Knowledge Zone · Card 13 of 13
Landing on Mars
Mars has air, but not much: about a sixtieth of Earth's density at the surface. It is enough to burn a shield off at orbital speed and nowhere near enough to land on. A 500 kg lander still arrives at the ground at 175 m/s with no engine at all, which is why parachutes alone genuinely do not work here.
So the retro engine has to do it, and the only real decision is when to light it. Too low and the burn runs out of altitude before it runs out of speed: below about 1 807 m the ship reaches the ground above 20 m/s and breaks. Higher is safer and costs more — 261 m/s of Δv from 4 000 m, 304 m/s from 8 000 m. Touch down under 5 m/s and it is a clean landing; between 5 and 20 you survive and the score charges you for every metre per second.