I'm new to the game and only just played my first game of TI3 last night.
Anyway, I have been under the impression that Space Docks cannot be destroyed. The reason for this notion is this: They have no attack value, and therefore do not directly participate in a Space Battle. Also, on page 26 of the main rulebook, it states that Space Docks are "under blockade" when enemy ships are present. And why would anyone bother blockading a Space Dock if they are able to destroy it?
However, all that went out the window when it was revealed that there is the existance of a Secret Objective card that refers to destroying Space Docks. So really, how do you destroy them? Or, can they be shot out of the sky just like any other ship?