Even though this is long far late I couldn't resist but, they can't go back to earth because Area 51 or No Man's Land was basically shot off from the earth. I know this because after going back to Area 51 if you look at the sky you can see earth meaning that they're trapped in Moon. Also Weasel probably can't go inside Richtofen's body because his soul is either still in Purgatory or is at peace depending on how you chose the Easter Egg.
Sigh... Not this sh*t again.
Are you REALLY that stupid!?! The Dev's actually confirmed that the Earth behind the Teleporter pad was just a graphical glitch, but even if they hadn't, do you really believe that it was possible that the area 51 section was floating above the Earth? It is a ridiculous thought. They would require the P.E.S. suit to breathe for a start, and if they were actually floating above space, the heat from the fireball surrounding them would cook them instantly.