The future of Mars exploration may have monster truck tires -- big bulbous beachball-type rollers that can carry a rover over almost any obstacle that the planet presents.

Rather than being delivered by complicated, expensive rocket descent thrusters, the vehicle could be set down by a solar-heated hot air balloon -- one that opens like a parachute high in the atmosphere, collects air as it drops, and buoyantly eases its payload to the surface.

That's what engineers at NASA's Jet Propulsion Laboratory (JPL) have in mind for a next-generation rover that could be used to carry scientific components across the rocky martian ground.

. . . It will only see a few hazards here or there and it will just go over everything.
     

Using just 18 watts of power -- less electricity than the average refrigerator light bulb, the rover can bump over ragged terrain at about 1.2 m.p.h. (2 kilometers per hour), the speed of a very slow walk.

Planners in NASA's Mars Exploration program would like to be able to explore an area of about 40 square miles (100 square kilometers) around a landing site, Jones said. He manages JPL's efforts to research and develop inflatable technologies that can be used to support robotic missions.

Right now, the inflatable rover is the only vehicle in development capable of covering those large distances, he said.

"The other rovers, when they come across rocks they have to kind of stop and go around them and think which way to go," Jones said. "The big rover will have a hazard avoidance system, but it will only look at big rocks, over half a meter (20 inches) high, and that's believed to be far less than 1 percent of the surface of Mars. So basically in its field of view it will only see a few hazards here or there and it will just go over everything."

In tests during the past six months, the rover has performed "extremely well" in tests on sand dunes, over boulder fields, in rocky canyons and across dry lakebeds, Jones said.

The craft even rolls well across water, making it the only amphibious planetary rover being developed. This feature could be important for missions to Saturn's moon Titan, which is anticipated to have liquid methane and ethane seas, and perhaps ethane snows on frozen ground.

The rover's only obvious Achilles heel is precisely the large profile and light weight that makes it so capable. NASA would need to develop some method to keep the bubble car from being blown away or destroyed by windstorms or dust devils.

The rover could be delivered to the surface by many means, but Jones envisions a mission that would use a solar-heated hot-air balloon to set down the craft. After dropping its cargo, the black balloon might rise into the sky buoyed by the warmed air inside. It could carry a gondola with a camera or weather experiments, floating above the landscape until the end of the day.

Meanwhile, the far-ranging rover might be used to carry a radar device that searches for sub-surface water on Mars, or for a variety of other tasks. In the faraway future, such vehicles might be larger and more robust. One rover might be designed to rumble across the ground delivering other rovers to points across the landscape for coordinated science or exploration missions.

Rovers also might eventually be used to deliver polar ice to mid-latitudes, if the ice becomes a desired resource for producing rocket fuel, for instance. And one day astronauts might use a bigger version of this vehicle to drive around on the planet.

Engineers at NASA's Johnson Space Center are moving ahead with development of a crew-carrier prototype that could be used on the moon or Mars. Always interested in making payloads lighter, those engineers are examining the possibility of using blow-up tires on a vehicle that would carry astronauts, said Steven Fredrickson, of the robotics group at Johnson.

The rover that Jones is working on at JPL will be solar-powered -- perhaps by a solar array that is itself inflatable. ILC Dover, of Dover Delaware, the same company that manufactured the vehicle's tires, is testing a version of an inflatable solar array.

It opens like an umbrella, about 5 feet across. Its top is covered with light, flexible solar cells.

"This fairly large . . . parasol-shaped structure fits into something about the size of a coffee can that is part of the chassis when it's all packed up," said David Cadogan a senior design engineer at ILC Dover.

To deploy the solar parasol, air fills its central tube, and a ring-shaped tube around the perimeter. The array opens almost like an umbrella, standing up when it is fully inflated. The structure would then be made rigid so that it could stand on its own without air pressure.

This can be accomplished a few different ways, Cadogan said. The structure of the umbrella might be made of a fabric that hardens when heated. The fabric might be embedded with heating elements like the wires in an electric blanket. When heated to a certain temperature, the solar parasol would be made permanently hardened.

The rover is far enough in development that it could be ready to fly in five years, if NASA decides to send such a vehicle.