If someone plays a Bomber squadron on you, and you counter with a Fighter squadron, the Bomber squadron is discarded with no effect. Now, if that opponent plays a second Bomber squadron on you, does that same Fighter squadron get to cancel it too?

about Concussive Blast, can i move a ship even if she have speed 0?

Can I play the Action Card with "Ramming speed" on the same ship I played a blast with first? If the ship that i use Ramming speed hit another, can i use direct hit or direct hit effect? Can i use Ramming speed in a ship with speed 0?

Can I play asteroids or Minefields without a ship present?

Can you do this: Play a Blast card then Direct Hit play another Blast card then another Direct Hit then another Blast card, many times?

Well you have probably figured this out or moved on, but for posterity, I'll post some answers.

When a fighter intercepts a bomber, it then can't do anything else for the rest of the turn. So the second bomber would get through unless you play another card to cancel it.

Concussive Blast can move any ships, regardless of their speed.

A ship can be sacrificed with ramming speed after it has already fired a blast. I think you can play a direct hit and direct hit effect after using ramming speed, although I don't think you can use direct hit to fire another blast since there is no ship to fire it. You can sacrifice a ship with zero speed, but it will do no damage so the only reason would be to use a direct hit effect.

Asteroid and Mine fields can always be played on any player.

You can chain together a series of blast, direct hit, blast, direct hit, etc… until you run out of cards or one is canceled.