by Rod Pyle
Like other recent Mars orbital probes, MRO was built for JPL/NASA by Lockheed Martin. It was large by Martian orbiter standards and about the weight of a small car (2,300 pounds dry). In a nod to modern materials engineering, much of the structure was made up of carbon composites in addition to the more traditional aluminum and titanium. It was also the first spacecraft designed from the ground up specifically for the stresses unique to aerobraking. The computer brain of the craft was somewhat behind the cutting edge for off-the-shelf hardware of the time, a radiation-hardened, military-grade version of Motorola's Power PC® chip (the same unit that drove older Macintosh® G3 computers). The use of a commercially retired chip (the Mac was onto an advanced G5 by then) was not due to budget constraints or lack of foresight; rather, it reflected the need for a robust, proven, and evolved version of what would do the job. By planetary-exploration standards, this was still cutting-edge. Reliability and the ability to handle intense radiation from solar flares was key (thankfully, the military had already proven the chip's ability to handle radiation from nuclear attack, so space-borne radiation should be child's play). Few such chips are guaranteed to withstand the rigors of interplanetary space travel.
The eyes and ears of the spacecraft involved improvements on existing technology and some new designs as well. The High Resolution Imaging Science Experiment (HIRISE) was a new generation of camera and lens technology that would look down on the planet from on high. Previous cameras had boasted of being able to see things in visible light as small as about twelve feet across; the new camera would resolve items as small as three feet. The camera was also able to image in near infrared (much like Odyssey), which would allow it to see things not visible in normal light, such as dirt-covered gullies, channels, and other water-created artifacts. Mars was getting ever closer.
No Mars mission would be complete without a spectrometer. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) device sought out minerals that, as usual, extended the search for a watery past for Mars. Visible landforms were one thing; CRISM would search for the invisible indicators of features from the past, now buried or covered, such as dried pools, former thermal vents, dead hot springs, or dry lakes.
A device called the Context Imager (CTX) would create lower-resolution images to augment those of CRISM and HIRISE. The idea was to take wider-angle pictures of the exact locales observed by the other higher-magnification devices to provide a context (hence the name) for those observations. An example might be if one of the other devices showed evidence of water-deposited sediments such as a streambed or dried pond. The CTX camera would snap a wider-scale image (about twenty-five miles wide) that might show the former pond to be a part of a wider feature created by watery sediments or volcanic activity. It was a simple and elegant way of collecting data concurrent to the more powerful instruments. In addition, both CTX and HIRISE would be able to produce stereo (3-D) images of the surface below, continuing a trend of stereo imaging begun with the Viking probes.
Yet another device to take visible pictures was the Mars Color Imager (MARCI). This camera's job was simple: create regularly updated images of the broader planet for use in building seasonal climate and weather maps of Mars over time. It was a bit like a multispectrum stop-action camera trained on a busy street in Hollywood, providing a parade of otherwise pedestrian images that, over time, would create a fascinating picture of changes and trends on Mars.
The Mars Climate Sounder (MCS) was a modified camera that was able to sense in both visible and infrared. Its job was to measure, in deep “slices” of the atmosphere below, a profile that included temperature, humidity, and dust content, in high to low altitudes. Over time this would build up a more complete story about Martian weather in great detail.
A Shallow Subsurface Radar (SHARAD), supplied by Italy, probed for more water on Mars, as did so many other devices. What made this one remarkable was its reach: it could look almost three thousand feet deep! Where previous instruments had been able to see a yard or so into the dirt, rocks, and soil below, SHARAD promised to reveal much more complete results, albeit at less fidelity (features less than about four hundred feet wide would not be seen).
A gravity-field instrument would allow MRO to measure the Martian gravitational field in some detail, something not taken for granted since the Apollo program detected large variations in lunar gravity fields during the moon missions. These variations, dubbed “mascons” (mass concentrations) threatened to throw off orbital calculations and became a prime concern during that program. Future planetary missions would include devices to measure such things, no longer assuming that planets would have a nice, even gravitational field such as Earth's.
Finally, a device lyrically named Electra (for once, a proper name and not an acronym) was a radio that would act as a Martian GPS for the Mars rovers and also relay information from them and future landers to Earth. It would also be able to help track newer spacecraft on their way to Mars from Earth.
It is worth noting that a craft such as MRO has very critical navigational and positioning needs. With such high-magnification instruments aboard, small errors would be rapidly compounded. One of the quiet heroes of such a mission was the collection of devices that would orient and point the craft to specific spots for imaging and data collection. No less than sixteen sun sensors were affixed, along with two advanced star trackers (which had come a long, long way since the Canopus star tracker of Mariner 4). Additionally, two inertial measurement units, not dissimilar from the technology that allows an iPhone® to sense its orientation, were included to provide precise measurements of velocity in any direction, augmented by laser-enhanced gyroscopes. If nothing else, MRO would know exactly where it was at all times.
To best utilize this data, twenty small rocket motors were onboard, along with six larger thrusters for gross changes in speed. These tiny thrusters would allow for incredibly precise positioning of the spacecraft, and were further augmented by a system of flywheels called reaction wheels, used to stabilize the probe during operations.
MRO left Earth in 2005 for the now-familiar trip to Mars. Slinging into Martian space too fast for immediate proper orbit, as was now traditional for Mars probes launching on smaller, affordable rockets, it began aerobraking operations in March 2006. After the successes of the Mars Global Surveyor and Mars Odyssey, you might think that aerobraking was old hat at the lab. But not so, for, as it turns out, JPL had been planning these maneuvers under some mistaken assumptions about atmospheric density and had been somewhat lucky on previous missions.
Let's review. When a probe like MRO reaches Mars on a trajectory that requires aerobraking, it still fires the same slowing rockets that a mission like Viking, which did not require aerobraking, did. But it has less fuel than Viking and is therefore unable to slow as much, so it cannot force itself into a nice, round orbit. It enters an elongated orbit, shaped like a large egg. In MRO's case, aerobraking saved over 1,300 pounds of fuel—a huge amount when you are launching things from Earth.
The orbit the probe entered upon reaching Martian space was lopsided, with the farthest point being twenty-eight thousand miles from Mars. The nearest point during aerobraking was about sixty miles. The final orbit, the lowest yet attempted (which would allow for close-in use of the new high-powered cameras) would be 196 miles and circular.
To accomplish this aerobraking maneuver, one must plan very precisely. And to do that, you must know all the variables, such as the atmospheric density of Mars, exactly. Otherwise, you will not slow the craft sufficiently as it dips into the friction of the atmosphere, or you will slow it too much or perhaps even damage it. So JPL uses a mathematical model to calculate this value, which had worked well in the past. But MRO was to be a lower, more carefully positioned orbit, and extreme care would have to be taken to trim the aerobraking orbit just right.
When dealing with lower maneuvers in the atmosphere, one must take into account that the polar regions are very cold, resulting in denser air there.
Another reason that density can vary from day to day (even at the same altitude) is the lack of oceans on Mars. On Earth, the oceans are able to store large quantities of heat during the day and then release them slowly at night. On Mars, with no liquid water, no such balancing mechanism exists. Temperatures can fluctuate quickly and dramatically, by well over 100°F per day. Although the atmospheric pressure is just a tiny fraction of Earth's, these large temperature fluctuations affect the air density very quickly.
And there was yet another reason that the planning for these maneuvers was critical and was keeping controllers up at night: the space around Mars was getting crowded. There is the Mars Odyssey probe, Mars Global Surveyor, the European Mars Express, and a host of older orbiters slinging around the planet. During aerobraking, when you are changing altitude continually, you must plan to miss these other craft, as mission managers tend to get very cranky if your spacecraft smashes into their spacecraft during an orbital maneuver. Of the twenty-six rocket firings to alter the aerobraking maneuvers, six were designed to avoid colliding with other probes. It was a very complex piece of mathematical wizardry to pull off from many, many millions of miles away.
Nonetheless, five months later, MRO had reached a stable, circular orbit almost two hundred miles above Mars and began its primary mission. The probe began investigations intended to send home about five thousand images yearly. The primary mission was intended to be two years, then optional mission extensions would be considered. Ultimately, as with Mars Odyssey, researchers will have to split time between their own projects and large amounts of data being relayed home from the Mars Science Laboratory rover. MRO is already in extended-mission mode, probably one of many.
On to the science.
Since all Mars missions have a goal, one way or the other, of searching for water, MRO did its part. With all the experience gained over the years, the complex of instruments the designers had clustered on the spacecraft—the most powerful to date—worked in brilliant harmony to provide answers. And the results have been rewarding.
One mystery of Mars has been to decipher the distant past of the northern regions. The southern hemisphere of Mars is heavily cratered and represents an older, less disturbed surface. But the northern regions, with their enormous volcanoes, had at some time been flooded with volcanic eruptions; lava had flowed, hiding much of the ancient surface there. And with that went much of the possible evidence of the watery past researchers had been seeking. But from its low-altitude vantage point, MRO was able to point its high-resolution cameras at these regions, seeking minute geological detail, and send back amazingly defined images of the terrain. And though it has thoroughly mapped only about 1 percent of the planet to date, the finds began accumulating quickly.
But, as always, there were issues. About two months after the science operations began, MRO experienced difficulty with the Mars Climate Sounder instrument. A so-called stepping mechanism malfunctioned, and the aim of the instrument became slightly off-axis. By December the problem had not corrected itself, and regular use of the device was abandoned. A partial work-around was later devised, and the MCS was pressed back into use, but not without compromises.
Additionally, the image-sensing electronics in the HIRISE camera, the CCDs (much like those in your home video camera) began to lose individual pixels and some electronic noise was found in the incoming images. While this was not a deal breaker, it didn't make anyone on Earth happy. Again, a work-around (utilizing a longer warm-up period for the electronics) minimized the problem, but it still remains as a weak point for the telescopic imager.
But perhaps the most alarming issue presented itself in mid-2009 when the onboard computer began resetting itself—shades of Pathfinder and Spirit. Perhaps the Great Galactic Ghoul has adapted itself to the modern era and figured out how to fool spacecraft computers into thinking that they are faulty, triggering a shutdown and restart. In any event, software changes seem to have alleviated the problem for the present. But the computer team remains vigilant. It took over two months to eventually solve the problem.
These issues did not stop the mission, however, and discovery after discovery continued to stream in. Wide measurements of the northern ice cap revealed more water than could have been hoped for, almost two hundred thousand cubic miles (not square miles, but cubic miles!) of water ice. This is equivalent to almost a third of Greenland's ice sheet and accounts for a lot of the “lost” water on Mars.
Some of the evidence of a wet past had come from the observation of phyllosilicates, heavily hydrated minerals formed in water. But the lava flows in the north had covered any evidence of these. The researchers needed a hole punched in this lava layer, and through extensive observation with their new orbiting toy, they got one. Nine craters in the lava fields were investigated, and each of the craters revealed hydrated minerals in the older layers below.
More craters were targeted, and a number of them showed bright blue-white materials on the surrounding ground. A few passes of the orbiter later, the material was slowly disappearing. From the rate of evaporation and the colors seen, it was clear that, once again, water ice had been found (the color indicated that it was almost 100 percent pure water ice). Just to be sure, the spectrometer was pressed into service, and, sure enough, the spectral signature matched that of water. To make it even more dramatic, some of these were about forty-five degrees south of the pole. By now, spotting ice in the polar regions of Mars, and as far south as sixty degrees, which is roughly equivalent to Anchorage, was not a shock. But finding it at the latitudes equivalent to Paris or Seattle was. It just kept getting better and better.
All this talk of water may cause the untrained ear to become a bit jaded. So there is water on Mars…big deal. But it may be just that—a very big deal. How this ice is formed so close to the surface is still a bit of a mystery. Mars's atmosphere is far too thin to support liquid water on the surface, and the formation mechanism for these ice patches is still not well understood. One theory involves a process that on Earth is called frost heave, in which small amounts of water can remain liquid around a grain of solid ice, even at temperatures below which it should freeze. Pressure causes this liquid water to migrate upward, where it then freezes, forming a lens-shaped structure on top of the soil below. Why this is important (beyond the pure geological implications) is that this process, which keeps water temporarily liquid in certain places near the surface, could form environments where bacteriological organisms could thrive. And as biological studies on Earth continue to find basic life-forms colonizing areas as diverse as undersea hot vents and the frozen dry valleys of Antarctica, the idea of water-bearing areas on Mars is tantalizing.
Continuing the search for water, in 2009 MRO used its camera cluster and spectrometer to image the vast reaches of Valles Marineris, the huge, hemisphere-girdling canyon that straddles Mars. In a region named Noctis Labyrinthus, scientists were looking for light-toned deposits (LTDs in the vernacular) indicative of water activity. They examined ten LTDs, which turned out to be troughs in the canyon, and found things they did not expect to see. The instruments on MRO, working in well-planned harmony, identified clays, hydrated silicas, and sulfates—all of which pointed to yet more watery activity sometime in the past. Some of the formations were dozens of miles across.
Small differences assumed large proportions. An example was one trough where most of the water-affected minerals were buried under later, wind-driven soil, but some was visible in the upper walls of the trough: a sure sign that the water-affected area was older than the trough itself. Another featured (water-derived) clays buried beneath newer plains. These and other findings, while seemingly innocuous, indicate a confused and jumbled timeline of multiple “wettings” of the area, multiple water-inundating events, which is in itself a major discovery. In short, Mars had not had just one watery time, but a number of them. This bodes well for a complex geological history, and again offers a possibility for the existence of past life.
Just how the wate
r arrived in these troughs was not evident. It has been hypothesized that it may have been melted ice from the volcanoes nearby, or some subterranean hydrothermal event. Since then, clays (representing, again, water processes) have been identified throughout Martian bedrock, so it is not a unique occurrence.
These discoveries resulted from the elegant and coordinated use of the context camera and the higher-resolution HIRISE imager. First the low-magnification CTX would spot something interesting, then the HIRISE imager would zero-in with its telescopic lenses and take a hi-res picture for detailed investigation. Finally, the spectrometer—the CRISM instrument—would take a careful look at the area and provide its chemical analysis. In this way, working in perfect three-part harmony, MRO was able to first identify, then analyze in detail, these kinds of soil deposits in high-resolution. The deposits dated back somewhere between 3.5 and 1.8 billion years ago—hardly recent, but important nonetheless. The troughs themselves seemed to have developed somewhere in the middle.
Glaciers were discovered in regions that first sparked interest among researchers in the days of the Viking orbiters; they surrounded the edges of cliffs in Martian valleys. They are lobe-shaped and gently sloping, and many are covered with debris and soil. Again, these are areas that apparently store huge volumes of water ice.
MRO also provided some of the first looks at earthly artifacts as well. When imaging Victoria Crater, the HIRISE camera captured an image of the plucky Opportunity as it went about its long traverse of the edge of the crater, making out the body of the rover and even the shadow of the camera mast. The HIRISE was also able to later photograph the Mars Phoenix Lander as it slowly descended toward the Martian north polar region in May 2008, dangling from its parachute. This was particularly exciting due to the fact that the landing event is short-lived and was snapped from an oblique angle—not the camera's strongest mode of operation. Both images were evocative and, besides being strong technological accomplishments, highlighted the infinitesimal human footprint on the Red Planet.
