Along with the 4 "regular" enemies described here, there are 4 oversized "boss" ships to destroy.
Dumb UFO These are the standard issue alien enemy fighters, these ships
are relatively weak and carry only a small amount of ammo. They should make perfect targets to up your kill count and earn
more power ups.
Bird Head Large slow ships that pack a punch, you'd do best to stay
away from these if they come out more than one at a time. They have moderately strong shields and carry a good deal of ammo.
Interceptor Part of the Elite Alien Attack Fleet (EAAF for short),
these large, well shielded ships weave back and forth while firing a heavy blaster. If you manage to let one expend all of it's
ammo, these ships become sitting ducks.
EAAF Skel Ship A fast, versatile attack ship making up the majority of
the EAAF. These ships not only pack a punch with heavy blasters, but sport the strongest class of alien shields and a
moderate payload of ammo. Do not attempt an assault on these ships without having a powered up blaster.