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N. Thomas (Max-Planck-Institut fuer Aeronomie, Katlenburg-Lindau, Germany.), M.F. A'Hearn (Dept. of Physics and Astronomy, Univ. of Maryland), D.C. Boice (Southwest Research Institute, San Antonio), D.T. Britt (Dept. of Geological Science, Univ. of Tennessee), K.J. Meech (Institute for Astronomy, Univ. Hawaii), B.R. Sandel (Lunar and Planetary Lab., Univ. of Arizona), L.A. Soderblom (Astrogeology Branch, USGS, Flagstaff.), R.V. Yelle (Dept. of Physics and Astronomy, Northern Arizona University, Flagstaff)
The MICAS imaging system on board NASA's Deep Space One spacecraft provided 53 images of the nucleus of comet 19P/Borrelly within a 30 minute period immediately prior to closest approach. These observations showed the innermost coma out to a distance of at least 100 km. Another set of observations was acquired 10 hours before encounter. Assuming that the rotation period is close to 25 hours (Lamy et al., Astron. Astrophys., 337, 945, 1998), the appearance of a persistent jet feature almost invariant in projected direction in these two observing sets suggests that the jet feature is from a constantly illuminated pole. The fact that the direction of the emission is perpendicular to the long axis of the nucleus also provides circumstantial evidence for this hypothesis.
The jet itself is well collimated and provisional fits suggest a full-width half-maximum of around 20 degrees. The jet is superimposed upon a background emission which is mostly hemispheric in nature centered upon the sub-solar point. Some emission into the nightside hemisphere is also evident.
Beyond 2-3 nucleus radii, the emission appears to follow a 1/r distribution consistent with force-free radial outflow out to distances greater than 50 km.
The presentation will provide an initial survey of the dust distribution near Borrelly. Comparisons with ground-based data and to inner coma observations of comet Halley acquired by Giotto will be made.