Nuclear-powered spacecraft at Ganymede, computer artwork. Ganymede is one of the moons of Jupiter. The spacecraft is powered by nuclear fusion, the same process that takes place in the Sun. This is a cleaner source of energy than nuclear fission, though it is thought it will take many years to develop controlled nuclear fusion reactors. Travel to the outer planets, which involves much greater distances and travel times than for travel to the inner planets and the Moon, will probably require nuclear fusion. The crew's living quarters are at right, away from the nuclear propulsion systems at left. This minimises the exposure of the crew to dangerous radiation from the nuclear reactor.