Cosmic Defender

From A.N.A.L.O.G. magazine issue 32 from July 1985. For thousands of years, the evil Lyrean race has ruled the galaxy. As soon as another race reaches technological levels advanced enough to achieve star travel, the Lyreans' ultimate weapon — the Mesotron Cannon — is trained on them. Now, mankind's turn has come. The cannon is in- side Pluto's orbit and approaching rapidly, gathering asteroids to use as fuel as it comes. Things couldn't look more desperate.

Yet all is not lost. While on a routine maintenance stop on asteroid A37THETA, your ship is swallowed by the cannon. Your duty is clear. You must blast the asteroids which the cannon uses as fuel, and fire through force fields that grow ever closer. You must render this huge weapon inoperative.