Having a Ball on Mars
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 of JPL's Inflatable Rover Program.
Above: This is not your parent's Mars rover! A giant inflatable ball -- bearing little resemblance to better-known rovers like Sojourner -- might someday prowl the surface of the Red Planet. A 6-meter diameter ball on Mars could accelerate to about 10 m/s (22.4 mph) pushed along by Martian afternoon winds of 20 m/s (44.7 mph).
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"This is preliminary work," Jones cautions. But he is enthusiastic about the promise this technology might hold for exploration of Mars and other worlds 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.
Below: To a certain extent, a "tumbleweed" rover would be at the mercy of the whimsical wind, but it would have the ability to "throw anchor" by partially deflating.
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.
Left: The spherically-wheeled rover that inadvertently gave birth to the idea for a giant tumbleweed ball. When one of the wheels broke loose during a test, it traveled across the terrain only too well!
"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."
Toys, Balloons, and Serious Science
In a lab that appears to mix serious R&D with the ambiance of Santa's festive workshop, Jones and his colleagues are surrounded by shiny Mylar balloons of various sizes, pink and yellow beach balls, heavy-duty nylon tumbleweed ball prototypes, tall tanks of compressed gas and worktables full of mechanical and electronic devices. The team, which includes senior engineer Sam Kim and design engineer Jay Wu, is now preparing for desert tests later this month that will incorporate a radar into the ball's center to test the prototype's ability to find underground water. Such instrumentation could eventually be used to search for possible water hidden beneath Mars' surface.
The ball is weighted so that it has a preferred axis of rotation. It tends to roll with the heaviest part down, so two weights opposite each other send the ball along a straight path. The upcoming tests will also try out a center-of-mass control device that would allow the ball to be steered by pumping contained fluid to the left, right or center of the tire, which will be slightly oblong.
"Again, this is experimental, so we're trying different things," says Jones. "But I'm pretty confident it will work."
"With a 20 kilogram ball and 20 kilogram payload, the 6-meter diameter tumbleweed ball is light enough that it could be added on to another lander and deployed from the ground, or it could be in its own delivery vehicle," he added. The large, lightweight ball could possibly also serve as its own parachute and landing airbag able to withstand the bounce following a 30-meter per second terminal velocity descent to Mars. The ball shares the same heritage as the airbag used for the 1996 Pathfinder mission.
Other work being planned for coming months include desert drop tests with a prototype tumbleweed ball made of super rugged Vectran, the same material used for the Mars Pathfinder's airbag landing system. In the coming year, Jones hopes to arrange for long-range testing of hundreds or thousands of kilometers in the harsh, challenging, Mars-like environment of the Arctic or Antarctic.
For more information about Mars exploration -- including the tumbleweed rover and other innovative ideas -- please visit http://mars.jpl.nasa.gov.
Exploring Mars: Blowing in the Wind -- JPL article about the tumbleweed project
Mars Exploration Program -- from the NASA Jet Propulsion Laboratory
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