Sensor-equipped robots are humans’ eyes on Mars
Rick Nelson, Chief Editor -- Test & Measurement World, 6/15/2007 7:37:00 AM
Rosemont, IL. Sensor and vision experts got a close-up look at a couple of out-of-this-world robots this week when Steve Squyres addressed the jointly convened Robots & Vision Show and Sensors Expo. Speaking Tuesday, Squyres, the Goldwin Smith Professor of Astronomy at Cornell, described the history and present status of what he called his two favorite robots, the Martian rovers Spirit and Opportunity.
Spirit and Opportunity operate in what he called a terrible place. Temperature is -60ºC, and it’s so dry that if all the available water were condensed and equally distributed across the Martian surface, it would only be sufficient to create a layer of frost 0.01 mm thick. Mars, he said, is cold and dry, but he asks, “Was it different in the past?” He believes so, and he showed photographs of a valley apparently carved by liquid water. In the past, therefore, Mars, he said, might have been warmer and wetter.
He suggested a way of demonstrating that it was. Water-based erosion, he said, implies deposition, leading to the formation of layered sedimentary rock. Properly equipped robots, he said, could read the story embodied in the hypothesized sedimentary rock.
Enter Spirit and Opportunity, launched toward Mars four years ago, in the summer of 2003, from Cape Canaveral. They reached their destination January 2004, bringing with them cameras, alpha-particle x-ray spectrometers, microscopic imagers, infrared spectrometers, and Mössbauer spectrometers, all of which help serve as our eyes on Mars. They also arrived equipped with rock abrasion tools, or RATs, which are designed to grind into Martian rock an open window into which the rovers’ various sensors can look.
All that equipment, Squyres said, resulted in a payload mass about 10% higher than originally planned, putting strain on the parachutes and airbags that would safely decelerate the craft from Mach 25 to zero in a six-minute trip through the Martian atmosphere that Squyres described as a hell of a ride. The supersonic parachutes, which deploy at mach 2, posed particular problems. To meet the launch window, engineers embarked on three parallel parachute design projects, he said, adding that fortunately, one worked.
Once on the surface, the rovers needed to unfold themselves in a process Squyres called reverse origami. This was the process that scared him most, he said, because of the many gears and motors that had to work flawlessly. If this process doesn’t take place the first day, he said, the rovers don’t survive the first night, and the mission is over.
Once unfolded, he said, the vehicles face the challenge of driving off their lander without flipping over—then they must contend with the fact that the Martian surface isn’t exactly paved. The vehicles, he said, are equipped with a six-wheel system that passively conforms to the surface. The success of the system, he said, is attested to by the 1200 days of mostly trouble-free operation.
Ideally, said Squyres, he would like to be able to sit in his office and operate the vehicles with a joystick. Such real-time control is impossible, however, because of the 20 minutes or more that a radio signal would require for a round trip. Consequently, he said, the designers had to endow the rovers with sufficient intelligence to make their own decisions. He said they are also endowed with programmable levels of courage, or cowardice, depending on how dangerous the Martian surface appears to be.
The RAT, he said, is one of most important pieces of the mission’s scientific payload. He called geology is a forensic science, adding that it takes digging to get beneath the outer and perhaps contaminated outer rind to pristine rock underneath. He described the 3000-rpm diamond-tipped RAT as the first ever interplanetary power tool.
He described Spirit’s primary Martian destination as a formation called Goose Head crater, of interest because it appears to be associated with a big dried-up riverbed. If indeed water had flowed, he said, Spirit should have found sedimentary rock. In fact, what Spirit found was igneous basalt. “It turns out Mars faked us out,” he said, adding, “To this day I’m convinced that sedimentary rock is down there, covered by lava. It was a bitter disappointment at first.”
But then the luck changed. There was nothing but lava in every direction, he said, except one. In that direction lay the Columbia hills, named in memory of the crew of the fallen space shuttle, where Spirit found iron oxyhydroxide, which would have required water to make.
And then just within the last few weeks, he said, a serendipitous mechanical problem resulted in Spirit discovering even more evidence of a wet Martian past. Because of a motor brush failure, Spirit has had to proceed backwards, dragging one bad wheel behind it. The dragging wheel created a trench, exposing soil as bright as snow. X-ray spectrometry determined that the “snow” was 90% silicon, and such nearly pure silicon forms only as a consequence of hot water or volcanic steam interacting with rock.
Spirit continues to operate but has had its share of problems in addition to the broken wheel. For example, its solar panels had become coated with dust, reducing its power production from 900 Whr per day down to 350—near the death level of 250. “But one wonderful day,” he said, “a lucky gust of wind blew the dust away, returning power back to 850.”
The rover Opportunity has found additional evidence of a wet Martian past in the form of 4- to 5-mm diameter pebbles of hematite, the iron-oxide mineral found in rust. Squyres likened the pebbles to blueberries in a muffin, saying they remain on the Martian surface after the outcropping—the muffin—that contained them eroded away. Those pebbles would have formed originally, he said, in sedimentary rock supersaturated with water until they eventually precipitated out.
As does Spirit, Opportunity continues to operate, despite a risky 32-degree climb and having once got stuck, with all six wheels buried in a dune. Squyres said earth-bound simulations helped determine that a 32-degree climb would be safe. As for the buried wheels, he said, JPL had trucks fan out to home-improvement stores throughout southern California, buying up play sand and a diatomaceous earth used in swimming-pool filters—the best combination of readily available earth-bound materials to mimic the Martian surface. After a couple of weeks experimenting with rocking back and forth and employing varying wheel speeds, the engineers learned the best way to get unstuck: “Put it in reverse and gun it!”
Squyres concluded his talk by acknowledging the 4000 team members who have made the rover mission possible. For all the participants, he said, “It has been the experience of lifetime.”
You can find the latest news on Spirit and Opportunity here.
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