Designing planet rovers that tumble.

As soon as 2009, Mars could become a playground for earthlings wielding giant beach balls. The earthlings would actually stay home, but from there they could control 6-meter-diameter inflatable spheres sent to roam the Red Planet.

NASA researchers are currently testing such spheres, dubbed tumbleweed balls, in the Mojave Desert. Counting on Martian winds to blow the balls along, the researchers are hoping that the giant, carbon dioxide-filled balloons will be able to travel much greater distances than wheeled rovers can.

The head of the project, Jack A. Jones of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., predicts that the balls could cover thousands of kilometers on Mars. In contrast, the little, wheeled Sojourner vehicle, which arrived on Mars in 1997, wandered no more than a few meters from its lander.

With such an extensive range, the instrument-laden balls could conduct far-reaching X-ray searches for signs of underground water and perhaps life, Jones says. They might also search for variations in magnetic fields in the Martian terrain that could reveal ancient movements of the planet's crust.

Earlier this summer, tests by the JPL team indicated that tumbleweed balls can roll right over meter-tall rocks and climb 25-degree slopes. They are so mobile that the researchers must devise ways to control where the orbs go and when.

One possibility is for mission controllers to partially deflate a ball when it reaches an interesting spot. Another is to shift the position of the heavy payload of cameras and other equipment suspended inside the ball as a way to steer it, Jones adds.