D. McDavid (Limber Observatory), D.C. Boice (SwRI)
AAS 187th Meeting, San Antonio, TX – January 1996

Unfiltered CCD images of comet P/Swift-Tuttle were acquired on six consecutive nights from 25 November through 30 November 1992 with the 0.4 m 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 +/- 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.

Fig. 1. The field of Comet P/Swift-Tuttle at 01:12 UT, 25 November 1992 (5 arcmin square). The enlarged inset shows the inner 32 arcsec of the coma, where the measurements were made.