Halley's Comet is an arcade video game created by Pacific Manufacturing Ltd and licensed to Taito for manufacture. Receiving a world wide release in 1986, Halley's Comet is a vertical scrolling shoot 'em up where the player defends various planets around the solar system from a direct collision with Halley's Comet and the various alien swarms which accompany it.
Each planet is visited individually. You start above the planet you are rescuing and the first two levels consist of the approach towards the comet split in the middle by a mini boss enemy. Once you have reached the comet you must split the comet open and proceed inside the comet where an alien race have hollowed out the comet and are currently residing inside. Proceed through the waves of enemies will result in meeting the end of level big boss. On defeating this you can proceed to the next planet which essentially is more of the same with some new enemy ships dropped in.
To assist you in your fight, many power ups are available. These are obtained by shooting smaller rocks which float past you. Power ups are triple front cannon, mini ships, wide spread cannon, increased speed, increased maneuverability, forward shield, and at the end of each sequence of power ups a wild card special in the form of a smart bomb, a complete power up of cannons or a full complement of mini ships.
The power ups are also multi-staged in that the first triple front cannon you pick up will give you standard size triple shots. Picking up additional triple cannons will increase the shot size.
Picking up mini-ships increases the player's fire power such that when they are picked up they fall in on the player's left and right wings and fire single shots as the player fires. By pressing the Hyperspace button, a mini-ship can be used as a screen clearing smartbomb.
Source: Wikipedia, "Halley's Comet (video game)", available under the CC-BY-SA License.