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D. J. Tholen (Univ. of Hawaii), S. R. Chesley (JPL)
Fifty three images of comet 81P/Wild 2 were obtained during morning twilight and at extremely high airmass on 2003 Dec 19-21 UT using the ESI instrument on Keck II in support of the StarDust spacecraft encounter with that comet on 2004 Jan 2. Two-dimensional Gaussian fits to the cometary images using a dozen synthetic aperture sizes from 1.5 to 18.6 arcsec generally showed a monotonic eastward shift in the centroid as the aperture size decreased, with the total shift from largest to smallest aperture amounting to 0.9 arcsec. It has been known since at least the time of the spacecraft encounters with comet Halley in 1986 that the center of light measured for a comet does not necessarily coincide with the location of the nucleus of that comet, due to the tailward bias an asymmetric coma can have on the computed centroid. The monotonic shift observed in the centroids for Wild 2 suggested that an extrapolation to a theoretical aperture size of zero could remove at least some, if not most, of the bias, leading to more reliable astrometry of the comet's nucleus than the alternative technique of using the position corresponding to the brightest pixel in the image of the comet. The late December astrometry produced a 115 km shift in the location of the comet along the time-of-flight direction in the final ephemeris delivery, with a one-sigma uncertainty of 300 km, which allowed the science team to pursue a more aggressive imaging sequence, one that yielded spectacular results.
The photometry extracted from the astrometric images revealed less activity than expected, which caused the science team to reduce the intended flyby distance from 300 km to 250 km. A posteriori spacecraft ephemeris reconstruction showed the actual flyby distance to be 235 km.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.