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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