Gravity Assists Enable or Facilitate Many Missions
A spacecraft arrives within the sphere of influence of a body with a so-called hyperbolic excess velocity equal to the vector sum of its incoming velocity and the planet's velocity.
In the planet's frame of reference, the direction of the spacecraft's velocity changes, but not its magnitude. In the spacecraft's frame of reference, the net result of this trade-off of momentum is a small change in the planet's velocity and a very large delta-v for the spacecraft.
- Starting from an Earth-Jupiter Hohmann trajectory and performing a Jupiter flyby at one Jovian radius, as shown above, the hyperbolic excess velocity vh is approximately 5.6 km/s and the angular change in direction is about 160o.