Home / Topics / Gravitation & Orbits
Gravity assist
Advanced
A spacecraft steals momentum from a planet to speed up.
A spacecraft passing a moving planet can gain or lose speed relative to the Sun, taking momentum from the planet (negligibly changing it).
Voyager used Jupiter and Saturn.
It's an elastic collision at a distance.
In more detail
A gravity assist uses a planet's motion to change a spacecraft's speed and direction. It gains energy from the planet's orbital motion, without using extra fuel.
Real-world uses
- Voyager probes used gravity assists to reach the outer planets.
- Mission designers plan gravity assists to reach distant destinations economically.
- NASA's Voyager probes used gravity assists from Jupiter and Saturn to gain speed without carrying extra fuel.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Does the spacecraft slow the planet?
Yes, but the effect on the planet is negligible because of its enormous mass.
Where does the extra energy come from?
From the planet's orbital motion around the Sun.
Does a gravity assist really create energy?
No. It exchanges momentum with the moving planet, so the planet loses a tiny amount of its own orbital energy.
Related topics in Gravitation & Orbits
Eccentricity · Perihelion and aphelion · Orbital transfers · Tides · Tidal locking · Roche limit