Fifty Years of Space Mobile Nuclear Power

Fifty years ago this week, on June 29, 1961, an electrical generator driven by nuclear energy was launched into space for the first time.

The SNAP-3 radioisotope thermoelectric generator (RTG) powered by the natural decay heat of plutonium-238 provided a minuscule 2.7 watts of power to the Navy’s Transit 4A navigational satellite, which was placed in orbit around the Earth at a mean altitude of 930 kilometers.

The event was commemorated in this advert (shown above) for Martin Marietta, as the device’s manufacturer was then known, which appeared in the December 1962 issue of Astronautics magazine (thanks to Gary L. Bennett).

Since that time, plutonium power sources have enabled a series of ambitious missions into deep space that may rank among the grandest adventures of all time, extending human cognition into domains that were previously accessible only by imagination.

Voyager 1 and 2, for example, twin RTG-powered probes which were launched in 1977, are now on the threshold of becoming the first spacecraft to leave the solar system and to enter interstellar space.

“The men and women involved in Voyager did something that is absolutely the equal of Magellan or Columbus or any of the great explorers of terrestrial discovery,” said project contributor (and FAS sponsor) Ann Druyan.

She and Voyager project scientist Ed Stone offered “Perspectives on More Than 3 Decades of the Voyager Mission” (pdf) in an article by Randy Showstack in the May 10 issue of Eos, the weekly newspaper of the American Geophysical Union (scroll down to the middle of the first page).

Unfortunately, the plutonium 238 power sources that are used to power these missions are not only expensive, they are dirty and dangerous to produce and to launch.

The first launch accident (pdf) involving an RTG occurred as early as 1964 and distributed 17,000 curies of plutonium-238 around the globe, a 4% increase in the total environmental burden (measured in curies) from all plutonium isotopes (mostly fallout from atmospheric nuclear weapons testing).

A plutonium fueled RTG that was deployed in 1965 by the CIA not in space but on a mountaintop in the Himalayas (to help monitor Chinese nuclear tests) continues to generate anxiety, not electricity, more than four decades after it was lost in place. See, most recently, “River Deep Mountain High” by Vinod K. Jose, The Caravan magazine, December 1, 2010.

A good deal of effort has been invested to make today’s RTGs more or less impervious to the most likely launch accident scenarios. But they will be never be perfectly safe.

To minimise the health and safety risks involved in space nuclear power while still taking advantage of the benefits it can offer for space exploration, the Federation of American Scientists years ago proposed (pdf) that nuclear power — both plutonium-fueled RTGs and uranium-fueled reactors — be used only for deep space missions and not in Earth orbit.

Although this proposal was never officially adopted, it represents the de facto policy of spacefaring nations today.

The next RTG enabled space mission, the Mars Science Laboratory (MSL), is scheduled to be launched from Cape Canaveral between November 25 and December 18 of this year.

The MSL rover, known as “Curiosity,” will be fueled with 4.8 kilograms of plutonium dioxide. It will be, NASA says, “the largest, most capable rover ever sent to another planet.”