Definition
A conjunction occurs when two orbiting objects are predicted to pass close to each other. Not all conjunctions are dangerous — thousands occur daily at distances of several kilometres. The concern is when the predicted miss distance is small enough that the uncertainty in the objects' positions overlaps, creating a non-negligible probability of collision.
Conjunction Assessment
The 18th Space Defense Squadron screens the catalogue daily, producing Conjunction Data Messages (CDMs) for close approaches. Operators receive warnings with predicted miss distance, time of closest approach, and collision probability. NASA's CARA (Conjunction Assessment Risk Analysis) team handles screening for NASA missions.
Collision Avoidance Manoeuvres
When the probability of collision exceeds a threshold (typically 1 in 10,000 for crewed vehicles like the ISS), the active spacecraft performs an avoidance manoeuvre — a small thruster firing to shift its orbit enough to increase the miss distance. The ISS performs several such manoeuvres per year. Starlink satellites have autonomous collision avoidance using onboard AI and propulsion.
The Growing Challenge
With over 44,800 tracked objects and rising, the number of conjunction alerts has increased dramatically. SpaceX reported that Starlink satellites were involved in thousands of close approach events per month. As the debris and satellite population grows, conjunction assessment becomes an ever more critical — and computationally demanding — function.