Here's a challenge: How to traverse remote, unknown territory with huge boulders, steep slopes and deep gullies?
The solution: Roll with it.
NASA engineers are designing a tough, oversized beach ball that could someday roll over broad, rocky expanses of Mars. After bouncing to a landing, the ball would skid and roll around the Red Planet, propelled by Martian winds. The ball could accelerate to about 22 mph in 45 mile-an-hour gusts.
As envisioned, the craft, dubbed Tumbleweed, measures about two stories high or 20 feet in diameter and weighs a mere 44 pounds. Like the big wheels on jacked-up trucks, the ball is designed to keep on rolling.
"If you want to do science over thousands of kilometers, a giant ball makes sense," said Jack Jones, test manager for the Tumbleweed project at NASA's Jet Propulsion Laboratory. "A huge ball can go over anything."
Deflate to Stop, Inflate to Roll
Tension cords at the core of the giant rolling ball would carry water-seeking radar, cameras and other instruments. After drifting to a desired location, engineers back on Earth could "park" the giant ball for a while by partially deflating it.
The sagging ball would be less likely to budge in the wind. Tools and instruments would then exit the ball through a tube in the ball's design. Once tests are finished, the tools would retract and the ball would suck in thin Martian air to become rotund again.
To prevent punctures, the ball's outer layer would be made of a super-tough fabric, possibly Vectran, a material used in some bulletproof vests. The ball would also host multiple layers of inflatable bladders underneath its outer layer so if an outer rip occurred, an inner bladder could inflate to patch the hole.
Although scientists may not have precise control over the giant ball's direction, Jones is hopeful it could cover record territory. The first NASA Mars rover to land on Mars in 1997 relayed back a wealth of information and images, but the six-wheeled, 23-pound Sojourner covered only 100 yards over its entire lifetime of one month.
"When the Sojourner came up to any type of rock, it had to go around the rock and do a lot of thinking in the process," said Jones. "This ball could travel the total ground that Sojourner covered in seconds."
Tumbleweed may be more swift on Mars' surface, but there are some factors Jones and his team still need to work out.
Controlling the Roll
It could be tricky, for example, preventing the ball from tumbling into an impossibly deep crater. One possibility is to land the ball in a strategically flat location — such as Mars' North Pole — and then let the ball blow where it will. But Jones is also looking at ways of directing the ball using a device that would alter its center of mass.
The unit would pump fluid to one side of the ball or another and cause the ball to lean and turn. To straighten out the ball's path, liquid would be pumped back to its core. And, to ensure the ball drifts in a straight line, Jones is considering changing the ball's spherical shape to a slightly oblong shape. Like a rolling barrel, it would be more likely to tumble in one direction.
To test such new features, Jones has built a smaller-scale Tumbleweed prototype that's about 5 feet in diameter and bright yellow.
A beach ball rover may seem like a strange idea, but it isn't the first time scientists have pondered using a giant ball as a rover. In the late 1970s, a French visiting scientist at the University of Arizona proposed building a giant ball to explore Mars, but the model was motorized and much heftier, weighing more than 1,000 pounds.
Jones and his team have designed a three-wheeled rover that rolls on very fat inflatable wheels. The Inflatable Rover is now being tested for a possible launch to Mars in 2007. And it was actually this rover model that led to the design of the Tumbleweed.
Jones and his team had developed smaller-sized balls as rovers, but when they rolled them out in the Mojave Desert, they found the small balls became snagged behind rocks. Then, while testing the Inflatable Rover in the Mojave Desert, one of the rover's giant wheels spun off and began rolling briskly across the sand. That's when Jones imagined the Tumbleweed.
"We realized, 'Oh! We just have to make the balls bigger!'" he said.
Tumbleweed may be nearly ready in the laboratory, but it's still uncertain whether or not it will ever be launched to the Red Planet. The next robots due for Mars are a pair of twin rovers poised to launch in 2003. These rubber-wheeled rovers are much larger than the 1997 Pathfinder Sojourner and are expected to travel up to 110 yards each Martian day, which is 24 hours, 37 minutes.