Rosetta target asteroid 2867 Steins: An unusual E-type asteroid

ESA's Rosetta spacecraft will fly by main-belt asteroid 2867 Steins on September 5, 2008. We obtained new visible wavelength spectra of 2867 Steins on December 19, 2006 (UT), using the Palomar 5 in telescope and the facility Double Spectrograph. Two sets of spectra, taken similar to 3 h apart, one half of the rotation period for 2867 Steins, show It to be an E-type asteroid. The asteroid displays a 0.50 mu m feature that is considered diagnostic of the E(II) sub-class, but is deeper than any previously observed E-type. This feature Is most likely due to the presence of oldhamite (CaS) on the asteroid's surface. Also, the observed Steins spectra are far redder than any other known E-types. There is potential evidence for heterogeneity on hemispheric scales, one side of the asteroid appearing to be significantly redder than the other. No known recovered meteorite sample matches the Unusual spectra of 2867 Steins, but the closest analog would be similar to an enstatite achondrite (aubrite).