A.J.S. Rayl • Oct 22, 2004
Mars Exploration Rovers Update: Spirit Climbs toward Uchben Opportunity Picks up Power while at Wopmay
As winter gives way to spring on the Red Planet, the Mars Explorations Rovers are maintaining their 5-day a week work schedules and continuing to send surprises home to Earth. Despite a recurring 'ache' in one of her steering motors, Spirit is continuing her climb in the Columbia Hills toward a rock called Uchben, while her twin, Opportunity, is completing her work at Wopmay.
Overall, the rovers -- which were designed and built, and are being operated for NASA out of the Jet Propulsion Laboratory (JPL) -- are in generally good health, especially considering each is closing in on her 300th day of operation, more than three times the planned lifetimes. While Spirit seems to be suffering more aches and pains than Opportunity, she has roved nearly 2.5 miles over pretty rough terrain. Opportunity has traveled less than half that much, spending most of her time inside craters on the plains of Meridiani Planum. Now with the transition to spring, both of these robot field geologists will benefit from increased vitality.
Spring on Mars isn't much like spring on Earth mainly because there's nothing to bloom up there, but the season is bringing new life to the rovers. The daily power supply for Spirit and Opportunity comes from the Sun -- the 14 square feet [1.3 square meters] of solar panels each rover boasts convert the sunlight into electricity. Just after the landings in January, the output was about 900 watt-hours per day for each rover, enough to run a 100-watt bulb for nine hours. As anticipated, output gradually declined due to dust buildup and the Martian seasonal change, with fewer hours of sunlight and a lower angle of the Sun in the sky.
By July, Spirit's daily output had declined to about 400 watt-hours per day, but during the last couple of months, it has increased slowly, ranging between 400 and 500 watt-hours per day. Opportunity, which is closer to Mars' equator, and has had the advantage of a sunward-facing tilt as she explored inside the southern half of Endurance Crater, maintained an output level between 500 and 600 watt-hours per day in through June, July, and August. Since early September, however, the amount of electricity from Opportunity's solar panels has increased markedly -- and unexpectedly -- to more than 700 watt-hours per day at times, a level not seen since the first 10 weeks of the mission.
"We've been surprised but pleased to see this increase," said Jim Erickson, Mars Exploration Rover (MER) project manager at JPL. "The team is evaluating ways to determine which of a few different theories is the best explanation." It may be that the wind has blown some of the dust off the solar arrays or it might be that the action of frost might have caused some of the dust to clump and fall off. Either way, he said, "[w]e seem to have had several substantial cleanings of the solar panels."
Spirit from Gusev Crater
During the last couple of weeks, Spirit continued climbing up the West Spur of the Columbia Hills, up a peak called Husband Hill, named after shuttle commander Rick Husband. There, she stopped to investigate a rock called Tetl, a word that in the language of the ancient Mayans means 'stone.' The robot field geologist took measurements of the rock with her alpha particle X-ray spectrometer (APXS) and her Mössbauer spectrometer, and used her microscopic imager (MI) to create a mosaic of Tetl's layered rock face.
"Tetl was very interesting," Joy Crisp, MER project scientist, told the Planetary Society today. "We got some nice microscopic imager images of the layered rock, but when we look at the pictures it's still not obvious what kind of rock it is. We're still struggling with whether these rocks are sedimentary, whether they were deposited in liquid water, or are volcanic -- or something else. We're still intrigued, but we haven't seen anything that's diagnostic enough to tell us what we want to know."
The rover also used her miniature thermal emission spectrometer (mini-TES) to check out some nearby rocks named Zackuk and Palenque, possible future targets for in-depth observations; and performed daily atmospheric observations.
"We're seeing similar sorts of chemistries in the hills," Crisp noted. "It [still] looks like [these rocks] may have been altered by water. All these rocks in the hills are not identical, but they are similar, and all of them are quite different from the basaltic lavas that we saw on the plains of Gusev, at the landing site, at Bonneville Crater, and during that long drive over to the Columbia Hills. Those were all very hard rocks, rich in olivine. Now that we're in the hills we don't see olivine anymore, and the rocks now are soft when we grind into them and they look different. We're continuing to struggle with what they are, but we're very excited that they probably were affected by liquid water."
For now, these layered rocks remain a mystery. "But we're trying to work through what these rocks could be, and we'll be looking at more rocks to try and put this puzzle together," said Crisp.
Last Wednesday, as Spirit was beginning her drive to the next rock target, Uchben, which means 'ancient' in the old Mayan language, she experienced a repeat of the malfunction in the steering motor control system that prevents her right-front and left-rear wheels from being jostled in unwanted directions while driving, something the engineers first saw about two weeks ago.
Each of the front and rear wheels of the rover has a motor called a steering actuator that sets the direction in which the wheel is headed. Different from the motors that make the wheels roll, the steering actuators hold the wheels in a specific direction while the rover is driving. In essence, a dynamic brake on the steering motors affecting her right-front and left-rear wheels 'locks,' causing the those two wheels to 'freeze.'
Diagnostic tests for the problem indicated that a glitch in the relay used in turning these steering actuators on and off is most likely the cause. That would certainly explain the intermittent nature of the anomaly. The electronics relay in question, which operates Spirit's right-front and left-rear wheels concurrently, is still functional, but appears to be operating now only intermittently. The engineers are trying to figure out exactly what's going on so they can devise work-arounds.
Although this problem was not necessarily expected, it was not really unexpected, as the rovers will eventually break down one way or another. In any case, it hasn't kept Spirit from working -- the rover is continuing to rove, with engineers, for the time being, steering her in a tank-like fashion, and not using the steering actuator in question.
Spirit first experienced this steering anomaly on October 1. Since the glitch did not emerge in subsequent tests in the rover test bed at JPL, the engineers concluded then it may have been a one-time incident. But, after the anomaly occurred again last week, October 13, the problem appeared intermittently in ground tests.
"We are continuing tests on Spirit and in our testbed here at JPL," said Erickson. Since the rovers can be operated without that steering actuator, one possible work-around that the team discussed before would be to deliberately blow a fuse controlling the relay, disabling the brake action of that particular actuator. "The only change might be driving in shorter steps when the rover is in rugged terrain," Erickson said.
Opportunity from Meridiani Planum
Meanwhile, over at Meridiani Planum, Opportunity has logged her first mile and is still studying the strange, lumpy boulder Wopmay, inside Endurance Crater. This creviced rock with a brain-like appearance intrigued the science team form the moment they spotted it, and the rover is thoroughly investigating it.
"We have spent a lot of time looking very closely at Wopmay with the panorama camera (PanCam), and the microscopic imager, the APXS, and mini-TES," Crisp said. "The rock is the same kind of basic rock that we have been examining as we have come down into Endurance Crater -- it has the little spherules or 'blueberries' in it and the fine layering, and the same kind of chemistry. It just has an unusual exterior shape and more extensive polygonal cracking, sort of like we saw in the rock Escher, only it's a rounded, odd-shaped rock compared to the outcrop that's in the side of the crater."
Wopmay's appearance held special allure from a distance, Crisp said, because the science team thought it might be some other kind of rock that had somehow rolled into there. "But when we got up close we could see that it was the same kind of evaporite rock," she said. "The thing is, it may tie in with Escher and the unusual shaped, broken-up or frosted 'mutant' spheres, what we are calling 'popcorn.' We started seeing these different things as we got towards the bottom of the crater, and so Wopmay's texture sort of fits in with those textures, having possibly been exposed to a second episode of water, but we still don't fully understand it yet."
Nevertheless, Wopmay is intriguing enough, Crisp said, for the team to have decided to instruct Opportunity to move around the rock to the other side and she is currently beginning another in-depth study there.
Opportunity's sustained boost in power generation through her solar panels means that she is now averaging more than 660 watt-hours per sol from the solar arrays. And that means there's no time for a hiatus of any kind -- although the rover is currently on a schedule of going into Deep Sleep mode every other night because of the heater that has been stuck on and draining power since just after she landed in late January. With this arrangement, the rover can be up for an early morning communications pass with Mars Odyssey on the non-deep-sleep nights.
After Opportunity completes her work at Wopmay, she will rove toward Burn's Cliff, a tall stack of layered rock, where she will examine the surroundings. "From the base of the cliff, we'll look at the cliff using our cameras, and do a little bit of robotic arm-work at the base of the cliff," Crisp said.
Following that fieldwork, the rover will then climb out of Endurance Crater, hopefully from a route right next to Burn's Cliff. If that route proves too steep and treacherous, the team has decided they will head over to the spot where they first entered Endurance and exit the way they came in. And once she's back out on the plains, the plan still calls for Opportunity to head south to the heat shield she jettisoned during entry, and to the rugged terrain nearby, where deeper rock layers may be exposed.
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