Understanding Conjunction Assessment
How conjunctions are screened
Conjunction assessment begins by propagating the orbit of every catalogued object forward in time. Around each protected satellite the software sweeps a screening volume — an elongated region, typically a few kilometres across but tens of kilometres long in the along-track direction, where position error is greatest. Any object predicted to pierce that volume triggers a CDM. The US Space Force's 18th Space Defense Squadron runs this screening about three times a day against the whole catalogue, looking roughly seven days ahead, and distributes the messages through Space-Track.org. Because orbits are uncertain, the collision probability — not the miss distance alone — decides how serious an event is: a tightly-known 300-metre pass can be safer than a poorly-known 2-kilometre one. Typical (NASA CARA-style) risk tiers look like this, though exact thresholds vary by mission:
| Risk tier | Collision probability | Typical response |
|---|---|---|
| Green | Below 1 in 10 million | Routine monitoring only |
| Yellow | 1 in 10 million to 1 in 10,000 | Track closely, refine orbits, prepare options |
| Red | Above 1 in 10,000 | Plan and, if needed, execute an avoidance manoeuvre |
Who performs conjunction assessment
Conjunction assessment is a core function of space situational awareness, and several organisations carry it out, each protecting its own assets:
- 18th Space Defense Squadron (US Space Force) — maintains the public catalogue and issues CDMs to operators worldwide; currently the principal global provider.
- NASA CARA — the Conjunction Assessment Risk Analysis team at Goddard, safeguarding NASA's robotic missions since 2005.
- ESA Space Debris Office — a highly automated service at ESOC for European missions, refreshed several times a day.
- US Office of Space Commerce — now taking over civil screening through its Traffic Coordination System for Space (TraCSS), which began delivering CDMs to commercial operators in 2024–2025.
From CDM to a manoeuvre decision
A CDM is a forecast, not a verdict. When an event is first flagged days ahead, the position uncertainty is large and the collision probability is often low; as fresh tracking data arrive the covariance shrinks and the probability may climb or — far more often — fall away. Operators therefore watch an event evolve and usually make the final call one to two days before the time of closest approach, when the geometry is best understood. If the probability crosses the mission's threshold, they plan a collision-avoidance manoeuvre, spending delta-v and briefly interrupting the mission. The stakes are set by history: in 2009 the defunct Cosmos 2251 struck the active Iridium 33 at about 11.7 km/s — the first accidental collision between two intact satellites — scattering more than 2,000 trackable fragments. You can watch active manoeuvres on our satellite manoeuvre tracker and the debris they dodge on the space debris map.