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Parachutes and legs

Rockets and landing · level 1, lesson 6 of 6

A question to start

A spacecraft arrives at Mars going about 20,000 km an hour, much faster than a bullet. It has to touch the ground going about as fast as you walk. How can it lose all that speed?

The idea

It does it in steps, using a different trick for each one.

1. heat shield: the air slows it2. parachute3. rocket engines push up4. legs, airbags or a sky crane
  1. Heat shield. The ship hits the top of the air so fast that the air in front of it gets hotter than a furnace. A thick heat shield on the bottom takes the heat, and rubbing through the air slows the ship down a lot.
  2. Parachute. When it is slow enough, a huge parachute pops out. On Mars the air is very thin, so the parachute has to be enormous, and still it cannot slow the ship enough on its own.
  3. Rockets. Then rocket engines fire downwards, pushing up against the fall, to slow it down more.
  4. The touch. Last, something soft for the landing: squashy legs, big bouncy airbags, or, for the Perseverance rover, a sky crane that hovered on rockets and lowered the rover to the ground on cables.

All of this takes about seven minutes. It happens so fast, and Mars is so far away, that nobody on Earth can steer it. The ship does it all by itself.

Worked examples

1. Airbags. In 1997 the Pathfinder lander, and in 2004 the rovers Spirit and Opportunity, landed inside big balloons. They bounced across the ground, more than a dozen times, before rolling to a stop.

2. Legs. In 2008 the Phoenix lander came down on rocket engines and three legs, near the north pole of Mars.

Try it

  1. What are the four steps a ship uses to slow down at Mars?
  2. Why does a heat shield get so hot?
  3. Why can't a parachute alone land a ship on Mars?
  4. Name a lander that used airbags, and one that used a sky crane.
Answers
  1. Heat shield, parachute, rocket engines, and legs, airbags or a sky crane.
  2. The ship is going so fast that it squashes and heats the air in front of it.
  3. The air is so thin that even a huge parachute leaves the ship going too fast.
  4. Airbags: Pathfinder, Spirit or Opportunity. Sky crane: Perseverance (or Curiosity).

Common mistake

"Landing on Mars is like landing on the Moon." The Moon has no air, so landers there use only rockets. Mars has just enough air to burn a ship that comes in too fast, but not enough to slow it with parachutes alone. It needs every trick.

Where it leads