Each space station is crawling with aliens. In most rooms of the station, at least one alien will attack. When you have defeated that alien, another may take its place, or the room's bounty may be revealed, in the form of an energy pod or an oxygen bottle.

Living, chomping Bear Traps are the least offensive aliens, and the easiest to kill.

(This screenshot was taken with the Stella Atari 2600 emulator.)