The harvester is a small mining ship: a corvette-size ship that mines the body it is at and carries the ore in its hold. It mines at the same rate as the hull, with a much smaller hold.
It is the only ship the hull can build, and it costs only iron. A Shipyard can also build it. It fits every station's dock.
It wears as it works. A worn harvester moves at half speed and mines at half rate until it is repaired at a dock. The Excavator is the larger miner.