Main-Belt and Near-Earth Asteroid Results from the IRTF Using SpeX P.S. Hardersen* and P.A. Abell Department of Earth and Environmental Sciences Rensselaer Polytechnic Institute Troy, New York 12180 *Current Affiliation: Space Studies Department University of North Dakota Grand Forks, North Dakota 58202 Main-belt Asteroid Results Introduction: The detection of weak mafic silicate features in the near-infrared spectra (NIR, ~0.8 to 2.5 micron) of M-type main-belt asteroids can help constrain the likely mineralogies and petrologic histories of this group of asteroids. The absence of spectral features can also help constrain potential surface compositions. A subset of the M-type asteroids was observed in April and May 2001 using the SpeX instrument at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. Of the asteroids observed, 69 Hesperia, 110 Lydia, 201 Penelope and 216 Kleopatra were all observed over the course of multiple nights and nearly full rotational coverage was obtained for all four asteroids. 325 Heidelberga and the S-asteroid 305 Gordonia were also observed, but received only limited rotational coverage. Taxonomic Classification: The M-type asteroids are compositionally ambiguous due to their general lack of spectral features at visible and NIR wavelengths. Assignment to the ³M² taxonomic class is based on albedo, the lack of identified spectral features and a flat or slightly red spectral curve across the 0.3 to 1.1 micron wavelength range. However, the detection of weak spectral features in the NIR has been hampered due to the low spectral resolution of previous survey programs and instruments. With the recent introduction of SpeX, the capability exists to detect weak silicate features in the spectra of this group of main-belt asteroids. Confirmation of the presence of weak silicate features on some of these asteroids would not only help constrain their actual surface compositions, but also highlight the weaknesses of asteroid taxonomic classification schemes that attempt to classify geologic bodies based on observational parameters. Results: The four main asteroids from this work, 69 Hesperia, 110 Lydia, 201 Penelope and 216 Kleopatra, have all been found to exhibit weak spectral features in the 1- and 2-micron regions of their NIR spectra. Specifically, the Band I centers for these asteroids fall in the range from 0.90 to 0.93 microns and the Band II centers are in the range from 1.76 to 1.84 microns. Spectral calibrations by Adams (1974) and modified by Gaffey et al. (2002) show that the Band I vs. Band II center plots for 110 Lydia, 201 Penelope and 216 Kleopatra place these asteroids in the region of the orthopyroxenes. Calibrations from this work reveal that the orthopyroxenes on these asteroids are Fs < 5 and Wo < 5. The weakness of the Band II feature for 69 Hesperia has prevented determination of its Band II center position, although it is suspected to be similar to that of the other three M-asteroids. The petrologic compositions of the orthopyroxenes on these asteroids suggest that their parent bodies were formed in regions of the main asteroid belt that experienced relatively reducing chemical conditions. Conversely, these reducing chemical conditions would not allow the presence of abundant water within these asteroids during their formation epoch. This lends less credence to the interpretations of Rivkin et al. (1995, 2000) that suggest that the surface of 110 Lydia and 201 Penelope are composed of hydrated minerals (i.e., phyllosilicates). Near-Earth Asteroid Results Introduction: Very little is known about the actual geological properties of the Near Earth Asteroid (NEA) population, but such information is of great scientific value in understanding the source regions and delivery mechanisms of these bodies. In addition, these asteroids can provide crucial data on the compositional structure, thermal history, chemical evolution and collisional processes of the asteroid belt. Such detailed studies are also useful in assessing the threat from specific potentially hazardous asteroids (PHAs) since the physical properties of the asteroid will affect the nature of the impact itself and would constrain the potential mitigation strategies. Given the scientific importance of these objects, an observing program was started in October of 2001 to characterize PHAs and other NEAs in terms of their geologic compositions. Asteroid 1998 ST27: This NEA was observed for four nights in October 2001 using the low-resolution or asteroid-mode of the SpeX instrument at the NASA Infrared Telescope Facility on Mauna Kea, Hawai¹i. Since no rotation period had yet been determined for it, over one hundred and twenty spectra were taken during the course of the observing run (October 6-7 and October 9-10 Universal Time) in an attempt to obtain complete rotational coverage of the object. Asteroid 1998 ST27 was observed at phase angles of 28ƒ to 42ƒ and at Vmag ~15.3 during this period, which was well within the optimum limits of the SpeX instrument. Interest in this target increased after the analysis of Arecibo radar images taken on October 7 and 9, 2001 showed that it was actually a binary asteroid with at least a 4 km separation between the two components. Further analysis of the radar observations by Benner et al. (2001) have demonstrated that 1998 ST27 has a primary of 800 m in diameter and a secondary of < 100 m in diameter. Results: Reduction of the data from 1998 ST27 indicates a relatively flat spectrum with an exponential rise at wavelengths greater than 2.2 mm and the presence of a weak absorption feature at 0.9 mm, which changes in depth with the rotational phase of the asteroid. The flat spectrum and enhanced reflectance at wavelengths greater than 2.2 micron suggest that 1998 ST27 may be a dark carbonaceous body with a low albedo. The 0.9 micron absorption band is most likely due to the presence of phyllosilicates on the surface of the asteroid and its change in depth indicates that 1998 ST27 has some degree of heterogeneity on its surface. Conclusions Data obtained from the IRTF using the relatively new SpeX instrument on both main-belt and near-Earth asteroids has proven to be of much higher quality than that from previous instruments. Main-belt asteroids that have been previously studied can now be re-examined in more detail, and faint, fast moving near-Earth asteroids once considered unattainable can now be investigated. Through the use of SpeX, new insights can be formulated about both of these asteroid populations.