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Parachutes in thin air

Rockets and landing · level 4, lesson 5 of 6

A question to start

Perseverance's parachute was 21.5 m across, opened in less than a second, and pulled with the force of about 30 tonnes. Yet it left the lander still falling at about 320 km/h. Why is a parachute so hard to use on Mars?

The idea

A parachute works by catching air. The force it gets depends on how thick the air is and on how fast the air hits it. At the ground on Mars, the air is about sixty times thinner than at Earth's ground. To get the same push from thin air, a parachute must be much bigger, or go much faster, or both.

Mars parachutes do both:

  • Big. Perseverance's was 21.5 m across, the same size as Curiosity's, the largest yet opened on another world.
  • Fast. It opened while going faster than sound, about 1.7 times the speed of sound in Mars air. Most parachutes on Earth open at a few hundred km/h at most. A parachute opening supersonically is slammed open in a fraction of a second, and it can tear or flap itself to pieces.
Earth: small, slow fallMars: huge, still fast

Engineers test Mars parachutes on Earth by carrying them up on rockets to about 40 to 50 km, where the air is as thin as at Mars's ground, and opening them at Mars speeds. Perseverance's parachute was tested this way in 2017 and 2018, in tests called ASPIRE. In an earlier set of tests, in 2014 and 2015, two new parachute designs had torn apart.

Why it still is not enough. Even this huge parachute can only slow a lander of a few tonnes to about 300 km/h or so in Mars's thin air. Then the lander must let it go and use rockets. A parachute that could land a rover gently on Mars by itself would have to be far bigger than a football field.

Heavier landers for people, of tens of tonnes, could not use parachutes at all. They would need rockets firing from high up, while still going faster than sound (level 7).

Worked examples

1. A parachute on Earth. A skydiver with a 7 m parachute lands at about 5 m/s. On Mars, the same skydiver with the same parachute would land at roughly 25 m/s, about 90 km/h.

2. How big to land gently? To land the same lander five times slower, a parachute needs about 25 times the area, five times as wide. (You will see why in level 7.)

Try it

  1. Why does a parachute catch less force on Mars?
  2. Name two things Mars parachutes do to make up for the thin air.
  3. Why is opening faster than sound dangerous for a parachute?
  4. How are Mars parachutes tested on Earth?
Answers
  1. The air is much thinner, so there is less of it to push on the parachute.
  2. They are very big, and they open at very high speed.
  3. It slams open in a fraction of a second, with huge forces, and it can tear or flap apart.
  4. They are carried by rockets to 40 to 50 km up, where the air is as thin as on Mars, and opened at high speed.

Common mistake

"A bigger parachute always solves it." A parachute twice as wide only halves the landing speed, and it is four times as heavy and much harder to open safely. For big landers on Mars, rockets have to take over.

Where it leads