The United States and Russia have used dozens of nuclear-powered spacecraft in orbit around Earth and in deep space for more than 40 years.

That power is needed to run the spacecraft in place of solar panels once it is beyond the orbit of Mars. At that distance, the sunlight is too weak to keep a spacecraft going on solar panels.

But nuclear power supplies a constant source of electricity over a long lifetime with high reliability. As a power source, nuclear material is unaffected by the chilling cold of deep space or the high radiation belts found around planets like Jupiter and Earth.

The energy from an RTG is much different from that produced by nuclear power plants on Earth.

Power plants use a process call "fission" that splits unstable radioactive materials like uranium into smaller parts. Fission generates great heat, uses fuel rapidly and produces a huge release of energy. But the process is much more complex and not as reliable as simply using the heat produced by radioactive decay in the case of an RTG.

Each RTG is about four feet long (1.2 meters), weighs 121 pounds (55 kg) and contains about 24 pounds (11 kg) of plutonium dioxide pressed into 72 solid ceramic-like pellets each about the size of marshmallow. The half-life of the plutonium is 87 years.

In federal court cases brought by anti-nuclear activists over the Galileo and Cassini missions, NASA has argued that the risk is tiny -- less than 1 in 1 million -- of any human being injured by the plutonium in the event of a launch catastrophe. The plutonium would have to be pulverized and inhaled in order for it to cause any harm, NASA has said.

NASA has sent up a total of 25 spacecraft with RTGs, including the Apollo, Pioneer, Viking and Voyager programs. Cassini was the latest, launching in 1997. RTGs also have flown on Earth-orbiting weather, communications and navigational satellites for both NASA and the U.S. military.