The Moon was -- and remains -- the most popular target for any spacefaring nation's first adventurous steps into planetary exploration. It was, of course, the target of the space race between the United States and Soviet Union in the 1960s and 1970s. It is the only place in the solar system other than Earth walked upon by humans. Lately, the Moon has become a prime destination for efforts by many nations seeking to expand into space: Japan (Kaguya), China (Chang'E), India (Chandrayaan-1), and the United States (Lunar Reconnaissance Orbiter).