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 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.

Jack A. Jones and the quarter-size version of his "Tumbleweed Ball."

"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."