MSL: Landing site downselections
Posted by Emily Lakdawalla
2007/10/29 02:19 CDT
My blog entries have been kind of long and exhausting of late. For those of you who prefer them short and sweet, with pretty pictures, my apologies; I'll return to more of that style this week, I promise.
After three days of presentations, voting, and extended discussions, the "Mars community," as represented by something over 100 scientists who decided to attend the second Mars Science Laboratory (MSL) landing site selection meeting in a process that was open to all, have narrowed down to six the number of potential MSL landing sites. A few others were relegated to a sort of "wait list," standing by in case engineering constraints force too many of the top choices from the list.
The six potential sites are:Nili Fossae Trough (22°N, 75°E, -0.6 km elevation). A so-called "phyllosilicate"site, Nili Fossae Trough is a tectonic feature, a fracture concentric to the ancient Isidis impact basin to its southeast. Following the formation of the trough, it was eroded and partially infilled by sediments and then paved with volcanic rocks from Syrtis Major. A smaller (65-kilometer-diameter) crater outside the trough tossed phyllosilicate-bearing ejecta on top of this. MSL would land on the trough floor, on top of the volcanics and phyllosilicate-bearing ejecta, then travel northwest toward the trough wall, where ancient phyllosilicate-rich Noachian-age rocks are exposed. After this first phase, it would drive up into a "Monument Valley"-type terrain of flat erosional basin with carved mesas to the northwest, exploring more mineralogically diverse, ancient rocks, and seeing some truly spectacular vistas.
Sites in "purgatory" include Eberswalde, Northeast Syrtis, some interesting "chloride" sites, and a site called East Meridiani. You can look these up, if you're interested, on the workshop page, CRISM page, and THEMIS page.
Each of these sites is scientifically compelling, but they all present problems, and it may even happen that MSL won't be sent to any of them. There is a host of potential problems facing each one of these sites.
- Latitude. A number of factors influences the latitude within which MSL can land and operate. During landing, MSL must be able to communicate with Mars Reconnaissance Orbiter (MRO); for this to occur, it must land between about 30 degrees south and 30 degrees north. Furthermore, in order to guarantee that MRO will be in position to hear MSL during landing, the launch team has to know before MSL launches which one of two latitude bands the lander is headed for: (1) 30°N to 15°S and (2) 15°S to 30°S. Once MSL lands, latitude also has a strong effect on how much it can operate throughout the year -- this is a new problem that arose because of the failure to develop low-temperature actuators to run the rover's wheels, arm, and mast. Because southern winters are longer and more severe than northern winters, in the southern of these two latitude bands, MSL may only be warm enough to be able to operate about 50 to 70 percent of the time that it would operate at more northern latitudes.
To be honest, I was amazed that, given this operational constraint, the scientists and engineers were willing to even consider sites in the southern latitude band. Why build a capable rover and then fly it to a place where you can only operate it half the time? It seemed crazy to me and I think a bunch of the scientists in the room had the same feeling. But when a vote was taken on this very topic -- should we even consider southern latitude sites? -- the room was split almost exactly in half. And then mission managers stepped in and advised the scientists against tossing out southern latitude sites, saying that if there were no southern sites in the mix, the mission simply wouldn't investigate whether southern sites were possible, and they didn't want to narrow the choices so much just yet. So Holden and Terby remain in the list, as the two representatives of the southern latitude band. It occurred to me that maybe they could save money by not exploring southern latitude sites, perhaps enough money to return some of the descoped science instruments. Someone in the room asked this very question, but the mission managers hedged. Anyway, to return to the list of engineering constraints:
- Elevation. MSL's landing system is supposed to be more capable than previous ones, allowing landing sites with elevations of up to 1 kilometer above the mean elevation measured by MOLA, the Mars Orbiter Laser Altimeter. However, the engineers emphasized that it is significantly riskier to land at higher than lower elevations. In fact, for any given landing site, the engineers want a "safe haven" site designated within the same latitude band at an elevation below 1 kilometer below the Martian mean, plus an "uuml;ber safe haven" site with an elevation below 2 kilometers below the mean. As it turned out, all but one of the six chosen sites is "safe" with regard to elevation.
- Landing ellipse. Because of uncertainty about the direction and speed of winds during the parachute-assisted descent, the following requirements need to be satisfied for an ellipse that is 25 kilometers long in the down-track direction and 20 kilometers wide in the cross-track direction. "Safe" sites have an ellipse that is 7 kilometers larger in each direction, 32 by 27 kilometers.
- Slope and relief in terrain within landing ellipse. These requirements are complex, having to do with how much elevation change there is within the landing ellipse, measured along many different-length baselines. None of the sites satisfies all of the engineers' slope requirements, but apparently these requirements are a bit "squishy" -- they can be relaxed a bit if other risk factors are benign, especially if the elevation is relatively low, and if the slope requirements are only exceeded in small portions and/or at the edges of the ellipse. "Safe haven" sites must satisfy more stringent requirements.
- Rock abundance within the landing ellipse. Historically, rock abundance for landing sites could only be guessed at based upon thermal characteristics of the site, but with the advent of HiRISE, we can now see from space whether a landscape contains many small rocks, not visible to previous orbiters, that would be large enough to damage the rover upon landing. Future HiRISE imaging will likely make or break several of these landing sites on the basis of how many rover-killing rocks are present. Again, "safe haven" sites must satisfy more stringent requirements.
- Trafficability. Once landed, the rover has to be able to get around to interesting science. This mainly translates to a requirement of a relatively dust-free landing site. There is also some concern about interesting rocks located outside the landing ellipse; canyon walls may be interesting, but how far up them will the rover actually be able to drive?
All along, MSL has been touted for its capability to land in places that the Mars Exploration Rovers could not hope to reach -- inside a canyon, maybe, or within a steep-walled crater; the cameras would gaze upon dramatic terrain like that found in America's desert southwest. Once landed, MSL is supposed to be able to rove 10, maybe even 20 kilometers or more over its lifetime, so it could potentially land in a less-interesting place and drive into rugged, rocky exposures of ancient rocks, places that will also produce spectacular landscape photos. Each scientist proposing a site selected his or her site with this capability in mind.
But as MSL's development has progressed, the engineers have been getting more conservative. At this workshop, we heard how it could not tolerate low temperatures. We heard that "go-to" sites -- where the interesting stuff lies at the end of a long initial traverse -- may be considered too risky. We heard that MSL will probably be able to drive no more each day than Spirit and Opportunity could, limited by battery power to 100 to 150 meters at most each day. We heard that the landing ellipse is a little larger than previously thought, making some of the proposed sites too tight a squeeze into narrow spaces; and that the site may be required, late in the game, possibly even after launch, to have a much larger safe landing ellipse, forcing the mission to consider sites that are off of this list of six.
It was emphasized repeatedly at the workshop that there is only one proposed site that we are sure is "safe" by every definition, one called "North Meridiani." Located only a few hundred kilometers from where Opportunity landed (at 1.5°N, 2.6°W, and an elevation of -1.5 kilometers), the North Meridiani landing site would look quite familiar to fans of Opportunity's mission. It was proposed by Ken Edgett and Mike Malin as the answer to the question: "What if all [selected high-priority science sites] failed the various tests and we couldn't land at any of them. Where would we suggest to land MsL?" (Note that the "MsL" here is intentional, not a typo; since the descoping of several of MSL's instruments including Malin's MARDI camera and the zoom capability from Malin's Mastcam, several scientists have taken to calling Mars Science Laboratory "MsL," in protest of the reduction of its science capability.) At North Meridiani, MSL would begin at the bottom of the seven-meter-thick section of rocks explored by Opportunity and go much deeper into older rocks including ones that don't contain hematite but do contain phyllosilicates. It wouldn't be a terrible place to end up by any means, but it wouldn't be what they promised MSL could explore. Here's the presentation (PDF, 40 MB), Google map, CRISM page, and THEMIS page.
Conservatism is undertstandable with a $1.6-billion, one-shot rover. I want this mission to succeed, and if that means landing it in a less scientifically interesting place, well, as long as it can still meet its science goals to investigate past habitable environments, I guess there's no other choice. I just wish they hadn't promised so much in the beginning. Our close investigations of places we thought we would be able to send this rover have whetted our appetites for a meal we may not get to enjoy this time.
I suppose there's one good thing coming of this process. At the beginning of the meeting, Alfred McEwen (principal investigator on the sharp-eyed HiRISE camera on Mars Reconnaissance Orbiter) and his coworkers wallpapered the conference room and hallway with glorious high-resolution images of many of the proposed landing sites, many of them in 3D. He was rightfully proud of those images, and commented to me offhandedly that it was much more fun to be investigating potential MSL landing sites than it had been to map the flat, repetitive, and nearly featureless landing sites for Phoenix. No matter what, at least five of these six high-priority potential landing sites will not be visited by MSL, as the rover can only go to one landing site. But the process of preparing for a potential landing in these six places will focus the efforts of the orbiters to perform highly detailed study of six very scientifically interesting sites on Mars. Not only are they interesting, they're diverse; as part of the downselection process, the workshop attendees considered similar sites (for instance, sites in layered deposits within craters) and selected only the most compelling of each.
For each of these sites, they're going to get HiRISE images, maximum-resolution CRISM spectral maps, and they're going to develop highly detailed digital terrain models. It won't be as good as sending a rover, but it's hard to imagine doing much better from space. In the end, the MSL mission will only go to one spot -- or perhaps none of these, being forced to a safer spot -- but by preparing for the MSL mission, the orbiters will have produced fantastic data sets on many more exciting places on Mars.
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