Like any collection of electronic components, a spacecraft requires electrical power to operate. For long-lived missions there are only two currently available options: solar power and nuclear power. For spacecraft that must operate in hostile environments such as the cold, dark regions of the outer Solar System or on hostile planetary surfaces where there is little or no sunlight, nuclear power becomes the only currently available option. Nuclear power sources have enabled or enhanced some of the most challenging and exciting space missions yet conducted, including missions such as the Pioneer flights to Jupiter, Saturn, and beyond; the Voyager flights to Jupiter, Saturn, Uranus, Neptune, and beyond; the Apollo lunar surface experiments; the Viking Lander studies of Mars; the Ulysses mission to study the polar regions of the Sun; the Galileo mission that orbited Jupiter; the Cassini mission orbiting Saturn and the recently launched New Horizons mission to Pluto. In addition, radioisotope heater units have enhanced or enabled the Mars exploration rover missions (Sojourner, Spirit and Opportunity). Since 1961, the United States has successfully flown 41 radioisotope thermoelectric generators (RTGs) and one nuclear reactor to provide power for 24 space systems. The former Soviet Union has reportedly flown at least 35 nuclear reactors and at least two RTGs to power 37 space systems.