Cinq unités SNAP-27 fournissent l’énergie nécessaire au fonctionnement de lApollo Lunar Surface Experiments Package. Cette station transmet des informations sur les tremblements lunaires, les impacts de météores, les champs magnétiques et gravitationnels, la température interne et l’atmosphère de la Lune, pendant plusieurs années après les missions. Chacune de ces unités contient 3,8 kg de plutonium-238. (Photo d’un SNAP-27 RTG sur la Lune prise depuis l’astronaute Apollo.)

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The Systems Nuclear Auxiliary POWER (SNAP) program was a program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s by NASA.

SNAP-3 was the first RTG used in a space mission (1961). Launched aboard U.S. Navy Transit 4A and 4B navigation satellites. The electrical output of this RTG was 2.5 watts.

After SNAP-3 on Transit 4A/B, SNAP-9A units served aboard many of the Transit satellite series. In April 1964 a SNAP-9A failed to achieve orbit and disintegrated, dispersing roughly 1 kilogram (2.2 lb) of plutonium-238 over all continents. Most plutonium fell in the southern hemisphere. Estimated 6300GBq or 2100 person-Sv of radiation was released and led to NASA’s development of solar photovoltaic energy technology.

SNAP-19(B) was developed for the Nimbus-B satellite. Fueled with plutonium-238, two parallel lead telluride thermocouple generators produced an initial maximum of approximately 30 watts of electricity. Nimbus 3 used a SNAP-19B with the recovered fuel from the Nimbus-B1 attempt.

SNAP-19s powered the Pioneer 10 and Pioneer 11 missions. They used P and N doped ‘TAGS’ (Ag—Ge—Sb—Te) thermoelectric elements. Modified SNAP-19Bs were used for the Viking 1 and Viking 2 landers. A SNAP-19C was used to power a telemetry array at Nanda Devi in Uttarakhand for a CIA operation to track Chinese missile launches.

Five SNAP-27 units provided electric power for the Apollo Lunar Surface Experiments Packages (ALSEP) left on the Moon by Apollo 12, 14, 15, 16, and 17. The SNAP-27 power supply weighed about 20 kilograms, was 46 cm long and 40.6 cm in diameter. It consisted of a central fuel capsule surrounded by concentric rings of thermocouples. Outside of the thermocouples was a set of fins to provide for heat rejection from the cold side of the thermocouple. Each of the SNAP devices produced approximately 75 W of electrical power at 30 VDC. The energy source for each device was a rod of plutonium-238 providing a thermal power of approximately 1250 W. This fuel capsule, containing 3.8 kilograms (8.4 lb) 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 (6,100 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 arranged so that the cask would land in the trench. The cask survived re-entry, as it was designed to do, 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).

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Apollo astronaut photo of a SNAP-27 RTG on the Moon.