Glossary / Space
Gravity assist
Using a planet's gravity and orbital motion to change a spacecraft's speed or direction without fuel as it flies past.
From the planet's point of view the spacecraft simply approaches and recedes, but because the planet itself is moving around the Sun, the spacecraft picks up (or loses) some of that motion as seen from the Sun. In 1961 Michael Minovitch, a graduate student working at the Jet Propulsion Laboratory over the summer, showed how to find such trajectories systematically by computer[1].
Voyager, launched in 1977, used a planetary alignment that occurs once in 175 years to visit Jupiter, Saturn, Uranus and Neptune in turn. Nearly every mission to distant bodies relies on the technique.
Related terms
- Hohmann transferMoving between two circular orbits by riding an ellipse tangent to both, with just two burns — a fuel-efficient way to transfer.
- Kepler's lawsThree laws of planetary motion: planets move on ellipses with the Sun at one focus, sweep faster when closer to it, and have orbital periods tied to orbit size.
- HeliopauseThe boundary where the flow of particles from the Sun (the solar wind) is stopped by the gas between the stars — one definition of the outer edge of the solar system.
On the timeline
Sources
- M. A. Minovitch, “A method for determining interplanetary free-fall reconnaissance trajectories,” JPL Technical Memo 312-130 (1961).