Saturn

Saturn is a planet in the Solar System. Union Aerospace Corporation has facilities on the moons of Saturn including Titan.

Behind the scenes
Saturn or at least its moons are discussed in the "unused" backstories for some of the levels in the Master Levels for Doom II. These were Titan Manor, Express Elevator to Hell, and Trapped on Titan. This story was meant for when the wads were originally intended to be released for free as 'fan wads' but were instead licensed by ID for the expansion pack. Thus the story should be taken with a grain of salt (most of the backstories don't even seem to be related to the UAC or Doom universe anyways, IE reference to "Federation" in Titan Manor).