The comet mode of an Androsynth Guardian is called the Blazer Mode. It can be engaged as long as the Guardian has energy remaining at which point it transforms the ship into an high-speed, inertialess comet that burns fuel at a medium pace and does damage to foe on impact. Speed adjustment is not possible. When battery is exhausted, the ship returns to normal.
There was a bug that consisted in the following: When an Androsynth Guardian in Blazer mode is drained of its last energy by a DOGI, the Blazer mode isn't fully turned off; the ship retains it ability to do damage by impact. (bug #283).