From spencer@lowell.edu Wed Dec 15 12:23:22 1993
From spencer@lowell.edu Wed Dec 15 12:23:22 1993
Return-Path: <spencer@lowell.edu>
Date: Wed, 15 Dec 93 12:23:00 MST
From: spencer@lowell.edu (John R. Spencer)
To: spencer@galileo.IFA.Hawaii.Edu
Subject: Comet impact times
Content-Length: 2483
X-Lines: 61
Status: RO

Greetings:

    This is a brief message with important new information about the
impact times of the SL9 fragments.  New astrometry has now been done
by the Spacewatch team, following the comet's emergence from solar
conjunction, which gives a substantial improvement in impact time
predictions: Brian Marsden estimates "a couple of hours" uncertainty.
The information is in IAU circular 5906, sent out this morning.

    Impact times are about a day earlier than previous estimates-
adjust your observing plans accordingly!  Also, the estimated  
distance from Jupiter's center at the previous perijove is now
reduced to 90,000 km, or 1.25 Rj.  Previous estimates of the size
of the original nucleus based on tidal splitting models assumed
a closest distance of 1.6 Rj (Sekanina, Chodas, and Yeomans
preprint), and my *guess* is that the new, closer,
distance will reduce the estimated size of the nuclei.  

    I've taken the IAUC 5906 impact time estimates for selected
nuclei and extrapolated to get times for all nuclei, assuming
the relative separations of the nuclei given in Sekanina, Chodas,
and Yeomans are correct.  Results are summarised in the following
table:

-------------------------------------------
   Nucleus         UT date of
 Designation       impact (July)
----------------- -------------
Jewitt   Sekanina Fitted  IAUC   Brightness
DPS abs  Preprint Time    5906     Index
-------------------------------------------
  21        A 	  16.81              1
  20        B 	  17.08              1
  19        C 	  17.27              1
  18        D 	  17.46              1
  17        E 	  17.61    17.6      2
  16        F 	  17.98              2
  15        G 	  18.29    18.3      2
  14        H 	  18.79    18.8      2
  13        J 	  19.08              1
  12        K 	  19.41    19.4      2
  11        L 	  19.91    19.9      2
  10        M 	  20.24              1
   9        N 	  20.40              1
   8        P 	  20.61              2
   7        Q 	  20.82    20.8      3
   6        R 	  21.21    21.3      2
   5        S 	  21.63    21.6      2
   4        T 	  21.73              1
   3        U 	  21.91              1
   2        V 	  22.19              2
   1        W 	  22.33    22.3      2
-------------------------------------------

Again, times should be good to about 0.1 days.
Note that nucleus 8 is double.  The "brightness index" is
my subjective rating of the relative nucleus brightnesses,
with 3 being brightest.

John.