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 (Ha le-Bopp) in various bandpasses from the Space Shuttle on nine occasions dur ing 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 o bservations. Here we present some first results of the continuum and gas em ission measurements collected by SWUIS. We find that Hale-Bopp's dust-produ ction parameter, Afp, was (2.0 +/- 0.8) x 10(5) 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) x 10(29) s(-1). Combining this result with both other published H2O production rates and CO production rates, we find that our me asurements were made at the beginning of the period when the comet's activi ty 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 peri helion and 2.33 AU postperihelion was very close to r(h)(-4.0+/-0.6), but c oncerns over radio data indicate that it may have been shallower immediatel y postperihelion and then considerably steeper beyond about 2 AU. Such a be havior 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 tur noff occurred approximate to 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 red uction in illuminated area on Hale-Bopp's irregular nucleus between 1.5 and 2.3 AU outbound.