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T.L. Farnham, C.M. Lisse, M.F. A'Hearn, O. Groussin (U. MD), K.J. Meech (U. HI), U. Fink (U. AZ), D.G. Schleicher (Lowell Obs.)
The coma of comet Tempel~1 is of interest to the DI mission for a number of reasons: as a phenomenon unto itself; as a proxy for the nucleus composition; as a component of the flux measured in the DI observations; and as an obstacle through which the spacecraft must traverse. It is important to understand the normal baseline state of the coma, for planning purposes as well as to determine exactly how it is changed by the impact. To this end, we have collected all available observations, old, new, published and unpublished, and performed an analysis to produce as complete a picture of the coma as possible.
Our results indicate that Tempel~1 is not an unusual comet with respect to dust and gas. Narrowband photometry and spectral measurements show that the production rates of H2O, CN and dust (Af\rho) exhibit clear seasonal variations, with the peak production occurring about two months before perihelion. We expect the total outgassing rate at the time of the encounter to be about 2\times1028~mol~sec-1. The abundance ratio of carbon chain molecules is at the lower end of the range of ``typical'' in the A'Hearn et al. (1995) classification. Pre-perihelion images from 1994 show at least one jet in the coma, and changes in illumination of its source could be a major cause of the seasonal variations. The column density through the jet is only about twice that in the average coma, so the jet ejecta is not considered to be a significant additional threat to the spacecraft beyond the 0.1~g~m-2 total dust fluence already expected from traversing the coma. Thermophysical models of the nucleus' surface indicate that approximately 8% of the comet's surface is active, which is consistent with the existence of isolated active areas. We will present these and other coma properties in more detail, and discuss how they impact the DI mission.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.