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PUNCHING A HOLE IN THE MARTIAN SKY

Lifting a spacecraft off of the Earth takes enormous power and is a dramatic event. But just as dramatic, if not more so, is landing a spacecraft on another planet.

Preparations for the entry, descent, and landing (EDL) of the Mars Exploration Rovers have taken years. The basic EDL plan was inherited from the Pathfinder mission, but the MER landers are significantly larger and heavier than the Pathfinder lander. A new supersonic parachute had to be designed and airbags had to be made stronger. A six-minute drop to the surface of Mars includes extreme speeds, extreme heat, and extreme impact. Every step of the EDL process had to be reviewed and revised with the new lander specifications in mind.

Entry

As the MER spacecraft nears Mars, it will turn its heat shield toward the planet. The cruise stage will detach and the spacecraft will hit the top of the martian atmosphere at a speed of 5.4 kilometers per second (12,000 miles per hour). The martian atmosphere is thin, but it can still cause enough friction to scorch the heat shield with temperatures as high as 1,400 C (about 2, 500 F).

Descent

Aerodynamic forces will slow the spacecraft until it reaches the point where it is traveling about twice the speed of sound. Then a supersonic parachute will open to add more drag to the vehicle.

The heat shield will separate, and the lander will descend from its backshell on a tether. A radar system on the lander will measure its altitude, and a camera mounted beneath the lander will take three pictures of the ground to determine the spacecraft’s horizontal velocity. If strong winds are pushing the spacecraft sideways during its descent, small rockets on the backshell can be fired to reduce this horizontal speed.

Then, at about a thousand feet above the surface, gas generators will rapidly inflate a cluster of airbags that will cushion the lander and rover when they impact Mars. Rockets will fire to bring the lander to a near-stop. At about 15 meters (50 feet) above the ground, the tether will be cut and the lander will be released for a free fall to the martian surface.

Throughout EDL, a series of signal tones at 10-second intervals will indicate the completion of various steps during the descent. These tones will be transmitted to Earth through low-gain antennas on the cruise stage, backshell, lander, and rover. At about a minute before the vehicle falls to the surface of Mars, an antenna mounted on the lander will transmit status information to the orbiting Mars Global Surveyor. This information will continue to be transmitted until impact.

Landing

The initial bounce of the airbags, with lander and rover inside, could soar as high as 30 meters (100 feet) above the martian surface. The bouncing and rolling may continue for half a mile or more before the airbags finally come to rest.

Web content editor/writer: Pamela R. Smith

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