deorbit burn

A deorbit burn is a propulsive maneuver in which a spacecraft's engines fire to reduce orbital velocity enough to drop the vehicle out of stable orbit and into the atmosphere. The burn typically reduces velocity by 100 to 300 meters per second, depending on the target orbit altitude and reentry corridor. Without this velocity loss, a spacecraft remains in orbit; with it, the spacecraft follows a ballistic path that intersects the sensible atmosphere, where aerodynamic drag completes the work of slowing the vehicle to subsonic speeds.

The timing and direction of the burn are critical. The engine thrust must point nearly opposite to the velocity vector to be effective; a burn performed at the wrong orbital position or attitude wastes fuel and can result in a dangerously shallow or steep reentry corridor. Mission planners calculate the exact burn duration, start time, and spacecraft orientation weeks in advance. A 5-meter-per-second error in velocity change can move a reentry landing zone by tens of kilometers.

Variants and propulsion sources

Large crewed vehicles like the Soyuz use dedicated retro-thrusters; Skylab and early Space Shuttle operations used Orbital Maneuvering System engines; the International Space Station relies on Progress spacecraft or visiting vehicle propulsion to deorbit cargo modules. Unmanned satellites and spent upper stages often use monopropellant hydrazine thrusters or solid kick motors. The specific impulse and available delta-v of the spacecraft determine whether a controlled, long-duration burn or a quick retrograde impulse is possible. Some uncontrolled reentries occur when no fuel remains for a deliberate deorbit burn.

The burn must occur in the right half of the orbit, typically near apogee or at an equator crossing, so that the resulting trajectory intersects land or ocean in a designated impact zone. A burn performed on the wrong side of Earth can fail to lower perigee below atmospheric altitude, leaving the spacecraft in a highly elliptical orbit that reentries unpredictably or not at all. This is why deorbit burn sequencing is one of the most carefully monitored flight events in spaceflight operations.