Twin Rovers, Parallel Parking
Pasadena, Opportunity mission Sol 10
The Opportunity rover entered a crater from the east bouncing like a volleyball making seam marks in fine sand. So the latest full color panorama of its Meridiani site shows in a 360 degree view of its current position inside a 20 meter bowl. Jeff Johnson, USGS Rover science team member, estimated that the crater’s cause may be rather tiny to begin with: "A meter or two in size could cause the 20 meter crater. It could have come from a much larger impact, and this may be a secondary crater. It depends alot on the strength of the materials".
|The forward driving direction at Meridiani is less than 30 feet to the outcrop. What portions of the horizon are visible beyond the crater rim is spectroscopically similar so far to how the soil appears inside the crater|
Johnson presented the ‘mission success’ panorama on Opportunity’s ninth day on the planet (Sol 9). The complete view gives hints about what might be beyond the crater, as if scanning out across the martian surface: "It gives you the feeling that you are there. In this bowl, the depression we are in."
"On the left are the airbag retraction marks. The expanse to the right of the outcrop [shown in the banner image] is showing different colors we’re still trying to figure out," said Johnson "We rolled around some inside the crater. You can see the volleyball bounce marks where the rover first entered the depression."
"[The soil] is darker than Gusev", Johnson said, referring to the much larger crater, called Gusev crater which ishome for the Spirit rover on the opposite side of the martian equator.
"The entire outcrop [at Meridiani] is actually very tiny… There is a prevailing wind direction where [Meridiani is] seen from orbit, so the outcrop (against the crater’s northside) may just be a portion of bedrock that wraps all around the crater. So we may be half-a-wheel depth away from bedrock elsewhere in the crater."
The science team refers to interesting soil depths as if measured in units of wheel radii, partly because each six-wheeled rover is capable of an experiment called ‘trenching’. Five of the rover wheels are locked in place, and the six wheel is able to dig a trench about 20 centimeters into the soil. The rover then crawls all wheels out of the hole it dug for itself, and is able to maneuver its instrument turret back around to view its own trench.
The System Engineer for this robotic arm, Joe Melko, indicated that preparations began today for just such complex maneuvers. "Today was about the Instrument Deployment Device. The arm is the most complicated mechanical piece of the rover, and it performed beautifully. We moved all the motors around, checked out the rock abrasion tool (RAT). Moved the turret around and imaged all the instruments by removing their dust covers. We spun the turret again to look at the x-ray spectrometer [APXS]. Tonight we’re taking microscopic images of the soil. We’ll be putting the Mossbauer spectrometer down on the soil. Then we’re going to integrate [using the Mossbauer in place] for nearly 24 hours. The next day we’ll spend with APXS. At that point, we’ll make our decisions about what next."
The same engineering routine was carried out about two weeks earlier, when Spirit swiveled its own robotic arm and instruments into the pancam’s field of view. As was the case with Spirit, Opportunity’s first pictures of its four-instrument turret checked out.
In addition to this kind of remote self-diagnosis, the panorama serves many key roles for both the science and engineering team. It helps locate the rover relative to all sides, and will become increasingly used during some driving exercises. "Pancam is actually two cameras, to provide us with stereo coverage," said Johnson. "On each camera is a filter wheel. The navcams are right next to them. The pancam lenses are only about the size of your hand. Each pancam has 7 positions, with the option of 14 [color] filters to image with. Pancam is sensitive from blue, to out beyond the reds your eye can’t see."
"The dirt is red. How its brightness varies in each filter gives us the mineralogy, the geology of the surface we’re seeing," said Johnson. "Our main goal is to unravel the geological history of the site. So what does Mars look like in all these filters? The color calibration target, or sundial [has] color chips, each of which has lab spectra taken on earth. With this calibration, the Mars spectra will give us a very good picture of the true color."
Two Traveling Salemen Meet on Mars…
"In Meridiani, we extracted a few spectra," explained Johnson. "The [area] just beyond the outcrop and the [regions] on the horizon are likely the same material. ..A small cobble is visible of very dark material, which may be ejecta from the crater. [The foreground and background] is definitely looking the same to pancam. Its thermal sensitivity may be penetrating to a common deeper layer. We may be looking later in the mission to get out of the crater. There is alot more atmosphere that distorts the picture to the horizon. That was a great surprise when we drove off the landing platform, first to stand up and see the horizon by popping our head out."
Johnson compared the next stages of the science planning to a classic problem in optimization. "As of yesterday, the science team started thinking about how to attack the outcrop. We need to map it completely with all the tools we have, and those [tools] are considerable. It is a bit of a traveling salesman problem, to do the most science in the most efficient way." The traveling salesman problem Johnson referred to involves how to a salesman might have many cities to visit, and not wanting to double up over the same territory, plots a driving course that covers the most ground with the least total travel.
"There is a debate about starting at one side (left) and working down the edge," pointed out Johnson on the full panorama now available. "We’re figuring out how far out we have to be that the pancam and mini-TES can see the outcrop so we don’t hit our own solar panel."
Along that outcrop, said Johson, the science instruments may dissect the bedrock by moving down its length: "It is like parallel parking."
For Mars’ second traveling salesman — the Spirit rover–the forward course looks different than Opportunity. Jennifer Trosper, Mission Manager, confirmed that "Today we are doing Spirit science the same way we were about 11 days ago. We confirmed our suspicions about the first problem, as the number of files, and the amount of [computer] RAM [memory] needed to manage those files. The only residual concern, when we entered into that anomaly, we may have had side-effects we don’t fully understand. So we will reformat the flash later in the day."
|Top vertical slice through bedrock at Meridiani shows evidence for layering; Spirit’s Adirondack rock, bottom, an early science target|
"Today we are doing science on the Adirondack rock we started on," said Tropser. "We will put the microscopic imager, then use the RAT to clear any dust that may have accumulated… We have two operational rovers on the surface again."
Unlike Opportunity, which may stay put in its crater for some time, the science team is discussing Spirit as "the driving mission," according to Trosper, "so we are strategizing where to drive fast and furious…The long term mission from the science team, we will be driving towards the crater, which is quite a ways, so we will be looking to drive to that quickly.
Trosper said the first driving exercising will tell engineers much about the terrain and road conditions: "The one-way light team is 11 minutes from Earth to Mars, so the rover has a designated traverse and turn off the hazard avoidance in case it gets off track. As we get into terrain that is more difficult, we may want to turn on more of our hazard avoidance software. JPL has a Mars’ yard where we are testing, but it can’t simulate some of the [martian] shadows and terrain. The distance will start with 15 meters or so a day, and then increase gradually as we get more comfortable with this terrain. "
The specific problem that setback Spirit for a little more than a week and half may be common to Opportunity as its timeline gets closer to 18 days on Mars. The software problem, according to Trosper, was "the resource you need to manage the files is not just the amount of flash. What we have done with Opportunity is we are downlinking how much available RAM we are using with the software [and file manager]. Now that we understand what got us into the problem, the memo [to a future Mars mission] says: ‘keep your file number low, if you ever get above this number, [you] need to stop doing science and do something to reduce that number’."
When asked to speculate on the surface lifetime of both rovers, Trosper said the limits are both seasonal and weather related. "The dust accumulation on the solar panels is something that will take lifetime out of the mission. The design lifetime of 90-plus days is based on the Pathfinder dust deposit rate. When it starts getting colder, we need more survival heating at night. It doesn’t limit the life, we just need to save more energy to heat the electronics box and mini-TES, which is actually more exposed [to cold]."
"The dust deposition and the temperatures [those are the limits] over the course of time."