NASA Mars Visualization
Alliance
Meeting 3
Summary
May 14, 2002
10:30 a.m. CDT
Denver, Maryland, and MarsQuest (Space Science Institute) have been added to the Alliance.
Dr. Matt Golombek of JPL discussed the landing site selection process for MER:
Landing site selection is a very interesting mix of engineering and science. It involves the real world of engineering, which makes it incredibly un-academic, yet it involves scientists that basically are academic. Of course, landing a spacecraft on a planet is not; it’s real. That reality gives it a bit of a tinge. In addition, it’s changing activity that involves two parts, really, that go parallel and interact in a very interesting dynamic way. One is understanding about Mars and understanding what the surface is like and what the surface characteristics are like and what things are good for a particular kind of lander and what kind of things are bad. The other involves the actual building of the spacecraft and understanding how that spacecraft comes down and interacts with the surface in the landing situation.
Typically, while you’re building the spacecraft you’re also testing it. During those testing activities you typically will learn things that may change your view of what a safe landing site will be. We’ve had a very clear indication of that in the Mars Exploration Rover landing site selection activity recently, where Monte Carlo simulations and testing of airbags have shown a susceptibility to horizontal winds that were really not even recognized prior to the activity, prior to the first two years of the activity, which have thrown an enormous monkey wrench into the activity and had us scurrying around looking for so-called safe sites.
So the first thing, the way you get your arms around this gangly problem is you go to something you know and that’s, of course, that’s the engineering. Basically, what you do is you look at the spacecraft and you evaluate, in detail, how it comes down to the surface and what it does during that process. That means you have to talk to your friends, the engineers, who, of course, are designing that spacecraft. From Mars there are some fairly straightforward first-order things that really determine about 90% of the terrain that you can start searching for for a landing site.
The first is that the mass of the spacecraft and the ballistic co-efficient on the aeroshell effectively determine the elevation that you can land at because typically you slow yourself to first order, using a parachute. All spacecraft to Mars, or all landers, so far have used Viking Supersonic parachutes called Disk Gap Band parachutes, which pop out at supersonic speeds and are very efficient at the very high speeds, but much less efficient at the lower elevation.
Regardless of that, you need a certain minimum amount of time on that parachute to bring the spacecraft to a correct terminal velocity. The amount of time is related to the density of the atmosphere so you need to have a certain altitude, if you will, an elevation above the surface that you must be below or you won’t have enough atmosphere to slow yourself down.
That’s made even more interesting on Mars because the pressure of the atmosphere fluctuates by about 30% over the annual cycle, which actually changes that elevation bogie by about two to three kilometers, depending upon which portion of the year you come in. Actually, MER’s doing okay. They’re moderately lucky for the initial studies for the ’09 lander. We come in at the minimum pressure cycle, which is a really difficult time to use a parachute.
Then, of course, this spacecraft is slowed by a parachute. Typically, it senses the surface in some way to determine the closing velocity. Typically, that’s been a radar altimeter and these are fairly robust beasties that are used quite a bit here on the earth. They’re generally off-the-shelf kind of things, but they typically simply measure the closing velocity and when you get to a certain altitude above the surface, they’ll trigger a number of events.
Well, that radar altimeter obviously has to see the surface and there are some very interesting places on Mars where radar waves go in but don’t come out. That wouldn’t be a good place to target one of your landers, because it’d be looking for the surface and it’d never see it and, of course, that’s a bad thing.
So radar reflectivity becomes important; that’s sort of obvious, and then how the lander actually contacts the ground becomes perhaps the most interesting and dynamic of the whole suite because it’s sensitive to slopes and rocks and too much dust and a whole suite of things that could affect the way this thing lands or sinks, and what it’s susceptible to in terms of safety.
So you have to go through and evaluate each one of these against what’s really very imperfect knowledge, based on a whole wide variety of remote sensing data, that you’re really inferring, if you will. You’re inferring, to the best of your ability, what that surface is really like.
That’s the crux of the problem. You have very imperfect knowledge and yet the stakes couldn’t be higher. You have a $400 million spacecraft that you’re responsible for, making sure it lands safely. Everybody wants it to land safely. Nobody wins if it doesn’t succeed. There’s no science; there’s no nothing.
You have to be safe. That’s job number one; to make sure, to the best of your ability, that that surface is safe, even though your information is incredibly imperfect and the uncertainties imbedded in that information are very huge, so there’s this comfort and risk level. The space program, of course, has, historically, very different ways of dealing with risk, which can affect your landing site selection in a very major way.
MER is solar powered so to get maximum power it needs to be near the sub-solar latitude at the time of arrival and for the nominal mission, between ten to fifteen degrees north to about fifteen degrees south of the Martian equator. All of Mars has just been winnowed down to this thin little equatorial strip, which always happens for solar-powered spacecraft. Really, it kind of bugs the heck out of you. You want to explore all of Mars and you wind up going to the same little equatorial strip over and over..
A certain bogie elevation also needs to be considered to give the spacecraft enough atmosphere to properly slow itself down. The landing system is similar to Pathfinder. That’s at -1.3 kilometers with respect to the MOLA Geoid, which is roughly 0 kilometers for the old USGS geoid that used to exist but was very poorly known. This eliminates about 90% of the planet for possible landing.
So then, what else is important? Obviously you don’t want it to be too rocky. The airbags are qualified to a certain height rock with certain characteristics. If you have too many rocks on the surface you could poke a hole in them or actually bottom out through the airbags as this thing comes down. It’s also a solar-powered spacecraft and there are locations on Mars where the thermophysical properties, the temperature, indicates virtually no hard surface, just what I like to call “foo-foo dust;” huge, regions, in fact, that look like they’re… areas for this fine dust that’s often entrained in the atmosphere. A Viking 1 footpad actually disappeared out of sight, just sank right through this stuff; it was not load bearing. This is not a good place to send a solar-powered spacecraft-- So that wipes out another few percent of the surface, based on the thermal inertia measurements that we have.
Then, slopes. Clearly, if you land on legs and your slopes are over the angle or opposed, there’s some stability as this thing hits the ground and hops or bounces. In the case of airbags, slopes can accelerate the role of the airbags as they bounce and roll across the surface. There are a number of slope requirements that MER has set as well. That, to first order, gives you a hunting license because we know enough about Mars’ elevation and thermophysical properties to kind of rule out areas. We did that for the Mars Exploration Rovers.
We took the size of the ellipse, which is anywhere from 100 to 200 kilometers long, depending upon latitude. We look for the smoothest, flattest, most boring places that you could find to put those ellipses in. That’s the requirement: smooth, flat, and boring, because you don’t want any slopes. You don’t want any scarps; you don’t want too many rocks; you don’t want too much dust.
You go through various remote sensing data. You infer from them the properties that are acceptable and not. You draw a map of where you can look. Then you try to place these ellipses, which are given to you by the engineers that design and simulate the landing, and you try to place ellipses in large, smooth, flat areas. That’s the job.
We did that for MER. To our surprise it looks like the number of landing sites that you can consider initially scales with the size of the ellipse. For Pathfinder our ellipse was 300 kilometers by 100 kilometers and there were about 10 places that we could look that met all of those criteria. For Beagle, interestingly enough, the flight path angle was so poorly constrained where it enters the atmosphere, that the ellipse is 500 kilometers long. Given their latitude constraint, there’s only one place that you can land that thing. So landing site selection for Beagle was actually fairly simple.
MER, I am happy to report, had more landing sites to look at. Because the ellipse was down to 100 kilometers toward the southern latitude mark, we could fit in almost 200 landing sites for the two different rovers . Then the question is, okay, how do you triage them? You can’t possibly evaluate the detailed characteristics of all of those landing sites. You need some way to kind of narrow them down and what better way than to do the triage based on science?
So that’s what we did, In a sense, what you learn about Mars is critically dependent upon the material that’s available for you to look at after you come down on the surface. So to first order, you want the full expertise of the entire science community. Anybody who has a great idea, you want to hear it.
NASA has set up a formal outside committee science steering group made up of interested outside parties. The steering committee is led by me and John Brandt at the National Air and Space Museum to watch over the process and make sure that we’re doing everything we can to get all the information we can and to get the best science we can.
Out of 185 sites, we asked the community, “Tell us what are the great science sites among these 185; what are your favorites and why?” About 30 sites were recommended strongly by the science community. There was a real distribution. Some sites received multiple, multiple positive votes. One of them is the so-called hematite site, which is an area where the themass spectra show a grey coarse-grained hematite for which most formation models suggest that it formed either by precipitation from liquid water or by hydrothermal duration. You match the characteristics of that landing site to the science objectives of the mission and what the instruments on that spacecraft are capable of measuring and telling you.
One way that we scientists like to think about things is, how many testable hypotheses are there? You dream right down to the instrument measurement level, what will you measure with a thermal emission spectrometer at the surface that’s going to tell you something fundamentally important and what are the implications for that. The science objectives for the MER missions are to land at a place where water has been involved, to look for aqueous activity in the past dominantly, to try to find a place where biotic or pre-biotic activity could have occurred, and to try to learn something about the environment on Mars when that activity was going on. These objectives are directly relevant to the overarching goal of Mars exploration, which is follow the water and look for whether or not life could have ever started there. The first way you do that is by trying to understand what the environment was. Was liquid water stable at that time? Could life have even had a chance to have started there?
Given that, the hematite site’s obviously a pretty good-looking spot, because there’s this specific chemical or mineralogic information from orbit that’s lighting up like this red thumb saying, “Land here, there’s water.” So that’s a pretty good-looking spot and there are a variety of others as well.
We went through three landing site workshops to which we invited anybody and everybody [in the science community] to come and tell us what they think about great places on Mars to land and why they’re so compelling. We had a series of interactions during those workshops to actually have people, in some cases, vote for their favorite sites and in other cases, at the last workshop, evaluate the detailed characteristics of each of the sites with respect to things like safety but also in terms of science. What will you be testing; what will you potentially learn; why is it so important?
At the most recent, the third landing site workshop, there was actually a table that we produced; greens, yellows, and reds. It’s the Consumer Reports guide to MER landing sites. A lot of greens is good and a lot of reds is bad. We evaluated six landing sites that had made our short list and, in fact, wound up actually getting rid of getting some sites because of sensitivity to horizontal winds during touchdown.
The landing system has no way to compensate at present for horizontal winds. There was a system put on to try to take out wind shear at some level and they’re considering a system to try to take out the horizontal velocity. The landing system does not have chemical retrorockets that can fire and turn and then land and counteract a wind easily. Whatever the horizontal wind is, the parachute feels. Whatever the parachute feels, the lander feels. That’s exacerbated by a three-body system of the parachute over a back shell over a lander that is oscillating.
The evidence for Pathfinder is that oscillation actually imparted a horizontal velocity at final bridle cut, which allowed us to traverse, even in very mild winds, over a kilometer at one of the quietest times of the day, about 3:00 a.m. We believe our reconstructions have shown that we bounced about 1.2 kilometers across the surface. The whole point is to use the airbags to take out kinetic energy, not to add to it, so you want to try to go to a place where the winds are low and you’re going to bounce less, and you’ll have lower total velocity at impact. The higher your velocity, the more stroke the airbags have to endure, and the more they’re susceptible to damage by rocks and other things.
Winds are looking like a big deal, where we didn’t know about them being a big deal before. This factor looks like it’s going make three of the six landing sites potentially too hazardous to accept. One of the landing sites, it turns out, has very high radar return, which implies a very rough surface. It’s on the scale of the Rover wheels. That’s making that one look dubious and we’re basically down to 2.5 to 3 sites. Generally you like to have enough sites to make a selection and a still have a choice. You don’t want to get backed into a corner.
We’ve asked the atmospheric scientists to tell us the places where the winds are particularly low, to try to find more than two landing sites that would be acceptable and hopefully scientifically compelling.
That’s where we are in the process. We have about a year from now to select the landing site. The MERs launch in June ’03. We need to know where they’re going to go a couple months before then. It needs to get evaluated at a variety of levels. There’s a series of peer reviews. We want to make sure nobody’s making some grave error. Then NASA headquarters actually reserves the right to select the site, which involves a presentation of where you want to go and what your recommendation is. That would all happen in the March timeframe.
We’re looking at a final landing site workshop in the January timeframe, a series of workshops to evaluate the [Mars Odyssey] THEMIS data, which is radically changing our view of the surface properties of Mars, and to try to use that data to infer a lot about the surface characteristics at these specific sites. These sites are all being imaged by MOC, the high resolution camera on Mars Global Surveyor. Some of the sites have significant coverage from MOC.
THEMIS has about 18-meter per-pixel visible images and about 100-meter per-pixel thermal images. Those are all turning out to be incredibly interesting, in terms of trying to understand the surface properties. There’s a series of smaller-scale workshops that will evaluate that data in detail, leading up to that January workshop. Within a month or two after that, the project will select its two favorite sites and have all the rationale for that selection and then take it forward to NASA.
Question: I have a quick question based on your obvious keen interest in the science. How do you lead? What kind of machinations do you go through when you’re trying to lead the rest of the scientists on selecting a team, based on their priorities, and you’re anxious to say your own piece as well?
Matt: First of all, the process has enough gives and takes and groups involved that you can’t get away with partisan interest for very long. There are a lot of very smart people looking over your shoulder at what you’re doing. At four or five different levels there are checks and balances to the process. Let me just take a minute. There’s this outside group that we talked about that are people at universities in various places that have knowledge about the surface. There’s the project and the Project Science Group led by Dr. Joy Crisp of JPL and the Athena Science Team, which has a very specific interest in scientific goals in this.
I am actually the program level. I’m in the Mars Exploration program to facilitate the taking of images by other spacecraft. If you think about it, this is really the first time that we’ve had one spacecraft available to actually help another spacecraft. There’s very interesting dialog and negotiations that occur where you’re requesting, for example, the Mars Global Surveyor spacecraft to roll to take an image of a landing site, as well target locations for spacecraft. So we’re interacting with those science teams as well.
There’s a program level; there’s the project level; there’s the overall community. I guess you can think of them like the three branches of government; there are checks and balances. We have to be on our best behavior.
Partisan science: as much as you’d really like to go to some place that would be totally cool, you have to bite your tongue and do the right thing because, as I said, if the thing doesn’t land safely, nobody wins. There’s nobody who wins if the thing fails. It’s a very humbling activity. It’s not politics.
One example of of a very cool site might be the Valles Chasma site, which is at the bottom of the Valles Marineris. It has ten-kilometer cliffs to look at. It has what looks like layered sedimentary rocks at the bottom. Everybody’s salivating to go there, yet the winds are almost certainly too high to hazard.
Question: Is the hematite site still in the running?
Matt The hematite site is one of the favorites, although I’m not supposed to say that. It would be hard for me to imagine us not sending at least one of the spacecraft there for two reasons. One is that it has a compelling science rationale. The other is it’s incredibly smooth and flat and boring. At the scale of the ellipse you can move an ellipse in a whole number of places that are squarely on that hematite material and you have a high chance of getting that hematite material, whatever it is. It’s clean; it’s not a dusty location as far as we can tell, and the slopes are about as smooth and low as any place in the equatorial region so it has a series of characteristics that make it very, very positive.
Question: What strength of the winds is a danger point? Is there a cutoff?
Matt Yes. There are two kinds of winds; there are horizontal winds and then there’s a wind shear, which is a change in velocity
Joy Crisp: There’s a limit on the horizontal velocity that you hit the surface with. If you hit a slope, it’s a different wind speed that will hurt you. It’s on the order of 15 meters per second, but it depends on the slopes, so if you have a landing site that has a lot of slopes or steeper slopes then that number changes somewhat. They actually need to model the terrain that we’re going to, the actual slope distribution, to find out whether a site is safe or not.
Matt Obviously, a higher velocity on a very rocky area would conspire against you as well. Monte Carlo simulations try to take all of these factors into account. You might be able to accept one thing that might not be optimal, say a slightly rocky site, but you’d better be sure the winds are low. Or you might be able to accept a site that has low winds and some slopes, but not too many rocks. There’s still quite a bit of work by the engineers to understand the sensitivity of those various features.
Question: What’s the difference between wind shear and overall wind velocity?
Matt If there were a constant horizontal velocity, the system would still be effectively vertical, but traveling at whatever horizontal velocity is. The parachute, the back shell, and the lander are separated by the cords of the parachute and the bridle. The lander is hanging below the back shell. The rockets are on the back shell.
If there’s a wind shear, there will be an oscillation. It’s a three-body problem, effectively, where the back shell may not be aligned with vertical or the lander or the parachute. That actually imparts a horizontal velocity when you fire the rockets and cut the tether. You want to minimize that. Joy, correct me; there is a system to try to take out that wind shear.
Joy Yes, it takes out some of it.
Matt Some of the wind shear. They’ve added a few extra little rockets to try to take out some of that oscillation, which, of course, Pathfinder didn’t bother with.
Joy We actually run models of the winds at these different sites and then the engineers take the predicted average wind speed and run the system through the vertical profile of winds to find out if it’s a problem or not.
Comment: This actually was a fascinating discussion because I would have thought there was much more science, you know, scientific questions that go into this. It seems like there’s just so many other factors.
Matt Yes. In fact, I think the unfortunate situation, and maybe this has to do with risk as well, you have one spacecraft, you have two spacecraft. They cost hundreds of millions of dollars. I’m sorry; I don’t gamble. Who the heck wants to gamble with an investment like that? And not just money, but peoples’ careers. None of you guys would know who I was if Pathfinder had cratered. That’s six to eight years of your life. I didn’t do much else during that time. I can tell you, Joy’s not doing much else during her time. It’s not just her, but 100 or 200 people who are living, eating, and breathing this spacecraft.
If you had fifteen of these things, or twenty of them, I can see sending one to the bottom of Valles Marineris, where you might have a lower probability of success. I could see sending one to the foo-foo dust to see what it really is like. I could see doing a whole bunch of things. It’s just like investing; there’s some high risk, potentially high payoff, but the core of your investment is in safe stuff. There aren’t enough statistics to help you out. It’s all about safety. Science comes in at the very end.
That’s not going to change until we either get a lot of spacecraft or the attitude about the landing changes and maybe even the attitude about the exploration. That’s something you can have sort of a philosophical problem with because exploring is about understanding what you don’t know. If you try to make it as safe as you possibly can, you almost wipe that out. It almost becomes like human flight program, where safety is the absolute first thing and everything else doesn’t matter. So you’re not able to take chances.
Question: Can you compare this process with what you guys went through on Pathfinder?
Matt Yes. Even the Polar Lander, they’ve all been dramatically different processes. They’ve been different processes not only because of the personality of the project; each project has this personality that’s set at the very beginning by a handful of people that start it up and get it going; but also, perhaps much more dominantly, by the information that’s available at the time that you’re doing the activity. So for Pathfinder, it had been twenty years since we had gotten any information about Mars. We were dealing with 20-year-old Viking data and oh, we did get to bounce a few radar waves off the surface from the earth and that helped a lot.
We had virtually no information. We had thirty of the highest resolution images on the Pathfinder landing site; they were about 40 meters per pixel. The smallest thing you could see was the size of a football stadium. The thing you were worried about were rocks the size of the meter. How do you do that? How do you go from forty meters per pixel to a meter scale topography? Of course, you guess. You do the best you can with the models of information that you have and you try to eke out everything you can. One of the things you do is you write down a list of data in a data table that infer the properties that are important for the lander.
So Pathfinder landing site selection was one full-time equivalent job for two years because that’s all the data there was. We used the science community in that. We were incredibly fortunate to have the surface turn out to be pretty much the way we predicted it would be. Now, fast forward to MER and we have a fire hose of information coming in from Mars Global Surveyor and an even bigger fire hose that’s just starting with Odyssey and we need an army to even look at the information that’s relevant and try to infer something meaningful for the MER project.
There’s probably three full-time equivalents for three years working on this stuff, not even counting all the outside work and the work that’s buried in the project at Joy’s level and the Athena Science team. Now fast forward to what it’s going to be like for ’09, where we will have had these two fire hoses and now we’re going to get the biggest darn fire truck, this catastrophic flood of information from the 2005 Mars Reconnaissance Orbiter, that’s going to dwarf all the other data sets coming in. During the time that the selection is made, we’re going to need an army to look at all this information.
Question: This is just really fascinating because of the process you go through. Is this captured anywhere on any of your Web pages?
Matt There are two Web sites linked from the MUSE site that have quite a bit of information. It includes presentations at the various workshops. It includes our original map of 185 sites. It includes a lot of the memos that have led to the various decision points during the process and what happened when and why. It’s not all written down coherently. There are papers that describe the Viking landing site selection process and also a paper that I wrote in 1997 that describes the Pathfinder landing site selection process. Presumably, when we get to an actual site or two that we have decided on, I’ll write a paper that tries to summarize this whole process as well.
MUSE Website:
The MUSE page is up and those that are unable to access it need to provide Anita Sohus of JPL with their IP addresses. The page is a work in progress that works under Netscape and will work under Internet Explorer in the future. Other interesting links are being added. Stephanie Lievense of JPL is working on a meaningful calendar of events to be posted on the MUSE page for Alliance members. The Mars Public Engagement Plan that Michelle Viotti put together is linked from this page also.
The heart of the page is the Image Product Support page. There is a set of pull-down menus that allow the user to either select or replay a variety of different products The three important items are Action, Days, and Submit. The page’s instrument menu has a selection of cameras that are located on different places on the rover and are used for different purposes in terms of navigating the rover through the Martian terrain. The page allows the user to use these different cameras to create larger mosaics. EDR, Experimental Data Record, is the starting point for image products. The EDRs are stitched together to produce the larger mosaics to create the panoramas that allow the scientists to put together the scene around the rover. If you go to the Action section you can either select the products that have been created or you can replay. Replay is limited to the experimental data records, the single images that come back from the different cameras. If you wanted to simulate what it was like here at JPL to see the data coming back from the rover, you would select Replay and you could replay all of the images that came back from a particular day in the life of the rover on the Martian surface.
Event Scope is a NASA-funded project headed by Peter Copen at Carnegie Mellon University . It is not a web page itself, but an interface that allows the user to play around with 3D images. It is a Windows program, better run locally than from a server. It places the user in an actual 3D world with craters, depth, and detail. The topographical data has been included so that the user has a good sense of what it really looks like on Mars. A link to Event Scope isalso available through the MUSE page.A good overview is to look at the View Demo link. It’s another interface to view all the images and data sets that are available. Primarily, it is an educational tool for students, but it also contains some very interesting VRML technology that may prove useful to everyone, in terms of getting a better grasp of all the data available. There has been some discussion about integrating their technology with the data sets.
You can move your point of view over the data set and look at it from different heights and different positions.
We do have other areas where we’re working on making VRML 3D models of surfaces on the planet. Also, we are working somewhat with taking the actual data that we’re collecting on the ground during the mission operations and making VRML-type models of that. So if the kinds of models that you’re seeing in Event Scope are something that look like they would be interesting to you, there are more of those that we can probably generate.
It’s our intention to not only provide original data sets, but we are making combined sets for VRML purposes, and making them standard so that hopefully not just Event Scope, but a bunch of other viewers, can view this data in a standard VRML format. We’ll prepare the VRML data sets so they’ll be ready for primetime viewing by anybody, in other tools.
We’re actually talking with USGS and other people about what the best formats for commonality are and asking them then to produce data sets in the same format so that it’s not just us here, but a wider set of the community that will be producing model data with elevation and images in a standardize format for general use by viewers.