Knowledge Zone · Card 12 of 13
Capture burn or aerobraking
Reaching Mars is not the same as being kept by it. A ship that arrives is still moving at about 2 649 m/s relative to Mars — the arrival burn of the Hohmann transfer, seen from the other side — and unless something removes that speed it flies straight past on a hyperbola.
You can pay with fuel or with air. A capture burn is retrograde at the closest point, where the ship is fastest and the burn is worth most: 727 m/s buys a long ellipse, 2 070 m/s buys a circular 500 km orbit. Aerobraking is free but narrow: the corridor is only 61.7 to 73.6 km above the surface. Too shallow and the pass does not slow you enough, so Mars lets you go; too deep and the heating passes what the shield survives.