February 1960: SNAP - the dawn of nuclear satellite propulsion

A lightweight, high-temperature nuclear reactor was pivotal to satellite navigation systems. Jason Ford reports

The SNAP 2 reactor test core was successfully operated at full power in November 1959
The SNAP 2 reactor test core was successfully operated at full power in November 1959

On June 29, 1961 Transit IV-A became the first spacecraft to carry a radioisotope power supply into space, and in February of the previous year The Engineer was given a brief glimpse into the development of the nuclear power source.

Transit IV-A was one of several artificial Earth satellites designed at fabricated by Johns Hopkins University Applied Physics Laboratory (APL) in Maryland, USA as part of the ARPA-funded Transit programme, which established the basis for the wide acceptance of satellite navigation systems. As well as carrying a radioisotope power supply, Transit IV-A was notable also for breaking an APL mission-duration record and confirming that the Earth’s equator is elliptical.

A year before launch, The American Scene section of The Engineer took at look at the prototype of a lightweight, high-temperature nuclear reactor designed to generate heat to produce 3kW of electric power for spacecraft.

The reactor, dubbed the SNAP Experimental Reactor, weighed approximately 220lb without shielding and was fuelled with enriched uranium. Our American editor filed a report after it had been test-operated at design power and temperature.

The reactor was designed and constructed for the Atomic Energy Commission by Atomics International, a division of North American Aviation of Canoga Park, California, as part of a programme to develop systems for nuclear auxiliary power. Our correspondent noted that the reactor was situated in the Santa Susana Mountains, 25 miles from Los Angeles.

“In the ultimate operational device, heat from the reactor will be transferred by a liquid sodium coolant to a boiler containing mercury,” the American editor said. “The mercury vapour will be fed into a miniature turbogenerator. Such a conversion system has been developed and successfully operated at design conditions with an electrical heat source.”

The components of the power conversion system were developed by Thompson Ramo Wooldridge, of Cleveland, Ohio, a company whose defence division was bought by Northrop Grumman for $7.8bn in 2002.

The SNAP Experimental Reactor took its name from the Commission programme under which it was developed, namely Systems for Nuclear Auxiliary Power. The objective of this programme was to develop compact sources of auxiliary electric power for space vehicle systems, and two basic concepts were being followed.

“One concept, being developed for the Commission by the Martin Company of Baltimore, Maryland, will use the heat from a radioactive isotope to operate electrical power conversion equipment,” The Engineer’s American editor wrote. “Two devices, designated SNAP 1 and SNAP III, follow this concept. A proof of-principle model of SNAP Ill has been developed by Martin.

The author continued: “The other concept, designated SNAP Il, will use the heat from a reactor to operate electrical power conversion equipment. The present experimental reactor follows this concept. The objective of the SNAP reactor programme is to provide devices which will generate many kilowatts of electrical energy for a minimum period of one year in space and which will weigh no more than a few hundred pounds. The device must be capable of withstanding the shocks and vibrations of missile launch, must be capable of unattended, reliable, and automatic operation in a space environment, and must not present a radiation hazard. Such a device would make possible long-lived weather satellites, worldwide television communications, deep-space information transmission and, eventually, interplanetary travel.”


The core of the SNAP II reactor was made up of a hexagonal array of sixty-one cylindrical elements that delivered 50kW (thermal) to a liquid sodium coolant. The fuel elements, noted our American editor, were Zr H-U235 and were l0” long and 1” in diameter.

“The sodium enters the core at 1000oF and leaves at 1200 oF, carrying the heat to the mercury boiler, which drives the miniature mercury vapour turbo-generator,” The Engineer said. “The net electrical output is 3kW, which gives an overall energy conversion efficiency of six per cent.”

SNAP III-B went onto fly inside Transit IV-A and since that maiden voyage NASA has flown over 25 missions carrying a nuclear power system through its partnership with the US Department of Energy, which provides the power systems and plutonium-238 fuel.

NASA notes that there are only two practical ways to provide long-term electrical power in space: the light of the Sun or heat from a nuclear source. Currently, NASA’s Perseverance rover is making its way across the surface of Mars with the help of a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), and in 2027 the space agency will employ a MMRTG on Dragonfly, the mission to send a robotic rotorcraft to the surface of Titan, the largest moon of Saturn.