It's been ten years since NASA's Spirit rover landed on Mars' Gusev Crater. On Jan. 4, 2004, once the lander opened and the rover woke up, it took the series of images that were then stitched together into this stunning panorama showing the rover in its lander on the surface of Mars (view the hi-res version).
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To land on Mars, Spirit and its twin Mars Exploration Rover Opportunity (which landed 21 days later on Jan. 25) used a heat shield, aeroshell, parachute, and airbags; all of which were heritage technology used on previous missions.
NASA first landed on Mars with the twin Viking landers in 1976. These spacecraft, after separating from their respective orbiters, plunged through the Martian atmosphere in a 70-degree sphere-cone aeroshell. A parachute deployed to slow the aeroshell's fall, and when the heat shield fell away it uncovered the landing radar. Retrorockets slowed the lander to its final touchdown.
NASA's 1997 Mars Pathfinder mission used the same basic system in the early stages of landing, but in the final descent it pioneered the use of airbags to break its fall. The Pathfinder lander bounced and rolled along the Martian surface until it stopped.
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Spirit and Opportunity followed the same entry, descent, and landing profile as Pathfinder, though these rovers were both bigger and heavier.
Spirit was protected in its fall to Mars by four airbags each with six connected lobes each made from a durable synthetic material called Vectran. That the bags were connected was important: the designed helped dissipate landing forces and keep the whole structure flexible and responsive as it impacted the ground. The fabric of the airbags was also important. The bags weren't attached to the rover directly but rather by a series of ropes that gave them gave and simplified inflation by an onboard gas generator.
Because Spirit (and Opportunity) were so much heavier than Pathfinder, qualifying the airbags for flight meant a lot of test drops. And the testing paid off. Spirit reached Mars in good health, and it's planned 90 sol mission extended to 2,623 sols. The rover went silent in 2010.
Sources: NASA, Braun and Manning