At some point on their five-year journey, Mars rovers Spirit and Opportunity have both gotten their feet stuck in the soil, and NASA is taking notes for the design of the next generation of rovers.

In 2005, Opportunity spent five weeks spinning her wheels in a dune later dubbed "Purgatory." Last week, Spirit sank into a sandpit scientists are calling "Troy," and could stay there for weeks — or forever.

But rovers of the future may have an easier time of it. NASA scientists are building an army of prototypes with new and ever weirder ways to rove.

CLIFFBOT

____One of the toughest tasks for rovers is climbing steep slopes. Some of the most interesting bits of Martian geology, like exposed rocks on cliff faces or gullies in craters that might once have been flowing streams, are off-limits to Spirit and Opportunity. Anxious engineers fear a spill, or worry that once they're in, they won't be able to get back out.

The Cliffbot (known more formally as the Sample-Return Rover) gets around this by borrowing tricks from human mountaineers. It's tethered to two "anchorbots" that belay it from the top of the cliff using modified fishing reels. This configuration lets it climb down 80 degree slopes to take pictures and soil samples at the bottom.

Cliffbot is already getting its feet dirty: It spent the past three summers doing field tests in Svalbard, Norway, where it froze its batteries off and dodged polar bears.

LEMUR

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Another rover tackles the climbing problem with sheer dexterity. With a typically charming NASA acronym, the Lemur (Limbed Excursion Mechanical Utility Robots) was designed to help build things in orbit. It can crawl along a segmented mirror and climb the walls in a rock gym. Engineers hope it will be able to place "holds" in rock and soil, like rock climbers do. And at just 18 inches across, it's downright adorable.

ATHLETE

____The behemoth Athlete (All-Terrain Hex-Legged Extra-Terrestrial Explorer) rover is based on the Lemur, but it's anything but cuddly. Designed to carry people and equipment across the surface of the moon, it tackles tough terrain with sheer size. The prototype is four meters (about 13 feet) wide, and the rover is expected to be nearly twice that. It can roll up to six miles an hour on the lunar hills, while keeping the center of the vehicle perfectly level. That might not sound like much, but it's more than 100 times as fast as the Mars rovers, which have each traversed about five miles in as many years. And unlike other rovers, it doesn't just roll around. It can lift up its limbs to step over boulders (not to mention strike menacing scorpion-like poses).

Athlete would probably be used in tandem with a lunar rover like the one featured at Obama's inauguration.

"It's like retired people with their big Winnebago and the Jeep behind them," said Richard Volpe, manager of Mobility and Robotics Systems Section of NASA's Jet Propulsion Laboratory. "You park your Winnebago [in this case, Athlete] and it stays stationary for a week or two, and you do your little sorties in your Jeep."

One way engineers imagine getting these colossal insects to the moon is to make them collapsible. They'd fold down into discs, stack up for the flight, and self-deploy on landing like giant robot spiders popping out of a Pringles can. Another is to have them split into two three-legged "Tri-Athletes" that can click back together or dock to other robots on the moon.

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HOPPER

Able to leap small boulders in a single bound, this hopping robot doesn't waste time on navigation. The prototype is so new it doesn't have a catchy acronym yet, but it's the latest in a long line of hopping robots, all designed to save the time and energy lost tiptoeing around obstacles. Most earlier hoppers landed on their heads and needed helmets to survive, which meant they couldn't make long jumps or carry fragile equipment. This one deftly lands on its six spring-loaded feet. It can jump about a foot in the air on Earth, which would be six feet under lunar gravity. All six legs are also steerable, letting it take off and land at different angles. And it carries a small motorized gyroscope in its underbelly to keep it from tumbling mid-hop.

AXEL

The simplest proto-rover of them all, Axel is aptly named. It's just two wheels connected to an axle. Its symmetry means it's safe from one of the biggest rover worries on steep slopes: flipping over.

"Even if it’s turned upside-down it doesn’t matter, because upside-down is right-side-up," Volpe said.

Like the Cliffbot, Axel would be tethered to a bigger rover that would stand at the top of a cliff. But Axel can take unprecedented amounts of abuse. Its wheels can be foldable or inflatable, letting it absorb a lot of impact on landing. It's unperturbed by dangling in mid-air. It can carry scientific instruments in the cylinder that connects the two wheels, and could even take samples back the same way. All the tether rover needs to do is reel it back in when it's done.

__MARS SCIENCE LABORATORY

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____Most of these rovers are years away from seeing the dim light of space. According to Volpe, it typically takes 10 to 20 years from rover concept to deployment. But how will the next generation of Mars rovers handle the sand?

The Mars Science Laboratory, slated to launch in 2011, is based on the same basic system as Spirit and Opportunity, but twice as big.

"When it comes to rocky terrain, we can climb obstacles that are about twice as high or as deep as Spirit and Opportunity," said mobility engineer Jaime Waydo. "In that undulating terrain where there's rocks or holes, we do very well."

But they haven't solved the sand problem yet. "When it comes to sand, what you care about is something we call ground pressure, how much you float in the sand," Waydo said. "MSL has the same ground pressure as Spirit and Opportunity, so when we start driving in sand, we expect the performance to be about the same."

There's an obvious trade-off at work: The heavier your rover, the more it sinks. MSL could be more buoyant if it had bigger wheels (which would be harder to ship) or took fewer instruments, but "that would be really sad," Waydo said. "We could take less science, but that's the whole reason we go."

Images: NASA/JPL

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