How do you land a rover on Mars in the "seven minutes of terror"?

By doing almost everything automatically, in a single seven-minute sequence of heat shield, parachute, rockets and a hovering "sky crane". Radio signals from Mars take minutes to reach Earth, and as long again to come back, so any command sent in reply would arrive after the landing is over. Engineers nickname this phase "seven minutes of terror": the spacecraft is on its own once it enters the Martian atmosphere.

The timing is the heart of it. The one-way delay between Mars and Earth ranges from 4 to 22 minutes depending on where the planets are, and averages 12.5. When Perseverance landed on 18 February 2021, the news took 11 minutes to arrive. Try the slider: at Perseverance's 11 minutes, the descent was over before Earth even heard it had begun, and even at the shortest delay a reply would need about 8 minutes for the round trip, longer than the whole landing.

The landing is tricky because Mars's atmosphere is too thin for a parachute alone to do the job, yet thick enough to cause stability trouble for rockets. So the craft uses a chain of tools. An aeroshell with a heat shield leads the way in. A parachute opens while the craft is still moving at supersonic speed. Then, under 320 km/h and about 1.9 km up, the rover and a rocket-powered descent stage drop away from the parachute.

The final trick is the sky crane. Rockets on the descent stage hold it above the ground while it lowers the rover on cables. Once touchdown is confirmed, the cables are cut and the stage flies off to crash a safe distance away. Both Curiosity and Perseverance landed this way, and Perseverance came down within five metres of its target.