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M. S. Kelley (NASA-JSC, Mail Code SR), F. Vilas (NASA-HQ, Code SE), K. S. Jarvis (Lockheed Martin Space Operations), P. E. Painter (Dept. of Physics, Univ. of Texas)
Asteroid 25143 (1998 SF36) is currently the target of a cooperative Japanese-U.S. sample return spacecraft mission, MUSES-C, scheduled for launch in 2002. In preparation for that mission we conducted an extensive ground-based observing campaign of SF36 in March 2001. At that time, the asteroid made its closest approach to the Earth prior to the launch of the spacecraft. Observations were conducted at several observatories including the NASA Infrared Telescope Facility (IRTF), McDonald Observatory, Lowell Observatory and Kitt Peak National Observatory.
Two observing runs were carried out at the IRTF using the SpeX instrument in the near-infrared wavelength region of approximately 0.71-2.5 microns. The first run took place while the asteroid was at relatively low phase angles (24-26 degrees) and 150 spectra were gathered on 10-12 March. An additional 100 near-infrared spectra were obtained on 23-24 March when SF36 was at relatively high phase angles (47-51 degrees). The separation of the pairs of observing runs allows documentation and calibration of phase angle changes on the spectra. Since the MUSES-C spacecraft will view the asteroid at a wide range of phase angles during the mission, it is useful to understand the effects viewing geometry will have on the spectra.
Analyses of the near-infrared spectra for SF36 on 12 March indicate that the silicate portion of this asteroid has an Opx/(Opx + Ol) ratio of 0.239 with an Opx composition of approximately Fs41Wo8. Based on these results, there are currently two equally likely explanations for this asteroid: (1) It is an olivine-dominated assemblage from which significant partial melts have been extracted; (2) It is mostly undifferentiated, similar to olivine-rich L- or LL-chondrites. Analyses of the additional near-infrared and visible data are being conducted at present. These will allow us to confirm and refine the mineralogical composition of SF36 and document phase angle effects.