Martian Chronicles XIII: Elves
Three spacecrafts are now hurtling toward the Red Planet to look for evidence that it might once have been wet enough to sustain life. Orbital projections of where Europe’s Mars Express and the two NASA Mars Exploration Rovers (MER) are right now, can be continuously monitored over their half-year journeys. Experiments performed by the MERs will help to determine whether water might have once existed in volume on the red planet.
The two Mars Exploration Rovers are targeting what imagery indicates might have been ancient dry lake beds and other geologically interesting sites in early 2004.
The Martian Chronicles series gives an inside view of what it takes for scientists to deliver a complex mars mission. The journal entries are from Cornell’s Steve Squyres, the Principal Investigator for the Mars Exploration Rovers’ scientific package called Athena. The chronicles begin sequentially from the beginning of July 1999, four years before launch, and will culminate in the dramatic landing of the twin rovers on Mars in January 2004. The expected mission time roaming the red planet is ninety days, from January to April.
As Spirit and Opportunity speed toward Mars, more than three hundred scientists and engineers here on Earth will learn how to act in unison to master the art of commanding two very complex robots to do science on another world.
The chronicles include an insider’s view of hardware tests and site selection to problem solving and science planning on the surface of Mars.
October 25, 2003
In the operations readiness tests we’re running, we spend a lot of time talking about elves and gremlins. "Elves" are the people behind the scenes in our tests who make life easier for us by keeping the rovers running smoothly. But the same people are sometimes "gremlins". We call them that when they do something to challenge us, by making the test harder.
|A simulated image of the Mars rover carrying the Athena science instruments against the horizon of the Mars rust-colored sky. As Cornell’s Project Scientist, Steven Squyres, wrote about their simulations: "We’ve just finished a ten-day test using the rover to simulate twenty martian days. The guys took their baby out to a site somewhere out in the American Southwest, and we drove it from JPL just like we’ll drive the real MER rovers when they’re on Mars. The "landing site" was fantastic, with no vegetation, great rocks, and many geologic puzzles for us to solve. Very Mars-like! In our twenty "sols" of operations, we drove the rover more than 200 meters, made a bunch of detailed measurements of the chemistry of the rocks, and took a lot of spectacular pictures."|
The gremlins were hard at work during our latest operations readiness test. The last couple of times we’ve done this, it was pretty easy to drive the rover off the lander. Not so this time! This time we landed, put the mast up, took some pictures, and discovered that the lander was almost completely surrounded by large, dangerous-looking rocks.(Rocks that were put there by the gremlins, of course.) We couldn’t drive over rocks that big, so we had to find another way to get off the lander. The only reasonably clear path was the one directly behind the rover. So way we did it was to "hyper-extend" the lander petal that sticks out behind the rover, tilting it down at an angle to make something like a ramp. We then drove backwards down the ramp, sort of like backing out of a driveway onto the surface of Mars. Pretty cool.
And this was supposed to be our last "nominal" test. The next test, in November, will be an "off nominal" one. We’re all very curious to see what the gremlins come up with for us next time!
November 8, 2003
Good news about the Mössbauer Spectrometer on Spirit: We’ve figured out how to make it work.
You’ll recall that back in August we did an in-flight checkout of all of the instruments on both spacecraft. Most of them were fine, but there was something wrong with the Mössbauer Spectrometer on Spirit. We couldn’t tell exactly what was going wrong, but the instrument’s drive system the part that vibrates back and forth definitely was not working properly.
We’ve spent several months now troubleshooting it. The first rule in anything like this is "do no harm", so we worked through it very, very slowly. We’d do a very simple little checkout, take weeks to sift through the data, try another checkout, take more weeks to sift through more data, and so forth. It’s painstaking work, but with each checkout we learned a little bit more. And what we discovered, eventually, was that the drive system was bumping into something at one end of its motion. It wasn’t hitting it hard, and it wasn’t doing any damage to itself. But there was some minor obstruction that was keeping the drive from moving the way it’s supposed to.
So what to do? We couldn’t exactly head out after it with a screwdriver! But there are some tricks you use when you build space hardware, and we had used them on this instrument back when we built it. One trick is that you try to make an instrument as adustable as you can. This can be a good thing to do even if there’s no obvious reason why you’d ever want to make an adjustment… you never know what’ll happen. The second trick is that you try to design an instrument with lots of "margin". In other words, you try to make it better than it really needs to be. That way, if something goes wrong, it might still work okay after you make an adjustment.
We did both of these on our Mössbauer, and it paid off, bigtime. We built the drive system so that we could adjust both the velocity and the frequency with which it vibrates. By adjusting both, we were able to make it vibrate with less total motion… so that it wouldn’t hit the obstruction. And while we never expected to operate the instrument with a total motion that small, we built it with enough margin that the spectrometer still will still work properly on Mars.
So what was the mysterious obstruction it was hitting? Best guess is that it was a wire that was bent slightly during the intense vibrations of launch. Whatever it was, though, it’s not causing us any more problems. If the instrument works on Mars the way it’s working now, we’ll get all the science we were hoping for.
What a relief!
November 15, 2003
|Viking image of Gusev Crater, an ancient proposed lakebed that will be targeted in the forthcoming Mars Exploration Rover mission.|
It’s mid-November, and time for another ORT. These Operations Readiness Tests are intense, and they seem to be getting more so. As I write this it’s 2:00 AM at the Jet Propulsion Laboratory, and 2:30 AM (in our simulation) at Gusev Crater. The rover is asleep right now, and we’re in the process of getting the commands ready to make it do its thing when the sun comes up on Mars (again, in our simulation) tomorrow morning. Today’s main activities should be a checkout of the RAT followed by a short drive to a rock that the science team has, for some reason, nicknamed "Fromage".
Meanwhile, out in space… we just completed a final checkout of all the cameras on Spirit, and they all look great. The next time we’ll turn them on, they’ll be in Gusev Crater. For real.
November 29, 2003
Wow, what an ORT! And it’s a good thing, too, since this was our last one.
We had our final Operations Readiness Test last week, and this one was a real hummer. We simulated five martian days of motoring around Gusev Crater, with all the bells and whistles. The highlight of this one, by far, was that we used the RAT, or Rock Abrasion Tool. The RAT is our version of a geologist’s rock hammer… we use it to remove the outer layers of a rock so that we can see what lies underneath. In this ORT we spotted a rock that we really liked, and we decided to go after it with the RAT. (The science team nicknamed this particular rock "Fromage": bait for the RAT… get it?) We drove the rover to Fromage, stuck out the arm, and used the RAT to grind a beautiful circular hole in it. Then we stuck all three of the other instruments on the arm into the hole and got fantastic data. It was, by far, our coolest ORT yet. It was nice to finish up on such a high note.
We’re now just one month away from landing in Gusev Crater for real…