IMPACT EXPLOSION WILL BE SMALLER, 
          HIGHER THAN EARLIER PREDICTED, 
          UA's WILLY BENZ PREDICTS IN NATURE
          
          From Lori Stiles, (602) 621-1877
          University of Arizona, News Services
          -- Sent July 13, 1994
           
          (Science contact:  Willy Benz, UA Steward Observatory
                          phone:   41 22 755-2611
                          fax:   41 22 755-3983
                         email:  wbenz@ptolemy.as.arizona.edu)
          
          
               Comet Shoemaker-Levy 9 was originally "a rubble
          pile composed of hundreds, if not thousands, of smaller
          snowballs" no greater than 1.5 kilometers in diameter. 
          Observers will probably see  a smaller explosion higher in
          Jupiter's atmosphere than they now expect,  University of
          Arizona astronomer Willy Benz reports in the July 14 issue
          of Nature.
          
               Benz, an associate professor of astronomy at the UA
          Steward Observatory, and former UA graduate student Erik
          Asphaug, now with the NASA Ames Research Center,
          collaborated in modeling the structure of Comet SL9.  Benz
          is in Switzerland to observe the impact from the Geneva
          Observatory and possibly from the Observatoire de Haute-
          Provence, France.  
          
               "All the predictions and calculations of the impact
          itself hinge on knowing what is actually impacting Jupiter,"
          Benz said in an email message received yesterday.  "We
          tried to use the closest approach of SL9 with Jupiter in July
          1992 to constrain cometary models.  What we found was
          that the usual large, homogeneous dirty snowball model
          would not work.  By this, we mean it did not break-up in
          21-plus pieces when it flew by Jupiter. 
          
               "We investigated further and found that the model
          that worked best...the model that best fit observations...was
          one in which the comet was initially a rubble pile
          composed of hundreds, if not thousands, of smaller
          snowballs. The diameter of the parent object depends a bit
          on the initial rotation of the comet, but is on the order of
          1.5 kilometers. 
          
          
               "Our model implies that each of the 21 or so
          fragments is actually composed of a large number of
          smaller entities.  All these snowball clusters are loosely
          bound by their own gravity.  As a result, we believe the
          impact will be higher in the atmosphere than anticipated by
          most numerical simulations."  Some simulations assume
          that each fragment is an ice ball with a diameter of about
          two kilometers, Benz said.  "The parent comet being
          smaller, we also anticipate a somewhat smaller explosion."
          
               Benz said of the coming impact, "Even if this impact
          is somewhat less spectacular than the media would like to
          have it, I believe that potentially much can be learned about
          the structure of comets and the circulation in Jupiter's
          atmosphere.  And the best of all, we are going to learn all
          this for a lot less money than (the cost of) sending a
          spacecraft!"
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