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Seven minutes

Rockets and landing · level 4, lesson 4 of 6

A question to start

On 18 February 2021, engineers at NASA's Jet Propulsion Laboratory watched their screens in silence. Perseverance was already on the ground, or already wrecked; the news was still on its way, 11 minutes behind. The landing itself took about seven minutes. What happened in them?

The idea

The landing, called entry, descent and landing (EDL), had to take the rover from about 20,000 km/h to less than 3 km/h, all on its own, because there was no time for anyone on Earth to help.

0:00 entry, about 125 km up, about 20,000 km/h about 1:20 hottest, heat shield near 1,300 °C about 4:00 parachute opens, about 11 km up, about 1,500 km/h about 4:20 heat shield drops; radar and cameras look at the ground about 5:50 backshell lets go, about 2 km up, about 320 km/h powered descent: eight engines slow it sky crane, about 20 m up: rover lowered on cables about 7:00 touchdown, about 2.7 km/h
  1. Entry. The capsule hits the top of the air at about 20,000 km/h. Over the next four minutes, the heat shield takes the heat and the air brakes it. Small thrusters steer it, tilting its lift to aim for the target.
  2. Parachute. At about 11 km up and about 1,500 km/h, still faster than sound, a parachute 21.5 m across bursts open.
  3. Heat shield off. It drops away, and the rover's radar and cameras see the ground for the first time. A new system, terrain-relative navigation, compared the pictures with maps it carried and picked a safe spot to land.
  4. Backshell off. At about 2 km up, still at about 320 km/h, the parachute and the top half of the capsule are let go.
  5. Powered descent. The descent stage, with the rover tucked under it, fires eight rocket engines and slows down.
  6. Sky crane. About 20 m up, the descent stage hovers and lowers the rover on three nylon cables, about 6.4 m long.
  7. Touchdown. The rover's wheels touch the ground at about 0.75 m/s. The cables are cut, and the descent stage flies away to crash at a safe distance.

Worked examples

1. How much slowing. From 20,000 km/h to 2.7 km/h: the rover lost more than 99.98% of its speed in seven minutes.

2. What the air did. By the time the parachute opened, the capsule had slowed from 20,000 km/h to about 1,500 km/h using only its heat shield and the air: more than nine tenths of its speed, gone without a drop of fuel.

Try it

  1. Why must the landing happen with no help from Earth?
  2. Which step slows the capsule the most?
  3. What did terrain-relative navigation do?
  4. Why does the descent stage fly away after it lets go of the rover?
Answers
  1. Radio takes about 11 minutes each way, longer than the whole landing.
  2. Entry, with the heat shield and the air: from about 20,000 km/h to about 1,500 km/h.
  3. It matched camera pictures of the ground with maps, worked out where the rover was, and steered it to a safe spot.
  4. So it does not crash on top of the rover or blow dust over it.

Common mistake

"The parachute does most of the work." The heat shield and the air did most of it, before the parachute even opened. The parachute takes away most of what is left; the rockets finish the job.

Where it leads