Programming/math question, please see inside (if you're a math wizard)

I’m writing a tower defence game because I love tower defence games but I ran into some serious problems when I decided my projectiles needed to lead the target perfectly. So we have a moving target on the screen and a tower which would normally fire bullets but in my game the bullets orbit the tower until they are ready to be fired. That means I can’t fire them as soon as they’re in range of the tower, I have to wait until the bullet faces in the right direction to intercept the target.

Ok, so I have 3 objects: A, B and C.

A and C are in motion, B is stationary. A is in orbit of B and the path of C is known. A can escape from its orbit instantly and if it does it drifts in a straight line. How do I find the first possible moment at which the target and the bullet could collide?