Mars Curiosity’s “Sky Crane” Lander
Sky Crane is NASA’s descent module from the Mars Science Laboratory that will land a roving robot, Curiosity, on the Martian surface. It will lower Curiosity on a tether and the robot is supposed to land upright with its wheels ready to move. This operation is scheduled for August 16.
Scientists on Earth have been sending probes and robots to Mars since the 1960s. The failure rate of these missions has led to epithets such as the ‘Martian Curse’. There was a suggestion that a ‘Great Galactic Ghoul’ lives on Mars and feeds on a diet of probes from Earth.
Mars is a sphere with the surface area of the Earth’s continental masses put together. Its gravitational pull is one third of Earth’s and its carbon dioxide and nitrogen atmosphere is just one tenth the density of the Earth’s. The friction of a decent to Mars creates heat around a spacecraft, but brakes it only slightly.
Only half of the 38 Earth launches of spacecraft to Mars have reached the planet. Seven of 12 missions that landed on the surface managed to transmit information back to Earth. Most of the failures were early Soviet launches in the 1960s and 1970s. Just 13 of NASA’s 20 Mars missions were successful and six out of seven landing missions succeeded.
NASA has used heat shields, parachutes and airbags to assist probes landing on Mars (video). The Curiosity rover’s weight is three-quarters of a ton, the same as an SUV. Airbags that could cushion its landing would be too heavy for the transport vehicle and a parachute alone could not inflate in time before the rover lands.
Curiosity will begin its descent with its folded legs stored between a heat shield below and a parachute above. Its descent speed should be 1,000 mph at a height of four miles. The heat shield will burn away and the parachute will come into service. At three miles above the surface and a speed of 187 mph, the parachute and heat shield are jettisoned.
The rest of the landing will be determined by a ground-sensing radar on the underside of the rover. Retrorockets will fire from the side at half a mile above the surface. At 115 feet, the descent vehicle releases a 65-foot tether that should slow Curiosity’s descent to one mph as its hits the surface.
On touchdown, explosive bolts fire to sever the tether from the rover and send it some distance away. At this point, Curiosity should be able to start exploring.