by Rod Pyle
Altogether, it was a neat and tidy little science package, which owed a lot to the successes of its predecessor, Pathfinder's Sojourner rover.
The landing zone for Spirit, the first rover to descend, had been carefully selected. It would have to be smooth enough that the airbag landing method would work. It had to be low enough in elevation that there would be sufficient atmosphere to pass through for slowing of the heavier rovers to occur. It had to be near the equator, and not so potentially dusty that the solar panels would become disabled. Over 150 candidates were considered; for this first landing, the final choice was a large crater named Gusev.
About fifteen degrees south of the Martian equator, Gusev was named after a nineteenth-century Russian astronomer. It is almost one hundred miles wide and geologically speaking is a transition zone between the ancient highlands to the south and the smoother, younger plains to the north. And entering the crater from the southeast is a 550-mile-long meandering valley called Ma'adim Vallis, which appeared to have been massively eroded by water at some time in the distant past. The hope was that, since the water appeared to have emptied into the crater, the floor might have layers of sedimentation that could be explored.
It was a cleverly selected site, decided upon by a combination of acquired knowledge, deduction, and detective work. Its promise seemed clear.
Spirit arrived in a fireball on January 3, 2004, entering the Martian atmosphere at over 12,000 mph. Once again, airbags were used to cushion the blow of the high-speed entry, and the machine bounced a couple miles before settling into its final landing spot.
Upon arrival, the craft deflated the airbags, unfolded its petals, and took a preliminary image, again, just as its predecessor Pathfinder had. On the ground in Pasadena, those glued to the monitors were ecstatic. It was just what they had hoped for.
To the untutored eye, the flat expanse with a few rocks would seem desolate. But for a rover control team and the associated scientists, it was heaven. The rover had a perfect-looking surface to traverse, a wide selection of rocks to explore, and an open horizon to seek. And, perhaps most important, it was different than that encountered by either the Viking landers or Pathfinder.
For the first week, the rover sat in place and surveyed its surroundings. Nearby was a depression about thirty feet wide, soon dubbed Sleepy Hollow (features near landing sites never remained anonymous for long). It was either a wind-worn hole or a meteor crater; either way, it offered immediate access to subsurface geology. It was the proverbial hole in one.
But before this would be explored, a number of more symbolic gestures were to be made. First, there was a plaque aboard the rover, which was dedicated to the astronauts lost in the space shuttle Columbia accident with a moment of silent observation. Then a DVD, stored atop the spacecraft, was imaged by its camera. It was a funny moment: here was a DVD, with a little Lego®-style robot printed on it, and all of it held in place by what appeared to be Lego building bricks. It was all part of a sophisticated but seemingly simple effort to reach out to kids, to budding young scientists. This is something that JPL has done exceedingly well, especially since the Pathfinder mission with its huge Internet component. This DVD held the names of four million people, part of the “Send Your Name to Mars” outreach project, along with other student messages to future human or alien explorers. But not to forget the science, simple magnets had been attached to the edges of the disk to allow students around the world to study how much ferrous metal was contained in windblown dust. It was clever beyond measure, and fun to boot. Before too many years have passed, the first of these students will be entering careers in space science, many inspired in part by this simple gesture. It was an unusual moment of marketing genius by NASA, and it cost next to nothing to accomplish.
But now, it was time to explore. To the transmitted strains of Bob Marley's “Get Up, Stand Up,” Spirit flexed its robotic muscles and prepared to roll off its platform, a week after arrival. But before it went a-strolling, more images were sent home, these with the infrared spectrometer. After the rousing success of infrared imaging by Mars Odyssey, MER had taken a similar instrument along for the ride. Once again, materials invisible to the naked eye could be seen in the surrounding terrain, but this time, it was at ground level. Besides helping to identify the composition of the rock, the instrument also spotted dusty areas to be steered around. It was the best thing since putting wheels on Mars!
First the front wheels, then the rear ones, were extended. This was a carefully observed process. The suspension of the machine had acquired its DNA from the Sojourner rover, using the same “rocker-bogie” arrangement of swinging arms and six wheels.1 Once upright, a cable near the center of the rover had to be cut to allow it to move free of the lander. NASA had long ago learned the danger of plugs and connectors coming loose on spacecraft, so, since the dawn of the space age, when spacecraft connected by a wire to another craft or the ground needed to separate, this was accomplished with explosives and knifelike guillotines. While this may sound extreme, it works well in practice, and the failure rate has been very low. Bang went the pyrotechnic charges, a blade swung and the wires were cut, never to rejoin. Spirit said good-bye to its lander. It was the last of 126 pyrotechnic charges fired since the launch of Spirit almost a year earlier. Nobody said exploring space is simple…or quiet.
Slowly, so very slowly, Spirit embarked on the first step of its long drive to places unknown. Driving on Mars was a slowly evolving science. It had been done only once before, with the Pathfinder mission. And that rover, Sojourner, had gone only a few dozen feet away from the lander that was its link to home. Spirit and Opportunity would range far and wide, communicating (it was hoped) directly with JPL's orbiting spacecraft as they passed overhead every ninety minutes or so. But still, despite the wealth of experience, despite the somewhat-autonomous hazard-avoidance software onboard, and despite a long delay of outgoing radio commands (and a similar delay on verifications from Mars), the rovers needed to be driven by humans on Earth. It was the old mile-long drinking-straw analogy, the item through which mission controllers had to labor to make things work on Mars. It was not as simple as jumping into a car and driving off.
There were many layers in the process and many skills to be learned. First, the surrounding terrain was extensively imaged by Spirit to give the drivers a sense of place and a taste of the road ahead. Then, through communication with the Mars Odyssey orbiter overhead, Spirit got a better fix on its actual location. Finally, via the infrared images, it was possible to map out the difficult terrain and any “sand traps” in the surrounding area.
In the few years prior to the landing, JPL's “rover drivers” had been in a sort of extraterrestrial driver's ed class. Called Field Integrated Design and Operations, or FIDO, it involved taking a facsimile of the rover out to California's Mojave desert and simulating driving on the rock-strewn Martian plains. Like MER, FIDO moved slowly (less that 1 mph) and sported an onboard hazard-avoidance navigation scheme. The investigative tools were similar to Spirit's as well. As the program evolved, the rover was controlled from JPL just as it would be once on Mars. Commands were relayed via satellite with a built-in time delay to better simulate the Martian mission. In fact, the JPL personnel were not even advised as to where the rover had been dropped off, to better maintain the illusion of being on Mars.
The importance of simulating this mission was obvious. MER was a quantum leap beyond Pathfinder. The new rovers were much more autonomous, and indeed needed to be, for they would be covering up to three hundred feet in a day, which was farther than the Pathfinder rover ever got from home base during its entire mission. And with a planned mission duration of ninety days (and much more was hoped for), it was critical to gain experience with the machine.
While there were some basic differences between MER and FIDO—size, weight, and physical operations among them—they were essentially very similar and lessons learned would be of great value. Getting stuck in a crevasse or tipping over due to
an overly ambitious climb were a lot less expensive, not to mention final, on Earth than on Mars. On Earth, someone can wander over and kick the rover (and at times, someone did). On Mars, all one can do is invoke harsh language from afar.
The designers of the simulation had done their best to compress twenty days of Mars operations down to about ten days on Earth. This meant focusing on the most important operations and letting a few others slide. Like the grueling Apollo and Shuttle simulations of previous eras, planners deliberately inserted malfunctions and small emergencies into the program to make sure that the rover drivers back at JPL were on their toes.
The FIDO rover covered only about 450 feet during the ten days of the simulation, and the longest single traverse was about ninety feet. But it was enough: great pains had been taken to include problems and obstacles that would test the controllers' abilities and nerves. When asked, most of them felt as if they had already survived a compressed version of the mission.
As Spirit prepared to begin its historic travels, one programming team was sweating out the upcoming events more than the others. These were the authors of the autonomous hazard-avoidance software, essentially an advanced cruise control with the ability to avoid ramming into things or getting stuck, or so they hoped.
Beyond the driving-and-navigation software, programming was also used to make sure that the mechanical arm housing much of the instrumentation on the rover would not bang up against rocks and dunes. This required the rover to load 3-D images from its mast-mounted cameras into the computer, where it would build a 3-D map of the area being examined. This would then be compared to the projected path of the mechanical arm. If there was a conflict, changes to the arm's path (or suggestions to change the position of the rover) would be enacted to prevent damage to the sensitive electronics.
Overall, important lessons were learned, implemented in updates to software or procedures, then tried again. It's hard to say how many mishaps were avoided by the use of these simulations, but most would agree that they were worth many, many times their cost.
Back on Mars, it was January 15, 2004, and Spirit rolled onto Martian soil for the first time. Like a timid child heading off for a first day at school, the first image Spirit sent down was a (nervous?) look back at the now-empty landing stage. Its own tracks in the ruddy soil led to the bottom of the frame. Mission control erupted in cheers. In just over a minute, Spirit had moved ten feet, much faster than Sojourner. As a sign of how long things take when dealing with the lag time between Earth and Mars, including relay time from the assisting orbiter, the elapsed time between the sent command and reception of the confirmation from MER was over ninety minutes. You wouldn't want to drive in rush-hour traffic this way, autonomous software or not.
The first target, to be reached in about four days, was a rock called Adirondack. It was a low-lying football-sized chunk with flat sides that were fairly smooth. This made it a fine test candidate for the RAT, which could clean and, if needed, grind down the surface of the rock for closer examination. The rock and nearby dirt were examined, and almost immediately the surprises began. The first and most profound was the discovery of a mineral called olivine. This is a mineral that is easily altered by water, and the fact that it was here in relatively unaltered form meant that there had perhaps not been as much water in this area over the millennia as had been thought (and hoped for).
Then, just as things were getting interesting, Spirit went quiet. On January 21, eighteen days after its arrival, Spirit lapsed into silence. After a long and knuckle-biting day back home, the robot sent a message to its masters on Earth. It said, in effect, “I AM HERE. I RECEIVED YOUR LAST MESSAGE. I AM IN FAULT MODE—STANDBY.” For the next few days, technicians at JPL worked overtime trying to determine if this was a software or a hardware problem (one was probably recoverable, the other probably not, respectively). Then a few more short and cryptic messages came in from Spirit. Frustrated, JPL ordered the spacecraft to downlink an engineering data dump. It eventually did so, and it the problem revealed itself: the rover was having insomnia; it was dropping out of “sleep mode” and using excess battery power in the dark of the Martian night. This was also causing excess heating, which could be a danger to the craft.
Various theories were forwarded by increasingly desperate programmers and engineers. A leading candidate was that the machine was stuck in a “reboot loop,” an endless condition that occurs when the computer onboard the rover thinks there is a problem, restarts the computer, only to run across the problem when the computer is booting. It senses the fault and restarts again. The cycle can run forever, or at least until the rover's power supply or circuitry fails. It is a sort of cybernetic Möbius strip of the mind. Spirit had acquired a case of digital obsessive-compulsive disorder.
Programmers came up with a solution and it was sent up. The trick was to avoid a certain part of Spirit's cybernetic mind: the flash memory. It was not unlike a built-in version of the flash sticks we all use today. There was simply too much of the wrong kind of data in this area, and it was causing the faulty reboot sequence.
It took until the thirty-third day since landing, February 6, to resolve the issue, but resolve it they did, and Spirit regained consciousness. The historic mission of exploration was on once again.
Spirit was still parked near Adirondack; it was a bit like someone falling asleep in his soup and waking up, unfazed, to continue eating. Unsurprisingly, little had changed since it went comatose two weeks previous, and the rover resumed its chores as if nothing had happened. The RAT was brought to bear and began grinding away at the surface of the small rock. Controllers were careful—this was the first use of the tool on a Martian rock, and nobody wanted to be the person responsible for breaking it—but despite their caution, a nicely ground disk soon appeared on Adirondack, and the microscopic imager and spectroscopes on the robotic arm were brought into play. While the hole produced was only about one-tenth of an inch deep across an area just under two inches wide, it was enough. The results of this first-ever in-depth investigation of a Martian rock was like the first performance of a well-rehearsed ballet, and with the software issue resolved, things appeared to be back on track, and handsomely so.
The next challenge for the recently reawakened rover was to think on its own. A rock named White Boat was selected as its next target, and this time Spirit was told where to drive, but not specifically how to get there. It would have to use its hazard-avoidance cameras and software to plot a course and execute it. To do this, the stereo camera atop the mast on the rover provided images that the onboard computer used to build a 3-D map of the area ahead. Based on this, the rover would select the safest path, then continually update it with information gleaned from this and the hazard-avoidance cameras. It was a bit like handing your teen the keys to the car for the first time, with the attendant parental nervousness.
Meanwhile, there was more excitement afoot. On the other side of Mars, in a scene not witnessed since the twin Viking landings of 1976, another fireball appeared in the skies above a region called Meridiani Planum. The area had been selected, like Spirit's landing area, from a carefully parsed list of candidates. Of particular interest here was the presence of the mineral hematite, also known in some variations as magnetite, a form of iron that normally occurs in the presence of water. And since possible life, past or present, is the holy grail of Mars exploration, Opportunity, like its twin, would follow the water.
About six minutes after entering the thin Martian atmosphere, Opportunity had bounced to a halt, safe and sound, on Mars. But this rover, Opportunity, had been a bit luckier in its somewhat random final destination. JPL could pick the region, but the final resting place was at the whim of its Superball®-like arrival. For while mission planners had aimed for Meridiani, they had not known Opportunity would end up almost fourteen miles from the anticipated landing zone in an area soon known as Eagle Crater. While the rover could have later driven to such a crater, landing in one was a lucky stroke, and JPL consid
ered the shot a “hole in one.”
You see, craters are like holes punched into the crust of a planet to reveal the materials inside. Eagle Crater was no exception, and in addition to what the scientists might have expected to see, they also spotted layers of rock outcrops not far from the rover, about twenty-five feet away. Closer examination revealed apparent sedimentation…a possible sign of water-created processes. The layers, ranging from thick to thin (perhaps half an inch to paper thin), were presumed to be either a result of waterborne sediments or deposits of falling volcanic ash. Either pointed to a living, geologically active planet. And one possibility—water—was yet another indicator of the possibility of life. Meridiani was going to be an interesting place.
In early March there was another ripple of excitement through the MER team. Spirit had stopped at another rock, this one affectionately named Humphrey. It was about two feet tall and wide and appeared to be worth a closer examination. Spirit closed with Humphrey and got to grinding with the RAT. Once scientists got a look at the freshly revealed surface, there was a surprise in store. What they found literally sparkled in the sunlight: crystals. If found on Earth, this would assuredly indicate that water had moved through this volcanically formed rock at one time. The same could be true on Mars.
For anyone left from the Mariner 4 years, the scene would be somewhat surreal: a wheeled, mobile, drivable machine with semiautonomous computers onboard was parked next to a Martian rock, grinding away with a rock drill. It then employed devices of which folks from the 1960s could only have dreamed, at close range. The future had arrived.
