A gravity assist (or gravitational slingshot) uses a planet’s gravity and orbital motion to change a spacecraft’s speed and direction — without burning any fuel. It’s one of the most elegant tricks in orbital mechanics.

How it works

A spacecraft approaches a planet, swings around it, and departs. Relative to the planet, it arrives and leaves at the same speed — gravity just bends the path. But the planet is moving through the solar system, and this is where the magic happens.

In the planet’s frame: speed unchanged, direction changed. The encounter is symmetric.

In the Sun’s frame: the planet’s own orbital velocity gets added (or subtracted). The spacecraft can gain enormous speed from a planet that barely notices the interaction.

It’s like bouncing a tennis ball off the front of a moving train — the ball picks up the train’s speed.

Toggle between frames to see why the speeds differ. In the planet’s frame, the spacecraft’s speed is unchanged. In the Sun’s frame, it has gained (or lost) velocity.

Real missions

Voyager 2 (1977–): Jupiter → Saturn → Uranus → Neptune. Each flyby redirected the trajectory and boosted the speed. Without gravity assists, visiting all four outer planets would have been impossible.

Parker Solar Probe (2018–): Uses seven Venus flybys to gradually shrink its orbit. Each encounter slows the probe relative to the Sun, dropping the perihelion closer to the solar surface. By 2025 it passes within 6.1 million km of the Sun — closer than any spacecraft in history. Sometimes you want to lose speed.

DART / OSIRIS-APEX: After DART impacted asteroid Dimorphos (2022), the OSIRIS-REx spacecraft was redirected as OSIRIS-APEX. It uses an Earth gravity assist to reach the Apophis asteroid during its close flyby in 2029.

The energy has to come from somewhere

Where does the spacecraft’s kinetic energy come from? The planet. A gravity assist transfers orbital energy from the planet to the spacecraft (or vice versa). Jupiter’s orbit becomes infinitesimally larger with each Voyager flyby. But Jupiter is $1.9 \times 10^{27}$ kg — the effect is immeasurably small.

Why gravity assists aren’t free

They cost no fuel, but they cost time and trajectory constraints. You must fly past the right planet at the right time at the right angle. Mission design with gravity assists is a complex optimisation problem — finding a sequence of flybys that achieves the desired final orbit within the available launch windows.