Bounce game

I am making a physics game where the main purpose is to release the gravity for the highest bounce possible. I am having trouble with making a power up where the the ball gains speed when the spacebar is pressed and a fireball animation runs and when the key is released it goes back to normal. How do I make it so that as soon as the key is released the speed gradually decreases to its normal speed?

Regarding the fireball animation:
Is it possible to make this effect without using the timeline actions?
(Eye Candy - Trails section)