by Rod Pyle
More blueberries were found—they turned out to be quite common in Meridiani. One area was so densely adorned with the blueberries that they dubbed it “Berry Bowl.” Here at last were enough of them clustered in one spot for Opportunity's spectrometer to finally get a solid reading (individual berries are very small). They checked out as hematite—a water-formed mineral.
The presence of water was becoming almost boring…to the public. But not to the scientists working on MER.
“The whole theme of this Mars exploration program has been to follow the water, to understand the possibility of life on Mars. Clearly all life that we know of needs water in its cells and in its environment to survive, so it's always been the major goal of these Mars missions to understand the role of water in both the history of Mars and also the evolution of Mars,” said Squyres.
“The water was primarily beneath the ground,” he mentioned about the period when Opportunity was exploring a crater called Endeavor. “It was not nice, pure water, it was salty stuff, and it was probably very acidic.” That was not good for living things. But this was merely sauce for the goose to the planetary geologists. It would be one more thing to add to Curiosity's wish list—evidence of benign water in epochs past.
Across Mars, Spirit would not have it so easy. When the rover began sniffing the rocks in the Gusev area, one of the early discoveries was a mineral called olivine. It was not something they wanted to find, as it indicated a long-dry environment.
“Olivine is a mineral that tends to be present in unaltered igneous rocks,” said Squyres, “so finding it was a disappointment because that was one of the first things that made us realize that instead of landing on sedimentary rocks, we landed on a lava flow, one at least a billion years old. It took a while to sink in what we were dealing with, that the sediments we were looking for were completely buried in the lava.” He sighs at the memory. “Once we finally realized that, we had to move somewhere else.” That's when Spirit drove on to explore the Columbia Hills.
Once there, they had more luck. These rocks had been altered by water and were not just an endless expanse of flat basalt. As the amount of olivine declined, they found more indicators of a wet past, especially sulfates, which are indicative of clays and therefore water. Olivine decomposes in moist environments, so they were headed in the right direction.
The list of discoveries by the MER rovers can fill its own book; suffice it to say here that the list was long and at nearly every turn it led to the conclusion that Mars had once been very wet. This in turn dictated an even tighter focus for the mission of Curiosity: further refine the likelihood of an ancient habitable environment.
In July 2007, vast dust storms began swirling around the planet. With no oceans to intercept them, Martian dust storms become global events. The daytime illumination got dimmer and dimmer, and the rovers, which were dependent on solar power to provide electricity, were running down. The engineers began to worry; at one point, the sunlight was reduced by 99 percent. On July 20, 2007, NASA released a terse statement: “We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense.” The rovers were parked to wait out the storm, but if the power fell too low for too long, it would be fatal. They could switch to low-power fault mode but might not have enough energy to come back when the skies cleared. But clear they did, and by August 21 it was sunny enough to recharge the drained batteries and for driving to begin anew.
The use of solar panels for power had just demonstrated one of its disadvantages. Between the regular coatings of dust and the fact that the sun is fairly weak when you get as far from it as Mars is, a different source would be needed for Curiosity. The complexity of the new rover would demand one more reliable and consistent, that could not just power the rover through its duties but also help to supply current and heat through the long, cold nights.
Note #1 to metaphorical self: Find better, continuous source of power.
By early 2009, echoes of the Pathfinder computer glitches were reverberating through Spirit's brain. The onboard flash memory, not dissimilar from the thumb drive you might use to back up photos or transfer files to your own computer, began to behave erratically. It would download instructions and then fail to execute them or neglect to upload all the data gathered on a given day. It seemed to have developed a mind of its own. Then, like a tired dog curling up for a nap, it began to reboot spontaneously. These issues would crop up for the remainder of the rover's life.
Note #2 to metaphorical self: Supply a backup computer system.
Spirit managed to drive almost five miles across Mars in its six years of operation. Then, just past the five-year mark, it became bogged down in sand. And there Spirit sat, effectively functioning like a stationary lander, until the radio failed in March 2010. JPL worked on the problem until May 2011 and then, after a herculean effort, called it quits. They held a formal farewell ceremony for the rover on Memorial Day.
But very little is wasted on Mars. The long hours put into the attempted recovery of mobility—planning, software simulations, then mechanical simulations with the rover's earthbound twin in sand both at JPL and in the California desert—would be very educational. They would use this education for the continuing mission of Opportunity and in the ongoing preparations for the Mars Science Laboratory.
Note #3 to metaphorical self: Use larger wheels that spread the weight of the vehicle across a wider surface and have lots of traction.
Opportunity has had a better go of it. Now nearing its tenth anniversary on Mars, it has outlasted its three-month mission exponentially. In its long life, it discovered the blueberries scattered across Meridiani, located the first meteorite ever found on another planet, drove deep into vast craters, and studied the strata found there, which was a geological gift from the planet-drilling impact eons before.
Opportunity's long life and extended driving has allowed engineers to refine a technique they had only experimented with on the Pathfinder mission: autonomous driving. For much of the twenty-four-hour, forty-minute Martian day, the rovers were out of contact with Earth. They could either sit and wait until they could get live (albeit delayed) driving instructions, or they could learn to navigate on their own. This feature had been built in from the beginning, and the decade of operations gave the rover drivers precious experience in autonomous hazard avoidance and advance planning. Yesterday's images would be used to plan tomorrow's drive, or the next day's. A wonderful software program called RSVP—short for Rover Sequencing and Visualization Program—was developed. This tool allowed mission planners to download images from the rover, map them into a 3D model of the surface, then select routes to drive to specific targets. But that was not the amazing part. Similar software, living in the rover's limited onboard computer, allowed it to make its own surface maps. With some advance direction from the ground, the machine could then pick its way through the landscape, slowly at first, then with increasing confidence. Of course, it worked better in flat, open plains than it did in challenging, rocky, or cratered environments. But both MER rovers had plenty of flat terrain to cover, and the experience taught the JPL engineers a lot about autonomous driving. Pathfinder pioneered it, but MER matured it. Curiosity would perfect it.
Note #4 to metaphorical self: Apply more autonomy, both in driving and in surface operations.
In a related evolution, Opportunity's long and storied exploration of Mars has paid off in another, less direct way. With the fleet of orbiters that continue to circle Mars, providing ongoing maps of the surface far below, experience with the MER rovers has given MSL mission planners a far better understanding of how to correlate orbital imagery with what is actually on the ground. This would pay vast dividends when selecting a landing site for Curiosity…though it did little to dampen the passions, or temper the debate, about exactly where to set down.
Note #5 to metaphorical self: Having a pair of orbiters to relay information and provide updated surface maps is an incredible asset.
One final note: on May 16, 2013, Opportunity, having then driven 22.2 miles, officially broke NASA's off-world driving record set by the Apollo 17 lunar rover. That machine was piloted across the moon's surface by astronauts Gene Cernan and Harrison Schmitt, and covered twenty-two miles of hard, undulating lunar terrain. It did this during a three-day lunar stay, however, as compared to the Mars rover's decade. So, while Opportunity's record stands, there is still something to be said for human exploration. Once people are there, things will move a lot faster. (Yes, that is my hand you see raised in the corner, I am volunteering…)
Opportunity continues to rove the regions around Endeavor Crater. There have been ongoing problems with its aging robotic arm, and the motor driving its front right wheel has been using too much electricity for years. In fact, they have had to drive backward to compensate, and have done so for longer than they drove it as designed, arm forward. But the golf-cart-sized rover shows no signs of succumbing to the harsh Martian environment anytime soon and continues its slow but steady explorations to this day.
Note #6 to metaphorical self: How cool will it be to have two rovers working on Mars again once Curiosity arrives?
Lessons learned, time to step it up—a lot.
Even before the MER rovers landed, in fact, while they were still being built, JPL was tasked to come up with the next great thing. Based on what they hoped MER would achieve, they would spec out and design a new mission architecture that would go far, far beyond what even MER could do.
It was expected that much of what was learned with Pathfinder, and later with MER, would be applied to this new mission. And indeed, much was. But this new machine was so grand in its scope, so massive in its design, that much of what had gone before was not terribly informative. JPL’ers didn't just think outside the box, they threw it away.
In the late 1990s, a team of engineers, scientists, and managers were assembled to begin the process of identifying the shape and goals of such a new program. They were given a blue-sky mandate: think big and wow us. The result became known as the Mars Smart Lander, which would eventually morph into Mars Science Laboratory. Of course, blue-sky is fine until the price tag comes in, then budgets tend to reign in ambition.
What was known up front was that there was a keen desire to put ever-increasing levels of capability into the science packages on the rovers. The MER machines, Spirit and Opportunity, were already a quantum leap beyond Pathfinder's Sojourner. The MSL would be another huge step, but in exactly what direction and to what ends needed to be defined.
Overall, however, with over twenty years of technological advancement since the time of Viking, Curiosity would make the 1976 mission look like it had the capabilities of a sewing machine. The new rover would be more tightly focused and supplied with vastly better instrumentation than anything that had gone before, while building on past accomplishments.
Pathfinder had proved the concept of a roving vehicle that was delivered directly to Mars without settling into a parking orbit first. It also pioneered the a robotic arm and associated instrumentation, along with its innovative suspension system.
The Mars Exploration Rovers were in turn wildly successful and have gone far past NASA's wildest dreams in terms of accomplishment and longevity. The utility of improved instrumentation and the usefulness of orbiters for data relay were validated. Increased autonomy for future rovers was demonstrated.
Planetary-exploration programs build on experience and knowledge, and NASA is very good at incorporating lessons learned. JPL has made it into a science. While MSL had a lineage going back to Viking, it would not be tasked to look for microbial life as Viking had—that had turned out to be a far more complex undertaking than was understood back in the 1960s. In the time since Viking's mission goals were defined, truly weird life-forms have been discovered on Earth, such as the extremophiles, exotic critters that exist in hot, lightless environments like the “black smoker” geothermal vents on the deepest ocean floors. If things this odd, unexpected, and unusual could live on Earth, there was no telling what forms they might take on Mars. Other examples are living organisms found in places such as the Antarctic, that live inside rocks (near their surface), protected from harsh conditions but still able to draw sustenance from the environment. It appears that life is far more tenacious and clever than we had thought in the 1950s and 1960s, so another life-science mission to Mars would have to wait until habitable environments had been identified there. We would need good evidence to believe that life might currently survive on Mars to embark on such a costly and ambitious mission.
Once MSL's mission goals were defined (and constrained), this helped to inform choices for instrumentation to put on board (though there would still be plenty of debate over the final selections). The tools on Pathfinder had been minimal, and Spirit and Opportunity had expand on these greatly. However, MSL specifications were being planned well before the MER rovers landed and began operations, so decisions would have to be made largely without MER data and then would have to be refined later.
If you are beginning to sense nonideal circumstances here, I'll know you have been paying attention. Due to even this moderate pace of the Mars exploration program, two-year departure windows and the vagaries of congressional funding, planning for a current mission was usually based on data from missions two generations previous. So decisions about how and where to land Curiosity, and what to put on board, were hinging on what had been learned up to and including Pathfinder. Then, as data streamed in from the current mission, MER, they would be incorporated into MSL's plans. It's a system that has evolved to be tolerant of flux but can occasionally hold some surprises and make for some difficult choices. Fortunately, the 1990s and early 2000s were such a rich period for Mars orbital missions that the landscapes have been mapped at very high resolutions, with even small features visible, so mission planning for the future has become much more of an exact science.
Back to instrumentation. Pathfinder's lander had carried cameras with a spectroscopic ability. The Sojourner rover had cameras on board as well as the Alpha Proton X-ray Spectrometer (APXS) on the end of its arm, which could bombard target rocks with high-energy particles and read the resulting spectra.
The MER rovers’ onboard cameras were much higher resolution and far more capable. A variety of spectrometers, both active (like Sojourner's APXS) and passive, provided a huge improvement in the ability to understand the composition and nature of rocks and soil. The RAT (Rock Abrasion Tool), a rotary wire brush, allowed for the cleaning of rocks before they were investigated. A microscopic camera on the arm gave highly detailed close-ups of chosen targets. But as impressive as these improvements were, they paled next to what was planned for Curiosity. If one looks at the lineage from Sojourner through MER, it is a bit like comparing a little red wagon with a camera to a Jeep with a few scientific tools mounted to the front grille. Moving on to Curiosity equated to a school bus crammed with a full-on science lab. This was the challenge the JPL engineers, as driven by the scientists, placed before themselves. We will look at the onboard instrumentation for Curiosity soon. But first: where to land the massive rover?
The designers would build on data from previous rovers as well as the vast trove of new images and data coming in from the orbiters. By following the trail of ancient water features and geological evidence of the same, Spirit and Opportunity would provide data to better interpret the high-resolution orbital images and, by extension, help to determine MSL's landing zone.
Each Mars landing has been more precise than its predecessor. In the time of Viking, the safety of the lander had been of paramount concern, with interesting surface geology a distant second. When the landing area for that mission had been planned in the 1960s and early 1970s, the images available to do that had all come from Mariner and Earth-based telescopes. As we know, the detail from telescopic operation was pitifully low—even continent-sized areas were fuzzy. When the Viking planners were considering landing sites, much of their avail
able knowledge came from Mariner 4, which had mapped only 1 percent of the Martian surface—hardly enough to base a decision on. Mariners 6 and 7 had improved this, but they were still flyby missions that imaged only a swath of the planet as they dashed past. You could have flown over a Martian city and not known what it was.
Fig. 8.1. MEET THE FAMILY: For a sense of scale, JPL posed the three generations of Mars rovers together (along with a couple of willing volunteers in the ubiquitous lab coats). Pathfinder's Sojourner (1997) is to the lower left, a Mars Exploration Rover (2004) is to the upper left, and, dwarfing everything, Curiosity is to the right. The radioactive power source has not been attached yet. Image from NASA/JPL-Caltech.
By the time of Mariner 9 in 1971, image resolution was about 1,100 yards—anything smaller than that would be invisible. And yet it would take just one crater rim or one rock more than about eighteen inches high to destroy the lander. It seemed like a hopeless task, but with the momentum of the space race pushing them, they proceeded.
Of course, there was more to it than just the pictures. By observing areas with spectroscopes and looking for moisture and mapping surface temperatures, much could be inferred about surface characteristics that could not actually been seen with the eye.
The Viking orbiters improved image resolution, but we are still talking about twenty-five feet for the smallest surface features.
Flash-forward to the 1990s. The Mars Global Surveyor, a replacement for the failed Mars Observer, arrived in 1997 and began mapping the surface for the first time since Viking. A lot of surprises were in store. Objects as small as about five feet were now visible.
Additionally, scientists at JPL would compare these pictures to what were called “context images,” which were a wider view and showed the type of landscape the close-ups were located in. Between these two, researchers could infer a lot about the nature of the terrain. Of course, this was no help for the Pathfinder lander, which was arriving at Mars in the same year, but it was a tremendous help in planning for the MER rovers.
