Flying through space an object of natural origin from which gas can be produced by ordering the fleet to "recycle" using the formula recycling rate * [diameter of comet]/25000.

A comet can be found in an expedition of at least 6 hours duration or stumbled upon accidentally at local coordinates. Comets can be 13 to 33 km in diameter, flying at 0.05 to 0.5 km/sec, but they can be slowed down by a gravity gun beam. The rate of gas production on slowed comets increases by a factor of 10.

Once a comet is detected, its local coordinates are stored for 7 days. If necessary, the coordinates of an already detected comet can be updated by sending an expedition of any length. After the comet is observed by direct visibility from fleet or station, it stops being a hidden object and it's possible to send fleet to it.

Comet is constantly losing structure and disappears after about 1000-3000 hours after it appears. If damage is inflicted on the comet, this time is reduced. When a comet is destroyed or disappears there is no gravity wave, the associated fleets are not destroyed.