- Saturday, 01 September 2012
NASA Tech Briefs recently spoke with Doug McCuistion, Director of the Mars Exploration Program, and Michael Meyer, lead scientist for the Mars Exploration Program and Program Scientist for the Mars Science Laboratory (MSL). We talked about what NASA hopes to find, the technologies used onboard, and how the two-year mission is expected to progress.
NASA Tech Briefs: What are the science objectives for the Mars Science Laboratory?
Michael Meyer: The overarching goal of the Mars Science Laboratory and rover Curiosity is to understand whether Mars has ever been, or is capable today, of supporting microbial life. So that’s another way of saying we want to determine the habitability of Mars. There are other things that can be discovered by Curiosity as it roves about, but that’s the overall goal and how it was designed. NTB: Why was Gale Crater selected as the landing site?
Meyer: Over the past five years, the science team got together, people proposed what they considered were very interesting landing sites, and then there were discussions about how interesting it is to everybody else. As we narrowed it down, we also got into how safe it is, does the landing ellipse fit inside a good place, and are there rocks.
The science community had to be self-policing about what it could actually do and what it could reasonably speculate. This is one of the things we really benefit from — the amount of information we got from having a Mars program. We ended up picking Gale Crater because it has Mount Sharp in the middle — this huge mound that should have an extensive history of Mars starting from more than three billion years ago to whatever Mars is like at present.
NTB: This is the first time since the Viking landings in 1976 that NASA has used throttleable engines for landing a Mars spacecraft. Why was this method chosen for MSL?
Doug McCuistion: The engines are a new design based on a heritage unit. Because of the throttleable nature and the amount of thrust we can get from these, they make a great engine for orbiters for certain Mars orbit insertions as well. So, we’ll use these again, maybe next time on an orbiter.
There were a lot of things chosen because of the additional mass of MSL. Airbags max out around 200 kg, so the airbag technology couldn’t handle a rover of this mass. So we had to come up with a new technique. The concept was a larger parachute to get more drag, and obviously a larger entry shell that reduces our speed and also is volumetrically necessary. But once we got done with the parachute, the replacement for the airbags had to be something that could handle a 1,000-kg rover underneath it, to be able to take out both horizontal and vertical velocities. So instead of putting the engines underneath it like Viking, we decided to put the engines on top.
NTB: Curiosity is NASA’s largest and most complex rover. Other than size, how does it differ from Opportunity and Spirit?
McCuistion: It’s very different — probably the two biggest differences are the payload capability and the power source. Essentially, the plutonium 238-powered radioisotope thermal generator is a constant power source, regardless of time of day. We’re not dependent upon solar energy any longer. We’ve got a constant feed of power, with a constant output of about 110 Watts. That gives us a great capability to charge batteries overnight, to be able to rove farther, and to be able to last longer on the surface by design. That’s a fantastic capability because of the power source. For the instruments, we’ve gone from less than 6 kg of instruments to over 80 kg of instruments, comparing the MER (Mars Exploration Rover) rovers to the MSL rover.
Meyer: The key difference is that Curiosity is a roving analytical laboratory. There are two instruments in the interior of the rover that are major instruments. For Spirit and Opportunity, all of the instrumentation was remote and contact instruments, while Curiosity has two analytical instruments inside.
On the interior, we have an instrument called CheMin (Chemistry and Mineralogy), which is an x-ray diffraction/x-ray fluorescence instrument that measures the distance between atoms. This is the same kind of instrument you’d have in a laboratory. Mineralogy is important because it tells you the environment in which the rock was formed. The other instrument is SAM (Sample Analysis at Mars), and that’s a gas chromatograph mass spectrometer. This gives you composition — it tells you what things are made out of. It’s not the elements, but also the smaller compounds. SAM can also measure isotopes. In addition, SAM has what’s called a tunable laser spectrometer (TLS), which is a spectrometer that can measure certain things to an extreme degree. It can measure carbon dioxide, water, and also methane, which is probably the one we’re most excited about.
The other instrument that’s unique is the ChemCam (Chemistry and Camera suite), which is a laser-induced breakdown spectrometer. It fires a laser, creates a plasma, and then uses a spectrometer to look at the plasma and tell what the composition is. It’s a remote sensing instrument, so you don’t have to place the instrument against whatever you’re interested in. You can do it within 7 meters of the rover.
NTB: Are there other minerals you’re looking for besides carbon and methane?
McCuistion: This mission is highly unusual in that we’ve already targeted minerals that we see from orbit. We see sulfates and we see clays, both of which are minerals that form in water, and they also represent slightly different environments. Clays form in a neutral environment with a pH around 7, while sulfates tend to form in more acidic environments and you also find them, at least on Earth, in environments where the water is drying out. Those are good indicators that we’re going to go to a place where we have mineral deposits that were laid down when Mars was warmer and wetter, and mineral deposits that were laid down when Mars was drying out. As you go further up Mount Sharp, we’ll find things that are indicative of modern Mars, which is cold and dry.
NTB: What are the first steps in Curiosity’s commissioning phase?
Meyer: After it does its health check and everything’s working, it recalibrates its thermal model to make sure it has the right energy budget for managing things. It’s then going to move into a mode of first-time events. The team will move it a little bit and then say, “OK, we told it to move a foot — did it move a foot?” But these things come much later. Things won’t happen right away — this is all within the first 30 days. For each instrument, the team will turn it on and see if it’s working, and put it through its own personal health check. They’ll make a measurement, see what the measurement says, and if it corresponds to what’s expected. Pathfinder, Phoenix, and MER landed on the surface and they were expected to live 90 to 120 days. So it was, “We better get on with it, because we don’t have much time.” MSL is designed as a two-year mission. It’s a long-life mission and it’s going to take a couple of months to really get this rover fully commissioned before it’s fully operational.
NTB: What is Curiosity’s expected range of travel?
McCuistion: Spirit and Opportunity have proven to us that any predictions are completely useless. From an engineering perspective, it’s how long the mechanical systems last. That’s really the limiting factor. The power source will give us many, many years on the surface of nice, clean, consistent power. The rover is designed to be able to travel 20 kilometers. The reason for that is it’s designed to be able to get out of its landing ellipse. What that does is enable the mission to have a goal to go see something it can’t land on. And in fact, that’s Mount Sharp. It has to be able to travel a good distance to be able to get there.
NTB: Are the decisions of the science team as far as where Curiosity will go each day determined according to what findings were made the previous day?
McCuistion: Yes, this is actually unique and exciting at the same time. There is a Science Operations Working Group and essentially, every day, they find out what the rover did yesterday — did it do what it was supposed to do, and did it find something particularly exciting. They analyze the data and have a debate about what to do tomorrow. So filtering into the tactical decision about what to do each day will be “are we still headed in the right direction to meet our strategic objective?”
NTB: Curiosity has a payload of 10 instruments. Can you briefly describe some of them?
Meyer: We’ll characterize the modern environment of Mars very well. We have what’s basically a weather station contributed by the Spanish. We’ll also be measuring neutrons and return of neutrons from a neutron generator (Dynamic Albedo of Neutrons, DAN) that tells us how much water there is in about the upper meter of the regolith, and that’s contributed by the Russians. We have an alpha particle x-ray spectrometer (APXS) that is similar to what’s on the Mars Exploration Rovers that gives us elemental composition from contact. It’s provided by the Canadians.
The Mars Hand Lens Imager, MAHLI, is different in that it has its own light source, so it has a better magnification field to see things down to about 14 microns. It can see at night so if there are any fluorescent minerals, it will be able to detect those.
The MastCam is interesting in that not only is it stereo, but also it has a filter wheel so it gives you different colors. We’ll finally resolve the debate about if you’re on Mars, what color would the sky be? It has a huge amount of memory. It can take a high-resolution picture of everything and then send back thumbnails. The science team can say, “We really like this rock,” and instead of having to ask the camera to go look at that rock and take a picture, you just ask the system to send you back the highresolution picture of it. The picture’s already taken — it’s whether or not you request the data.
Curiosity also has a drill for sampling (PADS, Powder Acquisition Drill Sys tem). We’ll be able to get below the veneer that’s on rocks and sample the interior of the rock. That will be particularly useful for the analytical laboratory that’s in the rover. It will be able to take those, determine mineralogy, and also composition. Because we haven’t done that before, it may provide some real surprises.
NTB: Are there potential commercial applications for these types of instruments?
McCuistion: We already have one example, which is the CheMin. It already has a commercial version called Terra. It’s a suitcase you can carry into the field to measure minerals. I would expect that, for instance, ChemCam (the laser-induced breakdown spectrometer) might be very useful. Some people might be worried about carrying around a laser in a suitcase, but I can imagine that being a useful tool here on Earth.
Other things like SAM — there may be some commercial spinoffs just because of the efforts its gone through for miniaturization. It is taking a laboratory instrument that everybody’s happy with, and shrinking it down so that it fits in a box.
One of the things that’s unique about Curiosity is it will be able to measure organic compounds. One of the big surprises from Viking was not finding any organic compounds. You expect to find at least some because you get them from meteorites, if nothing else. So that’s going to be a big issue for Curiosity.
NTB: The Navcams and Hazcams enable Curiosity to navigate and see where it’s going. What other types of hazard avoidance measures are in place?
McCuistion: MSL has gained a lot from the Spirit and Opportunity rovers, and that’s in regard to autonomous software. Curiosity has a lot of software onboard that can actually navigate and recognize hazards autonomously and either navigate around them or decide it’s too complicated to do that, and just wait for Earth to help. The rover driver and navigation teams use the cameras on a regular basis to understand the rover’s surroundings and identify safe paths of traverse. The most important portion of that capability is the autonomous software aboard that helps us with navigation.
The rover also has accelerometers and inclinometers in the system, so it understands what its own tilt and roll angles are. As Curiosity climbs Mount Sharp and reaches limits of tilt and roll, the inclinometers tell the system that it is at the limit so it does not roll. The whole rocker-bogie system has a design that goes all the way back to the Sojourner rover, and is an extremely capable and flexible system.
Meyer: Just to add to what Doug said about software navigation, right now, you can take your images from a Mastcam or Navcam and plot out, safely, about 40 meters. And then after that, your imagery is too planar and you can’t really decide what would be the best path to go. So the rover itself has to decide that. One of the things that has been developed from MER is navigation software that is able to take images as the rover goes along and say, “OK, that’s a big rock — turn to the left.” That’s why the Mars Exploration Rovers have been able to go up to 100 meters at a time. Curiosity will benefit from that.
Meyer: That’s the advantage of the longevity of Spirit and Opportunity. I think people don’t realize the advancements in surface navigation that are only possible because Spirit and Opportunity survived for so long; that we could build new software tools, new concepts, new techniques, and then test them, upload them, and use them. It’s been spectacular, not just scientifically, but from an engineering perspective, what Spirit and Opportunity were able to do and port into MSL.
NTB: The Radiation Assessment Detection (RAD) instrument was taking measurements during the trip to Mars. What has it found that you didn’t know before?
McCuistion: The RAD is designed for a broad spectrum of high-energy radiation measurement, and it was turned on about a week after launch. It was turned off on July 13, getting ready for entry, descent, and landing. What’s interesting about RAD measuring in transit is that it sees what might be seen by an astronaut on its way to Mars. One of the concerns about high-energy radiation is what radiation is actually shielded by the spacecraft, and also what radiation is generated by the spacecraft. High-energy particles impinging on the cruise stage actually generate secondary particles that may be just as harmful, but of a different nature. RAD’s been able to measure those.
NTB: What have you already learned from MSL for future Mars missions?
McCuistion: Scientifically, we’ve already got a data set from RAD that we’ve never had before, which is the true radiation levels, dosages, etc., that astronauts might see in space in transit to Mars. From an engineering perspective, we’ve learned an enormous amount about how to build a system of this capacity and capability. The sky crane technique is a great technique for being able to put larger and larger masses on the surface, and frankly, as a feed-forward technology capability, you could foresee this putting all kinds of different scientific systems on the surface and potentially even re-supply for astronauts on the surface sometime in the future. We have learned to shrink instruments dramatically, changing their footprints significantly, which will always pay off in future scientific missions, whether they are on Mars or some other location.
Guided entry is another one – the ability to shrink the landing ellipse so significantly that we can get into areas that we couldn’t have imagined ten years ago. That opens up science portals that we can’t even fathom at this point. There are pretty exciting opportunities.
Meyer: With MARDI, the Mars Descent Imager, one of the big debates was whether or not, because of thruster plume, you’ll get useful images. So who cares, other than the scientists who have to figure out where you’re going? Well, one of the derived benefits of decent images would be for terrain recognition, which means in the future you could say, “I want to land right over here next to that rock,” and you can have the software look at the images and actually plan exactly where you want to go. That will make a big difference when we do sample return or we send humans to Mars, when you want them to land next to where we put the foodstuffs.
McCuistion: The other thing is the heat shield material. The heat shield material is called PICA (Phenolic Impregnated Carbon Ablator) and we adopted it for use when we saw what kind of mass we were dealing with and what the heating rates were. PICA was a lot safer and gave us a lot more margin. This will be the first time it’s actually been used. PICA is a potential heat shield material for human exploration in the future.