2060 Chiron: Colorimetry and cometary behavior

VRIJHK colorimetry of "asteriod" 2060 Chiron in 1988 resolves a long-standing ambiguity about the fit of the visible spectrum to the infrared spectrum confirming that Chiron's spectrum is very neutral, placing it in taxonomic class C. These observations also allowed us to discover that Chiron had brightened anomalously. Relative to an absolute magnitude determined from observations made between 1980 and 1983, Chiron was brighter by about 0.6 mag in February and March 1988, and by about 1.0 mag in September and October 1988. The onset of this episode of anomalous brightening apparently occurred in mid-1987 when Chiron was 13 AU from the Sun. The brightening has been confirmed by independent observers. We report negative searches for coma and emission features in 1988; however, in 1989, one of us (K.J.M.), with M.J.S. Belton, reported the first detection of a coma. Other data suggest that a previous brightening episode occurred in 1978 when Chiron was 17.5 AU from the Sun. The data are consistent with the hypothesis that Chiron undergoes weak, sporadic, cometary outbursts perhaps similar to those of comet P/Schwassmann-Wachmann 1. Chiron resembles Saturn's satelite S9 Phoebe (widely regarded as captured), in color, approximate size, and possibly dynamical history. Chiron may be an ice-rich object darkened by C-class carbonaceous soil, and may have been scattered inward from the Oort cloud recently in Solar System history. Although cataloged as an asteroid, Chiron is apparently the largest well-observed comet nucleus, about 180 km in diameter.