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From: kdq@marsupial.jpl.nasa.gov (Kevin D. Quitt)
Newsgroups: sci.skeptic
Subject: Re: Jupiter, Comets, and calamaty
Date: 16 Jun 1994 20:07:33 -0700
Organization: Jet Propellor Laboratory
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Thus wrote antonsen@cnd.hp.com (Tim Antonsen) 
>Just curious if anybody has heard any prophecies about Jupiter's pending
>encounter with cometary fragments, slated for July 16.  I can imagine people
>foretelling the birth of a binary star system, for example: Sol/Jupiter.

Not prophecy, but useful:

Newsgroups: jpl.shoemaker-levy
Subject: Comet/Jupiter Collision FAQ - 6/14/94

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

                     Frequently Asked Questions about
          the Collision of Comet Shoemaker-Levy 9 with Jupiter

                        Last Updated 14-Jun-1994
                            32 Days to Impact

    The following is a list of answers to frequently asked questions
concerning the collision of comet Shoemaker-Levy 9 with Jupiter.  Thanks
to all those who have contributed.  Contact Dan Bruton (astro@tamu.edu)
or John Harper (jharper@tamu.edu) with comments, additions, corrections,
etc.  A PostScript version and updates of this FAQ list are available via
anonymous ftp to tamsun.tamu.edu (128.194.15.32) in the /pub/comet
directory.  To subscribe to the "Comet/Jupiter Collision Mailing List",
send mail to listproc@seds.lpl.arizona.edu (no subject) with the message:
SUBSCRIBE SL9 Firstname Lastname.

RECENT CHANGES TO THIS FAQ LIST

        Question 1.1: Limb Crossing Times
        Question 1.3: HST March images
        Question 1.4: More predictions
        Question 2.1: Updated impact times and impact locations
        Question 2.3: Galilean satellite eclipses
        Question 2.4: Updated orbital parameters of the comet
        Question 2.5: Images of crater chains
       Question 2.10: Mail access to files
          REFERENCES: New Journal Articles

GENERAL QUESTIONS

 Q1.1: Is it true that a comet will collide with Jupiter in July 1994?
 Q1.2: Who are Shoemaker and Levy?
 Q1.3: Where can I find a GIF image of this comet?
 Q1.4: What will be the effects of the collision?
 Q1.5: Can I see the effects in my telescope?

SPECIFICS

Q2.1: What are the impact times and impact locations?
Q2.2: Can the collisions be observed with radio telescopes?
Q2.3: Will light from the explosions be reflected by any moons?
Q2.4: What are the orbital parameters of the comet?
Q2.5: Why did the comet break apart?
Q2.6: What are the sizes of the fragments?
Q2.7: How long is the fragment train?
Q2.8: Will Hubble, Galileo, etc. be able to observe the collisions?
Q2.9: To whom can I report my observations?
Q2.10: Where can I find more information?

REFERENCES
ACKNOWLEDGMENTS

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

GENERAL QUESTIONS

Q1.1: Is it true that a comet will collide with Jupiter in July 1994?

     Yes, the shattered comet Shoemaker-Levy 9 (1993e) is expected to
collide with Jupiter over a 5.6 day period in July 1994.  The first of 21
comet fragments is expected to hit Jupiter on July 16, 1994 and the last on
July 22, 1994.  The 21 major fragments are denoted A through W in order of
impact, with letters I and O not used.  All of the comet fragments will hit
on the dark farside of Jupiter.  The probability that all of the comet
fragments will hit Jupiter is greater that 99.9%.  The probability that any
fragment will impact on the near side as viewed from the Earth is < 0.01%.
     The impact of the center of the comet train is predicted to occur at a
Jupiter latitude of about -44 degrees at a point about 67 degrees east
(toward the sunrise terminator) from the midnight meridian.  These impact
point estimates from Chodas and Yeomans are only 5 to 9 degrees behind the
limb of Jupiter as seen from Earth.  About 8 to 18 minutes after each
fragment hits, the impact points will rotate past the limb.  The impact
sites of the later fragments are closer to the limb and will therefore
rotate into view sooner after impact.  After these points cross the limb it
will take another 18 minutes before they cross the morning terminator into
sunlight.


Q1.2: Who are Shoemaker and Levy?

     Eugene and Carolyn Shoemaker and David H. Levy found the 13.8 magnitude
comet on photographic plates taken on March 24, 1993.  The photographs were
taken at Palomar Mountain in Southern California with a 0.46 meter Schmidt
camera and were examined using a stereomicroscope to reveal the comet [2,14].
James V. Scotti confirmed their discovery with the Spacewatch Telescope at
Kitt Peak in Arizona.  See [11] for more information about the discovery.


Q1.3: Where can I find a GIF image of this comet?

     GIF images can be obtained from SEDS.LPL.Arizona.EDU (128.196.64.66)
in the /pub/astro/SL9/images directory.  Below is a list of the images at
this site.  The files are listed here in reverse chronological order:

Comet1993eA.gif    March HST image of SL9 comet train
Comet1993eB.gif    Time comparison image of SL9 comet fragment
sl90216.gif        February 16, 1994 Image from Kitt Peak (J. Scotti)
wfpclevy.gif       Photo Montage of January 24-27 Imaging of SL9 from HST
sl9hst.gif         Post-fix HST Mosaic of SL9 (Jan. 24-27)
sl90121.gif        January 21, 1994 Image from Kitt Peak (R. Jedicke)
sl9compl.gif       Six month comparison image of SL9
1993eha.gif        View of the comet from HST
1993ehb.gif        From HST, focused on the center of the train of fragments
1993esw.gif        Ground-based view of the comet
sl03302b.gif       March 30, 1993 Image from Kitt Peak (J. Scotti)
sl9_930330.gif     March 30, 1993 image of SL9, Spacewatch, J. Scotti
sl9w_930328.gif    March 28, 1993 image of SL9
shoelevy.gif       An early GIF image of SL9

     The following is a list of other collision related GIF graphics and
MPEG animations that are also available at SEDS.LPL.Arizona.EDU:

gipul.gif          Gipul Catena crater chain on Jupiter's moon, Callisto
chain.gif          Crater Chain on Jupiter's Moon, Callisto
orbtrain.gif       SL9's orbit showing the length of the train
schematic2.gif     Diagram of fragment positions as seen from HST; Mar'94
schematic1.gif     Diagram of fragment positions as seen from HST; Jan'94
approach.gif       View of last 18 hours of trajectory, from Earth
earthview.gif      Full-disk view from Earth
earthviewzoom.gif  Close-up view from Earth
vgr2view.gif       Full-disk view from Voyager 2
vgr2viewzoom.gif   Close-up view of impact sites from direction of Voyager 2
galview.gif        View from Galileo
ulysview.gif       View from Ulysses
poleview.gif       View from beneath Jupiter
orbjupplane.gif    SL9's orbit projected into Jupiter's orbit plane
orbsun.gif         SL9's orbit as seen from the Sun
earthjup.gif       Jupiter-Facing Hemispheres of Earth at Impact Times of SL9
impact.gif         Graph of impact times with moon eclipses
index.gif          Mosaic of some of the images in this directory
mitwave1.gif       Frame from MIT Flow Visualization Lab. Simulation
mitwave2.gif       Frame from MIT Flow Visualization Lab. Simulation
mitwave.mpg        MIT Flow Visualization Lab. Simulation Animation
vidjv.mpg          Animation of SL9 entering Jupiter's Atmosphere
visejz.mpg         Animation of explosion produced by SL9 entering Jupiter
sl9rend.gif        Rendering 3 views of comet-Jupiter collision
sl9rend1.gif       Larger rendering comet-Jupiter from Earth
sl9rend2.gif       Larger rendering comet-Jupiter from Voyager 2
sl9rend3.gif       Larger rendering comet-Jupiter from south pole


Q1.4: What will be the effects of the collision?

     As seen from the Earth, the fragments will disappear behind the limb of
Jupiter only 5 to 15 seconds before impact.  The later fragments will be
visible closer to impact.  Fragment W will disappear only 5 seconds before
impact, at an altitude of only about 200 km above the 1-bar pressure level:
it may well start its bolide phase while still in view.  Furthermore, any
sufficiently dense post-impact plume will have to rise only a few hundred
kilometers to be visible from Earth.
     Simulations by Mark Boslough and others indicate that when the fireball
resulting from an impact cools it will form a debris cloud that will rise
hundreds of kilometers above the Jovian cloudtops, and will enter sunlight
within minutes of the impact.  The arrival time of this giant cloud into
sunlight would provide data on its trajectory, which in turn would help us
know how big the comet fragment was.  It is possible that it would be big
(bright) enough to be seen by amateurs [42,43].  Mark Boslough of Sandia
National Laboratories also states that the probability is very high that
these effects will be visible for some of the later impacts (e.g. W and R,
visible from Hawaii, S, visible from India and the far East, Q1 and Q2,
visible from Africa, parts of eastern Europe and the Middle East, L, Brazil
and West Africa, K, South Pacific and Australia, and maybe even V, on the
final night, visible in the Western half of the U.S.).  Observers in these
locations are encouraged to anticipate the possibility of seeing the fireball
within tens of seconds after the impact, and a few minutes later after it has
cooled, condensed, and entered the sunlight.
     Jupiter will be about 770 million kilometers (480,000,000 miles) from
Earth, so it will be difficult to see the effects from Earth.  Also,
the comet fragments will not effect Jupiter as a whole very much.  It will
be like sticking 21 needles into an apple: "Locally, each needle does
significant damage but the whole apple isn't really modified very much." [35].
The energy deposited by the comet fragments fall well short of the energy
required to set off sustained thermonuclear fusion.  Jupiter would have to
be more than 10 times more massive to sustain a fusion reaction.
     Each comet fragment will enter the atmosphere at a speed of 130,000 mph
(60 km/s).  At an altitude of 100 km above the visible cloud decks,
aerodynamic forces will overwhelm the material strength of the comet,
beginning to squeeze it and tear it apart.  Five seconds after entry, the
comet fragment will deposit its kinetic energy of around 10^28 ergs
(equivalent to around 200,000 megatons of TNT) at 100-150 km below the
cloud layer [19].  Bigger fragments will have more energy and go deeper.
     The hot (30,000 K) gas resulting from the stopped comet will explode,
forming a fireball similar to a nuclear explosion, but much larger.
The visible fireball may only rise 100 km or so above the cloudtops.
Above that height the density may drop so that it will become transparent.
The fireball material will continue to rise, reaching a height of perhaps
1000 km before falling back down to 300 km.  The fireball will spread out
over the top of the stratosphere to a radius of 2000-3000 km from the point
of impact (or so the preliminary calculations say).  The top of the resulting
shock wave will accelerate up out of the Jovian atmosphere in less than two
minutes, while the fireball will be as bright as the entire sunlit surface of
Jupiter for around 45 sec [18].  The fireball will be somewhat red, with a
characteristic temperature of 2000 K - 4000 K (redder than the sun, which is
5800 K).  Virtually all of the shocked cometary material will rise behind the
shock wave, leaving the Jovian atmosphere and then splashing back down on top
of the stratosphere at an altitude of 300 km above the clouds [unpublished
simulations by Mac Low & Zahnle].  Not much mass is involved in this splash,
so it will not be directly observable.  The splash will be heavily enriched
with cometary volatiles such as water or ammonia, and so may contribute to
significant high hazes.
        Meanwhile, the downward moving shock wave will heat the local clouds,
causing them to buoyantly rise up into the stratosphere.  This will allow
spectroscopists to attempt to directly study cloud material, a unique
opportunity to confirm theories of the composition of the Jovian clouds.
Furthermore, the downward moving shock may drive seismic waves (similar to
those from terrestrial earthquakes) that might be detected over much of the
planet by infrared telescopes in the first hour or two after each impact.
The strength of these two effects remains a topic of research.  The
disturbance of the atmosphere will drive internal gravity waves ("ripples in
a pond") outwards.  Over the days following the impact, these waves will
travel over much of the planet, yielding information on the structure of
the atmosphere if they can be observed (as yet an open question).
     The "wings" of the comet will interact with the planet before and after
the collision of the major fragments.  The so-called "wings" are defined to
be the distinct boundary along the lines extending in both directions from
the line of the major fragments; some call these 'trails'.  Sekanina, Chodas
and Yeomans have shown that the trails consist of larger debris, not dust:
5-cm rock-sized material and bigger (boulder-sized and building-sized).
Dust gets swept back above (north) of the trail-fragment line due to solar
radiation pressure.  The tails emanating from the major fragments consist of
dust being swept in this manner.  Only the small portion of the eastern
debris trail nearest the main fragments will actually impact Jupiter,
according to the model, with impacts starting only a week before the major
impacts.  The western debris trail, on the other hand, will impact Jupiter
over a period of months following the main impacts, with the latter portion
of the trail actually impacting on the front side of Jupiter as viewed from
Earth.
        The injection of dust from the wings and tail into the Jovian system
may have several consequences.  First, the dust will absorb many of the
energetic particles that currently produce radio emissions in the Jovian
magnetosphere.  The expected decline and recovery of the radio emission may
occur over as long as several years, and yield information on the nature and
origin of the energetic particles.  Second, the dust may actually form a
second faint ring around the planet.


Q1.5: Can I see the effects in my telescope?

     One might be able to detect atmospheric changes on Jupiter using
photography or CCD imaging.  It is important, however, to observe Jupiter
for several months in advance in order to know which features are due to
impacts and which are naturally occurring.  It appears more and more likely
that most effects will be quite subtle.  Without a large ( > 15" ?) telescope
and good detector, little is likely to be seen.
     It is possible that the impacts may create a new, temporary storm at the
latitude of the impacts.  Modeling by Harrington et al. suggests this is
possible [30].  The fragments of comet Shoemaker-Levy 9 will strike just
south of the South South Temperate Belt of Jupiter.  If the nuclei penetrate
deep enough, water vapor may shoot high into the atmosphere where it could
turn into a bluish shroud over a portion of the South South Temperate Zone
[31].
     Impacts of the largest fragments may create one or two features.  A spot
might develop that could be a white or dark blue nodule and would likely have
a maximum diameter of 2,000 km to 2,500 km which in a telescope would be 1
to 1.5 arcseconds across.  This feature would be very short-lived with the
impact site probably returning to normal after just a few rotations of
Jupiter.  A plume might also develop that would look dark against the South
Temperate Zone's white clouds or could appear as a bright jet projected from
Jupiter limb [31].  The table below shows the approximate sizes of features
that already exist on Jupiter for comparison.

 +====================================================================+
 |          FEATURE                            SIZE ESTIMATES         |
 +====================================================================+
 |       Great Red Spot                       29000 by 12000 km [32]  |
 |     White Spots FA,BC,DE                   7500 by 3000 km         |
 |  Shadows of Io, Europa, and Ganymede       4300, 4200, 7100 km     |
 +====================================================================+

     Below is a list of files available at tamsun.tamu.edu in the /pub/comet
directory that may be helpful in identifying features on Jupiter:

tracker2.zip      MSDOS program that displays the location of impact sites
jupe.description  Description of a PC program showing features of Jupiter
jun1994.transit   Transit Times for Red Spot and White Spots for May 1994
jun1994.moons     Jovian Moon events for May 1994 (Shadows, Eclipses, etc.)

    Also, there are little anticyclonic ovals at latitudes of about -41
degrees which are typical of the South South Temperate domain.  There are
usually 6 or 7 around the planet and they move with the South South
Temperate current, i.e. faster than BC and DE.  See Sky & Telescope
for a photo of these ovals by Don Parker [38].


* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


SPECIFICS

Q2.1: What are the impact times and impact locations?

This information was provided P.W. Chodas and D.K. Yeomans:
============================================================================
Predicted Impact Parameters for Fragments of P/Shoemaker-Levy 9
P.W. Chodas and D.K. Yeomans (JPL/Caltech)
Predictions as of 1994 June 3
Date of last astrometric data in these solutions: 1994 June 1

Immediately to the right of the predicted impact times, we give the 1-sigma
uncertainties in those times for all fragments except Q2.  We have made an
effort to make these uncertainties realistic: they are not formal uncertainty
values.  NOTE: To obtain a 95% confidence level, one should use a +/- 2 sigma
window around the predicted impact time.  The uncertainties for fragment Q2
have not been quantified, but are probably comparable to those for P2.

The dynamical model used for the predictions includes perturbations due to
the Sun, planets, Galilean satellites and the oblateness of Jupiter.  The
planetary ephemeris used was DE245.

----------------------------------------------------------------
Fragment   Impact   1-sig   Jovicentric   Meridian  Angle
          Date/Time  Unc.   Lat.   Long.   Angle    E-J-F   Orbit
         July  (UT) (min)   (deg)  (deg)   (deg)    (deg)    Ref.
---------------h--m---------------------------------------------
 A = 21   16  19:55   26   -43.23   176    63.56    99.35    A10
 B = 20   17  03:07   23   -43.45    77    63.67    99.22    B11
 C = 19   17  06:59   24   -43.33   217    64.53    98.64    C8
 D = 18   17  11:18   28   -43.34    14    63.49    99.36    D9
 E = 17   17  15:30   17   -43.70   164    66.13    97.41    E25
 F = 16   18  00:40   23   -43.79   139    64.17    98.77    F16
 G = 15   18  07:52   16   -43.80    37    66.99    96.77    G25
 H = 14   18  19:47   16   -43.86   109    67.32    96.52    H23
 K = 12   19  10:39   16   -43.96   287    68.15    95.90    K24
 L = 11   19  22:40   16   -44.07     2    68.95    95.31    L25
 N = 9    20  10:21   26   -44.59    67    67.13    96.49    N12
 P2= 8b   20  15:27   25   -44.82   253    66.46    96.91    P11
 Q2= 7b   20  19:49        -44.48    48    69.27    95.00
 Q1= 7a   20  20:16   15   -44.20    64    69.69    94.75    Q27
 R = 6    21  05:59   19   -44.27    57    70.24    94.34    R22
 S = 5    21  15:46   17   -44.26    51    70.76    93.97    S32
 T = 4    21  18:16   44   -45.28   145    67.43    96.14    T7
 U = 3    22  00:25   85   -45.19     3    71.74    93.15    U6
 V = 2    22  04:06   31   -44.52   141    68.16    95.77    V8
 W = 1    22  08:34   19   -44.29   299    71.32    93.57    W25


Notes:
1. Fragments J=13 and M=10 are omitted because they have faded from view.
   Fragments P=8 and Q=7 each consist of multiple components.  The March'94
   HST image shows that P1=8a has almost completely faded away (so it too is
   omitted from the Table), and that P2=8b has split.  We do not as yet
   have sufficient data to obtain independent predictions for the two
   components of P2=8b.

2. The impact date/time is the time the impact would be seen at the Earth
   (if the limb of Jupiter were not in the way); the date is the day in
   July 1994; the time is given as hours and minutes of Universal Time.

3. The impact latitude is Jovicentric (latitude measured at the center of
   Jupiter); the Jovigraphic latitudes are about 3.84 deg more negative.

4. The impact longitude is in System II, measured westwards on the planet.

5. The meridian angle is the Jovicentric longitude of impact measured from
   the midnight meridian towards the morning terminator.  This relative
   longitude is known much more accurately than the absolute longitude.

6. Angle E-J-F is the Earth-Jupiter-Fragment angle at impact; values greater
   than 90 deg indicate a farside impact.  All impacts will be just on the
   farside as viewed from Earth; later impacts are closer to the limb.

7. The angle of incidence of the impacts is between 41 and 42 degrees
   for all the fragments.

============================================================================

     See "impacts.*" at SEDS.LPL.Arizona.EDU in the /pub/astro/SL9/info
directory for updates and more details about these predictions.  The
following are the 1-sigma (uncertainty) predictions for the fragment impact
times:
               on March 1         -  30 min
               on May 1           -  24 min
               on June 1          -  16 min
               on July 1          -  10 min
               on July 15         -   6 min
               at impact - 18 hr  -   3 min

The time between impacts is thought to be known with more certainty than the
actual impact times.  This means that if somehow the impact time of the first
fragment can be measured experimentally, then impact times of the fragments
that follow can be predicted with more accuracy.


Q2.2: Can the collision be observed with radio telescopes?

     The cutoff of radio emissions due to the entry of cometary dust into the
Jovian magnetosphere during the weeks around impact may be clear enough to be
detected by small radio telescopes.  Furthermore, impacts may be directly
detectable in radio frequencies.  Some suggest to listen in on 15-30 MHz
during the comet impact.  So it appears that one could use the same antenna
for both the Jupiter/Io phenomenon and the Jupiter/comet impact.  There is
an article in Sky & Telescope magazine which explains how to build a simple
antenna for observing the Jupiter/Io interaction [4,24,25].
     For those interested in radio observations during the SL9 impact,
Leonard Garcia of the University of Florida has made some information
available.  The following files are available via anonymous ftp on the
University of Florida, Department of Astronomy site astro.ufl.edu in the
/pub/jupiter directory:

  README.DOC      Explanation of predicted Jupiter radio storms tables
  jupradio.txt    Jovian Decametric Emission and the SL9/Jupiter Collision
  jupradio.ps     Postscript version of jupradio.txt
  hist.ps         Histogram of occurrence probabilities of Jupiter radio
                        emission at different frequencies.
  may94.txt       Tables of predicted Jupiter radio storms for May 1994
  june94.txt      Tables of predicted Jupiter radio storms for June 1994
  july94.txt      Tables of predicted Jupiter radio storms for July 1994

    The antenna required to observe Jupiter may be as simple as a dipole
antenna constructed with two pieces of wire 11 feet 8.4 inches in length,
connected to a 50 ohm coax cable.  This antenna should be laid out on a
East-West line and raised above the ground by at least seven feet.  A
Directional Discontinuity Ring Radiator (DDRR) antenna is also easy to
construct and can be made from 1/2 inch copper tubing 125.5 inches in
length (21Mhz).   The copper tube should be bent into a loop and placed 5
inches above a metallic screen.   A good preamp is required for
less sensitive shortwave receivers [39].


Q2.3: Will light from the explosions be reflected by any moons?

     One may be able to witness the collisions indirectly by monitoring the
brightness of the Galilean moons that may be behind Jupiter as seen from
Earth.  One could monitor the moons using a photometer, a CCD camera, or a
video camera.  However, current calculations suggest that the brightenings
may be as little as 0.05% of the sunlit brightness of the moon [18].  If a
moon can be caught in eclipse but visible from the earth during an impact,
prospects will improve significantly.  According to current predictions,
the only impact certain to occur during a satellite eclipse is K=12 with
Europa eclipsed.  However, H=14 and W=1 impact only about 2 sigma after Io
emerges from eclipse at longitude 20 deg, and B=20, E=17 and F=16 impact
0.5-2 sigma after Amalthea emerges from eclipse at longitude 34 deg.
     The following files contain information concerning the reflection of
light by Jupiter's moons and are available at SEDS.LPL.Arizona.EDU :

galsat52.zip      MSDOS Program that Displays relative positions of
                      Jupiter's Moons during times of impact
impact_24apr.ps   PostScript Plot of impact times at satellite availability
jsat8.*           Jovian satellite locations

The following information was provided by John Spencer, 11 Apr 1994:
===========================================================================

COMET IMPACT SATELLITE REFLECTIONS

Here's an updated table of satellite reflector availability, based on
the latest (1994/02/23) Chodas and Yeomans ephemerides.  Times haven't
changed much but uncertainties have gone down.  No impact times are yet
available for P1=8a and Q2=7b.

In the table, a "+" next to the orbital longitude means that the
impact will be visible from the satellite.  Longitudes greater than 90
degrees will be less useful as the phase angle of the reflected light
(to first order, equal to the longitude) will be high and its
brightness will be greatly reduced.  An "e" means the satellite will
be visible in eclipse, allowing higher sensitivity observations, and
an "o" means it will be occulted by Jupiter.  I've included Callisto
(J4) for completeness but it's likely to be too far out to be useful-
the same may be true for Ganymede (J3).

---------------------------------------------------------
          UT               Satellite Orbital Longitudes
        date of  Bright    (degrees past superior conj.)
        impact   -ness  ---------------------------------
Nucleus (July)   Index   J5     J1     J2     J3     J4
---------------------------------------------------------
  A=21  16.81      1    193    340    103+    75+    35+
  B=20  17.11      1     50+    41+   133+    90+    42+
  C=19  17.27      1    165     74+   149+    98+    45+
  D=18  17.48      1    317    116+   171    109+    50+
  E=17  17.61      2     51+   143+   184    116+    52+
  F=16  18.02      2    347o   226    225    136+    61+
  G=15  18.30      3    190    283    254    150+    67+
  H=14  18.78      3    177     21+   302    174     78+
  K=12  19.42      3    279    151+     7e   207     91+
  L=11  19.89      3    259    246     55+   230    101+
   N=9  20.41      1    274    352o   107+   256    113+
 P2=8b  20.61      2     59+    33+   128+   266    117+
 P1=8a             1
 Q2=7b             2
 Q1=7a  20.80      3    196     72+   147+   276    121+
   R=6  21.28      2    183    169+   196    300    131+
   S=5  21.61      3     61+   236    229    317    138+
   T=4  21.75      1    162    265    243    324    141+
   U=3  21.88      1    256    291    256    330    144+
   V=2  22.18      2    113+   352o   287    345    151+
   W=1  22.32      2    214     21+   301    352+   154+
Uncertainties (1-sigma):
         0.03      1     22      6      3    1.5      1
---------------------------------------------------------

The "brightness index" subjectively rates comet fragment
brightnesses, 3 being brightest. Brightnesses are eyeballed from
the press-released HST image where possible and are different from
those in previous versions of the table.

There's a good chance that reflections from the impact of the bright
fragment K=12 will be visible off Europa in eclipse, very close to
Jupiter: this impact can be seen in a dark sky from Australia, New
Zealand, and Hawaii.

Given the uncertainties, there's nearly a 50% chance that the impact
of either H=14 (a bright fragment) or W=1 will be seen reflected off Io
in eclipse.  The H=14 impact will be visible in darkness from East and
South Africa, and the middle East, the W=1 impact from New Zealand and
Hawaii.

The table below gives the orbital longitudes (in degrees) of
satellites when in Jupiter eclipse and occultation (used to annotate
the above table).  Values should be good to about one degree.

-------------------------------------
Satellite       Occulted   Eclipsed
-------------------------------------
J5: Amalthea   337 - 023  023 - 034
J1: Io         351 - 009  009 - 020
J2: Europa     355 - 005  005 - 016
J3: Ganymede   358 - 002  009 - 013
J4: Callisto   No occultations or eclipses
------------------------------------

     See "satellites.*" at SEDS.LPL.Arizona.EDU in the /pub/astro/SL9/info
directory for updates.

===========================================================================

     Also, monitoring the eclipses of the Galilean satellites after the
impacts may yield valuable scientific data with the moons serving as
sensitive probes of any cometary dust in Jupiter's atmosphere.  The geometry
of the eclipses is such that the satellites pass through the shadow at
roughly the same latitude as the predicted comet impacts.  There is an
article in the first issue of CCD Astronomy involving these observations.
The article says that if the dust were to obscure sunlight approximately
120 kilometers above Jupiter's cloud tops, Io could be more that 3 percent
(0.03 magnitudes) fainter than normal at mideclipse [40].


Q2.4: What are the orbital parameters of the comet?

     Comet Shoemaker-Levy 9 is actually orbiting Jupiter, which is most
unusual: comets usually just orbit the Sun.  Only two comets have ever been
known to orbit a planet (Jupiter in both cases), and this was inferred in
both cases by extrapolating their motion backwards to a time before they
were discovered.  S-L 9 is the first comet observed while orbiting a planet.
Shoemaker-Levy 9's previous closest approach to Jupiter (when it broke up)
was on July 7, 1992; the distance from the center of Jupiter was about
96,000 km, or about 1.3 Jupiter radii.  The comet is thought to have reached
apojove (farthest from Jupiter) on July 14, 1993 at a distance of about 0.33
Astronomical Units from Jupiter's center.  The orbit is very elliptical,
with an eccentricity of over 0.998.  Computations by Paul Chodas, Zdenek
Sekanina, and Don Yeomans, suggest that the comet has been orbiting Jupiter
for 20 years or more, but these backward extrapolations of motion are highly
uncertain.  See "elements.*" and "ephemeris.*" at SEDS.LPL.Arizona.EDU in
/pub/astro/SL9/info for more information.
     In the abstract "The Orbit of Comet Shoemaker-Levy 9 about Jupiter"
by D.K. Yeomans and P.W. Chodas (1994, BAAS, 26, 1022), the elements
for the brightest fragment Q are listed.  These elements are Jovicentric
and for Epoch 1994Jul15 (J2000 ecliptic):

1994 Periapses      Jul 20.7846
Eccentricity        0.9987338
Periapses dist.     34776.7 km
Arg. of periapses   43.47999
Long. of asc. node  290.87450
inclination         94.23333


Q2.5: Why did the comet break apart?

     The comet broke apart due to tidal forces on its closest approach to
Jupiter (perijove) on July 7, 1992 when it passed within the theoretical
Roche limit of Jupiter.  Shoemaker-Levy 9 is not the first comet observed
to break apart.  Comet West shattered in 1976 near the Sun [3].  Astronomers
believe that in 1886 Comet Brooks 2 was ripped apart by tidal forces near
Jupiter [2].  Several other comets have also been observed to have split
[41].
     Furthermore, images of Callisto and Ganymede show crater chains which
may have resulted from the impact of a shattered comet similar to Shoemaker-
Levy 9 [3,17].  The satellite with the best example of aligned craters is
Callisto with 13 crater chains.  There are three crater chains on Ganymede.
These were first thought to be from basin ejecta; in other words secondary
craters.  See Q1.3 and [27] for images of crater chains.
     There are also a few examples of crater chains on our Moon.  Jay Melosh
and Ewen Whitaker have identified 2 possible crater chains on the moon which
would be generated by near-Earth tidal breakup.  One is called the "Davy
chain" and it is very tiny but shows up as a small chain of craters aligned
back toward Ptolemaeus.  In near opposition images, it appears as a high
albedo line; in high phase angle images, you can see the craters themselves.
The second is between Almanon and Tacitus and is larger (comparable to the
Ganymede and Callisto chains in size and length).


Q2.6: What are the sizes of the fragments?

     Using measurements of the length of the train of fragments and a model
for the tidal disruption, J.V. Scotti and H.J. Melosh have estimated that the
parent nucleus of the comet (before breakup) was only about 2 km across [13].
This would imply that the individual fragments are no larger than about 500
meters across.  Images of the comet taken with the Hubble Space Telescope in
July 1993 indicate that the fragments are 3-4 km in diameter (3-4 km is an
upper limit based on their brightness).  A more elaborate tidal disruption
model by Sekanina, Chodas and Yeomans [20] predicts that the original comet
nucleus was at least 10 km in diameter.  This means the largest fragments
could be 3-4 km across, a size consistent with estimates derived from the
Hubble Space Telescope's July 1993 observations.
     The new images, taken with the Hubble telescope's new Wide Field and
Planetary Camera-II instrument on January 24-27, 1994, have given us an even
clearer view of this fascinating object, which should allow a refinement of
the size estimates.  In addition, the new images show strong evidence for
continuing fragmentation of some of the remaining nuclei, which will be
monitored by the Hubble telescope over the next month.


Q2.7: How long is the fragment train?

     The angular length of the train was about 51 arcseconds in March 1993
[2].  The length of the train then was about one half the Earth-Moon
distance.  In the day just prior to impact, the fragment train will stretch
across 20 arcminutes of the sky, more that half the Moon's angular diameter.
This translates to a physical length of about 5 million kilometers.  The
train expands in length due to differential orbital motion between the first
and last fragments.  Below is a table with data on train length based on
Sekanina, Chodas, and Yeomans's tidal disruption model:

           +=============================================+
           |    Date    Angular Length  Physical Length  |
           |               (arcsec)          (km)        |
           +=============================================+
           |  93 Mar 25       49            158,000      |
           |     Jul  1       67            265,000      |
           |  94 Jan  1      131            584,000      |
           |     Feb  1      161            669,000      |
           |     Mar  1      200            762,000      |
           |     Apr  1      255            893,000      |
           |     May  1      319          1,070,000      |
           |     Jun  1      400          1,366,000      |
           |     Jul  1      563          2,059,000      |
           |     Jul 15      944          3,593,000      |
           |    Impact A    1286          4,907,000      |
           +=============================================+


Q2.8: Will Hubble, Galileo, etc. be able to observe the collisions?

     The Hubble Space Telescope, like earthlings, will not be able to see the
collisions but will be able to monitor atmospheric changes on Jupiter.  The
new impact points are more favorable for viewing from spacecraft: it can now
be stated with certainty that the impacts will all be visible to Galileo,
and now at least some impacts will be visible to Ulysses.  Although Ulysses
does not have a camera, it will monitor the impacts at radio wavelengths.
        Galileo will get a direct view of the impacts rather than the grazing
limb view previously expected.  The Ida image data playback is scheduled to
end at the end of June, so there should be no tape recorder conflicts with
observing the comet fragments colliding with Jupiter.  The problem is how to
get the most data played back when Galileo will only be transmitting at 10
bps.  One solution is to have both Ulysses and Galileo record the event and
and store the data on their respective tape recorders.  Ulysses observations
of radio emissions data will be played back first and will at least give
the time of each comet fragment impact.  Using this information, data can
be selectively played back from Galileo's tape recorder.  From Galileo's
perspective, Jupiter will be 60 pixels wide and the impacts will only show
up at about 1 pixel, but valuable science data can still collected in the
visible and IR spectrum along with radio wave emissions from the impacts.
     The impact points are also viewable by both Voyager spacecraft,
especially Voyager 2.  Jupiter will appear as 2.5 pixels from Voyager 2's
viewpoint and 2.0 pixels for Voyager 1.  However, it is doubtful that the
Voyagers will image the impacts because the onboard software that controls
the cameras has been deleted, and there is insufficient time to restore and
test the camera software.  The only Voyager instruments likely to observe
the impacts are the ultraviolet spectrometer and planetary radio astronomy
instrument.  Voyager 1 will be 52 AU from Jupiter and will have a near-limb
observation viewpoint.  Voyager 2 will be in a better position to view the
collision from a perspective of looking directly down on the impacts, and
it is also closer at 41 AU.


Q2.9: To whom can I report my observations?

     Observation forms by Steve Lucas are available via ftp at
oak.oakland.edu in the /pub/msdos/astrnomy directory.  These forms also
contain addresses of "Jupiter Watch Program" section leaders.  jupcom02.zip
contains Microsoft Write files.  The Association of Lunar and Planetary
Observers (ALPO) will also distribute a handbook to interested observers.
The handbook "The Great Crash of 1994" is available for $10 by

                        ALPO Jupiter Recorder
                        Phillip W. Budine
                        R.D. 3, Box 145C
                        Walton, NY 13856 U.S.A.

The cost includes printing, postage and handling.
     John Rogers, the Jupiter Section Director for the British Astronomical
Association, will be collecting data from regular amateur Jupiter observers
in Britain and worldwide.  He can be reached via email (jr@mole.bio.cam.ac.U)
or fax (UK [223] 333840).  For other addresses see page 44 of the January
1994 issue of Sky & Telescope magazine [14].


Q2.10: Where can I find more information?

   The SL9 educator's book put out by JPL is in the /pub/astro/SL9/EDUCATOR
directory of SEDS.LPL.Arizona.edu.  There are two technical papers [18,19]
on the atmospheric consequences of the explosions available at
oddjob.uchicago.edu in the /pub/jupiter directory.  There are some
PostScript images and text files involving the results of fireball simulations
by Sandia National Laboratories at tamsun.tamu.edu (128.194.15.32) in the
/pub/comet/sandia directory.
    SEDS (Students for the Exploration and Development of Space) has set up
an anonymous account which allows you to use "lynx" - a VT100 WWW browser.
To access this service, telnet to SEDS.LPL.Arizona.EDU and login as "www"
(no password required).  This will place you at the SEDS home page, from
which you can select Shoemaker-Levy 9.  A similar "gopher" interface is
available at the same site.  Just login as "gopher".
     Below is a list of FTP and WWW sites with SL9 information:

===============================================================================
   FTP SITE NAME       IP ADDRESS       DIRECTORY                   CONTENTS
===============================================================================
SEDS.LPL.Arizona.EDU (128.196.64.66)  /pub/astro/SL9             Images & Info
jwd.ping.de                           /pub/people/hh/astro/comet   SEDS Mirror
oddjob.uchicago.edu                   /pub/jupiter                   Articles
jplinfo.jpl.nasa.gov (137.78.104.2)   /news and /images              Images
ftp.cicb.fr          (129.20.128.34)  /pub/Images/ASTRO/hst          Images
tamsun.tamu.edu      (128.194.15.32)  /pub/comet                 Images & Info

===============================================================================
     WORLD WIDE WEB SITES                                           CONTENTS
===============================================================================
http://info.cv.nrao.edu/staff/pmurphy/jove-comet-wham-2.html      Info & Images
http://seds.lpl.arizona.edu/sl9/sl9.html                          Images & Info
http://pscinfo.psc.edu/research/user_research/user_research.html  Animations

     If you have only mail access then try emailing the following message
(no subject) to bitftp@pucc.Princeton.edu:

ftp SEDS.LPL.Arizona.EDU
user anonymous guest
cd pub
cd astro
cd SL9
dir
cd info
get factsheet.txt

The file "factsheet.txt" should then be mailed to your account.  Other files
can be retrieved in a similar manner.  For more info email the following
message to bitftp@pucc.Princeton.edu:

help
howtoftp

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

REFERENCES

[1]  "Update on the Great Comet Crash", Astronomy, December 1993, page 18.
[2]  Levy, David H., "Pearls on a String", Sky & Telescope, July 1993,
      page 38-39.
[3]  Melosh, H. H. and P. Schenk, "Split comets and the origin of crater
      chains on Ganymede and Callisto"  Nature 365, 731-733 (1993).
[4]  "Jupiter on Your Shortwave", Sky & Telescope, December 1989, page 628.
[5]  "Comet on a String", Sky & Telescope, June 1993, page 8-9.
[6]  "Comet Shoemaker-Levy (1993e)", Astronomy, July 1993, page 18.
[7]  "A Chain of Nuclei", Astronomy, August 1993, page 18.
[8]  "When Worlds Collide : Comet will Hit Jupiter", Astronomy,
      September 1993, page 18.
[9]  Burnham, Robert "Jove's Hammer", Astronomy, October 1993, page 38-39.
[10] IAU Circulars : 5800, 5801, 5807, 5892, and 5893
[11] Observers Handbook 1994 of the R.A.S.C., Brian Marsden.
[12] Sekanina, Zdenek, "Disintegration Phenomena Expected During Collision
      of Comet Shoemaker-Levy 9 with Jupiter"  Science 262, 382-387 (1993).
[13] Scotti, J. V. and H. J. Melosh, "Estimate of the size of comet
      Shoemaker-Levy 9 from a tidal breakup model"  Nature 365, 733-735 (1993).
[14] Beatty, Kelly and Levy, David H., "Awaiting the Crash" Sky & Telescope,
      January 1994, page 40-44.
[15] Jewitt et al., Bull. Am. Astron. Soc. 25, 1042, (1993).
[16] "AstroNews", Astronomy, January 1994, page 19.
[17] "AstroNews", Astronomy, February 1994, page 16.
[18] Zahnle, Kevin and Mac Low, Mordecai-Mark, "The Collisions of Jupiter and
      Comet Shoemaker Levy 9", submitted to Icarus October 29, 1993.
[19] Mac Low, Mordecai-Mark and Zahnle, Kevin "Explosion of Comet
      Shoemaker-Levy 9 on Entry into the Jovian Atmosphere",
      submitted to Science on 10 February 1994.
[20] Sekanina, Z., Chodas, P.W., and Yeomans, D.K, "Tidal Disruption and the
     Appearance of Periodic Comet Shoemaker-Levy 9", Astronomy &
     Astrophysics, in press.
[21] "On a collision Course with Jupiter", Mercury, Nov-Dec 1993, page 15-16.
[22] "Timing the Crash", Sky & Telescope, February 1994, page 11.
[23] "Capturing Jupiter on Video" Sky & Telescope, September 1993, page 102.
[24] North, Gerald, "Advanced Amateur Astronomy", page 296-298, (1991).
[25] "Backyard Radio Astronomy", Astronomy, March 1983, page 75-77.
[26] Harrington, J., R. P. LeBeau, K. A. Backes, and T. E. Dowling,
     "Dynamic response of Jupiter's atmosphere to the impact of comet
     Shoemaker-Levy 9" Nature 368: 525-527 (1994).
[27] David Morrison, "Satellites of Jupiter", page 392, (1982).
[28] Weaver, H. A., P. D. Feldman, M. F. A'Hearn, C. Arpigny, R. A. Brown,
     E. F. Helin, D. H. Levy, B. G. Marsden, K. J. Meech, S. M. Larson,
     K. S. Knoll, J. V. Scotti, Z. Sekanina, C. S. Shoemaker, E. M. Shoemaker,
     T. E. Smith, A. D. Storrs, D. K. Yeomans, and B. Zellner,
     "Hubble Space Telescope Observations of Comet P/Shoemaker-Levy 9 (1993e)."
     Science 263, page 787-791, (1994).
[29] Duffy, T.S., W.L. Vos, C.S. Zha, H.K. Mao, and R.J. Hemley.  "Sound
     Velocities in Dense Hydrogen and the Interior of Jupiter"  Science 263,
     page 1590-1593, (1994).
[30] Harrington, J., R. P. LeBeau, K. A. Backes, & T. E. Dowling, "Dynamic
     response of Jupiter's atmosphere to the impact of comet P/Shoemaker-
     Levy 9", Nature 368, page 525-527, April 7, 1994.
[31] Olivares, Jose, "Jupiter's Magnificent Show", Astronomy, April 1994,
     page 74-79.
[32] Schmude, Richard W., "Observations of Jupiter During the 1989-90
     Apparition", The Strolling Astronomer: J.A.L.P.O., Vol. 35, No. 3.,
     September 1991.
[33] "Comet heads for collision with Jupiter",Aerospace America,April 1994,
      page 24-29.
[34] "Comet Shoemaker-Levy 9 and Galilean Eclipses", CCD Astronomy, Spring
     1994, page 18-19.
[35] Reston, James Jr., "Collision Course", TIME, May 23, 1994, page 54-61.
[36] "Boom or Bust", Physics Today, June 1994, page 19-21.
[37] Beatty, Kelly and Levy, David H., "Awaiting the Crash - Part II", Sky
     & Telescope, July 1994, page 18-23.
[38] Alan M. MacRobert, "Observing Jupiter at Impact Time", Sky & Telescope,
     July 1994, page 31-35.
[39] Van Horn, Larry, "Countdown to the Crash", Monitoring Times, June 1994,
     page 10-13.
[40] Mallama, Anthony, "Comet Shoemaker-Levy 9 and Galilean Satellite
     Eclipses", CCD Astronomy, Spring 1994, pages 18-19.
[41] B. M. Middlehurst and G. P. Kuiper, "The Moon, Meteorites
     and Comets",  Univ. Chicago Press, 1963.
[42] Boslough, Mark B., et al, "Determination of Mass and Penetration Depth
     of Shoemaker-Levy 9 fragments from time-resolved impact flashed signatures"
     submitted to Geophysical Research Letters, June 1994.
[43] Crawford, David A., et al, "The impact of Comet Shoemaker-Levy 9 on
     Jupiter", submitted to Shock Waves, April 1994.
[44] Bruning, David, "The Comet Crash", Astronomy, June 1994, pages 41-45.

ACKNOWLEDGMENTS

     Thanks to Ross Smith for starting a FAQ and to all those who have
contributed :  Robb Linenschmidt, Mordecai-Mark Mac Low, Phil Stooke,
Rik Hill, Elizabeth Roettger, Ben Zellner, Kevin Zahnle, Ron Baalke,
David H. Levy, Eugene and Carolyn Shoemaker, Jim Scotti, Richard A.
Schumacher, Louis A. D'Amario, John McDonald, Michael Moroney, Byron
Han, Wayne Hayes, David Tholen, Patrick P. Murphy, Greg F Walz Chojnacki,
Jeffrey A. Foust, Paul Martz, Kathy Rages, Paul Chodas, Zdenek Sekanina,
Don Yeomans, Richard Schmude, Lenny Abbey, Chris Lewicki, the Students
for the Exploration and Development of Space (SEDS), David A. Seal,
Leonard Garcia, Raymond Doyle Benge, Mark Boslough, Dave Mehringer,
John Spencer, Erik Max Francis, and John Rogers.

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *



SL9 LIVE

    The following messages discuss how one could get live updates:

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

Hello, I'm Jim Moskowitz of the Franklin Institute Science Museum.  I was
(tangentially) involved in the 'Neptune All Night' live broadcast we did
over PBS during the '89 Voyager flyby, and I've just learned that we're
going to be doing a PBS broadcast for SL9 as well.
On Wednesday, July 20th, from 10:30 to 11:30 PM EST we'll be providing
PBS stations with a show talking about the impacts and their effects,
featuring images from Hubble, discussion with Levy and the Shoemakers,
a piece by Arthur C. Clarke, and probably much more.
My understanding is that it will be up to the local PBS stations to decide
whether to show this program (as opposed to, maybe, an 'Eastenders' rerun).
You may want to call your area station and express interest in having the
program shown in your area...

-Jim  (jimmosk@fi.edu)

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

The Fernbank Science Center in Atlanta, Georgia, USA is planning to have
a video link from their 36-inch telescope down to their planetarium,
so people can view the results of the impact of comet Shoemaker-Levy 9
with Jupiter. Currently, they are working on a press release and hope
to have their plans finalized in June.

For more information call or write:

     Fernbank Science Center
     156 Heaton Park Drive, NE
     Atlanta, Georgia 30307
     USA 404-378-4311

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

     There may be continuous discussion/updates on the IRC (Internet Relay
chat) channel #Astronomy during the impacts.

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\


Article: 4 of jpl.shoemaker-levy
Subject: Galileo's Observing Plans

           Preview of Galileo's Tentative SL-9 Observing Plans

                            Clark R. Chapman
                      (cchapman@nasamail.nasa.gov)
                              6 April 1994

      Nearly all observations of the SL-9 impacts will be made from the
Earth, or from Earth-orbiting observatories.  Several spacecraft will also
be observing Jupiter in mid-July.  However, the Galileo spacecraft, headed
for Jupiter orbit in late 1995, has the best seat in the house.  It will be
the only observatory capable of actually "seeing" (i.e. resolving) the
impacts directly: the impact sites will be on the side of Jupiter facing the
spacecraft, and Galileo's CCD camera can resolve phenomena on Jupiter as
well as can be done from most ground-based observatories (Jupiter will be
about 60 pixels across).  Other Galileo instruments will be observing, as
well.

      The sequences are currently being designed and there is little room
for further modification.  Moreover, the work is being done with a
shoestring budget on a "best effort" basis.  The information below is
tentative and may be inaccurate in some respects.  It is being provided, at
the request of the managers of the bulletin board, in the interest of
developing needed coordination.

      The premier data will be taken with Galileo's camera.  About 6 of the
19 impacts (tentatively D, E, K, N, V, and W) will be imaged in one of two
basic ways.  For half of the opportunities, the camera will snap pictures in
a time-lapse mode using a new on-chip mosaicking capability.  Images will
be recorded every 2 1/3 second (tentatively for fragment N) through the
period that includes the bolide flash and any subsequent fireball and
lasting until the impact site rotates past Jupiter's morning terminator
(typically 6 to 10 minutes later).  Pictures will be taken every 8 2/3
seconds for two other events (probably V and D), which would permit
successive images to be taken through a repeated cycle of 4 color filters
and will reduce the 33% dead-time that characterizes the 2 1/3 second
mode; this 8 2/3 second mode is more conservative of resources but would
permit a brief bolide to slip through undetected, so it is best for the
subsequent fireball phase.  The images will be recorded on Galileo's tape
recorder as arrays of 7 by 7 or 8 by 8 images per frame.

      A second scanning mode will be most useful for recording the time
history of the brief bolide flashes as the comet fragments plunge for a
couple seconds through Jupiter's upper atmosphere.  Galileo's scan
platform will be moved so that the image of Jupiter drifts across the CCD
detector with the shutter open (through a narrow filter).  The scans will
sacrifice one spatial dimension -- so the pictures won't be "pretty" -- but
the mode should permit the rise and fall of a bolide flash to be measured
every two-tenths of a second, or so.  Diagonal scans across half of the
CCD are tentatively planned for events E and W and should be sensitive to
very faint phenomena (e.g. bolides from fragments as small as 100 m,
meteor storms, aurorae, etc.) as well as to bright phenomena.  A more
efficient horizontal scanning approach is tentatively planned for fragment
K, but Jupiter will be superimposed on the flashes, so faint phenomena will
not be detected.

      For five of the 19 impacts (tentatively C, F, G, P, and R), the camera
will not be used.  Instead, the other scan platform instruments --
including the Near Infrared Mapping Spectrometer, the Photopolarimeter
Radiometer, and the Ultraviolet Spectrometer -- will try to record the
bolide and fireball phases of the impacts.  Together, they cover a much
wider range of wavelengths than the camera, and often with better time
resolution.  However, these instruments lack the camera's spatial resolution
and will record the impacts simply as enhancements to Jupiter's total
radiation at those wavelengths.

      The remaining impacts (A, B[?], H, L, Q, S, T, U) will not be
observed by either Galileo's camera or most of Galileo's scan platform
instruments.  For those events, the spacecraft will not be in sight of Deep
Space Network antennas on Earth; scan platform operations are not
permitted when engineers on Earth are not in contact with the spacecraft. 
However, some monitoring of Jupiter's long radio-wavelength radiation by
the Plasma Wave Subsystem can be done during those events without
operating the scan platform; a low-rate PPR mode may alternate with PWS. 

      Most of the SL-9 impact data will be recorded on Galileo's tape
recorder for later play-back to Earth.  It is expected that most of the
desired observations can be fit onto the tape recorder.  The trick will be
to identify the important data, because only a few percent of it can
eventually be transmitted back to Earth over Galileo's small, low-gain
antenna given the limited allocations by the Deep Space Network.  (Galileo's 
large, high-gain antenna failed to open several years ago.)  In order to
find the valuable data, Galileo engineers will rely on exact timings of
phenomena from Earth-based telescopic observers.  They also hope to use
the measurements of the impact of an early fragment by the PPR to
calibrate the timings; if the absolute timing of that impact can be
measured, then the relative times of the remaining impacts will be
predictable to within a few minutes (or so we think).

      It remains to be determined just how quickly Galileo will be able to
respond to late changes in the predicted (or observed) impact times.  The
basic commands will be radioed out to the spacecraft a week or so in
advance of the mid-July commencement of impacts.  They will tell the
spacecraft to observe (e.g. snap the camera shutter repeatedly) for an
hour on either side of the predicted impact times.  Later information from
telescopic astrometry is expected to narrow the predicted impact times to
plus-or-minus 10 minutes during the final days, and it is hoped that late
commands can be sent up to Galileo to record data on the tape recorder
for plus-or-minus 20 minutes centered on such late predictions (it would
overtax the capacity of the tape recorder to record data for an hour or
two at each impact).  Galileo may have the opportunity to change the
record times for the last several impacts based on observed times for the
early impacts.  Finally, it is hoped that the actual times of the events will
become known to a couple of minutes, so that the appropriate parts of the
recorded data can eventually be returned to Earth.

      The Galileo experiment will be GREATLY ENHANCED by the most rapid
communication of (a) updated, more accurate pre-impact predictions of
impact times from astrometry, (b) preliminary indications of when major
impacts may have occurred, and (c) refined estimates of impact times from
synthesis of all groundbased and spacebased data.



-- 
#include	<standard.disclaimer>
 _
Kevin D Quitt  USA 91351-4454		96.37% of all statistics are made up

