Exploration of the Moon essentially stopped for about two decades after the Apollo program ended in the early 1970s. In 1990 the Japanese twin Muses-A space probes reached orbit around the Moon, but failed to transmit any data. The Clementine probe was launched by the United States in 1994, and surprisingly detected signs of water ice mixed in with rock near the Moon’s south pole. A follow-up mission, called the Lunar Prospector, was launched in January 1998. By March 1998, data from the spacecraft suggested that this subterranean ice might be present in large quantities at both lunar poles. Chandrayaan-1, LCROSS, LRO, and GRAIL are four additional spacecraft which have gone to the Moon to study it scientifically and to map the Moon in detail. The data gathered with these missions will be very valuable if and when humans return to the Moon.