To land on a planet, one must slow down from orbital speed (tens of thousands of miles per hour), to a standstill on the surface. Re-entry friction with Earth's thick atmosphere reduces a spacecraft‘s speed enough for parachutes to provide a gentle descent. On the moon, which has no atmosphere, rockets are used all the way down to ensure a soft landing. Mars' very thin atmosphere cannot provide enough resistance to slow a spacecraft to a safe landing speed with just a heat shield and parachute.
These are some methods that have been used, usually in combination, to land vehicles on Mars:
Aeroshell –Fragile probes are encased in a heatproof shell for entry into the Mars atmosphere. Curiosity’s 14.8-foot (4.5 meters) shell was the largest yet.
Parachutes –Once past the high temperatures of atmospheric entry, parachutes are deployed to reduce speed.
Rockets –Near the ground, rocket engines can be fired to slow the probe to a gentle touchdown.
Sky crane– Curiosity used an innovative rocket platform to lower the rover to the surface on cables, then fly away safely.
At 1 ton (907 kilograms), Curiosity is the largest spacecraft yet landed on Mars. To land larger payloads, engineers are developing the Supersonic Inflatable Aerodynamic Decelerator (SIAD).
The 28-foot (8 m) SIAD includes an inflatable section to increase drag during re-entry. The decelerator was first tested on a rocket sled, and next will be tested at high altitude in Earth's atmosphere.
Eventually, versions of the decelerator could land multi-ton payloads on Mars' surface, which is a requirement for manned landings.
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