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Landing on Mars

Air and fluids · level 7, lesson 3 of 6

A question to start

Perseverance hit the top of Mars's air at nearly 20,000 km/h and was standing still on the ground about seven minutes later. How did it lose all that speed, and why did it need rockets as well as a parachute?

The idea

NASA's landing sequence for Perseverance, on 18 February 2021, went in three stages.

  1. The heat shield. Air drag on the blunt heat shield did most of the work, slowing the craft from nearly 20,000 km/h to about 1,512 km/h. The shield got extremely hot, because the lost kinetic energy became heat.
  2. The parachute. About 240 s after entry, at about 11 km up, a parachute 21.5 m across opened. It slowed the craft to about 320 km/h.
  3. Rockets. That was still far too fast to land. The craft dropped away from the parachute, and a rocket-powered descent stage slowed to about 2.7 km/h. About 20 m above the ground, it lowered the rover on cables about 6.4 m long, the "skycrane", and set it down.
Speed Kinetic energy left, as a share of entry
Entry: about 20,000 km/h 100%
Parachute opens: about 1,512 km/h 0.6%
Parachute's best: about 320 km/h 0.03%
Skycrane: about 2.7 km/h practically none
heat shieldparachuterockets

The parachute could not finish the job because Mars air is so thin. Its terminal velocity stayed high, just as the skydiver's does. On Earth, the same parachute and load would fall roughly five times more slowly.

Worked examples

1. Energy shed by the heat shield. Kinetic energy goes with \(v^2\): \((1512/20{,}000)^2 = 0.0057\). So 99.4% of the kinetic energy was gone before the parachute opened.

2. The parachute's area. \(\pi \times (21.5/2)^2 = 363\) m², bigger than the whole playing area of a tennis court.

3. Too fast. 320 km/h is 89 m/s. Hitting the ground at that speed is like falling from a height of \(v^2/2g = 89^2/(2 \times 3.71) \approx 1{,}070\) m on Mars.

Try it

  1. Which stage removed most of Perseverance's kinetic energy?
  2. What share of the entry kinetic energy was left at 320 km/h?
  3. Why could the parachute not slow the craft enough to land?
  4. Using the ratio from the last lesson, about how fast would the same parachute bring the load down on Earth?
Answers
  1. Drag on the heat shield, before the parachute opened.
  2. \((320/20{,}000)^2 = 0.000256\), about 0.03%.
  3. Mars air is so thin that, even with the parachute, the drag only balances the weight at a high speed.
  4. About \(89/5.4 \approx 16\) m/s, roughly.

Common mistake

"The parachute does the slowing." On Mars, the heat shield and the air did almost all of it. The parachute took the craft from fast to less fast, and rockets did the rest.

Where it leads