A rush of percussive blasts followed the display, prompting some people in the region to run inside and hide, while others watched in awe. Mohammed Elhassan, walking home from his local mosque in the Nile city of Wadi Halfa, took out his mobile phone and snapped a few photos. Head-on, from his position, the dust looked almost like a child's doodle. Some locals even told interviewers they divined a message in the pattern: a sign from Mohammed approving of their Ramadan fast.
Astronomers would later confirma heavenly portent of another kind—the last dance of an asteroid the size of a sedan that ripped through the atmosphere at 28,000 miles an hour and erupted into a fireball some 23 miles above the desert floor. Scientists had spotted the rock, dubbed 2008 TC3, en route to the planet 20 hours earlier during routine telescope observations of the night sky. Earth-bound objects of this size appear only a few times a year, typically smashing into the atmosphere unnoticed and exploding into harmless, untraceable bits of dust. TC3 was different. It wasn't that it was any more dangerous; it was too fragile to get through the atmosphere intact. But because astronomers found it early, while it was still 300,000 miles from Earth, out beyond the moon, it presented them with a rare opportunity to trace its path and predict exactly where and when it would crash to Earth. The feat would be a historic first. Moreover, samples, if they could be retrieved, might reveal new insights about the mysterious interior of asteroids—essential knowledge if you're trying to devise a way to keep them from obliterating a city.
Back in Sudan, the pebble-like meteorites formed by the explosion in the atmosphere had scattered miles over the desert, lost on the rocky plain. It was up to astronomer Peter Jenniskens to find them.
NASA's Near-Earth Object (NEO) Program, headquarted at the Jet Propulsion Laboratory in Pasadena, California, has been identifying and tracking asteroids within striking distance of Earth since 1998. Of the 6,326 rocks the NEO Program has so far catalogued, at least 783 measure more than half a mile wide. That's big enough to potentially end civilization. At present, the risk of a large asteroid slamming into Earth is slim—the rock with the best shot has a 1 in 2,900 chance of striking in 2048—but astronomers are keeping their telescopes tuned because these orbits could shift over time, increasing the chance of impact. There are an estimated half a million asteroids in the 150-foot range, which strike Earth roughly once every 300 years. These rocks may not wipe out humanity, but they can cause serious damage if they hit the ground or explode in the atmosphere. Scientists believe one may have burst above Siberia in 1908, unleashing a shock wave so powerful it flattened 800 square miles of forest. Imagine if it exploded above Manhattan.
Today, all that is known about doomsday asteroids is what they look like on the surface. Astronomers group asteroids into different classes based on how the rocks reflect light in space. They can infer an object's size and density, but they can't say with confidence what it's made of, or whether it's a solid mass or a loosely-held-together pile of rubble. Despite the fact that scientists have collected well over 50,000 bits of asteroid debris, or meteorites, our prodigious stockpile won't help solve the puzzle. The rocks are astronomical orphans: Scientists can't connect meteorites in the lab to the asteroid in space that spawned it, a knowledge gap known as the asteroid-meteorite connection problem. "In most cases, it's all kind of guesses because we don't have samples," says Scott Sandford, an astrophysicist at the NASA Ames Research Center.
One way to eliminate the guesswork would be to spot asteroids on the way in and then go pick them up for analysis. This is where the NEO Program and its orbit-projection expert, Steven Chesley, come in. Chesley, whose thick beard makes him look more like a grizzled sailor than an astro-geek, has helped develop, and is constantly updating, a software program called Sentry that searches for possible asteroid impacts and their probabilities up to a century into the future. On the morning after astronomers spotted TC3, 13 hours before the asteroid would hit Earth, Chesley was getting his kids ready for school when he got a call from Tim Spahr of the Smithsonian Astrophysical Observatory's Minor Planet Center. The MPC is tasked with gathering and collating telescope data on asteroid sightings. When Spahr sifted through the data that morning, he saw that the computer couldn't make sense of certain telescope observations—one object in particular was too close, moving too fast. He ran it through MPC's orbit-projection software, which can tell how close an asteroid will come to Earth, and phoned Chesley to alert him of an impending collision. Chesley nearly fell out of his chair when he heard the news. He hurried to work and began crunching data, eventually processing information from 26 different observatories, totaling 570 positional measurements of the asteroid.
In fact, Chesley had too much data, too much room for error. If the clocks on the different telescopes didn't agree precisely, for example, then their calculations of the asteroid's location at a given moment would conflict, distorting his trajectory. He tweaked his math, allowing each telescope just a dozen data points so that no single observatory (and consequently no single error) would dominate. Within a few hours, he was able to verify Spahr's prediction. "I reached the conclusion that it was absolutely, 100 percent going to impact, and that it was going to hit somewhere in northern Sudan," Chesley recalls. By the end of the day, he knew within a mile or so where it would hit and within a fraction of a second when it would slam into the atmosphere.
This precision is what makes NEO's Sentry system unique. Previously, the software simply predicted whether an asteroid would impact, not where it would do so. Upgrading it to give the exact latitude and longitude of the collision wasn't simple work, since Chesley had to factor in the spin, orientation and rotational wobble of the Earth at each moment. (One asteroid expert, who asked not to be identified so as not to offend other colleagues, said of Chesley, "He is so far beyond anyone else in the world in his ability to do this kind of calculation. He's smarter than everyone at this work.")
Chesley guessed that the asteroid's explosion would probably be too intense and high up for any fragments of the rock to make it to the ground. But if any did survive, he had critical information to offer about where they might land.
Peter Jenniskens, an astronomer with the Search for Extraterrestrial Intelligence Institute in Mountain View, California, is an expert on meteor showers and, more generally, things falling through the atmosphere. Tall and lanky with sloped shoulders, he works meteor-shower hours, driving his clunker to work around 11 a.m. and staying late into the night. This way, his internal clock is in sync with the dark hours best suited for observation.
When he learned of TC3 on October 7,he instantly recognized the rare opportunity it presented. Astronomers were compiling masses of data on what the object looked like in space. If he could find a sample and identify its class, he would be able to make new conclusions about the thousands of other asteroids in the same family. It would be not only a huge science find but also a major bargain for NASA—a poor man's sample-return mission. "It's a lot cheaper to go pick it up on the ground than to send a space probe to go get it," says the director of the NASA Ames Research Center, Simon "Pete" Worden.
Government and weather satellites suggested that the asteroid exploded somewhere between 14 and 23 miles above the surface. But if Jenniskens was going to figure out where those potential survivors would land, he needed to have a more precise idea of the asteroid's final path. He phoned Chesley knowing that Sentry offered the closest thing to an actual observation and asked if he knew where, exactly, the rock was headed. "Well," Chesley replied, "actually, I do."
Chesley extended his orbit trajectory right down to the ground. Then he sent Jenniskens figures detailing the address—latitude, longitude and altitude—of the asteroid every six miles en route to Earth, plus its speed and angle of approach.
Jenniskens estimated that the explosion took place 23 miles up—the number that best agreed with Chesley's track—and then worked out where along the line the surviving fragments would fall. Satellite data suggested that the impact explosion had kicked out the energy equivalent of one kiloton of TNT. Factoring in wind data, he assumed that the lighter pieces of debris would lose momentum fastest along Chesley's projected track and land closest to the explosion site. In the end, Jenniskens's "treasure map" consisted of a 200-square-mile patch of desert.
About two months later, Jenniskens was standing, map in hand, in the middle of the Nubian Desert near the end of a seemingly interminable day wondering whether he had made a mistake. He'd spent days interviewing eyewitnesses like Omar Fadul el Mula and Mohammed Elhassan, asking if they had seen anything coming out of those big smoke trails. Everyone said no. That meant that if debris did survive, it would have to be incredibly small. The task of looking for a few handfuls of rocks on a rock-covered surface roughly the size of Chicago, using nothing but their eyes and a small spotting telescope, began to seem hopeless. Jenniskens's colleague, Mauwia Shaddad, a physicist at the University of Khartoum in Sudan, and a team of 44 students and staff had been helping him scour the sand for hours, looking for fragments that had the telltale black, glassy crust that forms when an asteroid flash-heats in the atmosphere. They hadn't found a thing. "It all looked pretty desperate," Jenniskens admits.
The sun was setting, and it was a two-hour drive back to their camp. He'd begun to consider quitting for the day when a 21-year-old student, Mohammed Alameen, discovered a dark black specimen the size of a thumbnail. Success. Jenniskens knew immediately. There was, of course, the fusion crust, indisputable evidence that it came from space. And the rock's dark coloring proved that it hadn't been exposed to the elements for very long.
Perhaps the biggest clue that it might be a piece of TC3 was that it was exactly where Chesley's trajectory and his own calculations said it should be. Chesley was right. Scarily correct. Spahr calls the accuracy of the prediction "absurd." And Jenniskens was spot-on, too. Holding the fragment in a piece of aluminum foil, careful not to contaminate the sample with oils from his skin, Jenniskens looked at Alameen, told him he had found what they were looking for, and watched as the students began singing and dancing.
Ready for the Fall
Jenniskens, Shaddad and their group found 14 additional fragments on that trip, and 265 more on subsequent searches. Most are minute, roughly the size of that first find. The largest is the size of a tennis ball. Yet these fragments alone are turning up some surprising results, just as Jenniskens had hoped.
The good news for the rest of us is that now, if astronomers were to spot a larger, more dangerous asteroid in that same class en route, they will know that it's porous and fragile, because they have pieces of TC3. And if it's the type of dangerous asteroid that has to be diverted, given those properties, any attempt to change its path will have to be gentle to keep it in one piece.
The downside of the find, from the perspective of saving the planet, is that the TC3-like rocks make up a scant 1.3 percent of all known meteorites. The next step to understanding and planning for asteroids, astronomers say, is obvious. "We need to find more of them," says Yeomans.
Replicating the TC3 discovery won't be easy. Even though asteroids of roughly that size fall to Earth a few times a year, the NEO Program telescopes often miss them because the survey is tuned to search for larger, more distant rocks. In the next decade, this could change with the debut of observatories like the Pan-STAARS system, slated for completion in 2012, and the Large Synoptic Survey Telescope, set to come online in 2015. Both are designed to survey wide swaths of the sky and could spot far more of the small space rocks.
And if another one does appear in the next few years or even decades, there's a strong chance that Jenniskens will be there to witness its glorious death. He's still a bit depressed that he wasn't on the ground in Sudan to watch TC3 explode. Next time, he hopes to receive word earlier on, as soon as Chesley or another astronomer predicts the impact location, so that he can catch a flight to the place where debris is expected to fall.
One afternoon in his small office, after wistfully reviewing the photos of his meteorite hunt and pulling out a few of the sample fragments, he imagines what it would be like to actually arrive on the scene before the impact, to be in the same position as Mohammed Elhassan, the cellphone photographer, armed with a few telescopes, if not a catcher's mitt. His eyes brighten as he ponders the possibility. "To get sufficient warning to be there when the asteroid crashes through the atmosphere, then study that whole breakup process in detail," he says, "that's sort of my dream."