Voyager 1 and Voyager 2 spacecraft launched in August and September 1977 required multi-hundred-watt (MHW) RTG containing plutonium oxide fuel spheres for an operational life appropriate for exploration of Jupiter, Saturn, Uranus, and Neptune. Conversion of the decay heat of the plutonium to electrical power was accomplished through 312 silicon-germanium (SiGe) thermoelectric couples. A hot junction temperature of 1273 K (1832 °F) with a cold junction temperature of 573 K (572 °F) compose the temperature gradient in the thermoelectric couple in the RTG. This mechanism provided the total electrical power to operate the spacecrafts instruments, communications and other power demands. The RTG on Voyager will produce adequate electrical power for spacecraft operation until about the year 2020. Similar MHW-RTG models are also used on the two U.S. Air Force communications Lincoln Experimental Satellites 8 and 9 (LES-8/9').