The Nucleus of Comet Hyakutake (C/1996 B2)

Infrared, optical, and radio continuum observations were made of the long-period comet C/Hyakutake 1996 B2 during its close approach to Earth in March 1996. Using these observations to characterize the comet's nucleus, we find an estimated nuclear radius of 2.4±0.5 km (1σ) from photometric resolution of the nucleus in the thermal infrared at 8-20 μm on 25 March 1996, no detectable optical nuclear emission above that of the coma on 19-23 March 1996, and a 3σ upper limit to the radius of 2.7 km (assuming an emissivity of 0.9) in the radio at 3.6 cm on 27 March 1996. The infrared color temperature of the nucleus was consistent with a 320-K blackbody, and assuming a 2.4-km radius, the maximum effective temperature at 3.6 cm was 230 K. We explain the optical nondetection of the nucleus as due to excess emission from a halo of small, cold, and high optical albedo dust particles surrounding the nucleus; such particles would have low emissivity in the infrared and radio. A surrounding halo of icy dust grains emitting water in addition to the nucleus accounts for the small nuclear size but large production rate of water from C/Hyakutake; otherwise an anomalously large fraction of the nuclear surface, nearly 100%, must be active. A fast rotation period of 6.30±0.03 h due to coma dust cross-sectional variations was found from optical and infrared imaging on March 20-23. The minimum bulk tensile strength required to stabilize the comet against centrifugal breakup due to this rotation, ∼10³ dynes cm^-2, is similar to that found for other comets.