Rationalization of Comet Halley's periods

Abstract

The observed orientations of the nucleus of Comet Halley at the Vega/Giotto encounters are brought into accord with periodicities extracted from time-series data on the brightness of the comet. To accomplish this the sense of orientation of the long-axis, at Vega 1 encounter, must be reversed from that deduced by Sagdeev et al. (1986a, Nature 321, 262–266). Reasons why this is a plausible interpretation of the Vega 1 images are given. It is shown that Vega/Giotto observations can be satisfied by either a 2.2-day, or a 3.7-day, free-precession period of the long-axis. A consistent set of periodicities is established in a reanalysis of five independent data sets using a new fourier algorithm. Strong evidence is found for periods harmonically related to ≈7.4 days. short review of published ground-based investigations yields no convincing evidence for a 2.2-day periodicity. Candidate models of Halley's nuclear rotation are developed. The preferred models are characterized by a long-axis precession period of 3.7 days and have either oscillations, or full rotation, about the long-axis with a 7.3-day period. In one case the direction of the angular momentum vector deduced for the model is only 6° from the direction independently established by Sekanina et al. (1987, Nature 325, 326–328). Other candidate models, characterized by a long-axis precession period of 3.7 days and oscillations, or full rotation, about the long-axis with a period of 2.5 days, also satisfy spacecraft and photometric constraints, but, because they imply an extremely inhomogeneous nucleus, are less attractive.