Luna (often simply referred to as "The Moon") is Earth's only natural satellite and the first extra-terrestrial object reached by humans. Exploration of Luna has its origins in the mid-20th century, courtesy of the space race between the United States and USSR. While the subject of much study and manned exploration, these space programs had completely shut down by the late 22nd century due to drastically reduced budgets and incessant political sabotage. With the coming to power of the United Powers League however, interest in space exploration was renewed, ensuring that the moon had become colonized by the mid to late 23rd century.
Underwood, Peter, Bill Roper, Chris Metzen and Jeffrey Vaughn. StarCraft (Manual). Irvine, Calif.: Blizzard Entertainment, 1998