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Michael Sims MER Journal: Sol 24January 27, 2003 Today's topics for my journal:Sharing this journal Where are we?
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Can you share my MER Journal with others? Absolutely! A number of my friends have asked about sharing it with their families and I'm quite flattered by that. My sister, Susan, said she was sharing it with her children's teachers! Boy, for me that makes all of the effort worthwhile. On the other hand, I'll bet some of my friends out there cringe at someone telling me that, because it just might encourage me to keep writing! Where are we on Mars? [This was written a while ago and I'll bring it up-to-date at the end.] We want to know specifically where the rovers are on Mars for two reasons: it is interesting scientifically to know the location, and knowing where we are is vital input into what to do next. The Viking and Pathfinder missions were different. For those missions everything we examined was in easy view from where we landed. So, for those missions knowing where we landed was important for putting our science discoveries into context, but it was not important in determining what we did. However, for the MER missions knowing where we landed is vital in determining what we will do after we leave our initial landing location. Specifically, by looking at images from orbit we can find features we would like to visit if they are nearby, so we have to figure out what is nearby. Our strategy is fairly clear. Initially after rolling off the lander we will examine soil and rocks which are close at hand. Then we will pick a representative set of rocks and other features nearby which we will examine more closely. These will give us an understanding of the elements, chemistry and mineralogy of our site. This will hopefully give us an understanding of the origin and history of this location. Once we've characterized the local site (i.e., the things we can see from the landing site) we will then use Spirit and Opportunity rovers to travel beyond the immediate area. And this is the time when it becomes crucial to know which direction to go. To know which way we want to go we need to know where we are. For example, if there is an interesting crater that we've seen in photographs taken from orbit then we need to know whether it is close enough to get to and whether it is northeast or southeast of us, for example. Looking at the first images after Spirit's landing it appeared that the area called Sleepy Hollow was our best bet for what to do next. It was possibly a crater with exposed rocks on a far wall. Our ideas have changed and that stock you bought in the Sleepy Hollow real estate has lost quite a bit of value over the last few days. We got new data (mostly much better images from PanCam) and for some people what seems most interesting has changed. That's science for you! Not to worry though - MER has a well thought out method of reaching consensus that has served us well before and I'm sure will for the Sleepy Hollow detour. In any case, after a week or two (again, as always new data may change these plans) we would like to head either north or northeast or east or southeast. But which way? The missing fact is where are we sitting relative to the features around us, such as craters, mountains and things that we see in the images we have taken by satellites orbiting Mars, such as Mars Global Surveyor (MGS) and Odyssey. As soon as we know where we are the science team will quickly converge on a decision about direction we want to go in. Unfortunately, we don't have a GPS system for Mars - we can't just turn on a device and have it tell us where we are. On the other hand, there is a really amazing capability that radio engineers have developed to tell where one is within the solar system. They do a process called "two way doppler" measurements. It allows radio engineers to listen to radio signals from the MER rover and from a spacecraft orbiting Mars (e.g., Mars Global Surveyor (MGS) or Odyssey), and through a process I won't go into here, pinpoint the location of Spirit or Opportunity to approximately 10 meters east-west position and to within a few hundred meters north-south. We do this measurement at a great distance - even further than the distance the Sun is from Earth. To me this is truly amazing. This 'two way doppler' measurement tells us where the rover is, but only in terms of the gross features of the planet. Unfortunately, we don't know very accurately where those gross features are with respect to the features we see in orbital images. And it is those orbital images that show us the craters, hills, valleys, and other prominent features that we really care about. All of this means that in spite of this wonderful radio measurement we still only know where we are within a few hundred meters. Two things will help us more accurately define where we are. One is that Mike Malin may actually capture a picture of the lander and the rover with the MOC camera on the Mars Global Surveyor spacecraft. If he can manage to do this then we'll know exactly where we are because we will see ourselves in the image with the other features (craters and all that) that we care about. It is theoretically possible, but it is worth noting that we haven't seen any other lander (or crash...) in a photograph taken from orbit. Actually, Malin believes he sees the Pathfinder rover in an image but not everyone finds that evidence convincing yet. We are optimistic that our lander can be imaged from orbit but that isn't guaranteed. The second thing that will help us locate Spirit is that as we start to roam away from the lander we'll see more and more things around us which we may be able match up with features in the orbital images. I would anticipate that after we roam a few hundred meters we'll be able to pick out lots of landmarks to compare. Current plans for Spirit are to visit a large (roughly 200 meters in diameter) crater northeast of where Spirit is now. When we get there, we should be able to accurately match features seen in our traverse with those identified from orbital images, and then we should be able to nail down our landing site. Stay tuned! [Since writing this journal entry a few events have occurred. First, Spirit had a major upset which will probably cause us to reevaluate our plans regarding the next destination. It will be a while longer before we know for sure. Some of the mission team is working on diagnosing and repairing the problem and have some very encouraging progress so far. The other event was that Malin and his Mars Global Surveyor MOC camera took a picture that clearly showed our lander and Spirit! It also had the backshell and the parachute and the heatshield where they landed! This is a pretty astonishing accomplishment that unfortunately was overshadowed by discussions of the Spirit problem. Now we absolutely know where we are! Also, Opportunity had a great landing this week. All of the same issues with finding our location have arisen with Opportunity. Yesterday we thought we knew that we were in one of two small craters. Then we were told by the radio engineers said we couldn't be there! The mystery goes on and meanwhile Malin plans on trying to capture our picture yet again! Stay tuned!] For a great description of localization see Malin's site: http://www.msss.com/mer_mission/finding_mer/ Fatique management is a important issue in transportation systems. You can find details on this at: http://olias.arc.nasa.gov/zteam/fredi/home-page.html Warnings: This journal contains some ramblings by a MER (Mars Exploration Rover) team member and it is orientated toward my friends and family. There is nothing official, approved or necessarily even accurate about my ramblings. I apologize in advance for inaccuracies, political incorrectness and poor grammar that I'm sure will be there. My intention in this is to share as honestly as I can a bit of the flavor of what it is like to work on a NASA mission at the scale of a single human being and to share a bit about what we are up to. My intention is that if I (and you!) make it through these journals you will know a great deal more about Mars and what we are up to with MER. My hope is to convey a lot of information and make it accessible to even the youngest of readers. |