What is Active Debris Removal?
Active Debris Removal (ADR) is the practice of using dedicated spacecraft to rendezvous with non-cooperative space debris objects — defunct satellites, spent rocket upper stages, or large fragments — and either deorbit them through controlled atmospheric re-entry or manoeuvre them to a disposal orbit. Unlike passive mitigation (designing satellites to deorbit themselves at end of life), ADR targets existing debris that accumulated before mitigation rules existed.
Why ADR is urgent
Over 3,000 defunct satellites and 2,000 spent rocket bodies in LEO are too large (>10 cm) to ignore but non-cooperative — they have no functional propulsion, no attitude control, and are slowly tumbling. A single collision between two large objects at relative velocities of 7–15 km/s generates tens of thousands of sub-10 cm fragments too small to track but large enough to destroy an operational satellite. ESA estimates that removing the 10–50 most dangerous large debris objects per year from key altitude bands could stabilise the environment.
Pioneer missions
Astroscale (Japan): ELSA-d mission (2021) demonstrated magnetic capture of a cooperative target in orbit. ADRAS-J (2024) achieved the first-ever close approach and fly-around of actual uncooperative debris — JAXA's defunct H-IIA rocket body — with approaches within 15 metres. ClearSpace (ESA-backed, Swiss): ClearSpace-1 will attempt the first capture and deorbit of an actual piece of debris (VESPA adapter, 112 kg, 660 km orbit) using a spider-like quadrupedal gripper mechanism.
Capture technologies
Candidate capture mechanisms include robotic arms (most controllable, most complex), nets (simple, tolerant of tumbling targets), harpoons (penetrating), and electrodynamic tethers (using Earth's magnetic field for drag augmentation). The fundamental challenge is that all large debris objects are tumbling at unpredictable rates, requiring real-time adaptive control or passive capture mechanisms that can handle angular momentum.