undock

Undocking is the controlled separation of a spacecraft or aerospace vehicle from a docking fixture, airlock, or berthing mechanism while the vehicle maintains autonomous control of its own propulsion and attitude. Unlike simple detachment, undocking involves active maneuvering away from the structure to which the craft was attached, typically over a period of seconds to minutes.

In practice, undocking requires several prerequisites. Electrical connectors must be severed or disconnected in the correct sequence to avoid arcing or loss of power during critical stages. Fluid lines carrying coolant, propellant, or life support media must be purged and capped. Mechanical latches, clamps, or magnetic couplers holding the vehicles together must release in a controlled fashion, often sequenced by ground control or onboard flight computers to prevent collision or tumbling.

Variants and contexts

Undocking occurs in several aerospace environments. Spacecraft separating from space stations (such as Soyuz modules from the ISS) perform undocking maneuvers under their own RCS thrusters. Cargo vehicles and crewed capsules undock after berthing periods that may span weeks. In atmospheric operations, aircraft may undock from specialized tow systems or ground equipment that temporarily hold them in place. The term is also used in suborbital vehicle testing, where prototype craft separate from test stands or mounting rigs under power.

The process is hazardous because the two structures were designed to be rigidly coupled. Separation can cause unexpected forces, fluid spills, or electrical transients if sequencing goes wrong. Ground controllers and onboard abort systems monitor sensor data throughout undocking to detect anomalies. Relative velocity between the departing vehicle and the fixed structure must remain low, typically under 0.3 meters per second, to avoid hard contact or debris generation.

The term undocking is preferred over "detach" or "disconnect" in aerospace precisely because it implies active, powered separation under the vehicle's own authority, distinguishing it from passive decoupling or release of a docked object. It reflects the critical distinction between a spacecraft that can maneuver independently and an object that must be retrieved or repositioned by external means.