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D.K. Yeomans, S.R. Chesley, J.D. Giorgini (JPL/CalTech), Deep Impact Science Team
Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 11 returns to perihelion, including all its returns since 1967. The observations for the eight apparitions beginning in 1967 have been fit with an orbit that includes radial, transverse and normal nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary nucleus. The normal component was the largest suggesting that the comet's nucleus has jet activity on the side facing south of the orbit plane. The nucleus images from the Deep Impact spacecraft were included in the comet's orbit determination process and these images were key to understanding the significant observational biases in the ground-based data: these biases were especially evident during a June 14 outburst event that was observed by the Deep Impact spacecraft. The success of the time independent model over the 1967 - 2005 interval suggests that the comet's spin axis is not tumbling. While there is evidence that the water production rate during the 2005 apparition is down by a factor of 2.4 since 1983 (Schleicher, 2005, IAU Circ. 8546) and the observed gas production rate did not reach a maximum at perihelion, the nongravitational acceleration model for this comet is not sensitive to these effects. The comet's modest outgassing and large size/mass are the likely reasons for this model insensitivity. Rough calculations suggest that the collision of the impactor released by the Deep Impact spacecraft will not likely provide a noticeable perturbation on the comet's orbit nor will any new vent that is opened as a result of the impact provide a noticeable change in the comet's nongravitational acceleration history. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.