Space Dock is an add-on for a planet which allows a certain percentage of a destroyed fleet to be recovered after a large-scale battle (>150,000 units destroyed) over the planet or its moon. A wreckage field will be formed after the battle, and the space dock can start the recovery process unless 3 days have passed, at which point the wreckage burns up.
The construction of a low-level Space Dock requires very few resources (200Metal 50Deut 50Energy for lv1), and it does not occupy any space on the planet (since it is in orbit). Thus, it is reasonable to construct one, since it is sure to pay back its cost after the first significant battle.
A Space Dock's cost increases fivefold with each level.