical 2.5- by 2.5-cm (1- by 1-inch) pellets.
Each heat source consists of 18 separate modules, each of which multiply encases four Pu-238 pellets. The modules are designed to survive under a range of postulated accidents: launch vehicle explosion or fire, reentry into the atmosphere followed by land or water impact, and post-impact situations. Graphitic outer coverings provide protection against the structural, thermal, and ablative environments of a potential reentry; additional graphitic components provide impact protection, and iridium cladding of the actual fuel cells provides post-impact containment. The GPHS RTGs are designed to release the 18 modules individually in the event of an accidental reentry.
The RTGs for the Galileo project are identical to those to be used for the International Solar Polar Mission (ISPM) (now known as the Ulysses Project). The current development program includes RTGs for both Galileo and ISPM, as well as a spare.
The Office of Special Nuclear Projects of the U.S. Department of Energy (DOE) is responsible for the government RTG program, while the General Electric Company at Valley Forge, Pennsylvania, is the system contractor responsible for the design and development of the electrical converter and heat source. The fuel is fabricated and encapsulated in the iridium cladding at DOE's Savannah River Plant, South Carolina, and shipped to DOE's Mound Plant in Miamisburg, Ohio, where the pellets are loaded into the graphite modules. Here, the heat source modules are also installed into the generators and qualification and flight acceptance tests are conducted. Oak Ridge National Laboratory, Tennessee, provides graphite insulation and iridium for the post-impact containment structure. Safety testing is conducted at Los Alamos National Laboratory, New Mexico, with independent reliability and quality assurance support provided by Sandia National Laboratories, Albuquerque, New Mexico. Independent safety and technical support is supplied by the Applied Physics Laboratory, NUS Corporation, and Fairchild Industries.
The thermoelectric converter for the qualification unit has been fueled at Mound, and is currently undergoing testing. The flight thermoelectric converters for Galileo have been fabricated but will not be fueled until later. The RTGs will be stored until shipment to Kennedy Space Center, Florida, where they will be installed on the spacecraft and tested. They will then be removed and stored until final installation on the spacecraft and tested. They will then be removed and stored until final installation on the spacecraft in the Shuttle payload bay on the launch pad several days before launch.
Thanks to G. L. Bennett, DOE, and R. W. Campbell, JPL, for source material and review comments.
* The thermal power at the beginning of the mission will be 4,410 W per generator.
** United Nations Committee on the Peaceful Uses of Outer Space, "Report of the Working Group on the Use of Nuclear Power Sources in Outer Space on the work of its third Session," Annex II to "Report of the Scientific and Technical Subcommittee on the Work of its Eighteenth Session," U.N. document A/AC.105/287, 13 February 1981.
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