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M.J.S. Belton (Belton Space Exploration Initiatives)
The rotation state of 2P/Encke has been the subject of three major observing projects (Jewitt and Meech 1987; Luu and Jewitt 1990; and Fernandez 1999). There is common agreement that a rotational periodicity exists in the light curve near 3.175 inv. days (15.1 hr). Since there is a substantial probability that many active short period comets may be in an excited rotational state (Jewitt 1997) and 2P/Encke in particular (Samarasinha, priv. comm), we have analysed the above data sets as a group looking for a second periodicity, unrelated to 15.1 hr, that would indicate an excited rotation state (Belton 1991). Such a periodicity has been found, with low amplitude, at 8.68 inv. days. Using model simulations (Kryszczynska et al 1999; Meech et al 1993) as a guide to interpreting the periodicities, we find that the nucleus spin is excited in the long axis mode and the major to minor axis ratio is greater than 2.1 : 1 (for a homogeneous ellipsoidal model). If this ratio is 2.6, as recently suggested by Fernandez (1999), then the spin axis would precess around the angular momentum vector once every 15.12 hr at an angle near 33.5 deg. and roll about the long axis every 8.7 hrs. The net spin would be about 6 hrs. (Funded by the NASA/APL CONTOUR project).
(Jewitt and Meech, A.J. 93,1542 (1987); Luu and Jewitt, Icarus 86, 69 (1990); Fernandez, Thesis Univ. Maryland (1999); Kryszczynska et al., A & A, 345, 643 (1999); Meech et al., A.J. 106, 1222 (1993): Jewitt, Earth Moon & Planets 79, 35 (1997) (Belton. In Comets in the Post-Halley Era 2, 691 (1991)