The moon has been an object of wonder and the subject of art and poetry since people first kept written records or decorated their caves with paintings. Many ancient civilizations based their calendars on the cycles of the moon. Lunar tides in seas and harbors helped determine when ships could set off on journeys or float safely into port. Before the development of electric lights and gas lamps, the moon lit the way for travelers at night. People imagined they saw a "man in the moon" and other shapes in the lunar surface, speculated about what the moon was made of, and wondered if it was inhabited. Scientists, too, have been curious about the moon and have scrutinized its cratered landscape with their telescopes. But for a long time, few of them gave much thought to how the moon came to be.

By 1998, the attitudes of scientists had changed. Not only were more researchers interested in the origin of the moon, but nearly all had come to believe in one particular model of lunar origin—the giant-impact theory. According to this theory, the moon was formed when a body about the size of the planet Mars—or a few times larger—struck the young Earth with cataclysmic force. Much support for this theory has come from the study of rock and soil samples from the moon, brought to Earth by U.S. astronauts. In 1998, a satellite called Lunar Prospector, in orbit around the moon, supplied evidence consistent with the theory, as did computer simulations of the giant impact.