Surface Characteristics of Comet-Asteroid Transition Objects 944 Hidalgo and 162P/Siding Spring (2004 TU12)

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Conference Proceeding

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American Astronomical Society, Department of Planetary Sciences


We present initial results from infrared and visible observations of two comet-asteroid transition objects. 944 Hidalgo is one of the asteroids most likely to be an extinct or dormant comet. Comet 162P/Siding Spring (henceforth 162P), displayed intermittent cometary activity shortly after discovery in late 2004. This comet is the third inner solar system object observed to have intermittent cometary activity even when relatively close to the Sun (the others are 107P/Wilson-Harrington and 133P/Elst-Pizarro). Thermal infrared observations (at 8.7, 9.8, 11.7 and 24.5 microns) of 162P were obtained with MIRSI on NASA's infrared telescope facility (IRTF) on Mauna Kea, Hawaii on UT Dec. 27, 2004. These represent one the most complete samples of a comet nucleus' spectral energy distribution and yield an effective radius of 6.0 ± 1.0 km. Using an absolute visible magnitude of H=13.7, the V-band albedo is 0.045 ± 0.015. Near-infrared spectra in the 0.8-2.4 micron region were obtained on UT Oct. 23 (Hidalgo) and Dec. 11 (Hidalgo and 162P), 2004 using SpeX on IRTF (Oct.) and NICS on the Telescopio Nazionale Galileo (TNG) on La Palma, Spain (Dec.). Visible spectra of both objects from 0.3 to 0.9 microns were obtained on UT Dec. 12, 2004 with ALFOSC at the Nordic Optical Telescope (NOT) on La Palma, Spain. The visible and infrared reflectance spectra of both objects are nearly identical, with a mean slope typical of D-type asteroids, and they display no strong absorption features. The spectral similarity with 162P is consistent with, but does not prove, the cometary nature of Hidalgo. A search for spectral variability with rotational phase in Hidalgo yielded no obvious spectral changes. We also obtained spectra of Hidalgo in the 2 to 4 micron region with SpeX at the IRTF and detected no strong spectral features in this region due to ices, hydrated minerals or organics.