Martian Chronicles IV: T-minus Zero?
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.
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 14, 2000
It’s PDR Week… time for the Preliminary Design Review. Everybody on the project has been spending weeks in preparation for this. NASA has put together what sounds like a very high-powered review board to check us out, and we’re going to be spending three days showing them what we’ve got. Just about everything gets reviewed… hardware designs, schedules, budgets, the works. There won’t be any time to rest once it’s over, but no matter how it comes out it’s going to be good to get this one under our belts.
|Clouds over the Tharsis Volcanoes. 13 March 1998 — MOC2-39 |
Credit: NASA/ JPL/ MSSS MOC
October 21, 2000
Well, we survived our Preliminary Design Review. This was a very big deal… there were nearly 250 people there! (Someone was joking that we may have to rent out the Rose Bowl when it’s time for the Critical Design Review.) Anyway, it went pretty well. We’ve got a long way to go, and it’s going to be a challenge to get this whole thing to work… especially to land a spacecraft this heavy using the Pathfinder airbag system. But it looks like we’ll make it, and there was no question that the science payload is in pretty good shape for this stage of the game.
Work has been going on even while the review was happening. The biggest development of the past week is that we have changed the design of the Microscopic Imager optics slightly. This is stuff that’s familiar to any photographer: We’ve decided to "stop down" our lens to get more depth of field. We’ll need to take somewhat longer exposures to make it work, but the net result is that this camera should be a lot easier to focus than it would have been with the previous design.
November 4, 2000
We’ve been dealing with lots of odds and ends this week — nothing momentous, just the day-to-day stuff that in the end will determine whether or not this mission will really work.
|Martian atmosphere and weather awaits the Exploration rovers|
Credit: NASA/ JPL/ MSSS MOC
What kind of power converters to use in the Mössbauer spectrometer?
How to make sure that the interior of the Mini-TES stays nice and dry in the humid Florida summer when it’s time to launch?
How far apart should the Pancam cameras be spaced?
What kind of material to use in the RAT grinding wheels?
With the Preliminary Design Review behind us now, it’s now time to get all these issues settled. And it’s going to be time to start building hardware again before too long…
November 11, 2000
We’ve made an important change to the RAT… our Rock Abrasion Tool.
The RAT uses a set of grinding wheels to work its way into martian rocks, exposing subsurface materials for the instruments to look at. The old design had a spike down the center to hold the RAT in place while the grinding wheels did their work. This was fine, but it left a small "island" of unabraded rock still standing after the RAT was done.
Tom Myrick at Honeybee Robotics finally came up with a solution to this problem this week: a ring around the outside of the RAT instead, with a built-in pivot that allows it to adjust to the shape of the rock. With this change, the island is gone, and the instruments should see a smooth, flat surface of fresh rock each time the RAT has finished its work.
November 18, 2000
The big issue this week has been how we’re going to provide power to our two German instruments — the APXS and the Mössbauer Spectrometer. The rover solar arrays provide power at about 28 volts, but these instruments need other voltages: 6 volts for the Mössbauer, 5 and 12 volts for the APXS. So we have to convert from what the rover provides to what we need.
|Martian polar cap, seasonal changes |
Credit: NASA/ JPL/ MSSS MOC
Converting voltages isn’t particularly hard for most kinds of electronics, but we have to use something that will work reliably in the radiation environment of space, and that will use power as efficiently as possible… we don’t have much power to spare! After working on this for weeks, it looks like we’re finally on the track of something that will work.
December 2, 2000
We’re making real progress with the RAT — our Rock Abrasion Tool. Honeybee Robotics (where the RAT is being built) is in Manhattan, and a few weeks ago a bunch of the guys from Honeybee piled into a pickup truck, drove to New Jersey, and dug up a few hundred pounds of Palisades basalt. This is very tough rock… as tough as anything we expect to find on Mars.
We’ve now got an early version of the RAT working, and it can grind very nicely into this stuff. The next version of the RAT will be even nicer, and will get rid of that little "island" of rock.
December 9, 2000
The big issue this week has been what’s called the "purge" for the Mini-TES instrument. Mini-TES is a great piece of hardware, but it’s got some touchy components. One of the ones we have to worry about most is deep inside the instrument… a piece of optics called a beamsplitter. What it does is pretty simple, but it’s made out of a very special material called potassium bromide.
Potassium bromide is great stuff, but it has one very nasty characteristic: If it ever gets exposed to too much humidity, it turns cloudy and is ruined forever. That’s not a big deal in the laboratory or on Mars, but it’s a major problem at launch… the launch site in Florida is in the middle of a swamp! So we’re wrestling with the best way to make sure it’ll stay nice and dry while we’re waiting to light the rocket off.
Sending a steady "purge" of dry nitrogen gas through the instrument is the way to go. But do we take the easy way out and turn the purge off hours before launch? Or do we do it the hard way and try to keep the purge going right up to T-minus-zero? We’re still working on this one…
December 16, 2000
The big deal this week has been radiation. One of our instruments, the Mössbauer Spectrometer, has a little bit of radioactive cobalt-57 in it. The radiation output is small, but it turns out that it may be enough that we need to worry about what it could do to some of our other instruments. The Microscopic Imager sits very close to the Mössbauer, and it has some electronic components in it that don’t like radiation much.
|Mesa-like layered geology on Mars|
Credit: NASA/ JPL/ MSSS MOC
We’ve now measured how much radiation the Mössbauer produces, and the next step is going to be to figure out exactly where the Mössbauer will sit with respect to the Microscopic Imager. If it ends up so close that the MI doesn’t like it, we’ll wrap a little shielding around the Mössbauer to keep the MI safe from the radiation.
December 30, 2000
It’s been a quiet week for a change! Work doesn’t stop completely over the holidays, but things have slowed down a good deal. It’s the calm before the storm. January is wall-to-wall with design reviews. Highlights over the next few weeks include preliminary design reviews (we call ‘em PDRs) for the RAT and the instrument arm, along with what’s called the "delta" PDR for the whole MER project… a review where we’ll clean up the issues that were left open after the big project PDR we had back in October.
Another thing coming up soon is our first landing site workshop, where several dozen Mars scientists will get together to start to narrow down the many possibilities for where we’ll land. So we’re kicking off this century about like we ended the last one… in high gear.
January 6, 2001
Things are back in full swing after the holidays. There’s been a lot happening this past week. Perhaps the most significant thing is that we’ve now got a good idea how much radiation the radioactive material in the Mössbauer Spectrometer will produce, and how much damage the radiation might do to some of the cameras on the rover, including the Microscopic Imager.
The answer, unfortunately isn’t simple. The amount is not so tiny that we can ignore it, nor is it so great that we clearly have to fix it. It’s in that uncomfortable in-between zone, where we still don’t know for sure if we need to do something in the design to account for it. So there’s still more work to be done.
January 13, 2001
It was a very busy week, with two big events. The first was the Preliminary Design Review for the RAT. The review turned up the usual number of little technical issues, but no major problems. Our next step will be to build the "brassboard" RAT, which is a preliminary version that will look and work very much like the flight units.
|Frosted slopes on one side of embankment |
Credit: NASA/ JPL/ MSSS MOC
The second big event of the week was a workshop where we invited in a number of the world’s leading sleep researchers to meet with some of the team. The martian day is about 24.6 hours long — a little bit longer than the Earth’s 24 hours. During operations, the whole flight team is going to have to live on Mars time, not Earth time, and if we’re not careful it’s going to wreak havoc with people’s schedules. The sleep researchers gave us lots of good advice on how to adapt to that kind of crazy work schedule for months on end. From the sound if it, it’s going to be an interesting experience!
January 20, 2001
A big issue for us this week has been dust… or more specifically, how to protect the APXS instrument from dust. The APXS has some pretty sensitive parts in it, particularly the detectors that we’ll use to figure out what martian rocks are made of. If those detectors get too dirty then they won’t work… and Mars is a pretty dirty place.
Our plan has been to have some protective doors over the instrument that will open when we want to use it, and close to protect it from dust when we don’t. But how to open and close the doors? We thought about a motor, but motors tend to be heavy and also would require more wires running up the instrument arm.
So instead, it looks like we’re going to design a little gizmo that’s sort of like the clicker on a ballpoint pen. Use the arm to press the instrument once against a hard surface and click, the door will pop open. Make the measurement and then press it again and click, the door pops closed again. The hard surface can be someplace on the rover, or it can be a martian rock. This way we’ll get the dust protection we need, but we’ll avoid new wires and motors.
January 27, 2001
The big event of this past week was the Mars landing site workshop at NASA’s Ames Research Center. Nearly a hundred people came to this workshop to help select landing sites for our two rovers. At the beginning of the workshop there were 185 candidate sites. By the end, nine highest-priority sites had been chosen, along with a number of others.
|Martian moon, Phobos|
Credit: Malin Space Systems/ JPL/NASA
Among the nine are some fantastic places: sites with weird minerals on the surface, sites with old sediments that may have been laid down in ancient lakes, sites inside the biggest canyon in the solar system. The next step is going to be for the Mars Global Surveyor spacecraft to spend several months taking high-resolution pictures of all the top sites. Then we’ll see what we see.
February 3, 2001
With the landing site work done for now, the focus is back on hardware issues this week. One of the things we’re dealing with now is what’s called "contact sensing". Two of our instruments, Pancam and Mini-TES, will look at Mars from atop a mast on each rover. They don’t need to actually touch the rocks and soils that they’re looking at. The other four pieces of the payload, though, need to touch the surface, which is why they’re all on the end of an arm. But how do you tell when they’re actually in contact with the surface?
Taking pictures would do it, of course, but that would mean taking the time to transmit the pictures to Earth and look at them, which we don’t want to do. Instead, we’re equipping each instrument with contact sensors — little switches that close when the instrument gets pressed against a rock, telling the rover that the instrument is where it needs to be.
Of course, it’s not quite that simple. Each instrument is different, so the kinds of contact sensors tend to be different too. And we don’t know how strong some of our targets will be… you might press an instrument against a target, only to find that it’s very soft, fluffy soil, so that the switch doesn’t close. But we seem to be making good progress. As of this week, we’ve got all of the contact sensors figured out except for the one for the Microscopic Imager.
|Rugged terrain on Mars |
Credit: NASA/ JPL/ MSSS MOC
February 10, 2001
We’ve been starting to think hard lately about what’s called "fault protection". This is basically the business of making sure you don’t break a very expensive piece of equipment once you get it to Mars! A good example of what we’ve been working on is Mini-TES. It’s a great instrument, but if you accidentally point it straight at the Sun you could be in big trouble. Mini-TES isn’t built to handle that much heat and light, and if we stare right at the Sun for too long with it we could damage it, maybe permanently.
So we have to make the rover smart enough not to accidentally point Mini-TES at the Sun. We know how to do this: The rover has a special camera for figuring out where the Sun is, and the onboard computer uses this information to keep Mini-TES safe. But it’s one more thing to worry about on a very complicated machine.
February 17, 2001
With the recent round of reviews now behind us, we’re starting to turn our attention to the business of practicing rover operations. This spring we’re planning to do three tests with JPL’s FIDO rover. There will be a couple of two-day tests in the JPL "Mars Yard" in late March and mid-April. The Mars Yard is an outdoor facility at JPL that looks enough like Mars that it’s a good place to take a rover for a spin. Then, at the beginning of May, we’re going to do a 10-day field test.
We don’t know where the field site is — or at least most of us don’t. It’ll be a "blind" test, where the location is unknown to the operations team. We do it this way so that we’ll be almost as ignorant about the landing site as we’ll be with the real rovers once they land on Mars.