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Calar Alto Impact Times for SL9


Accurate Earth-based times for Fragments A, E, H, L, Q1, and Q2
Evidence for a small impact before L

Some time ago, several people wrote messages to the exploder comparing times for various SL9 impacts, in particular H and L, as observed by Galileo PPR and by ground-based telescopes. Some of the times quoted for various observations were taken from the original exploder results which, as Clark Chapman correctly points out, should be taken with a grain of salt. In particular, the times that we initially reported from the Calar Alto 3.5m telescope corresponded to times when we first saw "something" in our simple sky-subtracted infrared images. These times were obviously improved as more careful reductions were done -- flat-fielding, averaging sky frames, optimizing the "stretch" of displayed images, defining what "something" was, etc.

Here then, is the current status -- with accurate times -- of the ten impacts that were available to us at Calar Alto.

                    Calar Alto 3.5 meter SL9 Data
|Impact  Date      Impact Time     Start of      Start of    Precursor- |
|       (July)      (Accepted)     Precursor     Plume       Plume Delay|
|A        16      20:11    +/- 3     20:11       20:18          7 min   |
|E        17      15:11    +/- 3       ?         15:17:30         -     |
|H        18      19:31:59 +/- 1     19:33       19:38          5 min   |
|L        19      22:16:48 +/- 1     22:16:30    22:17:30       1 min   |
|Q2       20      19:44    +/- 6     19:44       19:52          8 min   |
|Q1       20      20:12    +/- 4     20:13       20:20          7 min   |
|S        21      15:15    +/- 5       ?         detected         -     |
|U        21      21:55    +/- 7       ?         detected         -     |
|P2       20      15:23    +/- 7       ?           ?              -     |
|T        21      18:10    +/- 7       ?           ?              -     |

Data were taken from the 3.5 meter telescope at 2.3 microns. Times are UT; those with seconds are accurate to +/- 15 seconds or so, those without seconds to +/- 1 min. The accepted impact times are from the orbital solutions of Yeomans and Chodas (errors are in minutes); H and L impact times are from Galileo PPR.

As a cautionary note, we see faint spots at the impact site BEFORE the precursor for both the H and L impacts. In the case of L, a faint spot was already present on the planet when our 2.3 micron observations began at 21:58. The spot was continuously observed until it was swamped by much brighter light from the L precursor. The longitude of the "bogey" does not correspond to a pre-existing spot; we suspect that it is due to the fresh impact of a very small, previously unknown SL9 fragment. For H, a faint spot first appeared at 19:29 but the B spot was scheduled to round the eastern limb at about this time.

We will have a poster on these and other Calar Alto results at the DPS meeting in Washington D.C.


Doug Hamilton
Tom Herbst

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