http://msnbc.msn.com/id/5353015/

By James Oberg, NBC News space analyst // Special to MSNBC

Updated: 4:29 p.m. ET July 06, 2004

Months after their warranties have expired, both of NASA’s Martian rovers continue to energetically poke, scan and roll over the rocks of the Red Planet. And with the ever-changing landscape in their television cameras comes ever-expanding insights into Mars, and ever enlarging hopes for even longer survival.

“That’s the beauty of a roving mission,” mission scientist John Callas told MSNBC.com. “Every day is a new day.”

Spirit, the first of the twin rovers, is completing its sixth month on Mars, twice its originally planned 90-day mission length. It is now preparing to ascend the so-called Columbia Hill in search of strange rocks and a nest where it can safely hold up for the coming winter.

Opportunity, which landed three weeks after its twin, is facing downslope inside a crater the size of a football stadium. Nibbling and sniffing at rock layers, Opportunity is examining samples that date to earlier in time than those studied earlier in the mission. The rover’s traction has been good and its instrument

placement perfect, despite operating on slopes as steep as 25 degrees.

Opportunity's operators have a new motto: “If we can drive on it, we can drill on it!'

While both rovers have their own independent research programs, they can reinforce each other’s results, Callas pointed out. Hardware or software problems on one can be used to forestall parallel problems on the other, for example. And the scientific results of each of them influence the interpretation of results of the other.

“Two rovers are much much better than one,” perhaps ten times as good, Callas said. Sending two may have just seemed a good plan to be assured of at least one success, but when both worked, it had far greater value. “It gives redundancy against Mars tricking us, as it has tricked us in the past," he said.

Being at two “dramatically different sites”, Callas continued, allows scientists to avoid thinking features of one site are widespread. And when some features are found at both sites, they can be identified as likely to be far more common elsewhere.

Time for a tune-up

Spirit is now at the base of the Columbia Hills, finishing its studies of unusual rocks that appear to have tumbled down the flanks. It’s examined bizarrely-eroded rocks such as “Pot-of-Gold” and “Bread Box”, as well as features called “String of Pearls” (bright specular material near Bread Box) and “Bright Tracks” (shiny material that has been stirred up by the rover wheels).

Detailed color images have also been made of the rocks called "Pudgy" and "Pancake," a few feet east of the current location.

In a recent daily report, rover official Leo Bister described the "3,000 meter tune-up" that Spirit will undergo before embarking on its climb up into the hills. There are four specific engineering activities that operators have decided to perform.

First, they will work on the rover’s right front wheel lubrication. “The right front wheel continues to draw roughly twice the current of the other wheels,” Bister noted. To try to repair it, Spirit will drive about seven meters to a relatively flat, hazard-free area that has been named “Engineering Flats”. There it will execute a series of diagnostic drive tests and heating sequences over the course of several days.

“We are hoping that heating will reflow the lubricants in this actuator,” Bister explained. If uncorrected, the problem will be an additional drain on the rover’s already tightly-stretched electrical power.

Second, Spirit will test a new control mode called “visual odometry,” which uses TV images taken during a drive to determine the rover location. Due to wheel slippage in sand and gravel, operators using the current “blind drive technique” sometimes need two or more days to make a short approach to a rock target.

“This rover feature has been improved and is ready for trial runs now,” Bister wrote. “We would like to use it on a regular basis to get where we want to go more quickly.”

Third, Spirit will borrow a page from Opportunity's book and try out a "deep sleep" power mode. This involves completely shutting down the rover’s electronics overnight, and depending on the light of the rising sun to generate the electricity needed to awaken it the next morning. Since even the heaters are off, the rover gets much colder than planned.

“We will need deep sleep to save energy in the coming [days]”, Bister explained. But there’s a catch: “Since deep sleep is potentially harmful to the MTES [mini thermal emission spectrometer, which characterizes mineral-types by their colors] instrument, we have identified two MTES observations that must be completed before this is attempted for the first time.”

If the internal temperature drops too low, a special crystal used in the MTES instrument is expected to shatter. But it’s worth the risk, Bister explained, pointing out that “Opportunity has been using deep sleep for several weeks now,” with no damage to the instrument as yet.

Lastly, Spirit will calibrate its forward-looking hazard cameras by moving its instrument arm in front of the cameras and taking precision pictures. “We are currently experiencing a [one inch] error in predicted versus actual target locations in the vicinity of the [arm],” Bister explained.

Mission scientist John Callas added that this misalignment had been enough to cause the instrument arm to miss some of the desired target points on rocks to be studied. “It’s possible the camera has shifted,” he explained, or that some other warping had occurred after nearly 200 severe day-night temperature cycles.

Beginning the ascent

After the tune-up, Spirit will head back along the face of the hillside, looking for a ridge crest that would be gentle enough to use as a path. The incline must be less than about 20 degrees for a safe ascent.

“We see some interesting formations up the hill,” Callas explained. “It looks like there are stratified layers below the summit.” Spirit will study these layers using the same instruments the rovers have used on other rock layers.

From its present position, Callas estimated that Spirit’s route to the summit is about 500 meters long, with an altitude rise of about 80 meters. Operators estimate it will take several weeks to get to the first outcrops, and perhaps a month and a half to get to the summit.

“It will be a dramatic elevation”, Callas continued. “We should be able to see the entire ring of Gusev Crater’s rim on the horizon”. Long exposures will be needed to pull the stark silhouette out of the haze, he admitted, but that view alone -– and panoramas of nearer regions –- will make the climb worthwhile.

Further, there may be a path down the other side of this hill, into intriguing regions of dark soil seen in photographs from orbiting spacecraft. The route could well be shorter and safer than earlier suggested paths that circled the base of the hill.

“It’s safer because we’re in more sunlit terrain,” he explained. “We have to avoid the shadows of the hills –- shadows are lethal to the rover,” he said. “We wouldn’t have enough battery power to get back out if we got into one.”

But that trip will have to wait for next year. By mid-September, winter and its low sun angles will have set in, and Spirit will probably spend a few months atop the hill. “It won’t be a total shutdown,” Callas explained. “We’ll do just a few activities -– maybe move a little each day, forward and back, to keep the gearboxes active.” Early in 2005, with sun angles higher, ‘Spirit’ might travel on, assuming it has survived.

Opportunity, meanwhile, may winter inside Endurance Crater, where the sloping walls actually help by tilting its solar panels more toward the sun.

Operators know that both rovers are only faking the appearance of immortality with their astonishing durability. At some point, their roving days will end, whether by accident, exhaustion, or internal breakdown. Until then, for them, the horizon is limitless.