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Re: Jupiter capture

>Actually, the comet could have been--and I think was--in orbit
>around Jupiter, in this case a very elongated orbit, such that
>perihelion (I know, but what's the combining form for Jupiter?
>anyway, closest approach) was less than 2.44 radii, at which
>point tidal forces would pull it apart (thus the string of pearls
>effect) and the orbit was further perturbed. Skylab may be a
>useful analogy here; it should also be remembered that there's
>a _lot_ of stuff around Jupiter, so this is not a simple problem.
>Are there any physicists or astronomers on this list?
>Vicki Rosenzweig
>New York, NY

Now that I'm back from vacation, I see a posting upon which I can probably
shed some light. [Wow, an astronomy discussion (my other love) on a dino
list.... What will happen next?]

Rather than answer the above directly, I have attached appropriate portions
of the Comet Shoemaker-Levy 9 FAQ. The complete FAQ can be had by anonymous
FTP to seds.lpl.arizona.edu or tamsun.tamu.edu. This and much more on the
impact event is available. For those who are interested, I would be pleased
to forward a copy of the latest site list (showing all known Internet sites
for SL9 comet info and GIFs). Write me....

You'll notice the term is "perijove."  :)

Numbers within brackets [nn] refer to a bibliography not included here.


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

     Comet Shoemaker-Levy 9 is actually in a temporary orbit of 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           Arg. of periapses   43.47999
   Eccentricity      0.9987338             Long. of asc. node  290.87450
   Periapses dist.   34776.7 km            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 
     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 [27].  See SEDS.LPL.Arizona.edu in /pub/astro/SL9/images for images 
of crater chains (gipul.gif and chain.gif).
     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).  There is an Apollo 11
image of a crater chain on the far side of the moon at SEDS.LPL.Arizona.edu
in /pub/astro/SL9/images (moonchain.gif).

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; the fragments have visual magnitudes of
around 23).  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 
     The new images, taken with the Hubble telescope's new Wide Field and
Planetary Camera-II instrument in 1994, have given us an even clearer view 
of this fascinating object, which should allow a refinement of the size 
estimates.  Some astronomers now suggest that the fragments are about 1 km or 
smaller.  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 two weeks.   One can get an idea of the 
relative sizes of the fragments by considering the relative brightnesses:
--------------------       --------------------       --------------------
          Brightness                 Brightness                 Brightness
 Nucleus    Index           Nucleus    Index           Nucleus    Index
--------------------       --------------------       --------------------
  A=21        1              J=13        0             Q1=7a        3
  B=20        1              K=12        3               R=6        2
  C=19        1              L=11        3               S=5        3
  D=18        1              M=10        0               T=4        1
  E=17        2               N=9        1               U=3        1
  F=16        2             P2=8b        2               V=2        2
  G=15        3             P1=8a        0               W=1        2
  H=14        3             Q2=7b        2
--------------------       --------------------       -------------------

   The "brightness index" subjectively rates comet fragment brightnesses, 3 
being brightest. Brightnesses are eyeballed from the press-released HST image 
where possible.


*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*=- -=*
 Douglas E. Goudie                  To know all things is not permitted.
 ac941@leo.nmc.edu                               -- Horace (65 - 8 B.C.)