Well according to the stats, the supply depot has 11,200 units of armor on each side, and a core strength of 20,000 hit points. That means four torps worth right there, and you'd be nuts to try flying that mission in a Longbow--you don't have enough missiles to keep off all the light fighters in the mission. Just go in with your Arrow/Hellcat/Thunderbolt, take out some of the fighters and make sure that the rest of the fighters are busy dogfighting your wingman, and sit there and pound it with guns till it blows. It'll take a while, but since asteroid depots have no shields (they're built for stealth, not defense), it'll only be a matter of time before it does go up in flames.