Curiosity

by Rod Pyle

  A reminder about Spirit and Opportunity: these rovers were expected to roll to a landing in areas covering seven by fifty-four miles. So even if you knew of specific objects within that landing ellipse that were potential rover killers, it would be a matter of probability whether or not those objects would interfere with the landing. The good news was that by using the airbag landing technique proved on Pathfinder, you could bounce and roll across the worst of them, and odds were that the machine would end up somewhere that was adequate to allow the rover to roll off the landing stage and proceed with the mission.

  Curiosity would approach the forbidding surface in an entirely different manner, and one that entailed its own gruesome challenges.

  One more major improvement in MSL's design was the way in which the rover would be powered. As I mentioned, Sojourner and both MER rovers used solar panels on their backs to gather the weak sunshine on Mars and convert it to electricity. This was routed to batteries and stored for later use. It's a great system in theory and can operate at least a decade as Opportunity has shown.

  But solar panels have their drawbacks. One major limitation is that that they work only in direct sunshine. It's hard enough to get a lot of power out of these things on Earth, which receives four times as much sunlight as does the surface of the red planet. Solar panels on Mars have to work even harder than earthbound panels to generate much power.

  Then there is the dust that is ubiquitous on Mars. Due to the geology and weathering processes of the planet, the place is lousy with talc-fine dust that the winds can accelerate to very high speeds and cause planet-girdling storms. Even lesser weather patterns carry thousands of tons of dust and dirt aloft in the wind. And as with the 2007 dust storm that threatened the MER rovers, these events can be rover killers. The dust storms darken the skies and can end up coating the solar panels with a layer of grime that reduces their ability to receive light.

  To overcome these limitations for Curiosity, the designers turned to a technology that, in spaceflight terms, dated back to the 1960s. In more brutal terms, it dated back fifteen years earlier. They would use nuclear fuel—plutonium—the very same stuff that incinerated Nagasaki in 1945 (the Hiroshima bomb carried uranium, a less-powerful nuclear source but still very destructive when used in a bomb).

  Plutonium fuel had been used on the Voyager spacecraft that traveled out to the distant planets of the solar system—Jupiter, Saturn, Uranus, and Neptune—where there is almost no sunlight. Plutonium was also used to fuel the lunar-surface science packages left behind by Apollo 12 through Apollo 17, as well as the Viking landers. Space budgets of the time were vast, and there was a lot of plutonium around from the 1950s and 1960s (guess why?).

  The device that uses this fuel for spacecraft power generation is called an RTG or Radioisotopic Thermal Generator (there are other acronyms used, but this one is the most descriptive). The “hot” nuclear fuel is placed inside a metal cask, and its slow radioactive decay causes heat. Surrounding this are thermocouples, metal strips that convert heat into electricity. So, in this use, the plutonium always remains subcritical, that is, it is not used for fission (atom-bomb) or fusion (hydrogen-bomb) reactions. It just sits and quietly gives off heat.

  The upside is a nearly permanent source of power that is immune to dust storms, nightfall, or winter darkness. The half-life of plutonium is fourteen years, and it can provide power far longer than that. The Voyagers have been traveling the solar system for over thirty-five years and, while their power supplies are declining, they are still operating, albeit in a reduced capacity.

  If there is a downside, it's primarily theoretical. Some people get upset when NASA launches nuclear generators into space—what if the rocket blows up? Won't it poison the atmosphere? Well, NASA's civil servants don't want to die a lingering, horrible death of radiation poisoning any more than you do. The plutonium is encased inside a containment vessel that is tested to be much stronger than is required for it to withstand an explosion and atmospheric reentry. If that event did occur, the cask-encased nuclear fuel would fall into the ocean, sink to the seafloor, and slowly radiate heat for about forty years. Of course, it's toxic much longer than that, but nuclear plants on the surface of our planet are far more dangerous. It's all a matter of containment.

  There is another problem however: the United States used up its available supplies of plutonium long ago, and making more is not a trivial thing. So rather than reopen the old bomb factories, we have been buying it from our former archenemies, the Russians. Irony upon irony—a substance created and first used in the rage of war to destroy entire cities is now purchased by Americans from an old enemy that made that same substance to turn the United States into a pool of molten goo. It's a far better use.

  Truly a whole new ball game.

  8.2. GOING NUCLEAR: A schematic of a Radioisotopic Thermal Generator, or RTG, similar to the one powering Curiosity. The gray rod at the center is the plutonium fuel, which on Curiosity weighs eleven pounds. The RTG is mounted upward at an angle on the back end of the rover. Image from NASA/JPL-Caltech.

  Remember the Mars exploration mantra: follow the water. The observations from Pathfinder regarding a once-wet Mars had been titillating, and those from the ongoing MER rovers were transformational. In the midst of this embarrassment of riches, the scientists had to decide where to send Curiosity. The process was far more public and open-ended than you might expect.

  There is a tension between the mission scientists and the engineers—and it's all about the potential for wonderful discoveries versus arriving and operating with a margin of safety. In addition, on the science side, the potential for scientific return is paramount, and various locations on Mars were under discussion from that perspective. On the engineering side were people who loved a good challenge and would put the rover wherever they were instructed while warning about possible consequences. The scientists wanted an abundance of rock types, variations of surface terrain, new and ever-richer potential examples of water-altered rocks, and so forth. On top of both groups was NASA headquarters. They, of course, wanted great science, but please…please…don't lose the rover.

  So, a bit of pressure here.

  Grotzinger was at the helm by the time this discussion got serious in 2007. As with so much of the mission, the landing-site discussion involved many viewpoints that needed to be reconciled. A multitude of potential landing sites, all offering something wonderful, were being discussed. Each site had its promoters and adherents, and all views were held with great passion. Somehow, a final decision would have to be reached, and soon.

  When interviewing Grotzinger about the process of narrowing down landing sites, I got a sense of great restraint at work in his retelling—the process must have been pretty tough. The scientific community was trying to decide where to drop their once-per-decade, $2.5 billion rover. It could not have been easy…or even always collegial. “There was a lot of discussion, some acrimonious. People had their views of what the planet should be. There was sort of a status-quo view that it was first and foremost a volcanic planet and anything that involves water is likely going to be a hydrothermal environment [i.e., one involving ancient hot-water processes]. That's a totally legitimate point of view. When you look at the geological context [and] some of the places that were advocated, it makes perfect sense. In other cases, like when Eberswalde Crater came along 2003, which was not accessible to MER in 2004 for the landing-site selection because the [landing] ellipse was too big, it came back for MSL.” Eberswalde is a geologically interesting, partially buried impact crater. It would end up in second place as a preferred landing site for MSL.

  “Holden Crater, right next to Eberswalde, also has a lot of evidence for what look like sedimentary deposits. Gale Crater had been considered in 2004, but nobody really knew what to make out of it. It's just kind of a weirdo. But more importantly, you couldn't fit a landing ellipse in there. Now everybody agrees that all these places have had water. The question became, which one of these w
ould have been the most habitable environment? If microbes would have been there, people can imagine that there will be certain metabolic pathways where certain types of organisms could grow. Then there are other places where other types of microorganisms might have grown. What everybody was really trying to come up with was what they thought might be the best scenario. I realized that the trick for this mission was not whether life had evolved on Mars, it was that there could be places that could have been habitable environments. The trick was to find the evidence of it. The evidence isn't always preserved,” Grotzinger concluded.

  Now it got tricky. A decision would have to be made based on the likelihood of finding things once the rover was on the ground that could not readily be identified from orbit. It was the Viking problem all over again…one had to infer, using other observations and a sense of context. For, while the images from the Mars orbiters were better than ever, not everything can be seen from the orbital perch. It is tough, for example, to observe carbon deposits from orbit. “We can see carbonate minerals rarely, very rarely, in one or two places,” Grotzinger added. But that in itself was not much to go on. “It was just a big uncertainty. So I basically said, ‘Look, guys, we're just going to have to go there and hope that some of these things that we imagine are actually preserved.’”

  There had already been two big meetings about this, called MSL landing workshops. Now, in 2007, they would have the third. “They had managed to pare down sixty targets to twenty. But it wasn't until the third workshop that people started to really work it because in a few years we're going to have to get down to one. It was a big challenge. The process is open, so anybody in the community can engage with it. So I arranged one part of the workshops for some people that had experience looking for habitable environments on the early Earth. We are looking for preservation of organic carbon.”

  A short list of candidates had been compiled and had to be evaluated on both scientific and technical merits. In the past, difficult terrain and the lower-resolution images from the orbiters (which forced more guesswork) would have combined with more inexact methods of landing the rovers (i.e., the airbag system) to eliminate some of the potential sites. But not this time.

  Grotzinger continued: “I discussed it with the program scientist at NASA headquarters, and we realized that for the first time in the history of planetary sciences, the engineers weren't going to kill a single potential landing site.” In a historical first, the folks handling the landing system and worrying about the mobility of Curiosity were not vetoing any of the top sites still standing. “All of our top four finalists were viable! Holden was viable, when it had never been viable before. Eberswalde was viable. Gale was viable, and this place called Mawrth [Vallis] was viable. All of them are viable. How the hell are we going to reduce it down to just one?” Holden was a vast crater that seemed to be an ancient lake, and Mawrth Vallis was an ancient outflow channel possibly littered with clay, also indicative of water and sedimentation.

  What Grotzinger and his NASA colleagues decided was to present the rest of the scientists with a short list of choices. They were all reasonably safe and could be precisely targeted, given the accuracy of MSL's landing system.

  NASA convened the working group again and tossed the options into the fray. “The community made a list of pros and cons and then people took a swipe at everybody else's chosen site,” Grotzinger recalled. “Then we sat down and looked at our portfolio like an investor would, as a science team, which worked incredibly well.” They pared the list down to four candidates, and took a vote. At last, they would have a winner.

  Or perhaps not.

  Fig. 9.1. FINAL FOUR: This global map shows the top four contenders for MSL's landing site. The potential sites are boxed—Holden, Eberswalde, Mawrth Vallis, and Gale. Image from NASA/JPL-Caltech.

  “It turned out that they [each site] received exactly the same number of votes!” Grotzinger chuckled as he recalled the conundrum. With almost one hundred scientists voting on four sites, the result had been a dead tie. What were the odds of that?

  “It was unbelievable,” he said. “Then I realized, wait a minute here. What we will do is to have people vote for secondary favorites as well. When we did that, there was a big difference. I realized something very important—it was clear that we are going to be divided if we picked one versus the other.” However, if the resulting selection was at worst everyone's second choice, there would be few ruffled feathers. So they moved into the final phase of the process.

  Grotzinger told them, “Pick your first favorite, pick your second favorite, pick your third favorite, and pick your fourth.” He continued, “One site fell away right off the bat. The third looked a lot like the second. Then we decided the best thing to do was to have nine of the principal investigators pick the site.” They were down to two candidates.

  The results of this final phase were remarkable. “Gale was the fifty-percent favorite as everyone's first choice and eighty-percent favorite as their second choice.”

  The other primary choice besides Gale Crater had been Eberswalde Crater. It was agreed that this was a former river delta and that there would be good science to be done there. But “the problem is, now that you are there, what do you? What do you do for two years? What do you do for ten years? We characterized this site as a one-trick pony. If the trick works, it will be a beautiful day in the history of Mars exploration; but if it didn't work, then you will be faced with this awkward situation to try to explain to the taxpayers. They could say, ‘We knew there was water on Mars before you landed, that's what MER showed. Now at the Eberswalde delta you [had] more water, you [had] a lake, but what good is your lake if it doesn't have organics in it?’”

  What good indeed? They realized that at Gale Crater they would have far more options. There was the alluvial fan at the projected landing site and lots of interesting areas around it. Then, at the center of the crater, projecting 18,000 feet into the thin Martian air, was Aeolus Mons, also known as Mount Sharp. This huge feature was highly unusual in that it was composed of layer after layer of sedimentary strata. How it came to be in the center of Gale is still a topic of some conversation, but the important thing was that it was there. The mountain could provide a look at a huge chunk of the geological history of Mars, billions of years of fossilized data. Driving through the foothills of Mount Sharp would be the frosting on the cake.

  “When you look at Gale, there's something for everybody. There [are] all kinds of different options. The risk was [that] there wasn't just one beacon gleaming with a definitive story there. Before we landed, people said, ‘Yeah we know you guys see that alluvial fan, but it's kind of small. It's not the best-looking alluvial fan. Maybe we are going to get burned…maybe it's a lava flow.’”

  While this sounds like a geological beauty contest, the point is that from orbit one thing can mimic another. He continued: “Lavas can spread out like cake batter. Every other site where we ever landed, we've been burned. Pathfinder was supposed to land in a field of what should have been rounded boulders. That team [later] published the interpretation that they were transported in water, but it was a hard sell. Then, Spirit lands on what is supposed to be a lake but turns out to be lava flows. Opportunity lands, and it's supposed to be on a volcanic edifice that's been oxidized and had its top crusted over with iron, but it's not. It's ancient, windblown sand, and it has goofy sulfate minerals that carry a lot of iron. Who the hell would have thought that?”

  Fig. 9.2. GALE'S GRACES: This chart shows the many potential rewards of investigating Gale Crater. From the top, we see large, shallow valleys coming from the crater's rim, the alluvial fan that John Grotzinger often refers to. At the bottom lie the suspected clays and sulfates at the base of Mount Sharp that the science team covet, along with the billions of years of strata in the mountain itself. Image from NASA/JPL-Caltech.

  Who indeed? Certainly not me, and apparently not a lot of much-smarter people, either.

  A clarification is in order: th
ese interviews were a lot of fun. Besides the interesting retelling of the early history of the program, and besides having the opportunity to chat with the people making the science happen, there are also moments like the above: “Who the hell would have thought that?” as Grotzinger sits, hands spread, asking what to him and the other scientists on the mission is probably as obviously odd as a Doberman driving a BMW is to the rest of us. I nod knowingly during moments like this, trying to imply that I get it, that of course I would have had the same reaction to the obvious. No chance, of course, but it's fun to feel like an insider, even for just a few hours.

  The overall message was, however, clear: Mars is never short on surprises, and it's not stinting in the creation of mysteries. What unexpected delights—or agonies—would Gale Crater hold for Curiosity?

  Curiosity is a mission with a purpose. Explaining that purpose to the public has been a bit of a challenge, however. With Pathfinder, it was easy: we wanted to send an inexpensive, quickly designed, experimental rover to Mars to drive around and look at some rocks. It's going to be a test-bed mission. MER was a follow-up to that first rover with a spectacular twin-rover mission. They would be bigger and more capable and would drive as far as they could. Their instrumentation allowed them to better see the landscape, drive through it with some autonomy, approach rocks, and examine them in greater detail. And, as always, continue the search for a wet, watery past.

  Now, here comes MSL, and it has a vastly greater reach in scientific capability. But that statement alone does not convey what a quantum leap the mission, and the Curiosity rover in particular, represent. To better understand this, a discussion of the instrumentation on board is in order.