Compositional Investigation of Binary Potentially-Hazardous Asteroid 2008 BT18: A Basaltic Achondrite

The binary nature of potentially-hazardous asteroid (PHA) 2008 BT₁₈ was discovered during radar observations on July 6 and 7, 2008, from the Arecibo observatory. The primary has a diameter of 600 meters with a rotation period 2.57 hours (Pravec, Pers. Comm.) and the secondary has a diameter of >200; meters (CBET 1450). Near-IR spectroscopic observations of 2008 BT₁₈ were obtained remotely using the SpeX instrument on NASA IRTF on July 26 UT, 2008. The data were reduced based on reduction techniques developed by Gaffey et al. (2003). The near-IR spectrum (0.7-2.5 µm) of 2008 BT₁₈ shows deep band I (band depth 62±1%) and an equally deep band II (band depth 48±1%). The estimated band I and II centers are 0.926±0.001 µm and 1.94±0.01 µm respectively, with a band area ratio (BAR) of 2.0±0.1. A weak inflection is also noted at 1.3 µm. Based on the spectral parameters it can be suggested that the PHA's surface assemblage is dominated by high-Fe orthopyroxene with little or no olivine. This suggests that the asteroid's parent body experienced at least partial melting temperatures producing large eutectic melt similar to basaltic achondrites. The spectral parameters of the PHA plot it in the basaltic achondrites region on the Gaffey S-asteroid subtype plot (Gaffey et al., 1993) suggesting a V-type taxonomic classification. This research was supported by NASA NEOO Program Grants NNG04GI17G and NNX07AL29G. VR would like to thank J. Iglesias (UC Berkeley) and H. Stewart (Villanova University) for their support.