Rendezvous

Catching another spacecraft, which requires slowing down to speed up.

Orbital mechanics is counter-intuitive at close range: thrusting forward raises your orbit, which makes you slower in angular terms and drops you behind. Catching a target therefore means dropping to a lower, faster orbit, closing the gap from below, and rising to meet it. Early flights took days to do it; a modern fast rendezvous can reach the station in about three hours, which is four orbits and a great deal of confidence in the computer. The last few hundred metres are a separate problem again, and it is one of relative rather than orbital motion. Approaches are flown along a corridor, usually from below and behind, with hold points where the vehicle can stop and be looked at — and every crewed programme keeps a manual takeover, because the failure mode nobody accepts is a computer flying a spacecraft into a station.