After nearly two weeks of sparse, infrequent communication, Spirit and Opportunity have survived winter solstice and resumed "reliable" contact with Earth and the Mars Exploration Rover team -- and NASA has extended funding for an additional six months of operations, as long as the little robot geologists keep working, space agency officials announced late Tuesday.
On September 16, Mars all but disappeared behind the Sun -- an astronomical event known as a conjunction. For a 12-day period -- Sept 8 to 20 for Spirit, Sept 9 to 21 for Opportunity -- the energetic environment close to the Sun interfered with radio communications between the two planets. In anticipation of this event, the MER team cut operations way back. Neither rover drove during this time and the team scheduled a hiatus in terms of sending up normal daily commands.
Although the rovers were able to successfully upload data to Mars Odyssey, the orbiter was unable to get much data down to Earth; therefore, transmissions were erratic and broken. "Generally, every day we would get snippets of data," Mark Adler, Opportunity mission manager, told The Planetary Society yesterday. "The rover would communicate fine to Odyssey, because the Sun wouldn't interfere with that at all. But when Odyssey would try to send the data back to Earth, the Sun would interfere with that, so we would only get some of the data." Nevertheless, they got enough information to be assured that the rovers were 'alive and well.' "We didn't really have any big concerns, because the power situation was pretty good for both rovers -- they were both tilted to face north and they had good solar energy coming into the panels."
Just because they took a hiatus from their everyday routines doesn't mean, however, that the rovers -- or the MER team -- stopped working. Science team members and engineers did manage to sneak in some down time during the last couple of weeks, but they didn't get much. Despite the rovers' pared down agenda, a full spacecraft team was on duty every day.
Spirit and Opportunity, meanwhile, followed a set of longer-term instructions and continued doing research on the atmosphere, using the Mössbauer spectrometer on targets within reach to search of iron-bearing minerals, or conducting wind or communications experiments. "We sent up about 16 sols worth of sequences so that the rovers would repeat the same science observations everyday without us having to command them and they were able to just run on autopilot for that whole time," Adler elaborated.
"Although we didn't have to command them to do anything, we decided to do some experiments to help characterize communications during this time, because we don't really fully understand what the environment is like there, or how [the conjunction] prevents us from communicating, or how much it prevents us from communicating," added Adler. To that end, they sent 'no operation' -- known as 'no-op' -- commands, which the rovers acknowledge, but which carry no instructions. "It was, basically, a conjunction experiment to help the telecommunications division here try and characterize the environment around the Sun during this time, so if you have some other spacecraft that really needs to communicate at that time you can better determine the performance."
The only real moment of concern, Adler said, happened "somewhere around the middle of conjunction when we sent a batch of 'no-ops' commands up to Opportunity that caused a software error that was caused by a bug that we hadn't noticed before," which, in turn, "caused the rover to stop running its pre-programmed science sequences." Since the rovers were designed to take care of themselves during this conjunction without the sequences running, the error was ultimately not of great consequence. "But we had to check to make sure the mini-TES [mini-thermal emission spectrometer] temperatures didn't get too low, and everything's working fine."
By Monday, the science teams were back to their planning boards and operations were resuming as slated. Opportunity was back on her regular schedule in Meridiani Planum on Tuesday, while Spirit resumed her duties yesterday at Gusev Crater.
As Mars emerges from behind the Sun, Spirit is more than 2 miles [3 kilometers] from her landing site, and roving up the west spur of the highlands called the Columbia Hills, in honor of the seven Columbia astronauts who lost their lives on the space shuttle in on February 1, 2003. She'll spend a couple of days taking images from two different positions to build a digital terrain model and then continue on up the West Spur towards the top of Husband Hill, named after Columbia Commander Rick Husband. "It's going to be a long haul, but on the way there are a lot of interesting looking rocks, several of which look layered, so we're going to investigate those and find out if they're water-lain sediments as we go," said Joy Crisp, MER project scientist.
Over on the other side of Mars, Opportunity is still inside the stadium-sized Endurance Crater, and is down about as far as she'll go. Next on her agenda is the examination of some sand that appears to have been blown up from the artfully rippled dunes on the bottom of the crater and piled up against a rock the team has dubbed Ellesmere. "Since we weren't able to safely drive into the dunes because our wheels could get stuck -- and because the sand looks interesting, we found a place where the sand has piled up in a small area next to this rock," Crisp explained. "We want to carefully document the properties of that sand pile. The idea is to even use a rock abrasion tool (RAT) brush to kind of dust off this dune material to really characterize the inner material with mini-TES and Pan Cam, and the Mössbauer spectrometer."
The science team is also intrigued with the rock Ellesmere. "It looks like it has some interesting textures, so after we finish with the dune material that's piled up against it, we're going to see if we can take a closer look at this rock," Crisp said. After that, Opportunity, like her twin Spirit, will spend a couple of days, she added, taking pictures from different angles to build an elevation model of the interior of the crater, and then this robot field geologist will head towards Burns Cliff, named after the late Mars scientist Roger Burns.
En route to Burn's Cliff, Opportunity will stop to check out an odd, lumpy rock that caught the collective eyes of the science team some weeks ago. "They've called it Wopmay -- I don't even know where that name came from," Crisp chuckled, "but this rock has a very different looking exterior texture. It could simply be that it's weathered in a different way and it's the same kind of evaporate rock, like we've been seeing at Eagle Crater and Endurance Crater. We want to find out."
From there, Opportunity will rove along the inside of the crater wall toward the south side and up to Burn's Cliff. There, she'll examine close-up what looks like high-angle crossbedding in a stack of exposed rock layers. "We'll characterize those layers and take lots of pictures and then try to drive out of the crater over there," Crisp said.
During this second mission extension, rover science team members will spend less time at JPL, attending daily planning meetings via teleconference from their home institutions in several states and in Europe. "All 150 science team members and collaborators have been provided the tools to be able to participate remotely," according to JPL's John Callas, science manager for the rover project.
Besides reducing costs, remote operations allow scientists to spend more time at home. "We get back to more normal lives, back to our families, and we still get to explore Mars every day," as principal science investigator Steve Squyres, of Cornell, put it.
Another change in operations is a shift from seven days per week to five days per week from October through December. This accommodates a temporary trim of about 20 percent in the project's engineering team to about 100 members. The rovers' reduced energy supply, during the rest of the Martian winter, makes the inactive days valuable for recharging batteries. By January, the energy situation will have improved for the solar-powered rovers, provided they are still operating. The team size will rebound to support daily operations.
Spirit and Opportunity are both well past warranty and while they have been showing a few signs of aging for some weeks now, the MER team is confident that they've each got another six months in them, at least. " When we go to NASA headquarters and ask for an extended, extended mission we have to show what the health of these rovers is like and we have to demonstrate that we could keep going for another six months," Crisp pointed out. "So, based on our predictions right now, we believe that is very possible. But they are going through temperature cycles that are pretty severe every day. Most of us know how complex these things are and they could break any day. The fact that we've gone this far and that everything is working right now -- I think everybody is mentally prepared for whatever happens. "
That said, Crisp and Adler acknowledge that Spirit and Opportunity have continued to surprise and impress just about everyone -- and they keep outliving their masters' bets. "I have stopped guessing how long they'd last," laughed Adler. "I was always proved wrong. Chances are they're going to last longer than I guess, whatever that is."
"We have gotten pretty good at orienting the solar panels, finding places for the rovers to go so that the solar panels are oriented towards the Sun," Crisp added. "Having more energy because of that has made our mission better than we had originally thought, and it's helped us overcome some of the concerns. At one time, we even thought that we might have to hibernate in the winter, but when we started looking at how we could be going up the Columbia Hills and have our solar panel oriented and we realized that maybe that's not necessary and now we see it's not necessary."
Perhaps even more interesting is the fact that the dust accumulation on the rovers' solar arrays isn't the problem the team thought it might be at this point. "If the dust accumulation had continued at the rate it had been going, we would have enough solar energy to last until May or July '05, however the dust accumulation has stopped increasing," Adler said. "We figured sooner or later it ought to level off -- there ought to be some equilibrium reached between the dust accumulating and the dust being blown off -- and we think we might be there now, in which case energy may not be the limiting factor." That means, Adler added, Spirit and Opportunity could go, theoretically, "well past" May or July. "Then again, anything can go wrong, things can break. There could be global dust storm that takes us out, or there could be a degradation of the components. We're already seen wear on right front wheel of Spirit."
Spirit and Opportunity bounced to a landing in January and successfully completed their primary three-month missions on the surface of Mars in April, when the space agency extended their missions for the first time. Since then, the twin robot geologists have added volumes of "bonus" data gathered from their explorations and all indicators now are that they have 'miles to go before they sleep.'
So when will the team call it a day for the two intrepid explorers?