In order to reach their destinations spacecraft use gravity assist to gain speed and momentum. The Cassini spacecraft now orbiting Saturn used assists from the Earth and Venus.
The concept seems straightforward, as the spacecraft approaches the planet its gravity causes the craft to accelerate. The craft then makes a near miss and is flung out into space with greater speed and momentum.
But I’m confused, wouldn’t the same gravity that caused the craft to accelerate on approach cause it to decelerate as it departed? It seems like it should be a zero sum gain.