The Excited Spin State of 2P/Encke

Ways to rationalize the different periods seen in near aphelion R-band light curves of comet 2P/Encke are explored. We show that the comet is usually active at aphelion and it's observed light curves contain signal from both the nucleus and an unresolved coma. The coma contribution provides from 28 to 87% of the total brightness. The amplitude of the observed variations cannot be explained by the nucleus alone and are due to coma activity. .The changing periodicities are probably due to changes in the relative strengths of the active areas. We work out possible excited states based on experience with model light curves and by using an analogy to light curve observations of comet 1P/Halley for which the spin state has been separately determined from spacecraft observations. The possibility of a fully relaxed principal axis spin state is discounted because it provides a poorer fit to the observed periodicities and because of the unlikelihood of such a rich spectrum of periodicities from a principal axis rotator. Two possible SAM states emerge that provide equally good fits to the observed periodicities, one with a precessional frequency for the long axis about the total angular momentum vector of 1.631 d^-1 (Pφ = 14.7 h) and an oscillation frequency around the long axis of 0.515 d^-1 (Pψ = 46.6 h) and a second with a precessional frequency of 2.164 d^-1 (Pφ = 11.09 h) combined with an oscillation around the long axis of 0.504 d^-1 (Pψ = 47.6 h). In both cases the direction of the total angular momentum vector (α _M, δ _M [J2000] = 198.6, -0.3 deg) is assumed to be defined by the evolving geometry and morphology of the coma (Sekanina 1988a, b; Festou and Barale 2000). We offer a new interpretation of the 10.7 μ m data by Fernández et al. (2000) which yields an axial ratio a/b = 2.04. This, with the two SAM states that we have found, requires that b/c > 1.16 or > 1.09 implying a significant asymmetry in the shape of the elongated nucleus