Most of the time we never find out what hit us. This time we did. The object now remembered as 2008 TC3 had been circling the sun for eons, on an orbit similar to Earth's, its existence unsuspected until the day before it struck. Measuring about six feet across (or maybe 12—deduced from brightness, such estimates are rough), the boulder was at the limit of detectability for ground-based telescopes that search for asteroids on a collision course with Earth. It was discovered on the morning of October 6, 2008, by the Catalina Sky Survey in Tucson, Arizona, the current pacesetter in finding near-Earth objects, having identified 565 new ones last year alone.
Catalina employees alerted the staff of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who quickly plotted an orbit. By their calculations, 2008 TC3 would hit northern Sudan within 20 hours—exactly the sort of event the Catalina Sky Survey was set up to warn against. Had the object been 10 times larger, there would have been hurried calls to world leaders and a state of emergency. But rocks the size of 2008 TC3 enter the atmosphere every few months without causing harm.
Among those who took note of the impending strike was Jacob Kuiper, a meteorologist at the Royal Netherlands Meteorological Institute whose job it is to inform airlines of hazardous weather and volcanic ash clouds along their routes. Kuiper immediately realized the unique opportunity for pilots flying over eastern Africa to observe the meteor, and notified the Dutch airline KLM, which put out the word. Early on the morning of October 7, 2008 TC3 blazed into the atmosphere, and some 15 minutes later, Kuiper got a call from the airline: Two of its 747 pilots on a course from Johannesburg to Amsterdam reported several bright flashes to the northeast while flying over Chad. Captain Ron de Poorter and copilot Coen van Uden likened the flashes to artillery fire or distant lightning.
Meteors—the "shooting stars" you're likely to see from your back yard on a moonless night—are particles the size of a grain of sand heating the air and vaporizing as they zip along at up to 40 miles per second. Bigger objects burn brighter. Those that outshine Venus, the brightest planet, are known as fireballs, and some super fireballs are as bright as the sun.
Fireballs are memorable sights, but not all that rare, not even for pilots. On November 20, only a few weeks after the Sudan meteor, another large object fell toward Earth, this time over Saskatchewan, Canada, east of Edmonton. Among the hundreds who reported seeing the meteor was J.R. Novak, a pilot for Spec Engineering of Calgary, who saw it from his altitude of 9,000 feet as a "flaming red trail" ending in an explosion. Pilot Mark Lavoie likened it to an emergency flare.
That evening, United Parcel Service pilots Mike Meyer and Paul Locraft were on their way home to Anchorage, Alaska, in an MD-11. They were at 37,000 feet near the border of Alberta and Saskatchewan when a fireball as bright as the sun appeared in their windscreen, heading away from them on a parallel course. "We went from a resting heart rate to max heart rate in about two seconds," says Meyer. "I thought it was an airplane that had just turned on its landing light before it was going to hit us. Paul thought it was a missile." It took them only a few seconds to realize that the bright light, already fading to red, was a meteor. But in those few seconds, says Meyer, "we both thought that was our last moment here on Earth."
To Meyer and Locraft, it seemed that the meteor was at their altitude, about seven miles. In fact, it was between 15 and 50 miles; the pilots' eyes were tricked by the curvature of Earth. According to Martin Beech, a meteor expert at the University of Regina in Saskatchewan, that "cruel optical effect" has frightened many a fireball witness. Fireballs always seem closer than they are, and the meteorites—the pieces that make it to Earth—land much farther away than "just over that hill," which eyewitnesses typically report.
Harvey Nininger, the foremost meteoriticist of the early 20th century and a champion meteorite finder, stated flatly in a published paper, "It is absolutely impossible for any single observer to judge the distance of a meteor." Maybe so, but were he alive today, he would be flabbergasted by the variety of telescopes, satellites, cameras, and other sensors that enable scientists to track incoming meteors with unprecedented accuracy.
In the 20 hours between the discovery of 2008 TC3 and its detonation over Sudan, 27 observatories tracked its orbit. When TC3 hit the atmosphere, visual and infrared sensors on the European Meteosat 8 satellite recorded the flash. So too, according to NASA's Near Earth Object office at the Jet Propulsion Laboratory in California, did unspecified U.S. government spacecraft—presumably the Defense Support Program satellites that watch for infrared signatures of ground-launched missiles.
Data from military satellites are in fact our best evidence for the actual rate of space rocks hitting the planet each year. In 2002, Peter Brown of the University of Western Ontario and colleagues published in Nature magazine an analysis of flashes that military satellites recorded from February 1994 to September 2002. Some 300 events were identified as probable meteors. The scientists were able to reconstruct the energy of the explosions in the atmosphere, and to extrapolate the rate of larger and smaller objects hitting Earth. By their estimate, an object with 50 kilotons of energy (the bomb that exploded over Hiroshima in 1945 yielded about 15 kilotons) appears on average every 10 years. An impact with the force of 0.33 kiloton occurs monthly. The Sudan fireball, about a kiloton, was on the lower end of the range.
Ground-based instruments are also useful in catching meteors in the act. Large bolides (another term for impactors) can cause atmospheric pressure waves strong enough to register on seismic detectors. The first, and still largest, of these extraterrestrial seismic impact signals to be captured was caused by a June 1908 blast near the Siberian river Tunguska. Recent estimates put the blast at about 3,000 to 5,000 kilotons of energy, from an object roughly 120 feet across. A reasonable guess of the frequency of such impacts is once every few hundred to a thousand years.
As arms control agencies have set up a global network to monitor compliance with the Comprehensive Nuclear Test Ban Treaty, another tool has appeared to help meteor trackers. A meteor's dying scream falls in the "infrasound" range, below the range of human hearing. Infrasound can be "heard" with microphones tuned very low or barometers tuned very high, and instruments designed to listen for nuclear explosions pick up the low rumble of incoming space rocks as they hurtle through Earth's atmosphere.
Add to these the dedicated networks of all-sky cameras (nearly the entire sky appears in a single frame) set up in Canada, Europe, and the United States over the past 50 years to watch for bright meteors. The most recent is under construction in Australia's Nullarbor desert, a project headed by Phil Bland, a planetary scientist at Imperial College London, with colleagues from the United Kingdom and the Czech Republic. These networks provide photographs taken at different locations, which help researchers triangulate the positions of meteors, plot their paths through the atmosphere, and reconstruct their original solar orbits—like running a movie backward to see how it started.
Only nine times have scientists managed to assemble enough information from cameras, infrasound, seismic detectors, eyewitness reports, and other sources to reconstruct an impactor's original orbit. As a result, only a handful of the estimated 30,000 meteorites in collections come from known orbits.
Bland hopes that his Australian network, which consists of four cameras but is expected to grow to at least 10, will better bridge the gap between the astronomical study of asteroids and the geological study of meteorites. Bland foresees his cameras tracking a fireball so accurately that a search team will be able to find any resulting meteorites quickly, enabling a reconstruction of the original object's solar orbit. For asteroid researchers, that's like mounting a cheap sample-return mission. "Fundamentally, you need to know where that rock came from to understand it," says Bland.
An estimated 40,000 rocks heavier than a half-dollar fall to Earth every year (impactors larger than a couple of feet typically explode from the pressure of ramming through the atmosphere at high speed, dropping meteorites to the ground). Only a small percentage are ever found.
Scientists aren't the only ones interested in more efficient searches: The total haul of meteorites from a "witnessed" fall can be worth tens of thousands of dollars on the collectors' market. Meteorite hunters now know it's possible to predict fireballs. And maybe next time they'll get more than 20 hours' notice.
In that case, says Wayne Hally, a New Jersey-based coordinator for the North American Meteor Network, "many dozens of people would get in their cars and start driving." Among them would likely be McCartney Taylor, a collector in Austin, Texas, who says that such predictions, if they become routine, will "change the meteorite business. We're going to have to pre-deploy if we're going to beat other guys to the fall."
Robert Jedicke of the University of Hawaii's Institute for Astronomy is in charge of asteroid observations for Pan-STARRS, a new telescope network headquartered in Hawaii that will provide fast, frequent sky surveys. Pan-STARRS will outperform today's asteroid searches, but, says Jedicke, finding objects as small as 2008 TC3 on a collision course with Earth is "not going to be something that happens all the time. It's a very rare occurrence. We're going to need bigger telescopes covering much more of the sky on a regular basis."
Bigger survey telescopes are planned. Clark Chapman, who studies asteroids at the Southwest Research Institute in Boulder, Colorado, predicts that by the 2020s, the next generation of asteroid surveys will have tracked a quarter-million orbiting objects 16 feet in diameter—not much bigger than 2008 TC3.
For now, though, the Sudan meteor remains unique: the first object ever tracked from space all the way to its demise in the atmosphere. Robert Haag of Tucson, Arizona, the self-proclaimed "Meteorite Man," knows that for this reason alone any meteorites it dropped would be valuable—if he could only get to them. The trouble is, they fell on the edge of Darfur, one of the most dangerous places on the planet. A University of Khartoum team has since reported finding meteorites, but at the time of our talk in December, Haag doesn't know their location. And he's mulling over his chances of getting there first. In fact, even as we talk, he's got Google Maps up on his computer screen, scouting for train stations near where 2008 TC3 fell.
Air & Space senior editor Tony Reichhardt wrote about the search for Apollo artifacts on the moon (Aug./Sept. 2008). He's never seen a fireball, but if a meteorite lands anywhere near his home in Fredericksburg, Virginia, he'll be among the first to jump in his car and go looking.