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A.J.S. RaylJanuary 13, 2004

Spirit Begins Maneuver to Roll onto Surface; Mars Rover will "Head for the Hills"

As Earthlings slept last night, Spirit completed the first part of a three-part maneuver that will take her down the ramp and onto the Martian surface early Thursday morning Earth time, while the Mars Exploration Rover (MER) science team reveled in its "embarrassment of riches" and completed a draft of its travel itinerary for the mission.

"We have cut our ties and are ready to rove," said Flight Director Chris Lewicki at the morning press briefing.

So where are they heading?

"We're heading for the hills," announced lead scientist Steve Squyres of Cornell University. Those would be the just-named Eastern Hills, situated to the east of Spirit and the Columbia Memorial Station. "This site is very rich in its science potential and also offers a shared adventure unprecedented in human history."

Late last night, early this morning Earth time, the mission team cut the rover's final cable, the last tie that bound rover to lander, in essence its umbilical cord. Free at last, Spirit then performed a 'bump,' a maneuver in which it backed up 25 centimeters, so that engineers could test out a couple of actuators, as well as the rover's mobility. With the 'bump,' Spirit was right on the money, properly aligned and in the right place, Lewicki reported.

That task and test accomplished, Spirit initiated and executed the next sequence -- the first part of a three-part, 115-degree turn-in-place -- by turning 45-degrees.

Late tonight and in the early morning hours Wednesday Earth time -- what will be Sol 11 on Mars, Spirit will continue the maneuver by executing the next two parts of that turn, which, if all goes as it has been going so far, will position the robot geologist for a straight, smooth roll down the lander platform. The team had wanted to roll right off the front, but an errant airbag that failed to completely deflate and retract could have threaten the rear solar panels, so last weekend engineers tested various alternative with ground models in the JPL 'sandbox,' and settled on the plan Spirit is now carrying out.

The roll off will most likely occur in the wee morning hours this Thursday Earth time, but given the consistent performance of the rover and its instruments, the event could well happen sometime late Wednesday night. On Mars, it will be Sol 12 -- and then the adventure will really begin.

Spirit will first examine the soil and a few of the small rocks in its immediate vicinity. "The goal of this mission at this site is to look at the materials here and determine whether or not Gusev Crater once held a lake and what the conditions were like," Squyres reminded. "We're looking now at a vast plain." Gusev Crater is about the size of the state of Connecticut.

"Soon we'll know what the material the lander is perched on is like, but we want to find what lies beneath our wheels for as deep as we can go," Squyres continued. The team's long-term plan is basically, he added, "find out what lies below and above and over toward the hills."

Once Spirit's study of the immediate area has met its objectives, the rover will begin moving to the northeast, to a crater northeast of the lander, which they now estimate is about that is about 270 yards (250 meters) away, and about 220 yards ( 200 meters) wide. If it is an impact crater, which they currently suspect, whatever impacted probably hurled rocks from as deep as 22 to 33 yards (20 to 30 meters) onto the surrounding surface, where Spirit may find and examine them. They've made no decisions about driving into it.

"We don't know what we'll do when we get there, but as we go closer we'll be finding stuff excavated out of surface," said Squyres. "It will provide window into the subsurface of Mars. Once we have done that, we will have seen deeper into a crater on Mars than ever before. Then we're heading for the hills."

Spirit and its twin Opportunity were designed to be able to traverse at least 600 meters, traveling more in one day than Sojourner did in its lifetime. Still, Spirit probably won't make it all the way to Eastern Hills -- they are estimated to be 5 times that distance or close to 300 kilometers away. "Who knows how close we'll get?" Squyres admitted. "We'll go as far as we can and investigate materials along the way. We probably won't reach them, but the view will get better and better."

For now, each sub-group within the science team is focusing on specific aspects of the mission. Atmospherics group is focused on analysis of Mini-TES observations, and looking upward to atmosphere of Mars, Squyres said.

The geology team and the long term planning group, meanwhile, is working on strategy of which rock to go to first before Spirit strike out across the countryside toward the hills. "We don't want to waste time so we want to get to the rocks that will tell us the most in the shortest amount of time," he added.

At the same time, the rock and soil physical properties group are planning a couple of drills to see how soil interacts with wheels. "In the not too distant future, we're going to do a couple little maneuvers with wheels, scratch and turn to get better sense of how strong the soil is -- not a true trenching maneuver but [movements] to see how wheels interact with soil."

The mineralogy and geochemistry groups are, Squyres continued, "anxiously" awaiting Spirit's roll out so they can begin looking at the data from the alpha particle x-ray spectrometer (APXS) and the Mössbauer spectrometers, although they are continuing to examine the mini-TES data. "One of the things they are working on is how to pick the drive direction to maximize our ability to determine materials in rocks," he explained.

Other members of the navigation and science teams have been working on analyzing data to determine just where exactly Spirit came to rest, while the entry, descent and landing team has been reviewing how she got there.

The navigation team was able "to exploit" a radiometric technique -- similar to satellite-based global positioning systems on Earth -- using Odyssey to refine Spirit's location" to within 30 meters, shortly after landing, according to Joe Guinn, a member of the navigation team. "We know where Spirit is to a patch that is the length of this room. The accuracy of the various techniques they now use are "so good," he added, "the biggest uncertainty is in the maps now."

That's because, he explained, even the best maps of Mars don't display all the topographical features and "the images we get back don't always match up." In fact, he said, "they can be off by as much as 300-400 meters."

The radio measurements acquired during Spirit's entry however allowed them to determine the landing area within a few kilometers of uncertainty. "Basically, we've got a really good system for quickly determining surface positions that is better than level of maps we have now," Guinn concluded.

In addition to the radiometric data collected during descent, the MER team is utilizing other techniques, such as reviewing optical data, to "squeeze out the uncertainty," as Guinn put it, and to further home in and tighten the radius of Spirit's exact location. Other researchers have correlated features seen on the horizon in Spirit's panoramic views with hills and craters also identifiable in images taken by Mars Global Surveyor and Odyssey.

"We've got a tremendous vista here with all kinds of features on the horizon helping us pin down our location," said science team member Tim Parker, landing site-mapping geologist, as the team presented the latest mosaic, of the East Hills. Any real discrepancies, he said, are "in the precision of mapping controls" to the landing site, but the features Gusev Crater will allow them "to pin this down and learn where in Mars network this is."

Determining exactly where the spacecraft landed, in the context of radiometric data, images taken from orbit, and with the new high-resolution images Spirit is sending home with its Pan Cam, has given the team a better map than ever previously existed of the area. As Squyres put it: "We know where we are now and we know where we're going."

Since Spirit landed on January 3, the entry, descent, and landing (EDL) team has been reviewing and analyzing all the data collected by Spirit during its fiery entrance, focusing specifically on the parachute deployment, cutting of the bridle, and the firing of landing rockets.

As it turns out, the parachute was released 4.6 miles (75 kilometers) above the ground. "The parachute inflation went well, except software decided to open it lower," offered Rob Manning, who headed development of the EDL system. "It did exactly what we asked it too, but because of the interesting atmosphere [Spirit encountered on the way in] it happened about a mile lower than the target altitude . . . which makes things a little more exciting towards the ground." Wind gusts near the bottom," he added, "took the parachute off the angle [descent trajectory] and so the rockets were not pointed straight down . . . which gave us a significant horizontal velocity." The fortunate thing, he said, is that there was thicker atmosphere further down which helped slow the speeding spacecraft.

"We came in at 152 miles per hour (mph), slower than we had expected and we don't know why that is," Manning said, speculating that it might have been the result of either the atmosphere density being thicker than model predicted or possible air drafts. "At 152 mph - that's 32 miles faster than a skydiver falls -- is when parachute opened. You realize robots have nerves of steel -- or copper as it was in this case."

The EDL team fired the transverse rockets to successfully slow the horizontal motion seconds before impact, at 342 feet (about 104 meters) above the ground - "again the software decides," said Manning, "and it did the job perfectly."

The bridle, it turns out, was cut also a little lower than the simulations had indicated. In fact, the airbag bundled spacecraft was just 27.9 feet (8.5 meters) off the ground when its bridle was cut for the final freefall to the surface, while the target was for 12 meters above ground. "Imagine you're driving in the Indy 500 and instead of going through checkered flag [at the end of the race] you have to slow down real quick.," analogized Manning.

But Spirit did slow down real quick, thanks in part to the transverse rockets, which corrected attitude. If the rockets hadn't fired, Manning said, "we would've wound up with east-bound trajectory. I wouldn't have said [it would have been] a disaster but it would have been -- more exciting. Fortunately, the little rockets fired and . . . we had a very easy landing." While Mars ever-changing atmosphere was not unexpected, "we were surprised by dynamics of winds."

The bundled spacecraft, they have determined, first bounced 27.6 feet (about 8.4 meters) high, then bounced 27 more times in a short 57 seconds before landing in its present position, around 270 to 330 yards (about 250 to 300 meters) southeast of its first impact.

"After debating this - what is bounce, roll - we came to conclude that 28 bounces is our official number of bounces," offered Manning. "The scuff marks [visible in images of] in Sleepy Hollow are bounces 25, 26, 27, 28 - and we bounced into position. It appears we really didn't roll. We were surprised the bouncing went that quickly, then again it was only 8 meters high," he rationalizes.

The EDL team is now taking all this data, Manning said, "and converting it into results we'll apply to Opportunity landing." While they haven't deemed any major changes to be necessary for the second rover's arrival, they are considering a number of "minor changes," Manning informed. Spirit's airbags, for example, were a little warm," so they are making adjustments with the generators for that. "And we are also making some changes in parameters - not based so much on specifically what we learned from Gusev Crater, but now that we have truth from Gusev it makes a difference in how we model the Meridiani site -- we can apply those numbers to assimilation," he explained.

Opportunity remains on track for landing on the opposite side of Mars, in Meridiani Planum, at 9:05 p.m. Pacific Standard Time on January 24, (on Jan. 25 Universal Time and EST).

Read more: Spirit, 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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