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Session 71 - Comets & Asteroids.
Display session, Friday, January 09
We present results of diffraction-limited MIRAC2 imaging of the two largest main-belt asteroids, Ceres and Vesta. The peak thermal emission for both of these objects occurs in the mid-infrared.
Ceres was observed at seven mid-infrared wavelengths from 8.6 to 20.6 \mu\/m\hskip 5pt on May 5, 1996, from 9:23 to 9:43 UT. The measured fluxes at these wavelengths increase from 344 to 1759 Jy and are well-fit by a 238 K blackbody. The images are resolved at all wavelengths except 20.6 \mu\/m. The apparent size of the asteroid decreases with increasing wavelength until about 11 \mu\/m, and then the size begins to increase again. The object appears football-shaped and oriented with its long axis (in 2-D projection) running NE to SW at an angle of 50 degrees east of north. There is a large projection to the SE at a position corresponding to the mid-plane of the object, and a fainter projection to the NW also at the position of the mid-plane. Our calculated color temperature map indicates a cooler region of material across the object mid-plane, and our opacity/emissivity map indicates that this lower-temperature region has a higher opacity/emissivity than other portions of the asteroid. The southwestern half of the asteroid exhibits the highest temperatures.
Vesta was observed at eight mid-infrared wavelengths from 8.6 to 20.6 \mu\/m\hskip 5pt on May 5, 1996, from 8:23 to 8:52 UT. The measured fluxes, excluding the 20.6 \mu\/m\hskip 5pt image, increase from 401 to 1036 Jy and are well-fit by a 271 K blackbody. The images are resolved at all wavelengths except 20.6 \mu\/m. The apparent size of the asteroid increases with wavelength. The object appears rounder than Ceres and somewhat elongated, with a large flattened edge to the NW in 2-D projection. Our calculated color temperature and opacity/emissivity maps show smoother distributions than found for Ceres.
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