In the video game Gyruss, at least the NES port thereof, the comet-cannon is your ship's superweapon. After a very brief charge-up period it discharges a comet-shaped blast of energy that streaks toward the center of the screen, demolishing anything in its path (except bosses, and even those are damaged). Even normally invulnerable enemies, like meteors, plasma globes, space seeds and star-jellies are destroyed.
The comet-cannon has limited shots. The player starts with three, and more can be gained by shooting the gold power-up pods that appear after killing enough enemies. It is a good way to save yourself from imminent death at the hands of an abnormally tough enemy, or to make boss fights easier.