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S.R. Chesley (JPL/Caltech)
The nongravitational accelerations of Comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission, reveal a strong seasonal signal in the motion of this comet, evidenced by a post-perihelion increase in activity and acceleration that peaks 40--50 days after perihelion. The nongravitational accelerations of this comet are modeled well as outgassing from body-fixed jets that thrust according to the level of insolation received, which depends upon the heliocentric distance and the solar sub-latitude. I assume a near-polar northern jet and a mid-latitude southern jet and, from the available astrometry, estimate the jet strengths at a wide range of pole positions. With this configuration two pole orientations, separated by 160\circ, are clearly favored. For Solution A the polar jet is responsible for the post-perihelion activity and for Solution B, which is moderately favored by the fit statistics, the mid-latitude jet is active post-perihelion. Corroborating information comes from Weiler et al. [1], who report a persistent fan shaped structure in March 2003 images. Their images are not consistent with solution A, but show a near-perfect match to the predicted fan orientation and width for solution B. Thus, the pole orientation of Churyumov-Gerasimenko is estimated to be within 10\circ of \alpha=90\circ, \delta=+75\circ, which implies an obliquity around 45\circ. This pole is also consistent with the near-equatorial observational aspect reported by Lamy et al. [2]. The estimate does not imply a sense of rotation, and so it could equally be the direction of the south pole, in which case the mid-latitude jet would instead be in the northern hemisphere.
[1] Weiler, H. Rauer and J. Helbert, A&A, v.414, p.749-755 (2004)
[2] P.L. Lamy et al., (35th DPS meeting, #30.04) BAAS (2003).
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.