From Wikipedia:

NASA takes both Homer and Barney to Cape Canaveral to train them into astronauts. They pit the two in competition against one another as they can only take one to space. With NASAs' alcohol ban, the training goes well for Barney (he even does a backflip and sings the opening lines of Gilbert and Sullivan's "Major General's Song"), but the future is grim for Homer when he learns that Barney has been chosen to go on board with Buzz Aldrin and the fictional astronaut Race Banyon (whose name parodies Race Bannon). However, when Barney has a toast with the people at NASA, he drinks champagne that was apparently non-alcoholic, goes berserk and fastens himself to a jet pack. After taking off, the jet pack fails and he bounces off the roof of a pillow factory and onto the road, where he is run over by a marshmallow truck. A scientist declares Homer the default winner of the competition, and he goes up into space with the two other astronauts.

Reader comment: Alan says,

You would be remiss in your Homer astronaut post if you didn't at least mention the genesis of the overlords meme. Link.