Whats a gravity assist?
The planets retain the vast majority of the solar system’s angular momentum. It is this momentum that is used to accelerate spacecraft on so-called “gravity-assist” trajectories. In a gravity-assist trajectory, angular momentum is transferred from the orbiting planet to a spacecraft approaching from behind. Gravity assists would be more accurately described as angular-momentum assists. Galileo’s arrival was carefully timed so that it would pass behind Venus in its orbit around the sun. As the spacecraft came into Venus’ gravitational influence, it fell toward the planet, increasing its speed toward maximum at closest approach to Venus. Since all masses in the universe attract each other, Venus sped up Galileo substantially, and the spacecraft slowed down the planet in its orbit by a tiny amount, since the spacecraft approached from behind. As Galileo passed by Venus, of course it slowed down again relative to Venus, climbing out of Venus’ gravitational field. Its Venus-relative velocit
