A McDonald observatory study of Comet 19P/Borrelly: Placing the Deep Space 1 observations into a broader context

We present imaging and spectroscopic data on Comet 19P/Borrelly that were obtained around the time of the Deep Space I encounter and in subsequent months. In the four months after perihelion, the comet showed a strong primary (sunward) jet that is aligned with the nucleus' spin axis. A weaker secondary jet on the opposite hemisphere appeared to become active around the end of 2001, when the primary jet was shutting down. We investigated the gas and dust distributions in the coma, which exhibited strong asymmetries in the sunward/antisunward direction. A comparison of the CN and C(2) distributions from 2001 and 1994 (during times when the viewing geometry was almost identical) shows that each species is remarkably similar, indicating that the comet's activity is essentially repeatable from one apparition to the next. We also measured the dust reflectivities as a function of wavelength and position in the coma, and though the dust was very red overall, we again found variations with respect to the solar direction. We used the primary jet's appearance on several dates to determine the orientation of the rotation pole to be alpha = 214degrees, delta = -5degrees. We compared this result to published images from 1994 to conclude that the nucleus is near a state of simple rotation. However, data from the 1911, 1918, and 1925 apparitions indicate that the pole might have shifted by 5-10degrees since the comet was discovered. Using our pole position and the published nongravitational acceleration terms, we computed a mass of the nucleus of 3.3 x 10(16) g and a bulk density of 0.49 g cm(-3) (with a range of 0.29 < p < 0.83 g cm(-3)). This result is the least model-dependent comet density known to date. (C) 2002 Elsevier Science (USA).