A Radioisotope Thermoelectric Generator (RTG) is a wonderful power system (Radioisotope Power System, RPS) for all kinds of space craft. Recently, the Cassini spacecraft and the Curiosity rover on Mars both use RTG power; however, RTGs are not new. RTGs were used for both the Apollo and Viking missions. They were also for the Pioneer, Voyager, Ulysses, Galileo, Cassini, and Pluto New Horizons missions.

RTG power plants are great power systems because they are lightweight, compact, and extraordinarily reliable. Current RTGs weigh less than 100 pounds and have a diameter of 25 inches and a total length of 26 inches. Plutonium is the fuel source inside of it, and only about 10 pounds is necessary to provide 2,000 watts of thermal power and 120 watts of electrical power. The heat generated by the natural decay of the plutonium is the key to an RTG.

The massive difference in temperature between the decaying plutonium and the cold of space is applied across thermocouples, and those thermocouples generate electrical current. All of this happens with no moving parts, and that is an absolute key to the reliability of RTGs. Moving parts experience friction and wear down. RTGs don't worry about that. Additionally, they generate their power for a very long time since the plutonium used has a half life of almost 90 years.

Regarding the drawbacks of using RTGs for manned space missions, there really aren't many drawbacks. RTGs have proven time and time again that they are very reliable and exceptionally safe.

NASA has a long history of safe launch and operations of spacecraft carrying RPS, and no member of the public or NASA employee has ever been injured by any previous use of an RPS system or a related launch.

One "drawback" is how long the RTG works for. They generate power well beyond a decade, and no manned space missions come anywhere close to that length of time; therefore, it could be a bit of a waste to use an RTG for short manned missions.