Luna

Luna is Earth's natural satellite. In the 1960s several mission to put a man/woman on the moon was done and the first two men to walk on the moon was Neil Armstrong and Buzz aldrin, It was later colonized in the late 21st/early 22nd century by the UCN, along with many other worlds of the Terran Solar System.

It is unknown if this moon was terraformed, had enclosed colonies or if it was just as simple as people needing space suits.