Early Photometry of Comet p / Halley : Development of the Coma 1

Broadband charge-coupled device photomet ry of Comet p/Halley at heliocentric d is tances R = 5.9 A U (1984 October) and R = 5.1 A U (1985 January) is presented. The mean brightness at R = 5.1 A U is greater than expected from an asteroidal br ightness model fitted to earlier photometry . It is likely that this br ightness increase is due to the release of dus t grains from the nucleus beginning at about R = 5.9 A U. Simple thermal equilibrium sublimation models of a water-ice nucleus are shown to be consis tent with weak activity even at R = 5.9 AU, provided the nucleus is dark (Bond albedo A < 0.15) and slowly rotating. The br ightness of the comet varies on time scales from hours to days , with a range of nearly 1.0 mag at R = 5.9 AU, reduced to about 0.3 mag at R = 5.1 AU. The decrease in the range of the shor t term variations is explained by the increased contr ibution from the coma to the total br ightness of the comet . We find no convincing evidence for a dominant period in the shor t term variations. © 1986 Academic Press, Inc.