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A.J.S. RaylJanuary 24, 2004

Second Mars Rover Opportunity Fast Approaching Mars

The second Mars Exploration Rover -- Opportunity -- Spirit's identical twin -- is approaching the atmosphere of Mars and is expected to land on the Red Planet tonight around 9:06 p.m. Pacific Standard Time, just after midnight Eastern Standard Time.

Opportunity is headed for a location on the opposite side of the planet from Spirit, a vast plains area called Meridiani Planum that boasts some of the smoothest terrain on Mars. 

The second Mars rover is charged with much the same mission as Spirit -- to look for signs of past water and the geologic conditions associated with it, and whether the area could have ever sustained life.

Opportunity's search in Meridiani Planum, however, will follow a different geologic path to the water most scientists believe must have been there is substantial quantities way back when.

Beyond the always prevalent engineering safety issues, the scientists liked Meridiani because of what distinctly appears to be a huge amount -- say, the size of the state of Oklahoma -- of gray crystalline hematite, a mineral that on Earth usually forms in the presence of water. The hematite was detected and mapped with instruments onboard the orbiters Mars Global Surveyor and Mars Odyssey about two years ago.

"Gusev Crater has evidence of past liquid water in shape of terrain - it's a crater and has a dry riverbed leading into it; whereas, Meridiani Planum has evidence of past liquid water in the form of this mineral," Project Scientist Joy Crisp explains. "Life as we know it requires liquid water, so we want to go to Meridiani to study rocks to determine whether liquid water was around in the past when these rocks formed and whether that past environment was favorable for life."

"Imagine outcrops of deposits of hematite the size of Oklahoma - I don't know any place like that on Earth or any place in Earth's history where we have that extensive set of deposits," enthuses Ray Arvidson, deputy principal science investigator on the project.

The payload on Opportunity -- which is identical to Spirit's -- is, says Arvidson, " beautifully suited" for exploring Meridiani Planum. "With this payload we will be doing imaging in color from .4 to a micrometer, and emission spectroscopy that will complement what TES [thermal emission spectroscopy] sees from orbit with [what Opportunity sees with] Mini-TES. Then, we'll be able to get down and into the soil-like materials and onto the rocks with Mössbauer spectroscopy which gets you the iron and how the iron is bound in the minerals, what it's oxidation state is -- and we'll be able to do chemistry with the alpha particle x-ray, and get close up and personal images with the microscopic imager. It just a beautiful synergistic payload for getting at whether there is hematite there."

The first order of scientific business for Opportunity is to figure out whether this outcropping is in fact hematite or "something else there producing this hematite signature," says Arvidson. "Then we want to find out what is the geologic association, the origin of this material. As geologists we make maps - measuring and looking at the units that are present. The hematite bearing material in Meridiani is on the top of a massive, 200 to 300-meter deposit of sedimentary layers that are sitting on the ancient crater drain. Most of deposit appears to have been eroded by wind, so in places we can see all the way down to the ancient crater terrain that this hematite material is draped over."

The debate amongst the science team members is between two theories. One hypothesis maintains that this is, " a big pile of volcanic sediments," says Arvidson. "It [puts forth] the idea that this is an area where the hematite may have been altered from some precursor mineral to hematite, by hydrothermal fluid." The other notion is that Meridiani Planum is a fixed stratum that was produced in an ancient ocean. "At this point, we don't know," he says.

The science teams will also command Opportunity to uncover and characterize the other minerals present, and how the hematite is bound up in the surface of those other materials, and look at how the land has formed. "The microscopic imager going down and getting personal with rocks and soils will tell us more through the grain sizes and textures the presence of crossbeds, and things like that," Arvidson adds.

They also expect as Opportunity traverses across the vast plain, that she'll find the materials that carried the hematite to its present locations, as well as underlying materials. "The combination -- looking through PanCam through Mini-TES to the MI and then getting the chemistry and iron bearing mineralogy -- will provide the data we need to test the two major hypotheses of marine or volcanic, and the role that water has played," he predicts.

"We're following the water directly, in a way that's entirely complementary to Gusev," Arvidson continues. "Gusev Crater called to us to go there because of the morphologic evidence for channel system and maybe ancient lakebed. Hematite called to us because it was this massive deposit of material that is MP and the geologic association of a couple hundred meters of layered sedimentary rock that has some origin. Within a few weeks we should have some interesting things to say about the role of water. We shouldn't be Earth chauvinists, however. We need to stick to observations together with out field knowledge and knowledge of chemistry and physics, [in order] to have the real Mars stand up."

Opportunity approaches the terrifying ride in

For now all attentions are turning to Opportunity's entry, descent, and landing tonight. The rush through the Martian atmosphere is described as 6 minutes of terror, and as the MER EDL team prepares for arrival at Meridiani Planum tonight they're also going for another first for NASA. "This will be the highest altitude Mars landing NASA has ever attempted, Wayne Lee chief engineer for entry, descent, and landing (EDL) said yesterday at JPL.

"The successful landing of Spirit has given us very good confidence that the odds are with us for Opportunity," he added. "However we are respectful that landing on Mars is a very risky endeavor and there are no guarantees with this. Landing Opportunity on Saturday night is going to be extremely challenging because [it] the highest altitude Mars landing that NASA has ever attempted."

To insure things go right, a team of 30 engineers has been working around the clock for the last three weeks to analyze all the possibilities and probabilities and to consider final last-minute preparations for Opportunity's landing. "One of the main tasks we've been working on is to reconstruct what happened with Spirit's entry, descent, and landing system," offered Lee. "We need to figure out is there anything different we would do this time around?"

The good news was plentiful. "Everything was found to be just great with the entry, descent, and landing system," Lee aid. "All the systems were analyzed and they performed exactly how we expected in almost all instances without approaching any of the redline performance limits. The entry capsule, the parachute, the retro rockets, radar, the airbags, the flight software for ED&L all performed exactly according to the flight plan. There was only one area where the device did not perform according to flight plan -- and that was the descent rate limiter.

While many, many factors go into this kind of analysis, not the least of which is the fact that Gusev Crater and Meridiani Planum are different places, the "bottom line" that emerged from that study, says Lee, was that the systems "worked so well" that they will do "almost nothing" to Opportunity in the way of adjustments. The team has decided on one change - to reprogram the flight software to deploy the parachute approximately 4500 feet higher than what "was achieved" on Spirit.

"The reason for that is three-fold, he says. "One -- this is the highest altitude landing ever attempted on Mars for NASA, and if you're landing at higher altitude, it makes sense to deploy the parachute higher; two -- although we think the dust storm has subsided, we're still seeing variability in the atmosphere and we want to guard against the atmosphere changing on us at the last minute, so by deploying the chute higher in altitude, we spend more time on the parachute heading down to the ground, and the system has more time to perform all the activities it needs to, to fire the retro rockets, and bring us to a safe landing. And three -- we want to buy some insurance against the descent rate limiter performance we saw on Spirit."

The descent rate limiter -- a device that limits the deployment speed of the lander out of the backshell as the flight team reconfigures the spacecraft software in preparation for landing -- showed a deployment rate of nearly twice as was expected.

"We had anticipated a deployment time of about 6 seconds and the telemetry shows 11 seconds of deployment," explained Adam Steltzner the lead mechanical systems engineer for EDL. "That forced us to examine very closely both the telemetry and that subsystem and its performance - and after the last couple of weeks, we have concluded that the most probable root cause for that increased deployment time is an increase in braking friction in the braking device that controls the speed."

The team ultimately came to conclude that the increase in braking friction was caused by a loss of volatiles in the brake pads during the 7-month journey to Mars. "It may have lost some of that material making it perform better than we anticipated, having higher friction and doing a better job of braking," he explained. "The improved braking slows the separation of the two pieces of the vehicle more than anticipated and forces the longer deployment time."

They now expect Opportunity "will display similar behavior," Steltzner said. "That, along with the atmosphere data and altitude all folded into the decision to raise the altitude of the parachute in deployment. We learned through the Spirit landing event, and then testing of a behavior we had not anticipated, and now we feel comfortable with our understanding of it and are looking forward to [Opportunity landing.]"

Tactical splits

While part of the MER team continues communicating with Spirit, another part of the team is turning all attention to Opportunity. "The good thing is we have so many people on this team that we can split them in half and work the strategy of how to work on the two rovers at the same time," says Crisp.

"We're going to be splitting the tactical operations team in half," she continues. "These two groups will be out of synch with each other by half a day. On January 25 the science team groups working on Spirit's 22nd day on Mars will be ready to analyze the data that comes back from that rover during the daytime shift; whereas, the science team groups working on Opportunity's 2nd day will be analyzing data that comes back from that rover during nighttime shift. These folks are working on Mars time," she noted. "That means they're going to come to work 39 and a half minutes later each day. People will not necessarily stick with just one rover the whole time but they also are not going to be shifting back and forth frequently because of 12-hour 'jet lag' that will incur."

With Opportunity now facing its own "six minutes of terror" as it heads for entry, descent, and landing, along with Spirit's new-found troubles, has put just a little bit of pressure on this team, it's not any more pressure, says Crisp, than they already feel. "Truly every mission is critical to us," she says, " because we've invested a lot in it," she says. "That pressure really can't go up much more than it is."

Read more: Opportunity, mission status, Mars Exploration Rovers, Mars

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Salley Rayl
A.J.S. Rayl

Contributing Editor for The Planetary Society
Read more articles by A.J.S. Rayl

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