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S.R. Chesley (JPL/Caltech)
There are some comets for which the prevailing (A1,A2)-model  for comet nongravitational accelerations performs very poorly. In at least a few cases this model can be improved by incorporating an acceleration in the direction normal to the orbit plane (A3) or by adding a time lag (DT)  for the peak acceleration, which occurs at perihelion under the standard model. An alternate formulation is based upon body-fixed jets rotating with the comet and thrusting in proportion to the amount of solar radiation they receive at any given instant. Averaging over the comet rotation period smooths the acceleration and eliminates the need to know the rotation rate and direction.
Ground-based and spacecraft astrometric observations taken around the 2001 perihelion passage of Comet 19P/Borrelly indicate that the (A1,A2)-model is inadequate for this comet. In contrast, the (A1,A2,A3,DT)-model performs remarkably well, with both A3 and DT having significant values. Images from numerous sources during this period clearly indicate what appears to be a pronounced polar jet that we propose is the dominant source of the comet's nongravitational acceleration. Under this hypothesis one can infer the pole/jet direction simply from the estimated values for (A1,A2,A3,DT). The resulting pole position estimate [(RA,DEC)=(208\circ ±2.5\circ, -4\circ ±2.5\circ)], which is based solely upon astrometric data, agrees well with other estimates. Applying the jet model to Borrelly gives results nearly as good as the (A1,A2,A3,DT)-model, with modestly improved short-term predictions. Estimating the pole position within the framework of the rotating jet model leads to values only 10-15 deg from the presumed pole position.
Astrometric fits for Comet 2P/Encke, the first target of the CONTOUR mission, reveal that the addition of A3 or DT do not improve upon the (A1,A2)-model for this object. However, Sekanina  reported two main source regions, at latitudes 35\circN and 65\circS. Applying the rotating jet model with these two sources, and using the pole position reported by Sekanina, does give a significant improvement in both fitting and predicting the comet's position. Using the available astrometry with this approach suggests that a pole position around (RA,DEC)=(220\circ, +40\circ) provides even better fits and predictions than those obtained using the Sekanina pole.
This research was carried out at JPL/Caltech under a contract with NASA.
 B. Marsden et al. (1973) Astron. J. 78, 211--225.
 D. Yeomans, P. Chodas (1989) Astron. J. 98, 1083--1093.
 Z. Sekanina (1988) Astron. J. 95, 911--971.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.