Again, my apologies for not being able to keep this page more current. :-(
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-that-obvious-word-again.edu (no subject) with the message:
SUBSCRIBE SL9 Firstname Lastname
These new impact point estimates from Sekanina, Chodas, and Yeomans are much closer to the morning terminator of Jupiter than the old estimates. Although they are still all on the far side as viewed from the Earth, they are now only 5-9 degrees behind the limb. About 20 - 40 minutes after each hit, the impact points will rotate past the limb of Jupiter as seen from Earth. After these points cross the limb it will take another 17 minutes before they cross the morning terminator into sunlight.About 1.5 hours after each hit, the impact points will rotate into view as seen from Earth.
Q1.2: Who are Shoemaker and Levy?
Eugene and Carolyn Shoemaker and David H. Levy found the 13.8 magnitude
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 [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 via anonymous ftp (Lunar
and Planetary Lab, University of Arizona; 128.196.64.66). Below is a list
of the images at this site. The files are listed here in reverse
chronological order. Most of the images are accompanied by a text file
giving a description of the image.
The following is a list of other collision related GIF images and MPEG animations that are also available at SEDS (LPL):
There is an older archive also available via anonymous ftp (University of Maryland). The GIF images here are named SL9*.GIF. Also there are a some Hubble Space Telescope images at a site in France. The GIF images here are named 1993e*.GIF. (Also see references for photos.)
Q1.4: What will be the effect of the collision?
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 will only rise 100 km or so above the cloudtops. Above that height the density will 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 (slightly redder than the sun, which is 5000 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.
Finally, 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.
There are now two technical papers [18,19] on the atmospheric consequences of the explosions available via anonymous ftp from the University of Chicago. The paper and figures are available in UNIX compressed Postscript format; a couple of the computational figures are also available in TIFF format.
Specifics
Q2.1: What are the impact times of each comet fragment?
The following table gives the latest impact predictions. The 21 major
fragments are denoted A through W in order of impact, with letters I and O
not used. The predicted impact times for F, G, K, Q, R, and W have moved
slightly later (by up to 0.02 days), while those for H, L, and P are now
slightly earlier. The impact points have generally moved a few tenths of a
degree closer to the limb as seen from the Earth. Fragments J and M have
been deleted from the table because they have disappeared: they are not
visible in the HST image taken in late January.
Here is some more information (probably outdated).
+=============================================================================+ | Fragment UT Date/Hour Jovicentric Meridian Angle Angle Probability of | | of Impact Latitude Angle S-F-J E-J-F viewing impact | | month dy hr mn (deg) (deg) (deg) (deg) Io Eu Ga Ca | +=============================================================================+ | A = 21 July 16 19 -43.26 64.43 73.95 98.73 0 3 3 3 | | B = 20 July 17 03 -43.34 64.73 74.18 98.49 3 3 3 3 | | C = 19 July 17 06 -43.37 64.90 74.30 98.36 3 2 3 3 | | D = 18 July 17 12 -43.42 65.11 74.46 98.20 3 0 3 3 | | E = 17 July 17 15 -43.80 64.98 74.50 98.21 1 0 3 3 | | F = 16 July 18 00 -44.24 63.09 73.40 99.42 0 0 3 3 | | G = 15 July 18 07 -44.19 65.00 74.66 98.09 0 0 3 3 | | H = 14 July 18 19 -44.04 64.99 74.60 98.13 3 0 0 3 | | J = 13 Disappeared | | K = 12 July 19 10 -44.43 66.16 75.51 97.22 0 3 0 3 | | L = 11 July 19 21 -44.52 65.60 75.17 97.58 0 3 0 3 | | M = 10 Disappeared | | N = 9 July 20 10 -44.78 66.40 75.79 96.97 0 3 0 3 | | P = 8 July 20 15 -45.00 65.61 75.34 97.47 3 3 0 3 | | Q = 7 July 20 19 -44.54 67.56 76.50 96.19 3 3 0 3 | | R = 6 July 21 07 -44.78 70.07 78.27 94.39 1 0 0 3 | | S = 5 July 21 15 -44.69 68.07 76.89 95.80 0 0 0 3 | | T = 4 July 21 18 -44.10 69.63 77.77 94.80 0 0 0 3 | | U = 3 July 21 21 -44.11 69.77 77.88 94.69 0 0 0 3 | | V = 2 July 22 04 -44.14 70.09 78.11 94.46 0 0 0 3 | | W = 1 July 22 08 -44.24 70.69 78.55 94.02 2 0 0 3 | +=============================================================================+ | Approximate | | Uncertainty 43 min 0.6 2.5 1.6 1.7 | | (1-sigma) | +=============================================================================+
The following are the 3-sigma (uncertainty) predictions for the fragment impact times:
on March 1 - 90 min
on May 1 - 71 min
on June 1 - 49 min
on July 1 - 30 min
on July 15 - 19 min
at impact - 18 hr - 10 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: 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 according to the new
solution; 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 0.995. 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 the Postscript plots called "fig*.ps" at SEDS.LPL.Arizona.EDU or
[14] for a visual representation of the orbit. The right ascension and
declination of the comet along with some orbital elements can also be
obtained via FTP at SEDS.LPL.Arizona.EDU in the /pub/astro/SL9 directory
in the file called "elements.nuc". This file is updated periodically.
Q2.3: 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) on July 7, 1992. Shoemaker-Levy 9
is not the first observed 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 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.
Q2.4: 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. However, 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 several months.
Q2.5: 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 |
+=============================================+
Q2.6: Will Hubble or Galileo 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. The impact points are also viewable by both
Voyager spacecraft, especially Voyager 2. 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.
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.
Q2.7: How can I observe the effects of the
collisions?
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. There is a
MSDOS program written by Lenny Abbey that displays information about
Jupiter's major features and predicts the locations of Jupiter's moons,
the Great Red Spot, and the central meridian System I and System II
longitudes. See this file for
more information about this program.
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. However, current calculations suggest that the brightenings may be as little as 0.05% of the sunlit brightness of the moon [18]. If Io can be caught in eclipse but visible from the earth during an impact, prospects will improve significantly. The MSDOS program "galsat51.zip" will calculate and display the locations of the Galilean satellites for the predicted impact times and can be obtained via anonymous ftp. One could monitor the moons using a photometer, a CCD camera, 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, biases, and flats) [23].
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 and Telescope magazine which explains how to build a simple antenna for observing the Jupiter/Io interaction [4,24,25]. This simple antenna is less directional that a dipole or long-wire antenna. See this file for more information. This file is updated periodically.
Q2.8: To whom do I report my observations?
The Association of Lunar and Planetary Observers (ALPO) will distribute
a handbook to interested observers. Also, observation forms by
Steve Lucas are available. These forms also contain addresses of "Jupiter
Watch Program" section leaders. "jupcom.zip" contains Microsoft Write
files. For addresses of ALPO, Steve Lucas and other Jupiter recorders see
the January 1994 issue of Sky & Telescope magazine [14].
Q2.9: Where can I find more information?
Jovian Moon Events for March 1994
+---------------------------------------------------------------+ | SATELLITE EVENT TYPE | +---------------------------------------------------------------+ | I -> Io Tr -> Transit D -> Disappear | | II -> Europa Sh -> Shadow R -> Reappear | | III -> Ganymede Ec -> Eclipse I -> Ingress | | IV -> Callisto Oc -> Occult E -> Egress | +---------------------------------------------------------------+
Mar. 21
5:24 I.Sh.I.
6:16 I.Tr.I.
7:34 I.Sh.E.
8:24 I.Tr.E.
Mar. 22
2:33 I.Ec.D.
5:32 I.Oc.R.
8:00 II.Sh.I.
9:43 II.Tr.I.
10:21 II.Sh.E.
11:57 II.Tr.E.
19:52 III.Ec.D.
20:02 III.Ec.R.
23:30 III.Oc.D.
23:52 I.Sh.I.
Mar. 23
0:42 I.Tr.I.
1:05 III.Oc.R.
2:02 I.Sh.E.
2:50 I.Tr.E.
21:01 I.Ec.D.
23:58 I.Oc.R.
Mar. 24
3:01 II.Ec.D.
6:58 II.Oc.R.
18:20 I.Sh.I.
19:09 I.Tr.I.
20:30 I.Sh.E.
21:17 I.Tr.E.
Mar. 25
15:29 I.Ec.D.
18:25 I.Oc.R.
21:17 II.Sh.I.
22:52 II.Tr.I.
23:37 II.Sh.E.
Mar. 26
1:06 II.Tr.E.
9:48 III.Sh.I.
11:55 III.Sh.E.
12:49 I.Sh.I.
13:11 III.Tr.I.
13:35 I.Tr.I.
14:43 III.Tr.E.
14:59 I.Sh.E.
15:43 I.Tr.E.
Mar. 27
9:58 I.Ec.D.
12:51 I.Oc.R.
16:19 II.Ec.D.
20:08 II.Oc.R.
Mar. 28
7:17 I.Sh.I.
8:02 I.Tr.I.
9:27 I.Sh.E.
10:10 I.Tr.E.
Mar. 29
4:26 I.Ec.D.
7:18 I.Oc.R.
10:34 II.Sh.I.
12:01 II.Tr.I.
12:54 II.Sh.E.
14:15 II.Tr.E.
23:50 III.Ec.D.
Mar. 30
1:46 I.Sh.I.
1:59 III.Ec.R.
2:28 I.Tr.I.
2:57 III.Oc.D.
3:56 I.Sh.E.
4:31 III.Oc.R.
4:36 I.Tr.E.
22:54 I.Ec.D.
Mar. 31
1:44 I.Oc.R.
5:36 II.Ec.D.
9:17 II.Oc.R.
20:14 I.Sh.I.
20:55 I.Tr.I.
22:24 I.Sh.E.
23:03 I.Tr.E.