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MER Update

Mars Exploration Rovers Update: Spirit Investigates New Outcrop Methuselah as Opportunity Roves into a Ripple and Gets Stuck

Posted by A.J.S. Rayl

28-04-2005 12:00 CDT

Topics: Opportunity, Spirit, mission status, Mars Exploration Rovers

The Mars Exploration Rovers have both encountered some truly challenging obstacles in recent days, but have also presented the team with some surprises, and continue to be in overall good health some 16 months after bouncing to a landing, and more than a year after completing their primary missions.

At Gusev Crater, Spirit had to divert from her original course up Husband Hill because the slopes were too steep and the sandy terrain too slippery. The rover, like any determined hiker, has not given up. Once she completes the extended investigation of the current target -- an outcrop called Methuselah, Spirit will continue on across the slope looking for another route up to the summit.

On the other side of the planet, Opportunity has been driving ever southward toward the Etched Terrain, making some stops to investigate interesting targets along the way. For the last three weeks, the rover has been cruising up and over ripples that have been getting "progressively larger," Steve Squyres, principal investigator for rover science, of Cornell University, said in an interview earlier today. The rover was 'steady as she goes' until a couple of sols ago when she roved right into a ripple. Although this berm looked like all the other ripples, it obviously wasn't. Now the rover is buried up to her "hubcaps," in a fine-grained, sand like soil, and will likely be there for awhile as the MER team works to get her unstuck.

Earlier this month, both rovers lost a couple of days' of driving when Mars Odyssey -- their main communications link between Mars and Earth -- unexpectedly entered 'safe mode.' For some as yet unexplained reason, the fault protection software instructed the spacecraft to disregard its onboard sequence of commands and wait for instructions from the ground. As a result, relay communication with the rovers was suspended, and neither Spirit nor Opportunity received any commands or returned any data for a couple of days. Even so, they continued working on remote sensing studies, while the Odyssey flight team scrambled to figure out what happened. By April 7, the orbiter was out of 'safe mode' and back in communicative action, and the rovers got back to roving.

Spirit and Opportunity have also each experienced some software glitches, and each continues to 'flirt' with aging, as parts work intermittently or break down. Still these rovers seem infused with good fortune. Spirit's right front wheel -- which had been out of full-time commission for many months last year, causing the rover to have to drive backward, continues to work smoothly after making a comeback about three and a half months ago.

Meanwhile, Opportunity's right front steering motor appears to have failed and has been disabled, but the rover's mini thermal emission spectrometer (mini-TES), which has been on the fritz since the beginning of March, surprised the instrument's principal investigator, Phil Christensen, of Arizona State University, and other science team members in recent weeks by returning several days worth of good, solid data. "It's been behaving like an intermittent electrical short for want of better words," Christensen said in a recent interview. "The good news is it's not dead."

Overall, both rovers remain remarkably robust and team members are confident that Spirit and Opportunity -- which NASA sent into their third overtime earlier this month -- have many more meters to go before they call it a sol.

Spirit from Gusev Crater

During the last couple of weeks, Spirit has really struggled on her journey up Husband Hill, located in the Columbia Hills region of Gusev Crater. The rover has not only persevered through tough terrain, but also command sequencing errors, and a few software glitches to return and more images of dust devils, and more data from newly examined targets.

The sandy terrain that had the rover slipping at the beginning of the month continues to present challenges, and ultimately forced the MER team a couple of weeks ago to divert Spirit from the planned path. The revised plan calls for the rover to use tried-and-true hiker techniques -- such as driving a zigzag course -- to ascend higher, and forge onward and upward to the summit of Husband Hill, conducting investigations of selected targets and remote sensing as she goes.

While the rover team was working on new drive plans, Spirit took some time to test the image brightness of the navigation camera -- the point of which was to establish the latest time that the rover can take images prior to sunset and still have viable images for the rover drivers' to use in planning future drives. Ultimately, the image analysis may allow the MER team to use later times for post-drive imaging and thus increase Spirit's drive time every sol.

From Sols 449/April 8 to Sol 456/April 15, Spirit had no end of troubles. On Sol 449, the robot field geologist was attempting to climb the steep and rock-strewn slope leading to the summit of Husband Hill when suddenly the flight software rebooted and the rover automatically went into 'safe mode,' a mode in which only systems vital to the rover's health operate. That caused Spirit to lose knowledge of where her high-gain antenna was pointed. So, on the following sol, when she tried to use the antenna during the communication uplink without knowing where it was pointed, a fault condition resulted, and the rover missed the uplink for the day.

In order to get out of fault, the MER uplink team came in on the weekend of April 9-10, put the spacecraft back into 'safe mode,' then commanded Spirit out of the high-gain antenna pointing error and left the rover in auto mode. In this mode, the robot is not running instructions from the Earth, but rather is taking care of itself.

Since Spirit was in auto mode, with no sequence of commands running, the rover essentially 'relaxed' and took care of herself on Sol 452, then got up the next morning and took fresh images of the surrounding terrain in preparation for driving on the following sol. The climb on Sol 454/April 13 did not, however, go very well because the rover kept slipping. Engineers and scientists decided then it was time to try a different tact. Instead of continuing up the course the team had plotted, Spirit is being directed to drive down a bit and cross the slope until an easier path to the summit is identified.

On Sol 456/April 15 or Tax Day down on Earth, Spirit used her 'downtime' to study a soil target the team nicknamed Crumble. Using her front wheels to churn up Crumble, the rover performed a maneuver the MER engineers and scientists call a 'scuff,' and then spent that weekend examining the soil underneath with the full complement of tools on her instrument deployment device (IDD) or robotic arm.

Since then, Spirit has taken lots of new images, done a little more driving, and on Sol 461/April 20 captured yet another image of dust devil passing by, using her panoramic camera (PanCam). After more than a year of looking, Spirit spotted the first dust devil of the MER mission last month, on the rover's Sol 421/March 10. Within days, the rover had snapped images of another one. Scientists believe the small cyclones are seasonal, perhaps linked to wind storms that occur in the Martian spring. And, indeed, what started out as a couple of dust devils a few weeks ago has now turned into something of a swarm of dust devils blowing across the plains of Gusev Crater almost every day. Although orbital images in past years have detected dust devils in many places on Mars that are up to several miles or kilometers tall, so far, the dust devils Spirit is capturing with her cameras are about the same size as those that whip up desert dust and sand in the southwestern United States.

Spirit then drove toward a piece of outcrop in the neighborhood that attracted the eye of science team, and spent last weekend and this week checking out the target the team has dubbed Methuselah. Several meters across, this section of bedrock presently forms a semi-circular boundary around the rover.

"Basically, the idea there was to cover the whole thing at a very high-resolution and then find the very best spot to move in and do some detail work with the IDD and that's what we're doing right now," Squyres told The Planetary Society.

The rover moved in on the spot the science team chose and nicknamed Keystone earlier this week and will be hunkered down there throughout this coming weekend. Currently, Spirit is in the process of taking 144 microscopic imager (MI) pictures for a "huge mosaic," as Squyres described it. "We planned [to take] two-thirds of those 144 images yesterday and so the rover should start executing that in just a few hours now," he elaborated. "And we're planning the remaining one-third of it today. And then we're going to do RAT brush, APXS, Mössbauer spectrometer, and a complete work over of this thing."

Opportunity from Meridiani Planum

Opportunity was in the midst of an up-close study of the dual craters the MER team nicknamed Voyager and Viking during the first week in April when Odyssey went into 'safe mode' and relay operations were suspended. With no post-drive imaging from that weekend, and very little data volume available in her onboard flash memory, the rover spent a few sols conducting low-volume remote sensing -- using her PanCam to assess the clarity of the atmosphere, the alpha particle X-ray spectrometer (APXS) to take readings of the air, the mini-TES to take some test readings.

On Sol 428/April 7, after downlinking data with the high-gain antenna, the rovers' direct-to-Earth link, Opportunity hit the Meridiani 'highway' once again, continuing her journey south to the Etched Terrain. By sol's end, the rover had driven 159 feet [48.4 meters], enough to put her over the 3-mile [5-kilometer] mark.

At this juncture, the terrain that lay ahead of Opportunity was visibly wavier than what she had just traversed. "That means the ripples are getting bigger and bigger as we head to the south," Squyres said, "and what that means we're still trying to figure out."

On Sol 433/April 12, after a long drive southward from Voyager crater -- 495 feet [151 meters] -- Opportunity's right-front steering motor stalled out during an end-of-drive turn. While performing tests to help the team diagnose the condition of that motor, the rover also continued to make remote-sensing observations to see if the right-front wheel had bumped up against anything to cause the steering-motor stall. No rock or other obstacle was there, so the rover attempted to straighten her wheels after backing up, but the right-front steering motor stalled again. That didn't stop this rover though. Opportunity resumed normal science and driving operations on Sol 436, but with restrictions on use of that particular motor. The failure had nothing to do with the wavy terrain -- "that particular steering actuator appears to have just failed," Squyres said -- and the rover will not be hindered much by this breakdown.

Opportunity and Spirit are capable of driving with one or more steering motors disabled, although turns will be less precise in this type of scenario. The latest revision in flight software on both rovers, uploaded in February, however, gives the rovers upgraded capabilities for dealing with exactly this type of condition, enabling them to repeatedly evaluate how well they are following the intended course during a drive, and to adjust the steering autonomously if appropriate. "We actually got a very lucky break here -- it failed {with the wheel} pointed almost straight forward," said Squyres. "It's toed in maybe [8] degrees [left of straight ahead], so it's nearly straight. We've got so many other ways to steer this vehicle, it's just not a big deal."

On Sol 435/April 14, the rover conducted some diagnostic tests in attempts to change the steering direction of the right-front wheel very slightly at different times of day and at different voltage levels, as well as some more remote sensing observations. The testing showed motion in the steering motor though analysis continues. In the meantime, the rover resumed normal science and driving activities with restrictions on the use of the right-front steering motor. Given the robustness and resiliency of the rovers to date, it is possible the motor will start back up again, but, said Squyres, "at the current time we're proceeding as if we can't use that actuator."

Opportunity spent tax day, her Sol 436, taking PanCam images for some ground and sky observations, and continued testing the mini-TES. But the following sol -- a day in which the team had planned a southward drive of about 147 feet [45 meters] -- the rover took an unexpected curve, literally. During the drive, the rover swerved to the left, and sensing she was off course, ended the drive after about 98 feet [30 meters]. The same driving commands produced the same results in a software testbed at JPL, which told the engineers that the curving happened as a result of how software parameters were set, and was not a more serious hardware problem.

On Sol 439/April 18, Opportunity took PanCam pictures of soil in a trench that was scooped out by a wheel when the rover turned to get a good communications orientation after the Sol 437 drive, then took off on a 262-foot [80-meter] drive south.

Last week, science team members were taken back a bit when the mini-TES began returning good data again. "We had been turning it on a few times a day just to get some statistics of when it would work and when it wouldn't," explained Christensen. "We did that for a week and it didn't work, and then all of a sudden it clicked and it started working again." But as it was in use last Tuesday, Opportunity's software unexpectedly reset at approximately 12:45 Mars local solar time, and that caused the rover to shut down.

"It took a couple of days to recover the rover and we're still trying to debug and figure out what the heck happened and we're not sure," Christensen elaborated. "The mini-TES was running when the reset occurred. It doesn't take a rocket scientist to conclude those may be related -- but they may not be. The initial reaction was to assume it was the mini-TES, and I have been trying to remind people there are other issues and there may be something lurking in there with the mast. We just don't know what happened."

On Sol 441/last Thursday, Opportunity was back up and running and began transmitting data acquired prior to and during the Sol 440 reset event back to Mission Control at JPL. That mini-TES data -- acquired over several days and all good -- brought big smiles to Christensen and the faces of the science team members. "Those data were perfect. The very last observation was perfect," he said.

Apparently, the "electrical short" theory that Christensen previously suggested is right on, and at the root of the problem are the extreme Martian temperatures to which the mini-TES has been exposed. "We've clearly got an intermittent problem with the electronics -- sometimes it's bad and sometimes it's good and recently it's been good," agreed Squyres. "The mini-TES is a seriously abused instrument that has been stressed in ways that it was never intended to be. It's had a very long lifetime and has [survived] very cold temperatures. Remember, this is the vehicle that we've been doing DeepSleep on and what that does is take mini-TES far outside its expected temperature range."

The mini-TES was designed to go to -40 Celsius, but with DeepSleep, the instrument has been below -50 Celsius some 150 times. "The fact that the mini-TES is working at all and that it has worked so long is to me astonishing," added Squyres, "and we'll take every little bit of science we can get out of it at this point."

The mini-TES has not been turned back on since the Opportunity's reboot event last Tuesday and no one is saying when it will be turned on again. "Part of the problem is a number of the engineers who are necessary have been swamped or unavailable to look into this," said Christensen. "The guy who designed this piece of mini-TES electronics in 1997, for example, has retired and is now on a cruise. So we've been calmly patiently waiting for the pieces to come together. The problem with the way the reset occurred is that it gave us very little diagnostic information to give us insight as to what caused the problem and that's where we sit today. We definitely need to find out what's going on. The mini-TES is on hold until we do. My fear is that it wasn't us that caused the rover to reboot, but by the time we figure that out mini-TES won't be working again. I am, however, completely understanding of the bind the engineers are in. For me, this is the kind of stuff that makes this whole business fun."

At the moment, the intermittently functioning mini-TES is the least of Opportunity's problems. The rover has plowed right into one of the rippled waves she had been traversing -- and is now buried up to her "hubcaps," as Squyres put it. "We've been driving over these large ripples, and every other ripple we've driven over we've gone up one side and down the other, right over the crest with hardly any sinkage at all," Squyres recounted. "Then a couple of sols ago, we encountered one we just drove headlong into. So, all six wheels are buried to the hubcaps."

No fear. It's not the end, said Squyres. "We're not concerned that we won't be able to get out, but we're going to approach it very, very carefully. This is something new -- we have not seen this before in these ripples." This particular ripple was different, "and we don't know why," he said. Right now, the team is focused on figuring out two things: what was different about this ripple that caused the rover to drive into it rather than over it; and the best way of getting the rover out of this ripple and all six wheels back up on top of the soil.

For the time being, Opportunity is stuck and will probably be at her current location for awhile, Squyres confirmed. "We're going to work our way through this very slowly and carefully. This is a very unexpected occurrence and we want to extricate ourselves from
 
See other posts from April 2005

 

Or read more blog entries about: Opportunity, Spirit, mission status, Mars Exploration Rovers

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