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Veering in a decidedly noncircular solar orbit between Earth, Mars and sometimes beyond, a small, potato-shaped asteroid named Eros quietly cruises through space. There is nothing innately remarkable about Eros: It is one of hundreds of asteroids that have been lurking around Earth's celestial backyard since the creation of the solar system. But Eros does have one unique feature. Perched among its ancient rocks and dusty craters is an artifact from another world, a robotic scout from Earth that ended a yearlong orbital reconnaissance mission with a most unlikely touchdown. On Feb. 12, 2001, dozens of men and a few women jostled for elbow room at computer screens lining a control center at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. This was to be their last day together, a sad but triumphant finale to one of NASA's most successful (and, at $224 million, least expensive) space exploration missions. Five years earlier — almost to the day — the team had gathered at the Cape Canaveral Air Force Station in Florida to watch a Delta rocket loft the Near Earth Asteroid Rendezvous probe into orbit. The spacecraft, known by its acronym NEAR, aligned its sun sensors and star tracker, fired its thrusters and set off on what was originally planned to be a three-year voyage to Eros. Although NEAR traveled alone, it was carefully tended by a dedicated team of scientists and engineers who had been selected by NASA and divided into groups based on their expertise in working with particular remote sensing equipment. NEAR was outfitted with six sophisticated scientific instruments, but the darling among them was the color electronic camera, which would bring these armchair explorers — and ultimately a worldwide television audience — along for an unprecedented and closer-than-imagined tour of an alien world. Mark Robinson, a Northwestern geologist and researcher, landed an enviable position on the camera science team. He proposed to use NEAR's camera, known as the Multispectral Imager, to try to figure out what type of rocks were on the asteroid and what sorts of geological occurrences from the past could explain its present condition. "Different rocks have different absorption features depending on how much iron they contain and how the iron is bound up in the crystal structure," explains Robinson, who has honed his ability to interpret remote sensing data over three previous space missions. Back before the space bug bit, Robinson was what he calls "a real geologist," hunting gold for a mining company in Alaska. He remembers seeing some pictures of Mars in a library book and was so impressed by how much information was available from remote observation of another world that he returned to school to study remote sensing geology. "The science is sort of like a jigsaw puzzle. You get all these different kinds of data sets, and you try to figure out how something formed on the surface of a planet or what the chemistry is," says Robinson. "You don't always know the answer from the data itself, and you have to make assumptions based on what you know about how geology works on the Earth." In planetary geology, pictures are often the only things available, so it pays to know as much about Earth as possible so observers can successfully interpret what they're seeing. "I spent eight years in Alaska looking for gold, trying to figure out the geology of the Earth, and I was always dumbfounded. Mother Nature likes to hide her secrets," Robinson says. Transitioning from Alaska to the University of Hawaii, Robinson got his first crack at real-time planetary exploration when he worked on the Galileo project as it flew by the moon and two asteroids on its way to Jupiter. During Galileo's 1991 flyby of Gaspra and 1993 visit to Ida, scientists were able to get the most detailed, highest-resolution pictures of an asteroid ever taken. The spacecraft passed 994 miles from the first asteroid and 1,491 miles from the second. NEAR shattered the records, capping a year's survey of the asteroid Eros with a final image taken from an altitude of 394 feet — less than one-tenth of a mile away. Throughout NEAR's long journey to Eros — a fourth year was tacked on when a technical glitch caused the spacecraft to miss its first opportunity to go into orbit around the asteroid — Robinson worried that Eros would turn out to be a huge geologic bore. "My biggest fear had been that we were going to get there and there was going to be this lumpy rock, just bare rock, and we'd say, 'OK, we've got Picture No. 50,000, Picture No. 50,001, so what?' I didn't know. That was my fear ... but I was holding out hope that it would be interesting," says Robinson. What the geologist and the rest of the science team found was beyond their wildest expectations: a rock-and-crater-strewn world, thickly coated with fragmented debris called regolith — a dusty and rocky soil similar to what the Apollo astronauts found on the moon. Despite Eros' puny gravitational grip — a 200-pound person would weigh only a couple of ounces on the asteroid's surface — scientists discovered the force was adequate not only to keep the NEAR spacecraft pinned in orbit but also to maintain thick layers of coating over Eros' solid core. "Ten to 15 years ago people predicted there would be no regolith on asteroids because their gravity is too low and it wouldn't hold the material there. It would always just go out into space," says Robinson. The science team found evidence of Eros-quakes: intriguing landslides that may have been triggered when the asteroid was whacked by flying objects. Those random encounters caused the entire 21-mile-long body to vibrate and shake the regolith into slopes and craters. One of the earliest puzzles Robinson pondered was why there were relatively few small craters on the asteroid's surface. "There are almost none, relative to the amount we see on the moon, smaller than about 50 meters in diameter," he says. "At first that was a real surprise and kind of a mystery. People threw around ideas like, 'Well, Eros doesn't get hit by small things,' which always seemed farfetched to me. I think it's clear to at least some of us the reason is because there is so much loose material on the surface, [small craters] get covered." Robinson believes two processes are responsible for the formation of the regolith: the occasional meteor strike that smashes into the asteroid, churning up the ground cover and spewing it around; and the constant bombardment of micrometeorites, which has the effect of sandblasting the surface of Eros. The micrometeorites are tiny — most are smaller than a grain of sand — but moving so fast, roughly three miles per second, that damage is inevitable. "What we see is a whole range of states of degradation, where some things are completely fresh, like they were just made yesterday, and other boulders and craters are so degraded you can hardly even tell they are there anymore," says Robinson. "Time just wears everything down. The regolith is always being formed." Other instruments on NEAR probed Eros' chemical composition, looked for a magnetic field and mapped the asteroid's topography. One of the primary scientific goals of the mission was to try to link a group of meteorites found on Earth with a type of asteroid like Eros. The team had a strong hunch that such a connection would provide insights into the formation of the solar system. "We have all these wonderful meteorites from which we've learned a lot about the formation of the solar system, but we'd really like to know their geologic context," Robinson says. "Where did they come from within the solar system? "A way of thinking of that is like someone going around Earth and collecting a couple of hundred different samples, taking them to Mars and giving them to Martian geologists who know nothing about Earth," he says. "If you didn't tell them where the rocks came from, it would be very difficult to put together a coherent story about Earth. ... If I told them exactly where they came from and put them in their context, they become much more valuable and much more useful. "Of course we need to get the chemistry of a whole bunch of asteroids. But this is a fantastic start." Asteroids, like comets and meteorites, are windows into the early solar system, says Jessica Sunshine, a senior scientist with Science Applications International Corp., in Chantilly, Va. "These fundamental materials and processes are what ultimately formed our planet and what we evolved from." The NEAR science team will spend months combing through the 160,000 images and other data collected during the spacecraft's 230 orbits of Eros. However, the final 69 pictures, snapped as the orbiter descended and made an unexpectedly gentle landing on the asteroid's surface, are the ones that will be permanently etched in Robinson's mind. "That was just an incredible day to be sitting there watching these highest-resolution images ever taken of an asteroid — or almost of any body — coming down as the spacecraft was landing," he says. "The last image was only six meters across — that's comparable to a picture being taken looking down from the roof of a building." With NEAR running low on fuel and operating funds coming to an end, project managers decided to try to bring the spacecraft as close as possible to the asteroid's surface in an attempt to wrest some highly detailed images from the probe's cameras. The spacecraft, which is about the size of a car, was never intended to be a lander, but managers figured they had nothing to lose. At worst NEAR would run short of fuel and crash to the surface before relaying any pictures. NEAR not only landed, fortuitously cushioning its descent by touching down on the only science instrument that was no longer working, it continued to operate from the asteroid's surface, prompting NASA to extend the mission by two weeks for additional studies. "When we learned that the spacecraft had not only landed on the surface but was still operational, we hardly knew what to think," says project scientist Andrew Cheng. "Watching that event was the most exciting experience of my life," he adds. "I was asked immediately afterward how I felt, and I mumbled something about being tired and happy, but I missed the point. I realized what I should have said: It was like watching Michael Jordan on the basketball court when the game was on the line and he was in the groove. One miracle after another unfolded, and we were left stunned and speechless." Robinson, for one, does not have time to savor the moment. He already is deeply immersed in his next space adventure: a virtual close encounter with the planet Mercury. The launch of the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft to Mercury is scheduled for 2004 (sidebar). Robinson is most looking forward to seeing the half of the planet that has never been viewed before. NASA's 1974 Mariner 10 mission to Mercury — the only spacecraft so far to have visited the innermost planet of the solar system — only imaged about 45 percent of Mercury. "Every time we get data back from a planetary mission, there is always a big surprise," says Robinson. "The science is great, but it's really being one of the first people to see something new, something that nobody else ever has seen before, that's really exciting," he concludes. "It would be much nicer to actually be there and go do it, but this is the best we can do right now." Irene Brown (J82) is a Florida-based freelance journalist who has covered space explorations for 14 years. Her work regularly appears on the Discovery Channel and in books and magazines. For more information about the NEAR mission, visit Mark Robinson's Web site at www.earth.northwestern.edu/research /robinson/near.html. |
