Sky and ground observations determine an asteroid’s composition

How an asteroid reflects sunlight as a function of wavelength reveals something about the asteroid’s make-up. Based on reflectance spectra, astronomers have developed a classification scheme for asteroids and noted that those of a given type tend to be formed in the same region of the asteroid belt. The composition of meteorites, most of which come from asteroids, can be determined in detail. Until recently, however, no meteorite could be unambiguously associated with a specific asteroid or even a spectral class. But now scientists have obtained reflectance and composition data for the same asteroid—2008 TC₃, which blew up over Sudan’s Nubian Desert shortly before dawn on 7 October 2008. Its story was told recently by an international team led by the SETI Institute’s Peter Jenniskens . The asteroid, also called Almahata Sitta, had been sighted 20 hours before it disintegrated—early enough for scientists to take reflectance measurements. And some of Almahata Sitta survived the explosion high above Earth and rained down to the surface. Students from the University of Khartoum , led by professor Muawia Shaddad, gathered 47 of the meteorites, one of which was studied in detail. That remnant’s high porosity and dark carbon-rich material are peculiar. Those anomalies may, in time, help physicists understand the processes that took place in the solar nebula in the region where Almahata Sitta and its spectral classmates formed. (P. Jenniskens et al., Nature 458, 485, 2009 . Photo courtesy of NASA/Peter Jenniskens.) — Steven K. Blau