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From: starrfie@hydro.la.asu.edu (Sumner Starrfield)
Subject: Comet/Jupiter Collision Frequently Asked Questions
Date: Thu, 2 Dec 93 21:27:12 MST
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* Comet/Jupiter Collision FAQ *
* *
* The following is a list of frequently asked questions concerning the *
* collision of comet Shoemaker-Levy 9 with Jupiter. Thanks to all those *
* who have responded to questions. Contact Danny Bruton (astro@tamu.edu) *
* John Harper (jharper@tamu.edu) with comments, additions, corrections, *
* etc. This FAQ and future updates can be obtained via anonymous ftp to *
* tamsun.tamu.edu in the /pub/comet directory. *
* *
* Last updated 02-Dec-1993 *
* *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
GENERAL QUESTIONS
Q1.1: Is it true that a comet will collide with Jupiter in 1994?
Q1.2: Who are Shoemaker and Levy?
Q1.3: Can I see the comet with my telescope?
Q1.4: Where can I find a GIF image of this comet?
Q1.5: What are the coordinates and orbital elements of the comet?
SPECIFICS
Q2.1: Why did the comet break apart?
Q2.2: What does the orbit of the comet look like?
Q2.3: What are the sizes of the fragments and how long is the fragment train?
Q2.4: Where will the comet fragments collide on Jupiter?
Q2.5: What will be the effect of the collision?
Q2.6: How can I observe the collisions?
Q2.7: Will Hubble, Galileo or Voyager be able to image the collisions?
Q2.8: Where can I find out more information?
GENERAL QUESTIONS
Q1.1: Is it true that a comet will collide with Jupiter in 1994?
Yes, the shattered comet Shoemaker-Levy 9 (1993e) is expected to
collide with Jupiter over a 5.6 day period. The first of 21 comet
fragments is expected to hit Jupiter at around 10 UT on July 18, 1994
and the last around 0 UT on July 24, 1994 [1].
As to the precision of time estimates, the central impact is now
predicted for 1994 July 21.2 plus or minus 0.9 days. With continuous
astrometry starting in January 1994, the following are for 3-sigma
(uncertainty) predictions for the fragment impact times:
at minus 2 months - 55 minutes From Don Yeomans
at minus 1 month - 40 minutes Division for
at minus 1 week - 21 minutes Planetary Sciences Meeting
at minus 6 hours - 9 minutes (Posted by Ben Zellner)
The dust "wings" of the comet will start interacting with the planet more
than a month ahead of the large fragments, and will continue to interact for
more than a month afterward. The long-term monitoring of Jupiter is therefore
even more important. Speculations are that the dust _could_ form a crown
shape or even a ring over the next 10 to 30 years.
Q1.2: Who are Shoemaker and Levy?
Eugene and Carolyn Shoemaker and David H. Levy found the "squashed"
comet on March 25, 1993 on photographic plates taken on March 22, 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. James V. Scotti confirmed their discovery with
the Spacewatch Telescope at Kitt Peak in Arizona [2]. See [11] for more
information about the discovery.
Q1.3: Can I see the comet with my telescope?
The comet had a magnitude of 13.8 at discovery. The glow of Jupiter makes
imaging the comet difficult. If you have a CCD camera, you should be able to
see the train of fragments with a telescope of about 20 to 25 centimeters;
a slightly larger telescope will probably be needed if you use conventional
photography. Jupiter will be far enough away from the sun (angular distance)
to begin imaging in late-December 1993 or early-January 1994.
Q1.4: Where can I find a GIF image of this comet?
Some GIF images can be obtained via anonymous ftp from pdssbn.astro.umd.edu
in the /ftp/pub/images directory. The GIF images here are named SL9*.GIF.
Also there are a some images at ftp.cicb.fr in the /pub/Images/ASTRO/hst
directory with explanations in /pub/Images/ASTRO/hst/docs. The GIF
images here are named 1993e*.GIF. (Also see references for photos.)
Q1.5: What are the coordinates and orbital elements of the comet?
The right ascension and declination of the comet along with some orbital
elements can be obtained via anonymous ftp at pdssbn.astro.umd.edu in the
/ephemeris directory. The filenames are elements.28, iauc5893 and iauc5892.
SPECIFICS
Q2.1: Why did the comet break apart?
The comet is thought to have broken apart due to tidal forces on its
closest approach to Jupiter (perijove) in July 8, 1992. Shoemaker-Levy 9
is not the first comet 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].
Furthermore, images of Callisto and Ganymede show crater chains
(lined up caters) which may have resulted from the impact of a comet similar
to Shoemaker-Levy 9 [3]. 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. There are also a few examples on our Moon. Davy Catena
for example, which may have been due to comets split by Earth.
Q2.2: What does the orbit of the comet look like?
Shoemaker-Levy 9 is thought to have made its closest approach to Jupiter
on July 8, 1992 at a distance of about 1.53 Jupiter radii from Jupiter's
center [8]. 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 looks somewhat elliptical. See postscript
files on pdssbn.astro.umd.edu in the /ftp/pub/images directory for a visual
representation.
Q2.3: What are the sizes of the fragments and how long is the fragment train?
Images taken with the Hubble Space Telescope suggest 3-5 km diameter
fragments. Models of Shoemaker-Levy's breakup in 1992 suggest that the
original intact comet may have been only 2 km across. If this is the case,
the largest fragments could be no more that 500 meters across [1].
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 12
arcminutes of the sky, slightly less that half the Moon's angular diameter.
The translates to a physical length of about 30 million kilometers [1].
Q2.4: Where will the comet fragments collide on Jupiter?
All components of the comet will hit on the dark farside of Jupiter,
out of sight from Earth. More precisely, the impact of the center of the
comet train is predicted to occur at -37 degrees Jupiter latitude at a
Sun-comet-Jupiter angle of 51 degrees on the morning side of the planet.
(Source: David Seal from JPL). About 1.5 hours after each hit, the impact
points will rotate into view as seen from Earth [1].
Q2.5: What will be the effect of the collision?
Kevin Zahnle from JPL predicts that each comet fragment will explode
into a fireball about 200 km below the cloudtops of Jupiter. The
fireball will initially be hidden underneath the cloudtops but will
expand rapidly with the plume material rising to about 1.02 to 1.25
Jupiter radii above the cloudtops. The temperature of the fireball may
reach up to 30,000 degrees C with a luminosity of 4x to 25x the brightness
of Jupiter.
The energy of collision is said to be between 200,000 and 100,000,000
megatons of TNT. The lower number is for 1 km diameter fragments; current
nuclei diameter estimates range from 1/2 km to as high as 3 or 4 km. The
fragments will be traveling with a speed of 60 km/sec relative to Jupiter [9].
According to New Mexico State University's Reta Beebe, "If the comet nuclei
excavate down to the 700 millibar level, the explosions should cause ammonia
ice to well up to the top layers of Jupiter's atmosphere. The ice crystals
will form high cirrus clouds that will create an abnormally bright equatorial
cloud zone." [8] She also said at the ALPO conference in August 1993, that
clouds may form that obscure the atmospheric activity such as the Red Spot
much like in the case of Uranus' atmosphere whose Voyager flyby images
revealed a near featureless disk.
Q2.6: How can I observe the collisions?
One can monitor the atmospheric changes on Jupiter using the naked eye,
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 comet
impacts and which are naturally occurring. A video camera may be able to
record real time atmospheric changes if your telescope is big enough. The
Red Spot and other features have been recorded using a video camera and a
14" telescope with eyepiece projection (2" eyepiece works best). With WWV
ticking away in the background one can obtain an excellent record of this
unusual event.
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. Some suggest that moons may brighten by as much as 0.2 to
2 magnitudes (ALPO conference, August 1993). The MSDOS program GALSAT will
calculate and display the locations of the Galilean satellites for a given
day and time and can be obtained via ftp from oak.oakland.edu in the
/pub/msdos/astronomy directory. One could monitor the moons using a
photometer, a CCD, or a video camera pointed directly into the eyepiece
of a telescope. If you do video you can get photometric information by
frame grabbing and treating these like CCD frames (applying darks,
bias', and flats).
Radio emissions due to the impacts may be strong enough to be detected by
small radio telescopes. Some suggest to listen in on 15-30 MHz during the
comet impact, but to avoid 27 MHz because this frequency is used for CB
communications (ALPO conference, August 1993). 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 which explains how to built a
simple antenna for observing the Jupiter/Io interaction [4].
Q2.7: Will Hubble, Galileo or Voyager be able to image the collision?
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 telescope is scheduled for repairs in December 1993.
The comet collisions are visible from both Voyager 1 and Voyager 2.
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. Jupiter will appear as 2.5 pixels from Voyager 2's
viewpoint and 2.0 pixels for Voyager 1. If there is any imaging to be
done by Voyager, it will only be by Voyager 2.
The impacts will occur on the limb of Jupiter as seen from the Galileo
Space Probe. The Ida 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 Voyager 2 and Galileo record the event and and store the data on their
respective tape recorders. The Voyager 2 data will be played back first at
something like 3500 bps. The images will be small, but at least the time of
each comet fragment impact can be determined. 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 would 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 imaging of the comet impacts by Voyager 2 and Galileo has not
been officially approved yet, but it is very likely to happen.
Q2.8: Where can I find out more information?
There is a technical paper on the consequences of the explosion available
via anonymous ftp from oddjob.uchicago.edu in the pub/jupiter directory.
The paper and figures are available in Postscript format; a couple of the
computational figures are also available in TIFF format. The paper is
written by Kevin Zahnle (NASA Ames) and Mordecai-Mark Mac Low (Astronomy &
Astrophysics Center, University of Chicago). Also there is lots of
information available via ftp from pdssbn.astro.umd.edu and in the
articles mentioned below.
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] Robert Burnham, "Jove's Hammer", Astronomy, October 1993, page 38-39.
[10] IAU Circulars : 5800, 5801, 5807, 5892, 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] Melosh, H. H. and P. Schenk, "Split comets and the origin of crater
chains on Ganymede and Callisto." Nature 365, 731-733 (1993).
ACKNOWLEDGMENTS
Thanks to Ross Smith for starting a FAQ and to all those who have posted
answers to questions: Mordecai-Mark Mac Low, Phil Stooke, Rik Hill,
Elizabeth Roettger, Ben Zellner, Kevin Zahnle, Ron Baalke.
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|| Danny Bruton || . . .
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