Images1–3 of the comet Halley nucleus by the Vega and Giotto spacecraft and a compilation4 of recurring events show a 'rotation' period of roughly 2.2 days. In contrast, molecular emission intensities5 (C2 and CN) in the inner coma exhibit 7.4-day periodic variations. In this report the comet Halley nucleus is modelled dynamically as a rigid homogeneous ellipsoid with three unequal principal axes in torque-free rotation about its centre of mass. Two qualitatively-distinct, free-precessional modes are found, each exhibiting both 2.2-day and 7.4-day periodicities. As viewed from the nucleus, the two modes are distinguished by the closed path (called a polhode) that the end of the angular velocity vector traces with a period of 7.4 days (see Fig. 1). The long-axis mode has a polhode that encloses only the long principal axis of the nucleus. It reduces to a motion proposed by Sekanina6,7 when the minor axes are equal. However, because the polhode encloses the long axis in a long-axis mode, the resulting motion with a 7.4-day period around that axis conflicts8 with the Vega and Giotto images of the nucleus. The short-axis mode occurs when the minor axes are unequal and the polhode encloses only the short principal axis of the nucleus. The resulting motion is around the short axis and does not conflict with the spacecraft images.
This is a preview of subscription content, access via your institution
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Rent or buy this article
Prices vary by article type
Prices may be subject to local taxes which are calculated during checkout
Sagdeev, R. Z. et al. Nature 321, 262–266 (1986).
Sekanina, Z. & Larson, S. M. Nature 321, 357–361 (1986).
Vaisberg, O. L. et al. Nature 321, 274–276 (1986).
Sekanina, Z. Adv. Space Res. 5, 307–316 (1986).
Millis, R. L. & Schleicher, D. G. Nature 324, 646–649 (1986).
Sekanina, Z. Nature 325, 326–328 (1987).
Sekanina, Z. IAU Circ. No. 4273 (1986).
Smith, B. A., Larson, S. M., Szegö, K. & Sagdeev, R. Z. Nature (in the press).
Landau, L. D. & Lifshitz, E. M. Mechanics (Pergamon, Oxford, 1960).
Julian, W. H. & Zund, J. D. Stud. appl. Math. 75, 171–177 (1986).
About this article
Cite this article
Julian, W. Free precession of the comet Halley nucleus. Nature 326, 57–58 (1987). https://doi.org/10.1038/326057a0
This article is cited by
Celestial Mechanics and Dynamical Astronomy (2008)
The Astronomy and Astrophysics Review (1993)
Earth, Moon and Planets (1990)