Curiosity

by Rod Pyle

  Pathfinder also reported Martian weather, the first meteorological reports from the planet in twenty years. The average day temperatures were below twenty degrees Fahrenheit, and the nights plunged to –104.

  The rover had been designed to last one week and the lander a month. But in a pattern that would become synonymous with JPL's missions, both were still going strong over two months later. By early October, however, the computer-reset and communications problems were becoming more frequent, and it was clear that the duo's days (or, to use Martian terminology, sols) were numbered. By September 27, communications were failing, and the lander breathed its last on October 7. The surface exploration had lasted almost three months.

  Fig. 6.2. HELLO, YOGI: In 1997, the Mars Pathfinder rover Sojourner spent almost three months exploring a small area near the lander. As seen here, it is investigating a rock they named Yogi (yes, after the cartoon bear) with the APXS instrument. Image from NASA/JPL-Caltech.

  In its brief life, Pathfinder and Sojourner examined sixteen rocks and returned seventeen thousand images. The first on-ground evidence of water in Mars's distant past was found, though the area seemed to have been dry for at least two billion years. The rocks were volcanic in origin, demonstrating a geologically busy past for the planet—long suspected but a welcome confirmation. Lots of other scientific data were returned. But the most important thing was what they learned about going to Mars and how to land, drive, and survive in the harsh Martian environment.

  It would all pay off in just a few years when the Mars Exploration Rovers headed off to the red planet.

  Pathfinder had outstripped even optimistic projections. The concept of a roving vehicle, delivered directly to Mars without settling into a parking orbit first, worked brilliantly, and on a budget.

  What was called for now was an ambitious follow-up: larger, more capable rovers to traverse far-greater distances and explore more targets in a much more detailed manner. But the year was 2000, and it was not yet time for Curiosity. An intermediary design took the form of the Mars Exploration Rovers. The MER rovers were extremely successful and have gone far past anyone's wildest dreams. The accomplishments of these machines, especially the still-operating Opportunity, nearly defy belief.

  But as these roving machines were being built, JPL got a black eye. First one, then a second mission failed in 1999. The Mars Climate Orbiter (MCO) had sped off to Mars in December 1998, arriving there in September 1999. Intended to aerobrake into a Martian orbit—that is, enter the atmosphere just enough to slow it but not enough to make it crash—the unfortunate spacecraft plunged into the Martian air at the wrong altitude and augered in. It the end, the embarrassing revelation was that somewhere along the development path of the software, there had been a failure to convert from English units to metric ones. The altitude settings were way too low. Oops. In a business where a few milliseconds or a fraction of a degree of trajectory error can cause calamity, this was huge—and damn embarrassing.

  On top of this, a second JPL mission, the Mars Polar Lander (MPL), failed in December of the same year. When it arrived at the red planet, the spacecraft retrobraked into its descent, ejected its parachute, fired its braking rockets—and went promptly and permanently off the air. It just vanished. No indication of landing, no final call for help, just silence. What actually killed the mission is not entirely clear, but it is suspected that one of the violent events that occur during a landing—firing of braking rockets, deployment of the landing gear, something—had caused the software to think it had reached the surface safe and sound. The computer shut off the rocket engines and the machine fell from perhaps 120 feet or so—too far to survive the impact. It was like a skydiver cutting loose his parachute at a couple hundred feet, and the results were similar. It died.

  JPL was zero for two with Mars in 1999. A lot of hope was being placed on MER's shoulders, and if for some reason this mission was unsuccessful, there would likely be no MSL. Those involved knew the stakes and rose to the occasion. They would absolutely not fail.

  The new millennium brought renewed hope even as it fostered seemingly endless and painful reviews. JPL's managers wanted to know what had gone wrong in the months previous, and atop them sat the NASA juggernaut, also waiting to know what had happened to its hundreds of millions of dollars. And atop them…some members of Congress, looking for someone to blame, or for excuses to further trim the space budget.

  Meanwhile, planetary science does not stand still…missions ready to go were allowed to continue. The Mars Odyssey orbiter reached the red planet in October 2001 and began returning spectacular results. It was fortunate for all concerned that it did.

  Throughout these trials and (more recent) successes, the Mars Exploration Rovers were being readied. The MER rovers, Spirit and Opportunity, were scaled up by an order of magnitude in every way over Pathfinder. Weighing well over ten times as much as the Sojourner rover at 410 pounds, with much more robust instrument packages, they would assault Mars as twins. Not since Viking had a mission to Mars had the luxury of a backup in case of single failure, nor are we likely to see it again. It has simply become too expensive.

  There was a new and unique twist to this mission besides the size and complexity. The twin rovers would be able to regularly use NASA's Mars orbiters, Mars Odyssey and the older Mars Global Surveyor, to relay messages back to Earth. This gave them a much wider time slot in which to talk to JPL and far more bandwidth than surface antennae such as that on Pathfinder. This blending of assets served MER perfectly and was a good training ground for the future MSL, then being planned. MSL would have much more demanding data-relay needs, so success in this with MER was crucial.

  As before, the machines would be powered by solar panels, except that rather than sitting as a small rectangle atop the microwave-sized Sojourner, these folded out like beetle wings, giving the rovers a decidedly insectoid look. Aboard were larger and far more sophisticated instrument packages, but as with the rest of this program, they were an evolution of what had gone before. It was a good case of trying, learning, and improving with each mission.

  Steven Squyres, a professor at Cornell University, was the principal investigator, in effect the science boss, of the Mars Exploration Rover mission. Tall and lanky, he brims with the confidence that only a lifetime spent as an astronomer and a decade of Mars-rover operations can inspire. He speaks plainly and easily, has a breezy sense of humor, and is the kind of guy you'd want as a geology (or calculus or chemistry) professor. MER, he said, had a far more ambitious plan than did Pathfinder, now that they had the basics licked. “The primary goal of MER was to go to two places on the Martian surface and try to learn what conditions were like in the past, and then discern if they might have been habitable.” He paused. “You know, Mars is a cold, dry, and desolate place, but in the past conditions were probably different. So we tried to choose two places that looked, from orbit, not just to be good places to land on, but that appeared to have traces of water in the past. We hoped to really read that story in the rocks and see how habitable it might have been.”

  His distinctive leadership of the mission can be sensed when he discussed the instrumentation of the rovers: “The way I chose the payload was by picking a set of tools that were as capable as possible. It was sort of like trying to design a Swiss army knife, finding the most capable tools you can and putting them on the vehicle. There were also some very serious practical considerations regarding availability of technology, its maturity, and the risks of the technology you would buy. Everything has got to work. As things have turned out, I'm quite happy with the payload that I chose.”

  While the MER rovers’ level of sophistication pales when compared to MSL, the machines were still stuffed with everything the planners could afford to fly. The most obvious was the camera mounted atop a tall, folding mast. Called Pancam, it offered a splendid, wide field view of the terrain ahead of the rover. Navigational cameras offered an even wider view for the rover drivers. A small camera mounted o
n the end of the robotic arm, the Microscopic Imager, allowed for close-up views of rocks and soil. Four more small cameras completed the collection, these also for driving and hazard avoidance. All these imagers would be refined and augmented for MSL.

  A Thermal Emission Spectrometer was mounted in such a way to see the rocks and soil ahead and help to identify promising areas for investigation. By observing the rocks in infrared, the device would be able to effectively see through the dust that clung to everything on Mars and also sense how quickly heat escaped over time. This would aid in determining the possible composition of a rock or outcrop.

  Out on the rover's arm, along with the microscopic camera, were four more tools. There was another spectrometer, this one designed to be held up close to interesting rocks. The Alpha Particle X-ray Spectrometer, or APXS instrument, would bombard targets with energy and observe the resulting reaction. This was a more-sophisticated version of what had already flown on Pathfinder and would in turn be improved for Curiosity. There were magnets to gather ferrous soil, and finally the Rock Abrasion Tool or RAT, a wire brush for cleaning off dusty rocks prior to examination.

  Deep in the chassis was the onboard computer similar to the one that had given Pathfinder's Sojourner rover such fits. This little CPU, an IBM RAD6000 in the jargon, was way out of date when compared to those then being used in consumer computers on Earth, but it was the most affordable unit that had been “hardened” against radiation and was of military quality. Similar units had powered Macintosh computers back in the early 1990s; now it would go to Mars. It ran at a now unimaginably slow 20 megahertz and was supported by 128 megabytes of RAM. Today it would cower in a corner if it met a modern smartphone. But the thing was designed to withstand a nuclear attack, which should make Mars child's play by comparison.

  Another evolutionary element of the MER rovers was mission management, which was in many ways a very scaled-up version Pathfinder's. While MER was an entirely different beast than Pathfinder, it had actually grown out of simpler origins. In a set of circumstances that will surely never be repeated at JPL, the entire mission—both rovers and all the science conducted by them for over a decade—fell under one man. Steve Squyres had been tapped early on for the mission. His pedigree was a profound one: he had worked on Voyager (as a very young man), the Magellan Venus mission, the NEAR–Shoemaker (Near Earth Asteroid Rendezvous–Shoemaker) mission, the Cassini Saturn probe, and a European Mars orbiter called Mars Express.

  When Squyres was assigned to MER, however, the mission had been planned as a single lander. Then it morphed into a rover…. Then two rovers. It just grew and grew, and soon it had 170 scientists working on it, all for one principal investigator—Squyres. It was a busy time that included his scientific work on the aforementioned Mars Express as well. As Grotzinger, who also worked on the MER mission, once commented, Squyres was often like “a flea on a hot griddle.” He seemed to be everywhere and involved in everything. Of course, Grotzinger would soon find himself in similar circumstances.

  During mission planning it had all seemed doable. After all, each MER rover was supposed to have a ninety-day primary mission, and, if they were lucky, the machines would survive a Martian winter and soldier on for a year. If they were lucky. Nobody at the time dared suggest that one of the rovers would still be busily driving along the Martian surface a decade later.

  But first they had to get the rovers to Mars.

  After a multimonth trek, and in a repeat of the techniques pioneered by Pathfinder, these two comparative behemoths would hurtle directly into the Martian atmosphere upon arrival. Within a few seconds, the heat shields would go from space-cold to incandescence as they slammed into the thin air high above the planet. Parachutes would plume open, braking rockets would fire, and in the bizarre ballet pioneered by Pathfinder, the tumorous beach-ball-like airbag cluster would inflate instantaneously when the rovers neared the ground.

  Upon impact, the spacecraft would bounce, nearly one hundred feet high and many times, rattling and shaking the rovers ensconced within. Yet within minutes they would both be stationary, with the airbags deflated and lying across them like dead skin.

  In order to use the airbag system, they had to first slow the spacecraft from its interplanetary cruise from Earth. This would require a parachute. But Pathfinder had weighed a lot less than MER, and the Vikings had orbited Mars before landing (which slowed them down). Like Pathfinder before it, the MER rovers would be coming straight from Earth, shot like artillery shells leaving Florida and aimed at a spot over a hundred million miles distant. The speed at which they would arrive at Mars was very high and would require a new parachute-system design…and we already know how difficult developing parachutes for Mars can be.

  Fig. 7.1. BOUNCING TO MARS: Like Pathfinder before them, the twin Mars Exploration Rovers arrived encased in inflated cloth bags that looked, and acted, like giant beach balls. Each rover rebounded over a dozen times but eventually landed safely. This landing system was not strong or accurate enough for Curiosity. Image from NASA/JPL-Caltech.

  Rob Manning had discussed the parachutes with me at some length, but I'll just repeat a bit of it here. He detailed the gruesome process of testing the parachutes for MER, as (in a repeat of Pathfinder's teething pains) they ripped, snarled their lines, and ripped again. “We were still testing the parachute in the wind tunnel with less than a year to launch, and still having failures. We attached big weights to the parachute and threw it out of a helicopter. Sometimes the parachute opened late, and at that point it was going so fast that it just ripped open. So we would retime the opening and it would still rip. The whole setup was just really hard to get right.” They tested and tested, until they felt they had wrestled the problems into compliance. But right up until landing, the parachutes were still a concern.

  These and a thousand other challenges were ground down to size and beaten into submission, and the twin rovers launched in 2003, arriving at Mars in 2004. They entered the Martian atmosphere at high speed as anticipated, the parachutes deployed and inflated as planned, and the airbags inflated on cue and cushioned the rovers as they bounced to a stop. Of course, all this happened a hundred million miles away on Mars, and the engineers had to wait over fifteen minutes for the confirmation signals to reach Earth. But when they came in, everything had worked damn near perfectly and each rover landed in the region at which it had been aimed. The landings were engineering tours de force.

  As it turned out, the area in which Spirit descended, a crater named Gusev, was a flat, billion-year-old basaltic plain. This was interesting in its own right but not what the scientists had hoped for—the basalt resulted from lava flows that obscured the older sedimentary material beneath. The rover would drive and explore, but its primary objective—finding evidence of a watery past and a possibly habitable environment—would be a challenge. But then Opportunity landed.

  This second rover came to rest in a region called Meridiani, which was much more geologically varied than Spirit's. And better still, Opportunity rolled to a stop inside a crater. Named Eagle Crater, it was in effect like a huge drill hole deep into the Martian surface, created by nature. Opportunity was the benefactor. “We lucked out!” Squyres enthused, “We discovered that we were in a giant impact crater that had all the things that we could have wanted exposed there in the wall of the crater. In two months, much of the important science was revealed to us.”

  The discoveries began to pour in: they had direct and verifiable evidence of copious amounts of water on Mars in the past. This was represented in the sedimentary layers found throughout Meridiani and was also evidenced by the small hematite spherules, dubbed “blueberries,” found everywhere the rover went. Grotzinger was a prominent member of the MER team, and this discovery just whetted his appetite for more.

  “I was the only sedimentologist on that mission,” he recalls, “but there were other geologists who were familiar with sedimentary rocks. When we rolled off the lander and went to the first outcrop and l
ooked at it, we said ‘Yeah, these are sedimentary rocks.’ Then the question was, do they have something to do with water? Soon we were seeing the ‘blueberries,’ the spherules, lying all over the place, and we saw that they were coming out of the outcrop. At that point, we labored night and day and debated what the various origins might be. The berries were a big part of it. One option could be that they were volcanic; the other option could be that they were produced by a big impact. The third option, which was considered to be the most exotic, was that they were sedimentary. We worked through this, and it was a lot of effort,” he remembered with a smile. “It was about three weeks. Seventy-five percent of the team was pretty well convinced that they were sedimentary in origin.”

  Fig. 7.2. ROVING ENDURANCE: This composite portrait of Opportunity driving at Endurance Crater shows the ubiquitous “blueberries” on the ground. The MER rovers pioneered much of the technology, especially the autonomous driving techniques, used for Curiosity. Opportunity's mission continues today. Image from NASA/JPL-Caltech.

  For Grotzinger it was a turning point. “For the next seven years [and] when we got to Endeavor Crater that's all we saw.” But one can take nothing for granted on Mars. “We decided to repeat the experiment. Every time we went to a new outcrop, we thought, ‘This is Mars, everything can totally change!’ But then we get to another outcrop and we say ‘Hmmm…here is the same thing.’ But it wasn't exactly the same thing. Sometimes the environment that it indicated was wetter, and at times it was dryer; in some cases, we had fields of ancient sand dunes that were frozen in time.” These are subtle differences, but they make a lot of difference in geological terms, especially on a currently arid world like Mars.

  The evidence of sedimentation was clear. They had seen cross-lamination, a sure indicator of water-deposited sediments. And these layers were deposited not by a lazy pool of standing water but by a flowing stream or river. Mars had not just been damp but had at one point been drenched in water.