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.