0.7- to 23-μm photometric observations of P/Halley 1986 III and six recent bright comets

We report 0.7- to 23-μm observations of P/Halley 1986 III and six other recent bright comets. P/Halley was measured on 47 occasions between 1985 December 12 UT and 1986 May 6 UT, several times within hours of the perihelion passage on 1986 February 9 UT. Our data show that the strength of the 10-μm silicate emission feature and the temperature excess (superheat; S = T_obs/T_BB) of the infrared continuum emission are strongly correlated. IR Type I comets have low continuum superheat and muted or undetectable silicate emission features, suggesting that the coma emission from these comets is produced by large grains with radii larger than 1 μm. IR Type II comets have superheated thermal infrared continua and high-contrast silicate emission features, indicating that the coma emission is from small grains with radii between 0.5 and 1 μm. Both types of behavior were exhibited by Comet P/Halley at various times. The relationship between superheat and 10-μm silicate emission may be complex, for although though the strength of these quantities was generally strongly correlated, several comets exhibited occasional episodes when superheat and silicate emission were not correlated. P/Halley's dust coma had an average albedo of 0.20 at a scattering angle of 130°. Our data show that the scattering phase function for typical comet dust is characterized by a moderately strong forward scattering peak, no appreciable backscattering peak, a mean bolometric albedo of ≈0.32, and an albedo of ≈0.15 for scattering angles between 120° and 180°. These characteristics are consistent with laboratory and theoretical results for nonspherical and "fluffy" core-mantle aggregate grains. P/Halley's 10-μm silicate signature showed significant variations in strength and was occasionally weak or absent at heliocentric distances both smaller and larger than 1 AU. Simultaneous measurements of P/Halley and Bradfield 1980 XV with different diaphragms are generally consistent with the steady-state model for nuclear ablation. P/Halley's coma luminosity fluctuated by a factor of nearly 10 on time scales of 1 to 2 days. These variations are consistent with jet-like activity probably associated with nuclear rotation. Dust mass loss rates for the comets studied here are estimated, and we conclude that P/Halley was losing ≥10⁶ g sec^-1 of dust at a heliocentric distance of 1 AU.