Operators predict close approaches from tracking data, work out the chance of a hit, and fire thrusters to shift the orbit when that chance is too high.
Thousands of objects share low Earth orbit, so near misses are common. The trick is to know about them days in advance. Tracking networks send warnings of predicted close approaches, called conjunctions, to satellite operators. ESA, for example, receives several hundred such messages every day and updates its predictions several times a day, usually looking seven days ahead.
Most warnings turn out to be harmless once the orbits are checked carefully. Operators calculate the probability that the two objects will actually collide. If it crosses a set limit, they plan a small engine burn — a collision avoidance manoeuvre, or CAM — that changes the satellite's timing or height enough to open up the gap.
For the International Space Station, NASA set two levels. Above a 1-in-10,000 chance, the station always manoeuvres if it can; under NASA's 2011 rules, if it could not, the crew moved into their docked Soyuz. Above 1 in 100,000, it manoeuvres if convenient. ESA carries out about 12 avoidance manoeuvres a year across its missions.
ISRO's 2024 situational assessment report gives real numbers. Its teams analysed more than 53,000 close-approach alerts for Indian satellites, most of them from the US Space Command's tracking network. Only 10 led to a dedicated avoidance manoeuvre: 6 in low Earth orbit and 4 in geostationary orbit. On several other occasions ISRO adjusted routine orbit-keeping burns so they also cleared the danger. In 2023 the count had been 23.
That is roughly one manoeuvre for every 5,300 alerts. Even the Moon is not empty: Chandrayaan-2's orbiter moved a planned burn to 11 November 2024 to avoid predicted close approaches with NASA's Lunar Reconnaissance Orbiter.
Rockets are checked too. Before each launch ISRO screens the lift-off time against known objects. PSLV-C60 was held for 2 minutes 15 seconds on 30 December 2024 because of that analysis.
Each manoeuvre uses fuel that would otherwise lengthen a satellite's life, and every new constellation adds objects to screen. There is still no accepted international space traffic management system, so close approaches are settled case by case between operators.
Watch the station's estimated altitude: a reboost or debris-avoidance burn shows up as a small step in height once fresh orbital data is published. Open the ISS tracker
The key number is Pc, the probability of collision. Each object's predicted position has an uncertainty; the bigger the uncertainty, the more spread out the possible miss distances. Pc is the chance that, given those uncertainties, the two objects come closer than the sum of their sizes. A very uncertain prediction can give a small Pc even for a close pass, which is why NASA added “validity criteria” so that decisions are made only on good data.
Before Pc methods, the Space Shuttle used a fixed “box” around the vehicle: any object predicted inside it triggered action. NASA judged that a box rule could not work for the ISS, which flies 24 hours a day, all year, and moved to Pc. The two ISS thresholds — 1 in 10,000 (red) and 1 in 100,000 (yellow) — were chosen to balance safety against the disruption of frequent burns.
The burn itself is small. Raising a circular orbit's speed by Δv changes its period by about ΔT = 3 T Δv / v, so the satellite drifts along its track by about v × ΔT = 3 T Δv every orbit. In low orbit, ESA gives v ≈ 7.8 km/s and T ≈ 90 minutes (5,400 s). A tiny Δv of 0.1 m/s therefore moves the satellite about 3 × 5,400 s × 0.1 m/s ≈ 1.6 km along its path per orbit. Planned a day or so ahead — about 16 orbits — that grows to roughly 26 km, turning a predicted hit into a safe miss.
ISRO's 2024 numbers show the filtering at work: 53,000 alerts ÷ 10 manoeuvres ≈ 5,300 alerts per dedicated burn.
A small planned thruster burn that changes a satellite's orbit so that a predicted close approach with another object becomes a safe miss.
When the chance of collision is greater than 1 in 10,000, NASA's rule is to manoeuvre if at all possible.
Ten for Earth-orbiting satellites — six in low Earth orbit and four in geostationary orbit — after analysing more than 53,000 alerts.