August 10, 2001
One answer to roving across the surface of Mars may be blowing in the wind. Literally.
Researchers exploring different methods to deliver scientific instruments to various Martian locales are studying the potential for a giant, lightweight, two-story tall beach ball. Equipped with scientific instruments, the so-called "tumbleweed ball" conceived by JPL researchers could potentially explore vast tracts of planetary terrain, blown by the wind.
The wind blowing across the face of the red planet would be the only engine needed to move the giant tumbleweed ball from place to place, said Jack A. Jones, who is leading JPL's research into various inflatable machines for exploring space. JPL's Inflatable Technology for Robotics Program aims to create rugged, all-terrain vehicles and other devices with low mass and low-packing volume.
A scientific payload, carrying instruments such as magnetometers or water-seeking radar, would be held in place by tension cords at the tumbleweed's center. Cameras mounted inside the ball would peer out at the local terrain. When scientists identify a promising spot and want the tumbleweed to put down roots and sit for a spell, the ball could be partially deflated. 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 admonishes as he prepares for more field tests. But he is enthusiastic about the promise this technology may hold for the exploration of Mars and other solar system bodies.
Much of Mars' terrain is sloping and littered with boulders, which makes tough going for most vehicles. But researchers were excited by the results of tests this summer of a 1.5 meter-tall version of the tumbleweed. The tests confirmed that 6-meter diameter (about 20-feet) balls should be able to climb over or around one-meter rocks and travel up slopes as high as 25-degrees in the thin, but breezy martian air.
Follow the Bouncing Ball
Serendipity and a busted wheel on an experimental rover played a roll 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 dominate much of Mars' terrain. 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."
Toys, Balloons, and Serious Science
In a lab that appears to mix three parts of serious R&D to one part 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 Connors conceived of which 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," said 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," said Jones. The large, lightweight ball could possibly also serve as its own parachute and landing airbag, he said, able to withstand the bounce following a 30-meter per second terminal velocity descent at Mars. The ball itself shares the same heritage as the airbag used for Pathfinder and that which will be used for the Mars Exploration Rover.
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, Marslike environment of the Arctic or Antarctic.