flyby

A flyby is a spaceflight maneuver in which a spacecraft passes close to a celestial body, typically to collect scientific data during the approach and to influence its trajectory using the body's gravity. A flyby does not involve entering orbit around the target or landing on it, though it may be used to set up future spacecraft maneuvers.

During a flyby, a spacecraft can perform a gravity assist (gravity slingshot) to change its velocity and direction. By carefully planning the encounter, mission designers can increase or decrease the spacecraft’s speed, adjust its trajectory to reach additional targets, or reduce fuel needs. The distance of closest approach, or pericenter, is chosen to balance science return, safety (avoiding radiation belts or atmospheric entry), and the desired change in velocity.

Many historic missions relied on planetary flybys. The Voyager program used multiple gravity assists to reach and study Jupiter, Saturn, Uranus, and Neptune. Pioneer 10 and 11 conducted early flybys of Jupiter and Saturn, respectively. More recently, New Horizons flew by Pluto in 2015 and by the Kuiper Belt object Arrokoth in 2019, following a path that included gravity assists to reach the outer solar system.

Flybys are distinct from orbital missions, which insert a spacecraft into sustained orbit around a target, or from landing missions, which touch down on the surface. They remain a fundamental technique in mission planning, enabling exploration of distant destinations with limited propulsion and extended scientific payoff.