Saturday, July 25, 2009

For Mars Rover, Really Remote Roadside Assistance

Past Warranty, It Survived Solar Flares and a Three-Story Bounce, but the Off-Road Buggy Is Belly-Deep in a Ditch

On Mars, NASA's robot rover Spirit is spinning its wheels on the soft shoulder of planetary exploration, up to its axles in silt millions of miles away from tense engineers who are struggling to extricate it by remote control.

At NASA's Jet Propulsion Laboratory in Pasadena, Calif., mission planners are testing ways to drive it free of the Martian sand trap in a last-ditch effort to save a $300 million robot that, after puttering across alien terrain for five years, may have rolled its last few feet.

To recreate the treacherous Martian ground, they built a tilted plywood sandbox and packed it with 5,400 pounds of diatomaceous earth and fire clay mixed to mimic soil with the properties of "slippery cake flour," says Sharon Laubach, head of the Mars rover project's integrated sequencing team. A full-scale duplicate of the 384-pound vehicle is parked on an artificial hillside, hub-deep in the powder.

Working through their first round of 11 rescue experiments, JPL rover engineers wearing protective masks and white "Free Spirit" lab coats gingerly gunned the test rover's engine, then periodically measured its progress with a yardstick. With each trial maneuver, though, the robot dug itself a little deeper.

This month, the world celebrates the 40th anniversary of the Apollo 11 moon mission and humanity's first footsteps on a world beyond our own. In the decades since, space flight has in so many ways become routine. Hundreds of satellites orbit our home planet, owned or shared by 115 countries.

Not since 1972, however, have people returned to the moon or ventured much beyond low Earth orbit. Generations of unmanned probes, though, have toured Mercury, Venus, the outer planets and beyond. For the robot pioneers of planetary exploration, every journey is a narrative of near misses and narrow escapes, of human ingenuity rising to a challenge.

"We know we have to diagnose these problems from millions of miles way," says Dr. Laubach. "It would be nice to be able to stand around Spirit and actually see the situation. It would be even nicer if we could push her out."

The Spirit rover has long been working on borrowed time. Designed for a 90-day mission, it has been sending back data 20 times longer than planned. At its best, Spirit has traveled further in a day than the 1997 Mars Sojourner rover covered in its entire three-month mission.

"We are way out of warranty," says project deputy principal investigator Ray Arvidson at Washington University in St. Louis. "Nobody expected us to be still doing this in July 2009."

Yet any one of Spirit's almost 2,000 days on Mars could easily have been its last.

Spirit's travails began the moment it was launched from Earth in 2003. During its 311-million-mile interplanetary voyage, it survived bombardment by charged particles from some of the highest energy solar flares on record. Wrapped in a cocoon of protective air bags, it hit Mars hard enough to bounce higher than a three-story building. It bounced up and down 27 times before skittering to a halt on a rust-colored plain strewn with volcanic rocks and pocked with impact craters.

A defective air bag briefly blocked its way onto the planet's surface. Once it rolled on to Martian soil, mission planners promptly lost control of the vehicle for two days. In the years since, Spirit's rover drivers at JPL have contended with faltering power levels, communications blackouts and periodic computer problems that have worsened in recent months.

To complicate matters, the Spirit by necessity has been driving backwards since 2006, when its right front wheel stopped working. JPL's rover drivers discovered they could still steer the vehicle more or less in the right direction, but only by driving it in reverse, dragging its locked wheel like an anchor. The scientists quickly came to value the broken wheel as a digging tool that churned up subsurface soil for analysis.

"Wherever you drive, you dig a big trench," says Dr. Arvidson. "The positive side is that it turns out to be a beautiful physical properties experiment."

All told, the JPL rover engineers have at least 60 maneuvers to consider, conceived during a recent brainstorming session involving hundreds of researchers around the country. It might be weeks before they can tell which approaches have the best chance of freeing the stranded rover in Mars' lighter gravity, where the vehicle weighs much less than it does on Earth. "It is our expectation that it will take a long time to get out," says JPL project manager John Callas. "This is the most serious event for the rover in its five-plus years on the surface of Mars."

With swept-back wings of solar panels, a gold-plated instrument box and clusters of cameras, the six-wheel Spirit rover resembles a mechanical moth with a gilded thorax. Spirit, one of two NASA rovers now on Mars, is the ultimate off-road vehicle. The nearest highway is more than 250 million miles away.

Rolling across this exotic terrain earlier this year, the Spirit rover broke through the thin crust of a volcanic rise called Home Plate. It has been mired there since May. "Unfortunately, when we tried to climb up that little slope, the rover couldn't make it," Dr. Laubach says. "The rover was digging into this material and the belly of the rover was getting closer to the surface," where it might easily have grounded itself permanently on rocks.

So far, the Spirit rover and its twin rover Opportunity together have transmitted 250,000 images and 36 gigabytes of technical data, equal to 2.5 million pages or so of text. They have uncovered mineral evidence that Mars once was warmer and wetter than today -- and buttressed the possibility that it might have supported the development of life.

"Both rovers, by digging in the dirt, provided absolute ironclad evidence of warm wet conditions on Mars several billion years ago," Dr. Arvidson says.

While engineers try to get Spirit moving again, project scientists are using its sensors to study the strange ground around it. "It turns out that the soil is scientifically very interesting," says Dr. Laubach.

At the same time, the wind has swept Spirit's solar panels free of dust, restoring the vehicle to full power for the first time in many months. "We've tripled the amount of energy," says Dr. Callas.

China and Russia plan to send satellites to Mars next year. NASA's next Mars rover mission is scheduled for launch in 2011. The U.S. space shuttle fleet, however, has reached retirement age. The International Space Station already is slated for de-orbit and demolition in 2016, NASA's space station program manager, Michael T. Suffredini, said this week.

For decades, NASA planners and space-exploration proponents dreamed of lunar settlements and manned missions to Mars, as a necessary prelude to even more ambitious human exploration of planets around other stars. In the weeks ahead, the U.S. human space flight plans committee, appointed by President Barack Obama, will be assessing NASA's human space exploration efforts. Are the fiscal, physical and psychological challenges of long duration space flight more than we can master?

Our machines boldly go, leaving tire tracks where footsteps may one day follow.

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