Annihilator is a game loosely based on Defender (what with a radar, smart bombs, scrolling terrain, etc.), but has some differences. There are no humans to rescue, and the aliens don't actually fire back... they just run into you. However, you have to hit them in pretty much the exact right place in order to kill them, making it quite challenging. It also contains hilariously politically incorrect (at least, in this day and age) directions, as you can see in the screen shot above to the right.
The author of this game, Doug Kelley, has been kind enough to release it to the public, so you can download it below.
Game Play Instructions:
Use the four arrow keys to move your saucer around. The <SPACEBAR> fires, the <CLEAR> key is your smart bomb, and the <P> key will pause/unpause a game in progress.
Title: Annihilator
Author: Doug Kelley
Publisher: Chromasette
Released: November 1983
Requires: Color Computer 1,2,3, 16K RAM, tape or disk.