What if it's the derelict that is trying to stop the prometheus getting away and not the other way around? Anyone thought of that?
Perhaps some of the crew of the Prometheus are trying to get back to Earth but there is already a mutation on board. The only way to bring it down is for someone left on the planet, to don the jockey suit, and fly the derelict into the Prometheus to stop it from getting away.
I know it's out there and what we have seen in the trailer goes against that...but you never know