Sweepers are both offensive and defensive ships. They are a variant of a fighter. The Sweeper has a greater combat rating than the fighter, but lacks the ability to be deployed. Sweepers provide both the best on planet protection and the best elminator of planetary defenses. Sweepers can also be used to clear Mining Platforms off of Asteroids. You can build a sweeper after level 16 of the Munitions Lab.
If you lead an attack with a wave of sweepers they will wipe out any defensive ships on the planet, thus making way for your assault ships to gain access to the planet. If a sweeper survives its encounter with the defensive ships, it will return home to be used in future battles. Sweepers, however, do not trigger SAWs like when escorting a colony ship to an uninhabited planet. They do trigger them when attacking the planet with the SAW directly.
There is no way to repair sweepers attack rating at this time.
The reset time for defending sweepers is calculated as it is with fighters. Sweepers also make for the best on planet defensive ship that there is. Both Drones and Fighters will have a lower Combat Rating than a Sweeper when they are all built on the same planet.
If you want to send a sweeper to one of your own colonies for defense, it will have to be moved there by one of your cargo carriers.