The Moon is the only natural satellite of the Earth. It is occasionally featured in MarissaTheWriter stories as a location, as opposed to just being a celestial object.
The Moon first appears in ITS MY LIFE!. Marrissa Roberts reveals that Wheatly was sent to lunar orbit by Chell, and after GLaDOS informs Marissa that she can fly to space, her first destination is there. Marrissa thinks that Chell is looking for Wheatly, but it turns out that GLaDOS was in disguise as Chell. GLaDOS, Marrissa and Wheatly then return to the Earth via an asteroid.
The Moon is also featured in Portal: The 4th Millennium: The Trollfic Edition. Principal Business Man travels there after avoiding rockets being shot by both GLaDOS and Portal High School, which have turned into following his Voot Cruiser instead. However, it is still lunar orbit that is visited, and not the Moon itself. However, later on Business Man's adoptive daughter Emprase Contone Ovasere visits the Moon with her motorcycle, chancing upon an Apollo landing site. She finds guns, which she assumes were used to kill Moon aliens, and supplies to be able to go back to the Earth.
The aliens from the Moon appear once again in Post-SCrash Session 3: Spectators of the Host. After Flashlight Girl flashes her flashlight at the Moon, the aliens abduct her. She is later found by Aranna Sorket, who used the rusty BMW that Skepness Man Beauregarde originally owned, and the two turn to visiting various other places in space.