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The Systems Nuclear Auxiliary Power Program (SNAP) was a program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s by NASA. Odd-numbered SNAPs were RTG tests and even-numbered SNAPs were reactor system tests.

In 1961, the first RTG used in a space mission was launched aboard a U.S. Navy Transit 4A and 4B navigation satellites. The electrical power output of this RTG, which was called (SNAP-3), was a mere 2.7 watts. SNAP-9A served aboard the rest of the Transit satellite series.

SNAP-19s powered Pioneer 10 and Pioneer 11 missions<ref>SNAP-19: Pioneer F & G, Final Report, Teledyne Isotopes, 1973</ref> as well as powering both Viking 1 and Viking 2 landers.

Five SNAP-27 units provided electric power for the Apollo Lunar Surface Experiment Packages (ALSEP) left on the Moon by Apollo 12, 14, 15, 16, and17. The fuel capsule, containing 3.8 kilograms (8.4 pounds) of plutonium-238 in oxide form (44,500 Ci or 1.65 PBq), was carried to the Moon in a separate Fuel Cask attached to the side of the Lunar Module. The fuel cask provided thermal insulation and added structural support to the fuel capsule. On the Moon, the Lunar Module pilot removed the fuel capsule from the cask and inserted it in the RTG.

These stations transmitted information about moonquakes and meteor impacts, lunar magnetic and gravitational fields, the Moon's internal temperature, and the Moon's atmosphere for several years after the missions. After ten years, a SNAP-27 still produced more than 90% of its initial output of 70 watts.

The fuel cask from the SNAP-27 unit carried by the Apollo 13 mission currently lies in 20,000 feet (6,500 m) of water at the bottom of the Tonga Trench in the Pacific Ocean. This mission failed to land on the moon, and the lunar module carrying its generator burnt up during re-entry into the Earth's atmosphere, with the trajectory was arranged so that the cask would land in the trench. The cask survived re-entry, as it was designed to do<ref>Apollo 12 ALSEP Off-load transcript, containing comment about re-entry survivability of fuel cask</ref>, and no release of plutonium has been detected. The corrosion resistant materials of the capsule are expected to contain it for 10 half-lives (870 years).<ref>Space FAQ 10/13 - Controversial Questions, faq.org</ref>

The United States flew only one complete nuclear reactor, aboard the SNAPSHOT mission. The SNAP-10A reactor flown on that mission produced 650 W of power from 1.3 kg of used uranium-235 fuel (embedded in uranium zirconium hydride) to run a small ion propulsion system.

[edit] References

• NUCLEAR POWER IN SPACE U.S. Department of Energy, Office of Nuclear Energy, Science & Technology

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