Wheels: Mars Pathfinder Rover
Talk about a cool
set of wheels: This baby had six of them, each powered by its own
independent motor. But it wasn't built for speed; it was built to
handle terrain that has given new meaning to the word "rough." Meet
Sojourner, Pathfinder's solar-powered go cart.
The rover was named Sojourner by 12-year-old Valerie Ambroise, from
Bridgeport, Connecticut, who won a worldwide contest to dub the little
buggy. The word sojourner means "traveler," but Valerie was
also thinking of Sojourner Truth, a former slave, abolitionist, and
champion of women's rights.
Sojourner was only 63 centimeters (25 inches) long -- about the size
of a microwave oven. On Earth its weight was 25 pounds, but thanks to
the lower gravity on Mars, it weighed in at only about 10 pounds on the
Red Planet.
The rover was ground tested at JPL on a specially designed "testbed"
that simulated the Martian terrain. It made its first solo run at the
mouth of Ares Vallis, a dry river on Mars, shortly after touchdown on
July 4, when it drove down a ramp from its mother ship onto the Martian
surface.
With an average speed of 6 millimeters (0.25 inch) per second,
Sojourner used an onboard control system to creep forward, back up, and
redirect itself in a circuit around its home base, the Pathfinder
lander that ferried it from Earth to Mars. To return
the favor, Sojourner took black-and-white snapshots of the lander and
transmited them back to Earth so mission scientists could check the
condition of the lander for any damage it may have suffered during
entry and landing.
Sojourner's laser navigation system could sense the distance of
obstacles in its path. Its six wheels, arranged without axles in a
"rocker-bogie" design provided maximum maneuverability, allowing the
rover to climb over obstacles half its height. In addition to the
lasers, the rover carried a black-and-white stereo camera to create
"virtual reality" scenes of its travels. It also had a color camera for
close-ups of rocks and soil. Its most important tool was an alpha
proton X-ray spectrometer which could "sniff" the atomic makeup of the
rocks it encounters, allowing scientists to identify the mineral types
on Mars.
No need to worry about miles per gallon. Sojourner's roof was
covered with solar panels, so it drew its fuel from the sun. For that
reason, though, hours of operation were limited to the four hours
surrounding high noon on Mars, with lithium batteries providing backup.
Sojourner was semi-autonomous. Many of its movements were controlled
from Earth, but because there was a time delay of more than 6 minutes,
depending on the relative position of Earth and Mars, Sojourner was
designed to make some decisions on its own. An Intel 8-bit processor
that could churn out 100,000 instructions per second kept it on target.
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