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Last October, asteroid monitors at the Catalina Sky Survey at the University of Arizona in Tucson picked up a small object on an immediate collision course with Earth. The asteroid was too small to present a real threat—just a few meters across, it stood little chance of penetrating the atmosphere intact. Indeed, it exploded in a stratospheric fireball over northern Sudan less than 24 hours later—an event witnessed by people on the ground as well as the pilots of a KLM airliner—conforming well to astronomers' predictions for its trajectory.
But the asteroid, dubbed 2008 TC3, was nonetheless a momentous discovery: Among the countless small objects that strike Earth's atmosphere every year, none had ever been detected and tracked before it impacted. Now the Sudan bolide has yielded yet another first: Researchers report in Nature today that they have recovered 47 meteorites from the object in the Nubian Desert. And lead author Peter Jenniskens, a meteor astronomer at the SETI Institute in Mountain View, Calif., says that another search completed earlier this month, after the paper was submitted, has upped the meteorite count to about 280.
Astronomer Donald Yeomans, manager of NASA's Near-Earth Object Program office at the Jet Propulsion Laboratory in Pasadena, Calif., calls 2008 TC3 "a perfect asteroid trifecta," referring to "pre-impact discovery, successful impact prediction, and successful sample return." (Yeomans did not contribute to the recovery research, but his office played a leading role in tracking the asteroid's entry.)
The find allows astronomers to connect the chemical composition of the meteorite to its orbit and reflectance in the sky during tracking. "The holy grail of asteroid science is to uniquely link a specific meteorite and its detailed composition to a specific asteroid type," Yeomans says. "And that has now been done without an expensive sample-return mission."
This object, which the study's authors call Almahata Sitta (Arabic for Station Six, a train station in the desert where eyewitnesses saw the fireball and that served as the researchers' base camp), appears to belong to a rare class of bodies called F-class asteroids, which constitute just 1.3 percent of all asteroids.
Chemically speaking, Almahata Sitta is a meteorite whose specific composition is unique among meteorite collections. It is a fragile, porous ureilite (a relatively rare kind of olivine- and pyroxene-bearing meteorite) containing graphite and nanodiamonds, among other materials. Its fragility, Jenniskens says, helps explain why it broke apart so high in the atmosphere.
With the benefit of the object's rarity as an F-class body and its orbit, tracked backward through time, the researchers established a possible link to a larger F-class asteroid, the 1.6-mile- (2.6-kilometer-) diameter 1998 KU2, which may have originated from the same parent body as Almahata Sitta.
"The orbit of the asteroid, by just tracking it for 20 hours, is 10,000 times better than anything you can get from just observing a fireball," Jenniskens says. "What's neat about this is that the big asteroid allows you to extend back in time the evolutionary history." He notes that scientists might be able to pinpoint the specific region of the asteroid belt that 2008 TC3 came from with more F-class asteroids from the same parent body.
Even the brief amount of time 2008 TC3 was tracked provided an excellent lead on where to look—and the desert surface provided an ideal surface for turning up the dark fragments. "The entry trajectory was very precisely known," Jenniskens says.
The first samples were found in early December by a 45-person search team from the University of Khartoum. (Three scientists from that university and one from the University of Juba in Sudan are among the co-authors of the study.) "We had many eyes and hands," Jenniskens says, trying "to find these."