Shape of Small Asteroid that Hit Earth Revealed

The asteroid that crashed in northern Sudan last year was shaped like a loaf of walnut-raisin bread, according to astronomer Peter Scheirich and colleagues at Ondrejov Observatory and Charles University in the Czech Republic.

Scheirich reported his findings at the Division for Planetary Sciences of the American Astronomical Society meeting in Puerto Rico on Oct. 5, 2009 in a special session dedicated to this asteroid one year after the fall. The small asteroid, designated "2008 TC3", was the first to have been spotted in space before hitting Earth.

Last December I traveled to Sudan. With the help of Sudan astronomer Muawia Shaddad, and 45 students of the University of Khartoum, we went to the crash site in the Nubian Desert and recovered 300 fragments (called meteorites) by carefully sweeping the gravely desert. We found many different looking meteorites, slightly south of the calculated impact trajectory.

We now have a gigantic jigsaw puzzle on our hands, from which we try to create a picture of the asteroid and its origins. Now, Scheirich and colleagues have provided us with a composite sketch of the culprit, cleverly using the eyewitness accounts of astronomers that saw the asteroid sneak up on us.

An irregular shape and rapid tumbling caused Asteroid 2008 TC3 to flicker when it reflected sunlight on approach to Earth. Astronomers Marek Kozubal and Ron Dantowitz of Clay Center Observatory in Brookline, Massachusetts, tracked the asteroid with a telescope and captured the flicker of light during a two-hour period just before impact. Scheirich combined these observations with others to work out the shape and orientation of the asteroid.

Other forensic evidence, which was presented during the special session at the AAS/DPS meeting, chaired by Jason S. Herrin of NASA Johnson Space Center and me, is based on analysis of the recovered meteorites. These are of an unusual "polymict ureilite" type. Herrin confirms that the meteorites still carry traces of being heated to 1150-1300 ?C, before rapidly cooling down at a rate of tens of ?C per hour, during which carbon in the asteroid turned part of the olivine mineral iron into metallic iron. Hence, Asteroid 2008 TC3 is the remains of a minor planet that endured massive collisions billions of years ago, melting some of the minerals, but not all, before a final collision shattered the planet into asteroids.

Mike Zolensky of NASA's Johnson Space Center first pointed out that, as far as ureilites are concerned, this meteorite is unusually rich in pores, with pore walls coated by crystals of the mineral olivine. He now reports from X-ray tomography work with Jon Friedrich of Fordham University in New York, that those pores appear to outline grains that have been incompletely welded together and the pore linings appear to be vapor phase deposits. According to Zolensky, "2008 TC3 may represent an agglomeration of coarse- to fine-grained incompletely reduced pellets formed during impact, and subsequently welded together at high temperature."

The carbon in the recovered meteorites is one of the most cooked of all known meteorites. Carbon crystals of graphite and nano-diamonds have been detected. Still, it turns out that some of the organic matter in the original material survived the heating. Amy Morrow, Hassan Sabbah, and Richard Zare of Stanford University have found polycyclic aromatic hydrocarbons in high abundances. Amazingly, Michael Callahan and colleagues of NASA Goddard Space Flight Center now report that even some amino acids have survived.

To find more puzzle pieces, Muawia Shaddad and I plan to re-visit the scene of the crash in the Nubian Desert on the one-year anniversary of the find this December 6-7, after sharing notes during a 2008 TC3 Workshop at the University of Khartoum.

• Closer Look at Asteroid 2008 TC3, Part 2
• Asteroid Exploded in Earth's Atmosphere
• Image Gallery - Asteroids