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M. R. Combi (U. Michigan), J. T. T. Makinen (FMI and ESTEC), J.-L. Bertaux, E. Quemerais (CNRS)
Comet 1996 B2 (Hyakutake) was noteworthy for having a very small geocentric distance at its closest approach (0.1 AU) in March 1996 and for displaying strong evidence for break-up, with a prominent antisunward dust spike and fragments traveling antisunward for many days after an eruptive event in late March. Because of its high orbital inclination and rapid southward motion after perihelion, its post-perihelion activity was not well monitored from the ground. The SWAN all-sky Lyman-alpha camera on the SOHO spacecraft observed the hydrogen coma of comet Hyakutake both before and after perihelion. These images were analyzed with a new time-resolved (TRM) model which provides daily averages of the water production rate and an estimate of the hydrogen atom lifetime during extended periods throughout the apparition. The daily average values of the production rate covered the March 19 outburst and two more outbursts seen in the April before perihelion, which each had progressively shorter durations at decreasing heliocentric distance than the March outburst. The long term variation of the production rate was found to be consistent with the seasonal effect predicted by the jet rotation model of Schleicher and Woodney (2003, Icarus 162, 190) when added to a more steady source which is about two-thirds of the maximum of the jet source. It appears that only a fraction of the original nucleus could have been lost during the pre-perihelion outbursts and that the post-perihelion period was free from large outbursts, and corresponding from emission by released fragments. The difference between the outbursts and fragmenting behavior from the jet source locations during the pre-perihelion period and the smoother variation from the rest of the surface, which dominated after perihelion, indicates a strong heterogeneity in the physical make-up of active areas on the nucleus.
Support for this work was provided by NASA grant NAG5-12415 from the Planetary Astronomy program.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.