A spacecraft trajectory that passes close to a target object without entering orbit, often for rapid data collection.A flyby is a spacecraft trajectory that swings close to a celestial body — a planet, moon, or asteroid — without entering orbit around it. The spacecraft uses the target's gravity to bend its path and change its speed, a technique called a gravity assist. Flybys are the workhorses of planetary exploration: they allow a single mission to visit multiple targets, or to reach distant destinations that would require too much fuel for a direct journey.
The concept of gravity assist was first proposed in the 1960s by Michael Minovitch, who calculated that a Jupiter flyby could accelerate a spacecraft enough to reach Saturn. The first successful planetary flyby was Mariner 2's pass of Venus in 1962.
As the spacecraft approaches the planet, it falls into the planet's gravitational well, gaining speed. It then swings around and exits, but because the planet is moving in its own orbit, the spacecraft's velocity relative to the Sun changes — it can gain or lose energy depending on the geometry. The spacecraft never touches the planet; it just uses its gravity as a slingshot. The key is precise timing: arriving too early or too late misses the window for the desired trajectory change.
Flybys are used in nearly every interplanetary mission — Voyager 1 and 2 used them to visit multiple outer planets, and New Horizons used a Jupiter flyby to reach Pluto. They are also essential for missions to Mercury, which requires multiple flybys to shed orbital energy.
A single Jupiter flyby can increase a spacecraft's speed by more than 10 kilometers per second — enough to cross the entire solar system in a few years instead of decades.
