The flying saucer is a small alien ship with a saucer-shaped body.
Before 2161, a flying saucer - captured by humans and after re-captured by aliens from Area 51 (evident from a note "property of Area 51, return if found" being seen on the body of the ship), crashed in California during flight, killing the two alien pilots.
Sometime later, the Vault Dweller can find the crashed flying saucer with the two alien pilots in skeletal form. They may find a special alien blaster with a different design and color compared to other alien weapons. It uses small energy cells, instead of alien power cells or alien power modules.