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Coming down gently

Rockets and landing · level 1, lesson 5 of 6

A question to start

Jump off a low step and land with straight, stiff legs: ouch. Now jump again and bend your knees as you land. Much better. Why does bending your knees help?

The idea

When you land, you have to stop. Stopping quickly gives a big, sharp jolt. Stopping slowly, over a longer time and distance, gives a small, gentle one.

Bending your knees makes your stop longer. Your body slows down over the whole time your knees are bending, instead of all at once. The same idea is everywhere:

  • A soft mat at the gym squashes as you land on it, so you stop slowly.
  • A cushion under a dropped egg can save it.
  • A car's crumple zone squashes in a crash, so the people inside stop more slowly.
hard floor: crack!cushion: safe

Landing a spacecraft is the same problem, but much harder. A ship coming back from space is going very, very fast. It must lose almost all of that speed before it touches the ground, or it will crash. So landers slow down a little at a time: first by rubbing through the air, then with parachutes, then often with rockets pushing against the fall, and last with soft legs or bouncy airbags for the touch.

Worked examples

1. Catching an egg. Toss an egg gently to a friend. If they hold their hands stiff, it may break. If they move their hands back with the egg as they catch it, it stops slowly and stays whole. (Try with a water balloon outside!)

2. Landing on Mars. In 2021 the Perseverance rover came down at the end, very gently, lowered on cables, at about the speed of a slow walk.

Try it

  1. Why should you bend your knees when you land from a jump?
  2. Why does a soft mat make landing safer?
  3. Would an egg dropped onto a pillow break? Why or why not?
  4. Why must a spacecraft slow down before it lands?
Answers
  1. You stop more slowly, so the jolt is smaller.
  2. It squashes, so you stop over a longer time.
  3. Probably not, from a small height: the pillow squashes and the egg stops slowly.
  4. It is going so fast that it would crash if it hit the ground at that speed.

Common mistake

"Soft landings are about falling slower." Falling more slowly helps, but even a fast thing can land safely if it stops slowly enough. That is why landers have squashy legs and airbags as well as parachutes.

Where it leads