Power in Space

Powering Space Exploration

If it weren't for nuclear-powered generators, space scientists wouldn't know a great deal of what's currently understood about far away planets like Mars and even the Moon.

The United States space program has used generators powered by radioisotopes in space for more than 40 years. These generators, called “radioisotope thermoelectric generators” or RTGs, have powered our navigational and weather satellites and have been essential in space communications. They enable us to conduct experiments on the Moon and other planets, and many of our Nation's outer planetary missions could not have been accomplished without the reliable power provided by these generators.

For instance, NASA's Galileo mission to Jupiter revealed volcanic activity on Jupiter's moon, Io. Launched in 1989 and powered by an RTG, Galileo was the first spacecraft to fly past an asteroid and the first to discover a moon of an asteroid. In 1997, the Cassini mission became the largest spacecraft ever launched to explore the outer planets. Powered by three RTGs, Cassini entered the orbit of Saturn in summer 2004. In 2006, NASA launched New Horizons, the fastest spacecraft to ever leave Earth. It has already sent back images and scientific data from Jupiter and will continue its journey three billion miles to study Pluto and its moon, Charon. It will take New Horizons until 2015 to get there!

In addition to generators, radioisotopes are also used to power heaters used in space. Known as “radioisotope heater units” or RHUs, these heaters help keep a spacecraft's instruments within their designed operating temperatures. In 2003, NASA launched the Mars Exploration Rovers Spirit and Opportunity to explore evidence of water on Mars. Each rover has eight RHUs to keep the rover instruments warm during the cold Martian nights. The rovers landed at separate sites on Mars in January 2004 and are still operating today.

Peaceful Plutonium

RTGs and RHUs are more broadly known as “radioisotope power systems” (RPS). RPS convert the heat from the decay of the radioactive isotope plutonium-238 (Pu-238) into electricity. Pu-238 is a type of plutonium that is not used for nuclear weapons.

There are two new RPS under development. One is the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which is being developed for operation in planetary atmospheres as well as in the vacuum of space. The first use of an MMRTG will be for the Mars Science Laboratory planned for launch in 2009.

The second new RPS in development is the Advanced Stirling Radioisotope Generator (ASRG). This is being created as a high-efficiency RPS technology to support future space missions on the Martian surface or the vacuum of space. The system uses a Stirling converter, which has moving parts, to mechanically convert heat to electricity.