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W. T. Reach (Caltech/IPAC), M. V. Sykes (U. Arizona), D. J. Lien (Wash. \& Jeff. College), J. K. Davies (JAC/Hawaii)
P/Encke was imaged by the Infrared Space Observatory ISOCAM on July 14, 1997 from a particularly favorable viewing geometry nearly perpendicular to the comet's orbital plane and at a distance of 0.25 AU. A structured coma was observed, along with a long, straight dust trail. For the first time, we are able to observe the path of particles as they evolve from the coma to the trail. The particles that produce the infrared coma are larger than dust grains, with radiation to gravitational force ratio \beta<10-3. For spherical particles, this corresponds to radii greater than 600/\rho \mum, where \rho is the mass density of an individual particle in g~cm-3. The dust trail follows the orbit of the comet across our image. This dust trail has a central core that is 2\times 104~km wide, due to particles with \beta<10-5 from previous apparitions. This corresponds to radii greater than 6/\rho cm, particles so large that they pose a severe hazard to spacecraft. There is no evidence of a dust tail due to particles with \beta<10-3, in marked contrast to other comets like P/Halley. The structure of the coma requires anisotropic emission and that the spin axis of the nucleus to be close to the orbital plane, so that there are strong seasonal variations of the particle emission. While some structures near the nucleus are due to young particles, emittted during the apparition of observation, the core of the dust trail requires emissions from previous apparitions. ISOCAM observations combined with ISOPHOT observations constrain the nucleus diameter to be between 1 and 3 kilometers.
The author(s) of this abstract have provided an email address for comments about the abstract: reach@ipac.caltech.edu