Session 42 - Solar Systems.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

## [42.09] The Rotation Period of Comet P/Swift-Tuttle

D. McDavid (Limber Obs.), D. C. Boice (SwRI)

Unfiltered CCD images of comet P/Swift-Tuttle were acquired on six consecutive nights from 25 November through 30 November 1992 with the 0.4m telescope at Limber Observatory. With an image scale of 0.804 arcsec/pixel, corresponding to about 775 km/pixel at the comet, details can be seen from dust jets'' in the coma to fine structure in the tail.

Rotation periods of 63.0, 66.9, 69.7, 68.1, and 68.6 hours with formal errors on the order of 5 hours were derived by linear least-squares fitting to time sequences of the position angles of elliptical regions of the inner coma measured using the IRAF task imexamine'' for fitting radii of 3, 5, 10, 15, and 20 pixels, respectively. The average is 67.3 \pm 2.6 hours, which agrees very well with most published results.

The apparent increase in rotation period with increasing radius shows up in contour plots as a twisting of the isophotes. There is a clear variation in the rate of change of the position angle with rotational phase which is related to the orientation of the rotation axis of the nucleus as projected onto the sky. These observations will be interpreted within the context of a model of the dusty environment surrounding comet P/Swift-Tuttle.