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Landing on rockets

Rockets and landing · level 4, lesson 6 of 6

A question to start

The first landers on Mars stood on three legs. Later rovers bounced down inside giant airbags. Curiosity and Perseverance were lowered on cables from a hovering rocket stage. Why so many different ways?

The idea

Every lander must finish its descent with rockets, because the air of Mars cannot slow it enough. How it touches down depends on how heavy it is and what it carries.

How it touched down Landers Mass on Mars at landing (about)
Rockets and legs Viking 1 and 2 (1976), Phoenix (2008), InSight (2018) 350 to 600 kg
Rockets, then airbags Mars Pathfinder (1997), Spirit and Opportunity (2004) 360 to 540 kg in their airbags
Rockets and sky crane Curiosity (2012), Perseverance (2021) about 900 to 1,025 kg (the rover)

Legs. The lander fires its engines all the way down and touches on three legs with shock absorbers, then shuts the engines off. Simple, but the hot exhaust blasts the ground right under the lander, and a rover would have to drive down a ramp afterwards.

Airbags. Rockets slow the lander to almost a stop about 10 to 15 m above the ground, then cut it loose. It falls inside a cluster of airbags, hits at about 50 km/h, and bounces, sometimes more than a dozen times, before rolling to a stop. It worked brilliantly for rovers up to about 200 kg, but airbags for a car-sized rover would be too heavy and would tear.

Sky crane. For a rover as big as a car, NASA flew the rockets on a separate descent stage that hovered high above the ground and lowered the rover on cables. The engines never got close to the ground, so they kicked up less dust and dug no holes, and the rover landed on its own wheels, ready to drive. Then the descent stage flew off to crash safely away.

Big landers for people would weigh tens of tonnes. They would land on rockets and legs, like Viking, but much bigger, and they would need landing pads so their engines did not blast holes and throw rocks.

Landing on Earth too. Since December 2015, SpaceX has landed Falcon 9 first stages upright on rockets, coming back from the edge of space to a pad or to a ship at sea, and flying them again.

Worked examples

1. Why not airbags for Perseverance? Spirit's rover weighed about 185 kg; Perseverance weighs 1,025 kg, more than five times as much. Airbags big enough would be too heavy to carry and would have to survive far harder hits.

2. The sky crane's cables. The rover was lowered about 6.4 m below the descent stage, which kept the stage's engines well above the surface while the rover touched down.

Try it

  1. Why must every Mars lander use rockets at the end?
  2. What are the good and bad points of landing on legs?
  3. Why were airbags not used for Curiosity and Perseverance?
  4. Why did the descent stage fly away after letting go of the rover?
Answers
  1. The air is too thin for heat shields and parachutes alone to slow it enough.
  2. Good: simple, and the lander ends up standing still. Bad: the exhaust blasts the ground under it, and a rover would need a ramp to drive off.
  3. They were too heavy: airbags that big would weigh too much and might tear.
  4. To crash far away from the rover, without hitting it or covering it in dust.

Common mistake

"The sky crane was a stunt." It solved a real problem: landing a heavy rover on its own wheels, with the hot engines kept high above the ground. It worked twice.

Where it leads