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M. K. Shepard (Bloomsburg University), B. E. Clark, S. Swetra (Ithaca College), F. Vilas, K. Jarvis, S. M. Lederer (Johnson Space Center)
We obtained rotationally resolved spectra of the near-Earth asteroid 2100 Ra-Shalom in Aug 2003 in the visible (0.5-0.9 micron) at McDonald Observatory and infrared (0.8-2.5 micron) at the Infrared Telescope Facility (IRTF). Ra-Shalom is one of the larger members of the Aten family. Its effective diameter is ~2.5 km (Harris et al. Icarus 135, 441-450, 1998; Shepard et al. Icarus 147, 520-529, 2000) and its rotation period is 19.8 h (Pravec et al. Icarus 136, 124-153, 1998). Ra-Shalom has been classified as C- (Binzel et al., Icarus 170, 259-294, 2004), S- (Harris et al. 1998), and Xc- class (Bus and Binzel, Icarus 158, 146-177, 2002). Our observations show significant spectral variations with rotation phase, however their proximity to the Milky Way makes interpretation difficult. Our hemispheric-averaged spectrum is an excellent match to the CV3 chondrite Grosnaja, suggesting Ra-Shalom to be its source (RELAB sample MR-MJG-119-P1; Gaffey, JGR 81, 905-920, 1976). Given the purported link between K-class asteroids and CV3 and CO3 chondrites (Bell, Meteoritics 23, 256-257, 1988; Doressoundiram, et al. Icarus 131, 15-31, 1998; Burbine et al. Meteoritics Planet. Sci 36, 245-253, 2001), we suggest that Ra-Shalom is K-class, making it the second known Aten of that class after 1999 JD6 (Binzel et al. Icarus 151, 139-149, 2001). This raises difficult dynamical questions. The major reservoir of K-class asteroids is the Eos family in the outer main-belt (~3 AU). Dynamical simulations by Bottke et al. (Icarus 156, 399-433, 2002) suggest that Atens originate from the 3:1 mean motion resonance with Jupiter (2.5 AU), the nu6 secular resonance (2.1-2.3 AU), or the Mars crossing population (2.1-2.8 AU). Specific computations for Ra-Shalom mirror this (Bottke, personal comm.). Either there is an unknown dynamical path or another K-class source to populate the Atens.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.