by Rod Pyle
Curiosity is currently crossing a flat plain, but progress has slowed due to the risk of wheel damage: “It's gotten to be nerve-racking in the last few weeks because we've noticed that we were doing damage to the wheels. We don't want to do that any more than we have to. We hadn't really considered that up until this point, we said ‘Well, we don't care too much about little rocks, we can drive over them.’ Of course we didn't want to go over a cliff or be stuck in the sand dune or a sand-filled crater. Those are the kinds of things we've been avoiding.
“But Mars throws monkey wrenches at you all the time.” Hmmm…seems like I've been hearing variations on this quite a lot. “Now we have to learn how to drive again, and we are starting over with shorter steps. We're trying to get out of this area and then we will have a lot fewer pointy rocks and we can make faster progress.” A few weeks after I spoke with him, Curiosity made a detour across a wide sand dune—a nerve-racking feat in itself—to get out of this wheel-killing terrain. To everyone's relief, the rover did not bog down.
Within the year, Curiosity will be making its way up into the foothills of Mount Sharp. Will operations change then? “As we get into the foothills and begin to go steeper and steeper, and then especially when we're doing arm operations, it's going to be challenging because there are limitations to what you do with the arm in various height and tilt regimes. Specifically, drilling and sample transfer and things like that.”
Arm operations is another specialized skill not found in your average video game. Vandi Tompkins is a rock-climbing, airplane-piloting adventuress who could give Lara Croft a run for her money, but fortunately for the MSL mission and us, she chose to be a rover driver, arm operator, and engineer. Coming from a fairly traditional home in India, accomplishing this was not a sure thing—there was an expectation from her family that she might pursue a more traditional path—marriage, kids, home, and hearth. But once you meet her, you can understand that most people, her parents possibly included, would make a suggestion and then step aside, for Tompkins is a determined person.
Fig. 26.2. A NEW GENERATION: Vandi Tompkins, rover driver and programmer, poses with her previous “ride,” a twin of the MER rover. Image from NASA/JPL-Caltech.
With a doctorate in robotics from Carnegie-Mellon, her first job with NASA was at the Ames Research Center in Northern California. Before too long she had relocated to JPL and was working with the Mars Exploration Rovers. What else would someone with a degree in space robotics want to do? Her husband of just a few years works over at SpaceX, Elon Musk's private rocket company in Hawthorne, about twenty-eight miles across LA from here.
“I do three primary roles on MSL, and as a rover driver we tend to have three specializations: driving or mobility, operating the robotic arm, and the sampling system. Each of these is such a rich domain. There are a couple of us [who] do all three right now and eventually they would like everyone to be able to do that. There are fourteen drivers as of now.”
So what is involved in “driving” the arm? How hard can it be? Pretty hard, as it turns out. “It's [a] five-degrees-of-freedom arm. There are all these instruments on the end of the arm: the drill, the CHIMRA instrument, and so forth. It's pretty heavy. So we have done a lot of testing related to that…trying to break it in various ways, all to make sure that that it will work on Mars.” It turns out that with a metal pole as long as the arm is, with joints and motors and heaters and so forth, there are a lot of variables: how much will it sag when extended? How much power will you need to move the arm? To run the devices on the turret? To keep it heated enough to not freeze up in the cold Martian night?
Tompkins continues: “What makes space robotics so different is that you don't have many second chances. You really have to dot all the I's and cross all the T's, so to speak. I'm interested in having the rover in situations that are not the nominal working situation.” This woman loves a challenge. “This giant, hundred-kilogram arm will drop a baby-aspirin-sized sample very precisely in these inlets and the instruments. An instrument like SAM, which is so capable, needs clean samples. We have to make sure the sample is not contaminated. Our general mode of operation is that instead of doing something risky, we want to have the time to stop and wait and call home. But there are times when we have to do something autonomously and that is also very interesting.”
Interesting is the term engineers often use for challenging and potentially risky operations. You have to love that.
Nothing is easy on Mars, and there are a lot of robotic acrobatics the arm must do to move a sample from one chamber to another inside the CHIMRA—a soil-holding and size-sorting chamber on the turret—and sift out the fine particles appropriate for the instruments inside the rover. “You have to very precisely position the arm because you have very close clearances. Once the arm is in position, you open the inlet [that leads to SAM and CheMin] just when you need to, and, for example, the SAM instrument has carousels of little cups built in. It places the equivalent of a little trash can under the opening, because when we bring the arm over, we're afraid there might be things on the outside of the turret that might fall, and we don't want to have the sample contaminated by them. So, once we get the arm into position, we move that trash can out of the way and then get the collection cup under it. Then you drop off the sample by vibrating the collection device. You then have to move the arm out of the way to an intermediate position because you can't yet close the cover on the inlet to the instrument.” The tolerances and distances are so close that you have a tension between getting the tiny sample into the cup without losing it and not banging into the rover. “It has to be close because wind and other weather effects could blow away the sample.”
On the Mars Phoenix mission in 2008, which landed near the Martian north pole and collected icy samples there, they discovered just how trying this could be. Remember that there is a long delay in two-way communications. When controllers positioned that spacecraft's arm above the receiving funnel and tried to drop the sample, prevailing winds kept blowing much of the sample away. They learned a lot in that mission.
There are other risks involved with arm operation and the instruments on the turret. “There can be a problem with the brushes,” she adds as an example of this. There are wire brushes on the arm that can be used to clean a rock before it's sampled—that's the DRT tool I discussed earlier. “If you leave them [the wire bristles] in contact with the rock for too long, they can get bent, so that's another example of a case where the arm will autonomously move itself just enough so that is not in contact. I think it's hugely interesting.” There's that word again. Demanding, maddening, complex…interesting. I envy the passion evident here.
“When the arm is making a sample deposit, it is software commanded, but it also has to be compensated because, for instance, the rover can be sitting on different tilts or inclines, so we have to do various moves to make the drop-offs work. That's something we simulate on the ground, then we expect it to do the same thing on Mars. We have a simulation which includes the rover's attitude so we know where it's going to go. We have to test all the possible situations and conditions under which the operation could be interrupted.”
At this point, I am a bit confused. There are software simulations of how the arm will operate in certain conditions, but there is also a physical clone of Curiosity at JPL that can be used to test arm motions as well. How do they decide when to use software, and when do they resort to testing with the physical arm on Earth? “For everything we have to do the first time, for instance, the first three months when we were working on Mars Time, we had many days like that…” that is to say, many days during which they needed to use both simulation modes—software and the physical twin of Curiosity—to test some of the arm's activities before sending up instructions to Mars. “But now that we have been on the ground for a while, some activities are repeats—we've done that before, and we really have very high-fidelity simulations. So, for example, let's say that the science team wants to get a MAHLI image o
f the drill hole, which is something we did recently. That is something we feel very comfortable doing because even though we haven't done that precise activity before, we have run it in simulation, and that's where experience and understanding comes into the picture.” When they drilled into Mars, they took close-up pictures of the drill hole from four angles to see exactly what it looked like all around its circumference.
So how do you translate a general simulated activity into a specific situation? “The Navcam will take a stereo image, and that gives us depth, and from that we can create a mesh.” It allows them to add that into the software simulator. “Now there's a certain amount of error in that, and the gyros can drift,” that is, the inertial measuring sensors—the gyroscopes—can incur some drift and show an erroneous location of the arm or even the rover itself; we are talking tiny fractions of an inch here, but that's enough to be a big problem. “There are also things you have to take into account regarding position accuracy of the arm. You have to have a good understanding of all that so you know how close you are getting to something. But we feel very comfortable with the understandings we've gotten from the simulations.”
I ask for an example. “Okay,…recently we wanted to dig a trench. We've done this with the other rovers. You spin one of the wheels around to dig a trench. That reveals the sub-layers, which have a more interesting composition. It's not been exposed to the surface and the elements. And then you want to be able to do contact science,” for example, to use the APXS or scoop or drill right at the rock or soil exposed by creating the trench. “Now our turret is so mechanism rich, it's very crowded; the clearances are very close. So if you want to use the APXS, you have to be careful where the drill is and also where the target rock is. When it's flat, it's not such an issue, but when the rover is in any kind of terrain with relief, it is.” Complicated, that is. “So in cases like that, we will simulate them in the test bed,” with the Earth-based rover twin. “In that case, you have to take into account that it's operating in Earth gravity. For example, with the motions we did to dig a trench, we know that because of the weight of the rover here on Earth we will go deeper…” In this case, they would have to take this into account and change the variables. “Part of the decision is a judgment of what do I need to use in this the particular situation, is it sufficient for me to have tested it in simulation? Or do I need to use the test bed?”
But it's even more complicated than that. “Another example is sample flow. Samples behave differently on Mars, so we have another test bed in a chamber that simulates some of those conditions with the rover in it. Part of the problem is figuring out which level of simulation we need to use. Over the years, we've learned a lot. Many of us also worked on MER, and some of us went to graduate school for robotics, as I did, and you have experience you bring in from that.”
“It's amazing how much we know about Mars now. Our team's approach works really well. To have people [whom] you can talk to, like soil experts, is very helpful. They can tell you what the soil is going to be like underneath, for example.”
So that's how you drive a rover, how you teach a rover to drive itself, and how you operate the robotic arm. It's complicated, and much of it has to be software driven and autonomous; there's just too much delay to do it in real time. Pathfinder blazed new trails with the slight autonomy that Sojourner had. With the MER rovers, ten more years of development and learning have been a huge benefit. Now, with Curiosity, all the hard work has paid off.
“I think we work very hard to try to eliminate any possibilities of failure,” says Tompkins “But you can't eliminate it entirely—it's a matter of probability, and you can't get it down to zero. So it's really interesting to work on development and testing. You like to think you've done everything you could, but there are still things that [you] could've done even more work on. So when it works, and it works better than expectations, you really realize that each day is still a new day, that anything can happen. The Martian environment is really harsh.” And probably…interesting.
If Opportunity's longevity is any indication, there could be a decade or more of exploration and discovery in store for Curiosity. And, being such an incredibly capable and advanced machine, every month of operation builds on MER's rich history and adds immense amounts of data to the books.
And then, of course, there is Curiosity's huge increase in capability for evaluating and gathering samples never available before. And that brings us to the long-dreamed-for ability to find materials not exposed to the harsh environment of Mars…
That is the magic of the drill.
There are a few things that set Curiosity well ahead of previous rovers.
It was bigger.
It was heavier.
It had a built-in lab to analyze soil and rock and atmospheric samples. In fact, it had 180 pounds of scientific instruments aboard its roughly one-ton chassis; the MER rovers had fifteen pounds of far less capable ones.
And, perhaps most notably, it had a drill.
This last item was just as remarkable as the onboard laboratory and almost as hard to engineer. I've talked about the science instruments—SAM and CheMin are fantastic machines—and discussed the scoop and the delivery system for Mars soil earlier, but I've been saving the drill until now because it's more interesting to hear about it when it is about to go into a rock.
A drill had been one of those Holy Grail, wish-we-could-have-that items on the planetary scientist's packing list since Viking. On that mission, if they wanted to investigate rocks or dirt that had not been bleached out and sterilized by the sun, they had to use the scoop on the end of the arm to shove things around as best they could. Viking was a static lander, so there was only so much that could be reached with the arm.
With Pathfinder's Sojourner, it was a different story, but with a similar outcome. The little rover was mobile and could travel an area tens of times that of Viking's reach, but Sojourner was the size of a toaster oven and barely outweighed many of the rocks it was looking at. So when JPL wanted a “fresh” sample to look at, they shoved a small rock over or dug a shallow trench (more like skimming off the surface dirt, really) by locking five wheels and spinning the sixth. It was better than nothing.
The MER rovers, Spirit and Opportunity, were more robust, weighing almost four hundred pounds each. They had robotic arms, more mass and larger wheels. Still, the best they could manage would be to flip some rocks, dig some trenches, and scrape off some surface patina on the rocks with a spinning wire brush. The technology showed progress, but, ingenious though the creators and operators were, the MER rovers had their limits.
The Mars Phoenix lander, which set down near the Martian north polar region in 2008, was also able to use a robotic arm to scoop and scrape through the icy crust up there, but it was at the mercy of soil variables, including hard ice, as well as the short span of the mission (about five months).
Then along came Curiosity. The geologists wanted a drill, which was a worthy goal. After the MER rovers and Phoenix, it was understood that the surface of Mars was a far-nastier place than had been suspected since the Vikings landed there. It was baked with solar radiation/UV and cosmic rays. There was bleach-like perchlorate in the soil, and the red hue of the sand and dirt and rocks was a product of billions of years of oxidation. In short, the surface was a sterile, radiation-blasted, bleached-out desert. It was possible that all of Mars, above the surface and below, was like that…but we would never really know until we could see inside a rock. And the only way to do that was to break one with a mechanical hammer or to drill into it.
In the end, the engineers really built a bit of both.
The drill on Curiosity is a masterpiece of ingenuity. It's a percussive drill, meaning that in addition to spinning like the hand drills we are all familiar with, it also hammers on the rocks as it spins. Just that mechanism is difficult enough to build to make it strong and reliable enough for use 150-million-plus miles away, but there is more. It also needed to collect the sample it dr
illed. Somehow, the powder that resulted from beating, chiseling, and grinding the rock had to make its way to the labs on board Curiosity. That's a challenge.
Oh, and one more thing. The rover had to carry more than one drill bit. Curiosity was designed to fly to Mars with the drill bits in a sterile box and the chuck of the drill empty, but as you will recall, a last-minute decision (and one that stirred much controversy within and outside of NASA) was made to place a bit in the chuck before flight, just to make sure that there was at least one ready to go. But if that drill bit broke, or got stuck in a rock, or just plain wore out (it would be drilling rocks, after all, and they tend to be hard), the engineers wanted to be able to have spares on board. So a rather-elaborate system was designed that allowed the drill head to let go of the worn, stuck, or broken drill bit and reach down and clamp onto another.
Whew.
So how do you build and test such a device? As with so much related to this mission, the process of designing and perfecting the drill for Curiosity could fill its own book. But we'll settle for part of a chapter.
In the broadest terms, the drill bit is attached to a chuck, or receiver, that is in turn attached to a mechanism that both twists (to drill the rock) and percusses (to beat the rock). So the drill assembly is attached to a thin metal diaphragm. There is a magnetic coil—much like a voice coil (for those who know how speakers or many microphones are made) and the diaphragm has enough “give” that this coil can cause it to flex in and out and cause the drill to hammer as it spins. This, in addition to the drive motor (that makes it spin), creates a drill that twists and hammers at the same time. It can be set to either drill or hammer alone, but it is really designed to do both, as that is the most effective way to drill a rock.
