Comet Hale-Bopp (C/1995 O1) Near 2.3 AU Postperihelion: Southwest Ultraviolet Imaging System Measurements of the H₂O and Dust Production

     The Southwest Ultraviolet Imaging System (SWUIS) imaged comet C/1995 O1 (Hale-Bopp) in various bandpasses from the Space Shuttle on nine occasions during 1997 August 9–15. These observations occurred when the solar elongation of the comet was too small to permit Hubble Space Telescope and other UV observations. Here we present some first results of the continuum and gas emission measurements collected by SWUIS. We find that Hale-Bopp's dust-production parameter, Afρ, was (2.0 ± 0.8) × 10⁵ cm when the comet was 2.33 AU from the Sun. Furthermore, we find that its water production rate, Q(H₂O), was (2.6 ± 0.4) × 10²⁹ s^-1. Combining this result with both other published H₂O production rates and CO production rates, we find that our measurements were made at the beginning of the period when the comet's activity was in transition from a H₂O dominated to a CO-dominated state. We also find that the average rate of decrease of the water production between perihelion and 2.33 AU postperihelion was very close to r, but concerns over radio data indicate that it may have been shallower immediately postperihelion and then considerably steeper beyond about 2 AU. Such a behavior could indicate a sharply declining H₂O production rate beyond 2 AU, but if this is the case, then the H₂O production curve's steepening and turnoff occurred ≈1 AU closer to the Sun postperihelion than did the H₂O turn-on preperihelion. An alternative explanation could be that a seasonal (i.e., obliquity-dependent shadowing) effect may have caused a reduction in illuminated area on Hale-Bopp's irregular nucleus between 1.5 and 2.3 AU outbound.