"Mars is very windy but the air is thin, which is why we need to use a big ballit acts like a huge sail and catches a lot of wind," said Jack Jones of the Jet Propulsion Laboratory, who is leading the research.
The "tumbleweed ball," as it is called, carries a payload of scientific instruments at its center, which are held in place with tension cords. Among the instruments is a radar for detecting underground water and a magnetometer for determining the location of tectonic plates. Neither task can be done from an orbiting craft.
The roving ball is also outfitted with cameras, which sit in recessed nooks on its outer surface. When the rover enters an interesting area that merits a closer look, scientists can send a signal to partially deflate the ball and stop it from rolling. When the ball has finishing taking measurements, it can be reinflated to enable it to roll onward.
Blowing in the Wind
The idea of inflatable rovers is not new, Jones pointed out.
Beach ball-size tumbleweeds, about 0.5 meter in diameter, were tested in the past but abandoned because they frequently became wedged between rocks, making them impractical as remotely operated rovers.
The idea of using a bigger ball came when Jones and his colleagues were testing a robot with three spherical wheels in the Mojave Desert.
"One wheel just fell off and the wind caught it&3151;carrying it up and down, and down and up, the sand dunes," said Jones. "It must have gone about a mile before we could catch up with it and stop it."
A tumbleweed ball 6 meters in diameter is unlikely to get stuck or wedged. It can easily roll over the mostly small rocks that litter the Martian terrain.
One concern the research team has about the tumbleweed ball rover it is its lack of controllability. "I hate to admit it, but these rovers are pretty dumb," said Jones. "They just go where the wind takes them."
The research team is developing a steering mechanism, which involves shifting the payload off center to force the ball to the left or the right.
Next year, if funding permits, Jones wants to test the tumbleweeds on Devon Island. "There are not many places on Earth where we can just unleash giant tumbleweeds and let them roam around," he said.
Jones has no doubt that the tumbleweeds will dramatically increase the potential for Mars exploration. The 1997 Sojourner rover was only able to move about 100 meters in one month, he noted, and the Mars rover planned for 2003 will travel only about one kilometer during the entire mission.
"But tumbleweeds," he said, "could potentially cover hundreds of kilometers per day."
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