Monday, September 28, 2009

40th Anniversary "SNAP"shot

This article is a continuation of "40th Anniversary 'SNAP'shot" from the Museum's Fall Newsletter and gives a look at the interesting device called a SNAP (Systems for Nuclear Auxiliary Power).

By Sarah Fair, curator

On January 16, 1959, a device that turned heat created from radioactivity into electricity was demonstrated for the first time on President Eisenhower’s desk. It was the size of a grapefruit, weighed 4 pounds and was capable of delivering 11,600 watt-hours for approximately 280 days. This device was called the SNAP-3. Two SNAP models will be on display in the Museum’s 40th Anniversary Exhibit. These models are an important part of the Museum’s collection and show how the Museum broadened its nuclear history and science horizons beyond weapons and into other nuclear technology uses.

The United States Atomic Energy Commission began developing a series of compact devices to supply power for space and terrestrial uses. These devices fell under the general title of Systems for Nuclear Auxiliary Power (SNAP). There were two different SNAP systems developed; both convert heat to electricity. In even-numbered SNAPs, heat is obtained from a small nuclear reactor. SNAP-10A, the first SNAP reactor power plant launched into space in 1965, is an example of that system. In odd-numbered SNAPs, the heat is obtained from the decay of certain radioisotopes such as Plutonium-238 (in the SNAP-27) and Polonium-210 (in the SNAP-29).

The early efforts on the SNAP began in 1956 with the objective of powering instruments in space satellites. By 1963, two satellites powered by SNAPs were already in orbit.

The SNAP-1 was a small turboelectric generator with high-speed rotating components to convert heat into electricity. It was abandoned for thermoelectric devices, which had no moving parts and a longer lifespan than SNAP-1, with its two-month life expectancy. Work began to develop a new and more efficient thermoelectric and thermionic conversion units for use with a radioisotope heat source.

The SNAP-3 generator was assembled and tested in January 1959. It was a success because it generated 2.5 watts of electricity with a half charge of Polonium-210 fuel. This radioisotope system was selected for several reasons. It provided a concentrated heat source, was readily available, and was safe to handle. Because of the SNAP-3’s success, work began on generators to power satellites, moon probes, automatic weather stations, and navigational aids.

Satellites need dependable, long-lived electrical supply. They also need to be lightweight because pounds are precious in payloads rocketing into space. They must withstand the rigors of a ride on a rocket. The generator must also be safe, that in the event of an accident there will be no serious consequences from radioactive contamination. The SNAP-3A was used in 1961 in the U.S. space program for the orbiting of a Department of Defense satellite. It used a radioisotope generator with a supplementary electricity source for its radio transmitters and was the first use of atomic power in space

The SNAP-27 was developed for use by NASA in lunar landing missions and was designed to power the Apollo Lunar Surface Experiment Package (ALSEP). Five SNAP-27 units provided electric power to ALSEP and were left on the moon by Apollo 12, 14,15, 16, and 17. The SNAP-27 from Apollo 13 is 20,000 feet under water in the Pacific Ocean. The systems 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. Even after 10 years a SNAP-27 still produces more than 90% of its 70-watt output.

Visit the Museum on October 17, 2009 to see models of the SNAP-27 and SNAP-29 on display in the Museum’s new 40th Anniversary Exhibit. Check out the Museum’s website,, for more info about the new exhibit.

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