Researchers at the Jet Propulsion Laboratory (JPL) have hit upon an idea for exploring the Red Planet that seems to be equal parts fun and serious science: it's a lightweight, two-story tall beach ball called "the tumbleweed rover." Equipped with scientific instruments and propelled by nothing more than the thin Martian breeze, the tumbleweed could potentially explore vast tracts of planetary terrain.
The wind blowing across the face of the Red Planet would be the only engine needed to move such a ball from place to place, says Jack Jones (shown in image below with prototype) of JPL's
. A payload carrying instruments such as cameras or a water-seeking radar could be held in place by tension cords at the tumbleweed's center. When it's time to stop for a while, perhaps to study an interesting spot or to wait for the wind to change direction, mission controllers would simply command the ball to partially deflate. Then, when it's time to move along again, the ball could be reinflated to roll on toward new frontiers.
"This is preliminary work," Jones cautions. But he is enthusiastic about the promise this technology might hold for exploration of Mars and
with thin atmospheres such as Pluto, Neptune's moon Triton or Jupiter's moon Io. 
For now, though, the focus is on Mars. Much of the Martian terrain is sloping and littered with boulders, which makes tough going for most vehicles. But researchers were encouraged by the results of tests this summer with a 1.5 meter-tall version of the tumbleweed. Their experiments in the Mojave Desert confirmed that 6-meter diameter balls should be able to climb over or around one-meter rocks and travel up slopes 25-degrees and higher in the thin, but breezy Martian air.
Follow the Bouncing Ball
Serendipity and a busted wheel on an experimental rover played a role in planting the idea that would grow into the tumbleweed ball.
Previous tests of beach ball-size tumbleweed prototypes had been disappointing. "They got stuck," Jones explained. Driven by the wind, the toy-size balls lodged against knee- and waist-high rocks like those that litter Mars. As rovers, the beach balls flopped.
But then, while conducting tests of an experimental inflatable rover in the Mojave Desert's Dumont Dunes, one of the bright yellow rover's shoulder-high spherical "tires" broke off the vehicle and blew away.
"It went a quarter of a mile in nothing flat," recalled technician Tim Connors, who quickly saddled up with the driver of a passing all-terrain recreational vehicle to chase down the runaway sphere. The moderate, 20-mile per hour afternoon winds drove the ball fast and far.
"It soared," Jones said of the big ball. Watching Connors in hot pursuit, the researchers marveled at the speed of the rogue sphere and the ease with which it moved across the desert, unimpeded by boulders. "Tim was flying over the sand dunes trying to catch it," he said. "The ball went up steep, steep cliffs of sand. Nothing stopped it." Until Connors, on the borrowed ATV, was able to catch up and corral the escapee.
"And therein was planted the seed," said Jones, "that if we make these things big enough, nothing will stop one."
Next page: Scientists fine-tune the tumbleweed rover.
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