With thousands of active satellites and a growing amount of debris in orbit, knowing where everything is has become essential. Space situational awareness, often shortened to SSA, covers the tracking of objects in orbit, the prediction of close approaches and the decisions operators take to avoid collisions.
How objects are tracked
Ground based radars and telescopes observe objects in orbit and determine their trajectories. Military and civil organisations maintain catalogues of these objects, and commercial companies operate their own sensor networks. Operators also share precise orbit data of their own satellites, which is usually much more accurate than external tracking.
Conjunction warnings
When the predicted paths of two objects come close, a conjunction warning is issued. It contains the time of closest approach, the predicted miss distance and an estimate of the collision probability. Because orbit predictions are uncertain, operators look at probability thresholds rather than distances alone.
Collision avoidance manoeuvres
If the risk exceeds the operator's threshold, the satellite performs a small manoeuvre to change its orbit slightly. ESA, for example, regularly performs collision avoidance manoeuvres with its satellites. Satellites without propulsion cannot avoid collisions, which is one reason why regulators and agencies increasingly expect satellites to be able to manoeuvre.
Coordination between operators
When both objects are active satellites, the operators need to agree who will manoeuvre. Without coordination, both might move and end up closer than before. Shared contact points and data exchange are therefore part of good practice.
Why this matters from the launch onwards
- After deployment, satellites must be identified and registered quickly so that they can be tracked.
- Launch providers check the ascent trajectory against known objects and may briefly delay a launch within its window.
- Operators should plan propulsion, orbit data sharing and operational procedures for collision avoidance before launch.
Related reading
- Space Debris Rules: What Satellite Operators Need to Plan For
- Satellite End of Life: Deorbiting, Graveyard Orbits and Passivation
- Space Weather Explained: How the Sun Affects Satellites
Sources
Cover image: Earth photographed from the International Space Station. Credit: NASA (iss040e117941), public domain.