The computer jumps in with 7 siege frigates, and I have 7 or 8 turrets defending the an asteroid, yet they can't even destroy 2 of them before the colony is destroyed completely, then they jump out...
I had the same problem when I played my first game. There is a thread with some very good tips pinned for you:
Asteroids have very limited life, so unless its a really strategic spot, why are you trying to defend an asteroid?
Make sure you research the Turret, Defense hanger and Reapir station tech. Even with an asteroid, you can have 6-7 turrets 8 bomber wings and a repair station. Emergency facilities will help a lot.
Place them yourself. You ought to know from where the enemies come from. I tend to keep them tight around the asteroid so all 6 have overlapping arcs
Also, whats with the Novalith Cannon? Its stationary... I had been lead to believe you could bring it with you and destroy planets (or rather make them unlivable/unable to colonize)
It does, but it doesn't have to move. It shoots across star systems. It causes a dely in re-colonizing so it you couple it with a good strike, you can do a lot of damage to an enemy. Try hitting thier home world with it right after to you start an assualt on their fleet.
where ALL of their ships are assembled, they simply run and jump to a different planet while I destroy their colony. I jump after them, and they jump away again... on and on
Why do that? That a really good way to get ambushed by another fleet. In addition it leaves your rear open to attack (which is why they cn just turn around and run past you. Use reserves. One should never commit their entire force.