A radioisotope thermoelectric generator (RTG, RITEG) is an electrical generator that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. This generator has no moving parts.

RTGs have been used as power sources in satellites, space probes, and unmanned remote facilities such as a series of lighthouses built by the former Soviet Union inside the Arctic Circle. RTGs are usually the most desirable power source for unmaintained situations that need a few hundred watts (or less) of power for durations too long for fuel cells, batteries, or generators to provide economically, and in places where solar cells are not practical. Safe use of RTGs requires containment of the radioisotopes long after the productive life of the unit.

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

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Note added at 9 mins (2016-12-31 09:15:05 GMT)
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Radioisotope Thermoelectric Generators, or RTGs, provide electrical power for spacecraft by converting the heat generated by the decay of plutonium-238 (Pu-238) fuel into electricity using devices called thermocouples. Since they have no moving parts that can fail or wear out, RTGs have historically been viewed as a highly reliable power option. Thermocouples have been used in RTGs for a total combined time of over 300 years, and a not a single thermocouple has ever ceased producing power.

https://solarsystem.nasa.gov/rps/rtg.cfm

Creating robotic spacecraft that could thrive in these extreme environments demands technical innovation. One of the most important components for such missions is their electrical power supply. For most space exploration missions where sunlight is abundant, solar power has been the preferred choice. But as the environments at chosen destinations grow harsher, and missions evolve to be more demanding, it becomes more likely that effective power and heating for a spacecraft would require a Radioisotope Power System (RPS).

http://www.jpl.nasa.gov/news/fact_sheets/radioisotope-power-...

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Note added at 13 mins (2016-12-31 09:19:37 GMT)
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A radioisotope thermoelectric generator, or RTG, uses the fact that radioactive materials (such as plutonium) generate heat as they decay into non-radioactive materials. The heat used is converted into electricity by an array of thermocouples which then power the spacecraft.

http://www.qrg.northwestern.edu/projects/vss/docs/power/1-wh...

'Space objects' could also be called 'spacecraft'.

A spacecraft is a vehicle, or machine designed to fly in outer space. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, space colonization, planetary exploration, and transportation of humans and cargo.

https://en.wikipedia.org/wiki/Spacecraft