This is a continuation of the pre-impact FAQ which contains information about the comet's discovery, orbit, tidal breakup, and fragment sizes. The answers to questions Q3.2 to Q3.6 below were provided by Mark Boslough and David Crawford. Send comments to astro@sfasu.edu.
Fragment A struck Jupiter with its kinetic energy equivalent to about 225,000 megatons of TNT creating plume which rose about 1000 km above the Jovian cloudtops. It was not long before the Hubble Space Telescope images of the fireball and impact site of fragment A were downloaded by thousand of observers. Many were surprised to see any effects from Earth. "We were thinking that we were going to have to go in with a microscope and you know stretch the image as hard as we could to pull out anything, but its just blasting away at us...unbelievable." - Hal Weaver
Fragment B was about the same brightness as fragment A in the pre-impact images but its impact left a small mark on Jupiter that was observed by only a few observatories. Then fragments C, D, and E left marks similar the impact of fragment A while the impact of F was difficult to detect. The real show-stopper was fragment G which struck Jupiter with an estimated energy equivalent to 6,000,000 megatons of TNT (about 600 times the estimated arsenal of the world). The fireball from fragment G rose about 3000 km above the Jovian cloudtops and was observed by many observatories (mostly in infrared). As it turned out, visible-light-radiating fireballs were seen with large telescopes and many observers reported seeing the dark impact scars with telescopes as small as 5 cm in diameter.
Observers have detected some of the collisions using radio telescopes [57] but it appears that the light emitted during the entry of the fragments into the Jovian atmosphere was not observed from the ground in reflection from Jupiter's moons as predicted. The Galileo spacecraft images of impact W have now been downloaded and do show a light flash that lasted a few seconds, but it was not particularly strong and would probably not have been detected in reflection from a moon by the available ground-based instruments [56].
Many of the later impacts hit near the sites of earlier ones and the resulting features soon became very complex. The development of the new features on Jupiter has since been followed by many observers. While the smaller features have almost disappeared the larger complexes are still visible even in small telescopes. Currently it appears that some of the dark impact sites are overlapping to form a partial band. No one really knows how long the features will continue to be visible, but perhaps this band will still be visible during the next observing season.
In summary, some of the fragments of comet Shoemaker-Levy 9 disappeared before they reached Jupiter (J, M and P1), some impacts were generally difficult to detect (B, F, N, P2, Q2, T, U, and V), some fragments created dark impact sites that measured about half of an Earth-diameter across (A, C, D, E, H, Q1, R, S, and W) and others created impact sites that were at least an Earth-diameter across (G, K, and L). Clearly not all of the fragments of comet Shoemaker-Levy 9 were the same. The size of fragment A is thought to have been about 1 km across and the diameter of fragment G is thought to have been about 3 km.
Q3.2: What causes the fireballs?
Most models have shown that the fireball is accelerated upward due to
the atmospheric pressure gradient, not by buoyancy. In fact, the fireball is
denser than the surrounding atmosphere, so its buoyancy is negative and it is
pulled back down by gravity. When the comet deposited its energy in Jupiter's
atmosphere, it created a zone of hot, high-pressure gas along its trajectory.
This channel of pressurized gas exploded because it had to expand into its
lower-pressure surroundings. However, it was more confined underneath and
along the sides by the higher ambient pressure of Jupiter's atmosphere.
The easiest direction for it to accelerate was upward, where it was unconfined.
It was also directed along its axis, because it must expand further in that
direction before its pressure can drop. The result was that the hot vaporized
cometary and atmospheric material was shot upward as if from a cannon. As it
rose, the driving pressure dropped and it went into a ballistic trajectory
(but because it was a gas it was also expanding and cooling). It went into free
fall, and like a fly ball it reached a maximum height and arched back down.
Because of this behavior, we call it a "ballistic fireball".
Q3.3: How does this differ from other phenomena called
"fireballs"?
There are two other related phenomena that go by the same name. When
the word "fireball" is used, it should be obvious from the context which
is being described:
Meteor fireball : a brilliant meteor that may trail bright sparks. This phenomenon is associated with the meteor, or entry phase of a bolide.
Nuclear fireball : a nuclear fireball is dominated by interior radiative transport at temperatures of tens of millions of degrees. It normally rises bouyantly, like an air bubble in water, and leads to the familiar mushroom cloud. In some high atmospheric tests, fireballs with diameters greater than the atmospheric scale height were generated. These fireballs rose ballistically in a manner very similar to the SL9 impact fireballs.
Q3.4: What is the difference between a "fireball" and a
"plume"?
There was some discussion among the modelers as to the most appropriate
term. Now that these are observational, rather than just theoretical
phenomena, it is more important to have concise definitions. Eventually
everyone will converge on the same terms by consensus and common usage. In
mean time, we recommend the following:
Fireball : The bubble of hot gasses consisting of a mixture of Jovian atmosphere and cometary material that is shot upward by the impact. In the first moments after impact it is very hot, incandescent, and radiating in the visible and near infrared (note that the term "fire" implies heat but not combustion).
Plume : The debris bubble after it has expanded and cooled adiabatically.
Obviously there is not a clear distinction between the two.
Q3.5: Why was there a gap between the apparent limb of
Jupiter and the plumes or fireballs in the Hubble images?
It is important to distinguish between the limb of Jupiter and the
terminator (or sunrise line) which was several degrees closer from our
perspective. In fact, the Hubble images show the luminescent fireball above
the dark limb of Jupiter only 1-2 minutes after the fragment G impact. The
apparent gap in this image is between the limb and the terminator. A few
minutes later, the fireball has risen high enough to be in sunlight.
There is also a gap due to the shadow of Jupiter on the plume. The debris cloud that makes up the plume has risen into sunlight as we predicted in our "Watching for Fireballs" paper (EOS, July 5). The earth, Jupiter and the Sun were not in a straight line at the time of impact, so the plume had to rise hundreds of kilometers higher than the limb to reach sunlight.
Q3.6: How deep did the fragments penetrate into Jupiter's
atmosphere?
That is currently one of the more controversial issues. We have not seen
enough data yet to come to any firm conclusions, but we believe that the lack
of spectral signatures for water is not sufficient evidence to conclude that
the penetration was shallower than expected. David Crawford's simulations
have indicated that, contrary to what many scientists have been saying, deep
penetrations do not necessarily bring up a lot of deep material into the
plume where it can be seen.
Q3.7: What is the dark material at the impact sites?
The dark semicircles south of the impact points are probably an ejecta
blanket composed of fine material condensed from the plume. This material
is either from the comet itself or from Jupiter and is suspended in the upper
atmosphere. Some scientist refer to it as soot. In the infrared they look
bright because of reflected sunlight and in the visible spectral region they
are generally darker than the Jovian clouds.
The composition of the plumes was investigated by spectroscopy in many different wavelengths. No new molecules have been found but it is expected that further analysis will eventually make it possible to document chemical processes that took place. The following elements and molecules have been seen in the spectra: Li, Na, Mg, Mn, Fe, Si and S; NH3, CO, H2O, HCN; H2S, CS, CS2, S2; CH4, C2H2, C2H6, and possible others [56].
Q3.8: How can the structure of the impact sites be
explained?
One of the most well know images is the
HST image of the G impact site just
after its appearance on the limb of Jupiter. There are a few features that
stand out on this image:
Q3.9: What were the impact times and locations?
Here is a summary of the SL9 impact and brightness times as reported
on the International Astronomical Union Circulars cited (IAU#). The IAU
circular number cited should be referenced for complete description of
brightness measurements and the wavelength observed. Not every observed
report of specific impact is listed. If anyone has a more complete list,
please send email to astro@sfasu.edu.
OBSERVED SL9 IMPACT TIMES
FRAG | IMPACT DAY / UTC | PEAK | FADED | OBSERVED BY IAU#
-----|------------------|-------|--------|---------------------------
A | JUL16.844 / 2015 | 2018 | ---- | Hubble Telescope 6023-24
| JUL16.845 / 2017 | | 2043 | Calar Alto Obs./Spain 6023
B | JUL17.122 / 0256 | | 0313 | KECK Obs./Mauna Kea 6024
C | JUL17.304 / 0718 | | 0739 | NASA/IR Telescope 6024
| JUL17.303 / 0716 | 0721 | +1 hr | Okayama Obs./Japan 6024
D | JUL17.496 / 1154 |faded in seconds| CASPIR/Austr. 6025
E | JUL17.637 / 1517 | | 1523 | Calar Alto Obs./Spain 6025
F | JUL18.060 / 0126 | 20min.| | ESO/Chile 6026
G | JUL18.315 / 0734 |**0738*| 0810 | SPIREX/South Pole 6026
H | JUL18.813 / 1931 |+10 min| | Calar Alto Obs. 6027
| JUL18.814 / 1932 | | | Galileo Spacecraft 6031
| JUL18.815 / 1934 | 1945 | | ESO/Chile 6027
K | JUL19.434 / 1025 | short flash | Okayama Obs./Japan 6028
L | JUL19.926 / 2213 | 2218 | | Calar Alto Obs./Spain 6029
| JUL19.933 / 2224 | 2226 | | Rio de Janero Obs. 6029
| JUL19.929 / 2218 | | | Galileo Spacecraft 6031
#M | JUL20.259 / 0613 | 0711 | | Mexican Natl. Obs. 6030
| JUL20.256 / 0609 | | | KECK Obs./Mauna Kea 6030
N | JUL20.441 / 1036 | 1037 | | IRIS 6030
| JUL20.441 / 1036 |faint flash 1038| CASPIR/Austr. 6030
P | --- No impact observed --- | 6031
Q2 | JUL20.822 / 1944 |faint flash only| Pic du Midi Obs. 6032
Q1 | JUL20.842 / 2012 | 2020 2nd flash| Pic du Midi Obs. 6032
| JUL20.848 / 2021 | | | La Palma/Nordic Team 6031
R | JUL21.237 / 0541 |**0543*| 0609 | CASPIR/Austr. 6032
| JUL21.233 / 0536 | 0546 | 0612 | Palomar Obs. 6032
S | JUL21.640 / 1522 | 1529 | 1537 | South African Obs. 6033
| JUL21.645 / 1529 | | 1533 | Calar Alto Obs./Spain 6033
T | --- No impact observed --- | 6034
U | --- No impact observed --- | 6034
V | --- No impact observed --- | 6034
W | JUL22.340 / 0810 |**0812*| | CASPIR/Austr. 6034
| JUL22.343 / 0812 | 0815 | | IRIS 6034
---------------------------------------------------------------------
NOTES: IMPACT - Time of impact or 1st light detected
PEAK - Time of peak brightness or start of Peak (reports varied)
** - Time detector saturated
FADED - Start of fade or when = Jup. brightness (reports varied)
#M - Missing fragment "M" impact observed
POST-CRASH IMPACT TIMES AND IMPACT LOCATIONS
FOR FRAGMENTS OF COMET SHOEMAKER-LEVY 9
PREDICTED ACCEPTED JOVICENTRIC SYSTEM II
FRAGMENT DATE TIME (UT) IMPACT TIME LATITUDE LONGITUDE
NAME JULY'94 HH:MM:SS & 1-sigma error (deg) (deg)
------------------------------------------------------------------------------
A 16 20:00:40 20:11:00 (3 min) -43 119
B 17 02:54:13 02:50:00 (6 min) -43 0
C 17 07:02:14 07:12:00 (4 min) -43 158
D 17 11:47:00 11:54:00 (3 min) -43 329
E 17 15:05:31 15:11:00 (3 min) -43 86
F 18 00:29:21 00:33:00 (5 min) -43 68
G 18 07:28:32 07:32:00 (2 min) -43 318
H 18 19:25:53 19:31:59 (1 min) -43 33
J 19 02:40 Missing since 12/93
K 19 10:18:32 10:21:00 (4 min) -43 209
L 19 22:08:53 22:16:48 (1 min) -43 281
M 20 05:45 Missing since 7/93
N 20 10:20:02 10:31:00 (4 min) -44 6
P2 20 15:16:20 15:23:00 (7 min) -44 184
P1 20 16:30 Missing since 3/94
Q2 20 19:47:11 19:44:00 (6 min) -44 338
Q1 20 20:04:09 20:12:00 (4 min) -44 355
R 21 05:28:50 05:33:00 (3 min) -44 334
S 21 15:12:49 15:15:00 (5 min) -44 325
T 21 18:03:45 18:10:00 (7 min) -44 75
U 21 21:48:30 21:55:00 (7 min) -44 209
V 22 04:16:53 04:22:00 (5 min) -44 84
W 22 07:59:45 08:05:30 (3 min) -44 215
---------------------------------------------------------------------------
Notes :
The impact times are from Don Yeomans and Paul Chodas. The impact
time given is the time the impact would be seen at the Earth, if the limb of
Jupiter were not in the way (i.e., the time listed is the time of impact plus
the light travel time to the Earth). The impact latitudes are from pre-impact
predictions and the system II impact longitudes were calculated using the
accepted impact times. Approximate system III longitudes can be calculated by
adding 67 degree to the system II longitudes. For information about how the
accepted impact times were obtained see:
http://www.jpl.nasa.gov/sl9/impacts2.html
Q3.10: Where can I find more information and GIF images?
Hartmut Frommert (spider@seds.org) has compiled a list of
GOPHER, FTP and WWW sites that contain images and information about the crash.
His list is available at
http://www.seds.org/~spider/sl9list.txt.
There is an article in the September 1994 issue of the ESO Messenger that
is an excellent overview of the events [56]. The article is also available
via the WWW at
http://www.jpl.nasa.gov/sl9/news35.html.
There are over 700 images available involving comet Shoemaker-Levy 9. Here is a list of just a few images and animations that are available at seds.lpl.arizona.edu (128.196.64.66) and some info about how to get them if you only have mail access to files:
If you have only mail access to files then mail the following message (no subject) to bitftp@pucc.Princeton.edu or ftpmail@seds.lpl.arizona.edu:
HELP
48. Boslough, M.B., Crawford, D.A., Robinson, A.C., and Trucano, T.G. "Mass and penetration depth of Shoemaker-Levy 9 fragments from time- resolved photometry", Geophys. Res. Lett., Vol. 21, No. 14, pp. 1555- 1558, July 1, 1994.
49. Boslough, M.B., Crawford, D.A., Robinson, A.C., and Trucano, T.G. "Watching for fireballs on Jupiter", EOS--Transactions of the American Geophysical Union, Vol. 75, No. 27, pages 305-310, July 5, 1994.
50. Crawford, D.A., Boslough, M.B., Trucano, T.G., and Robinson, A.C. "The impact of comet Shoemaker-Levy 9 on Jupiter", Shock Waves, in press.
51. Crawford, D.A., Boslough, M.B., Trucano, T.G., and Robinson, A.C. "The impact of periodic comet Shoemaker-Levy 9 on Jupiter", International Journal of Impact Engineering, accepted for publication.
52. Chapman, Clark R., "Dazzling demise of a comet", Nature 370, 245-246 July 28, 1994.
53. Glasstone, S. & Dolan, P.J. The Effects of Nuclear Weapons (U.S. Government Printing Office, 1977).
54. Beatty, J. Kelly and Goldman, Stuart J., "The Great Crash of 1994", Sky & Telescope, October 1994, pages 18-23.
55. MacRobert, Alan M. "Backyard Astronomy: Amateur Astronomy's Greatest Week", Sky & Telescope, October 1994, page 24-26.
56. West, R. M., "Comet Shoemaker-Levy 9 Collides with Jupiter: The continuation of a unique experience", ESO Messenger, September 1994.
57. O'Meara, Stephen James, "The Great Dark Spots of Jupiter", Sky & Telescope, November 1994, pages 30-35.
58. International Astronomical Union Circular No. 6020, July 15, 1994.
59. Eicher, David J., "Death of a Comet", Astronomy, October 1994, pages 40-45.
60. Burnham, Robert, "Jupiter's Smash Hit", Astronomy, November 1994, pages 34-39.
61. Eicher, David J., "Jupiter's Embattled Cloudtops", Astronomy, December 1994, pages 70-77.
Dan Bruton
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