The following is a list of answers to frequently asked questions concerning the collision of comet Shoemaker-Levy 9 with Jupiter Send comments to firstname.lastname@example.org.
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. After these points cross the limb it will take another 18 minutes before they cross the morning terminator into sunlight.
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Q1.2: Who are Shoemaker and Levy?
Comet Shoemaker-Levy 9 (1993e) was the ninth short-period comet discovered
by Eugene and Carolyn Shoemaker and David H. Levy and was first identified on
photographs taken on the night of 24 March 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]. The
13.8 magnitude comet appeared 'squashed' in the original image. Subsequent
photographs taken by Jim Scotti with the Spacewatch telescope on Kitt Peak
in Arizona showed that the comet was actually a shattered comet. Some
astronomers call the comet a "string of pearls" since the comet fragments
are strung out in a line or train.
Before the end of March 1993 it was realized that the comet had made a very close approach to Jupiter in the summer of 1992. At the beginning of April 1993, after sufficient observations had been made to determine the orbit more reliably, Brian Marsden found that the comet is in orbit around Jupiter. By late May 1993 it appeared that the comet was likely to impact Jupiter in 1994. Since then, the comet has been the subject of intensive study. Searches of archival photographs have identified pre-discovery images of the comet from earlier in March 1993 but searches for even earlier images have been unsuccessful. See  for more information about the discovery.
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Q1.3: Where can I find a GIF image of this comet?
There are many site that have images. Here are just
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Q1.4: What will be the effect of the collision?
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." .
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.
Calculations by Paul Chodas (JPL) indicate that 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 for 2-4 km fragments by Mark Boslough (Sandia National Laboratories) indicate when the fireball resulting from an impact cools it would form a debris cloud that will rise hundreds of kilometers above the Jovian cloudtops, and would 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].
If the fragments are 2-4 km in diameter then 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. The apparent visible magnitude of any fireball will be similar to that of a Galilean satellite at best, but it would appear redder. They would be much brighter at infrared wavelengths.
The following predictions by Mordecai-Mark Mac Low (University of Chicago) are based on simulations for 1 km fragments: Each comet fragment will enter the Jupiter's 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 fragment and tear it apart. Five seconds after entry, the comet fragment would 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 . Bigger fragments will have more energy and go deeper.
The hot (30,000 K) gas resulting from a 1 km 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 in this case. 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. 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 . 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 (July 8, 1994). 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.
At the University of Oregon astronomers plan to monitor the linearly and circular polarized light from Jupiter. They have a sensitive detector that can detect changes of one part in 10^5 and they expect to see some kind of change in the polarization signal as the magnetic field of Jupiter is disrupted by the impacts.
Due to the great distance between Jupiter and the Earth, the comet poses no threat to Earth. However, Eugene Shoemaker says that if a similar comet crashed on Earth it would be catastrophic: "...we're talking about a million megatons of kinetic energy. We're talking about the kind of event that is associated with mass extinction of species on Earth; really and truly a global catastrophe. It might not take out the human race but it would certainly be very bad times." (CNN, Headline News)
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Q1.5: Can I see the effects with 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 . 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 .
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 . The table below shows the approximate sizes of features that already exist on Jupiter for comparison.
+==================================================================+ | FEATURE SIZE ESTIMATES | +==================================================================+ | Great Red Spot 26000 by 11000 km | | White Spots FA,BC,DE 9000 km | | Shadows of Io, Europa, and Ganymede 4300, 4200, 7100 km | +==================================================================+
Below is a list of files available at ftp.tamu.edu in the /pub/comet directory that may be helpful in identifying features on Jupiter:
tracker3.zip MSDOS program that displays the location of impact sites and features of Jupiter jupe.description Description of a PC program showing features of Jupiter jul1994.transit Transit Times for Red Spot and White Spots for July 1994 jul1994.moons Jovian Moon events for July 1994 (Shadows, Eclipses, etc.)
Also, there are little anticyclonic ovals at jovicentric latitudes of about -45 degrees which are typical of the South South Temperate domain and are about 3500 km in diameter. 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 CCD image of these ovals by Don Parker .
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Q1.6: Will there be live TV coverage of the events?
NASA's coverage of the impact of Comet P/Shoemaker-Levy 9 during
the week of July 16-22 includes a series of live, televised press briefings
and a 24-hour newsroom operation at the Goddard Space Flight Center (GSFC),
Greenbelt, Md. The briefing panels will include Comet co-discoverers Drs.
Eugene and Carolyn Shoemaker and David Levy on most days as well as scientists
presenting images and information from the Hubble Space Telescope and other
spacecraft. Dr. Lucy McFadden will have a round-up of observations from
ground-based observatories around the world. The program and briefing
JULY DATE TIME (EDT) EVENT Sat. 16 10:00 p.m. Live from HST: First Impact Image Release Sun. 17 8:00 a.m. Press Briefing at GSFC Mon. 18 8:00 a.m. Press Briefing at GSFC Tue. 19 8:00 a.m. Press Briefing at GSFC Wed. 20 12:00 noon Press Briefing at GSFC Th. 21 8:00 a.m. Press Briefing at GSFC Fri. 22 9:30 a.m. Press Briefing at GSFC Sat. 23 8:00 a.m. Press Briefing at GSFC
Note: The above times are dependent on the STS-65 mission schedule. If there is a change in the launch or landing time of the Shuttle, the program times will change. Video Uplink Schedule: NASA will provide feeds of b-roll and animation of the comet impacts with Jupiter on the following schedule: July 15, 1:00 p.m. EDT. NASA TV is carried on Spacenet 2, transponder 5, channel 9, 69 degrees West, transponder frequency is 3880 MHz, audio subcarrier is 6.8 MHz, polarization is horizontal.
CNN also plans to offer live coverage at 10:00pm EDT on July 16th with comments and explanation from scientists at Goddard and the Space Telescope Science Institute. CNN plans live coverage of at least the first 2 briefings (Sunday and Monday) and then TBA depending on what the images look like. CNN also plans to have live reports at 9am and noon EDT throughout the week with a daily comet report for release in the afternoon.
Note also that your local PBS station will normally pick up one of the live feeds listed below so that you will see it at 10:30-11:30 local time. If you have a TVRO satellite dish you can watch it 5 times.
PBS : "THE GREAT COMET CRASH" Wednesday July 20, 1994 (into early Thursday morning 7/21) 10:30pm - 11:30pm Telstar 401-Ku 6 (PBS Schedule B) 11:30pm - 12:30am Telstar 401-Ku 6 (PBS Schedule B) 12:30am - 1:30 am Telstar 401-Ku 6 (PBS Schedule B) 1:30am - 2:30 am Telstar 401-Ku 7 (PBS Schedule C) 2:30am - 3:30 am Telstar 401-Ku 5 (PBS Schedule A) 2:30am - 3:30 am Telstar 401-C band 8 (PBS Schedule X)
Also, there is a gathering of professional and amateur astronomers every week on the IRC (Internet Relay chat) channel #Astronomy for real time discussions. Friday and Sunday sessions are held at 20:00 UT. There may be continuous discussion/updates on the IRC channel #Astronomy during the impacts.
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============================================================================== Predicted Impact Parameters for Fragments of P/Shoemaker-Levy 9 --------------------------------------------------------------- P.W. Chodas, D.K. Yeomans and Z. Sekanina (JPL/Caltech) P.D. Nicholson (Cornell) Predictions as of 1994 July 11 Date of last astrometric data in these solutions: 1994 July 10 The predictions for all fragments except Q2 are based on independent orbit solutions; our orbit reference identifier is given. The orbit solution for fragment Q2 was obtained by applying a disruption model to the orbit for Q1, and using astrometric measurements of Q2 relative to Q1. Except for fragment Q2, uncertainties in the impact parameters are given immediately below the predicted values. These uncertainties are 1-sigma values obtained from Monte Carlo analyses; we have made an effort to make them realistic: they are not formal uncertainty values. NOTE: To obtain a 95% confidence level, one should use a +/- 2 sigma window around the predicted values. The uncertainties for Q2 have not been quantified, but are probably comparable to those for fragment T. The dynamical model used for these predictions includes perturbations due to the Sun, planets, Galilean satellites and the oblateness of Jupiter. The planetary ephemeris used was iDE245. ------------------------------------------------------------------------------- Frag- Impact Jovicentric Merid. Angle Satellite Longitudes ment Date/Time Lat. Long. Angle E-J-F Orbit at Impact (deg) July (UTC) (deg) (deg) (deg) (deg) Ref. Amal Io Eur Gany ------------h--m--s------------------------------------------------------------ A = 21 16 19:57:34 -43.11 177 64.28 98.87 A20 205 345 107+ 77+ 8.2 .19 5 .76 .56 4 1 1 0 B = 20 17 02:54:02 -43.15 70 63.78 99.21 B19 55+ 43+ 136+ 91+ 6.8 .20 4 .75 .56 3 1 0 0 C = 19 17 06:59:25 -43.31 217 65.05 98.27 C16 177t 77+ 153+ 100+ 7.0 .17 4 .74 .55 4 1 0 0 D = 18 17 11:45:30 -43.40 29 65.47 97.95 D17 321 118+ 173 11+ 7.5 .18 4 .77 .57 4 1 1 0 E = 17 17 15:05:00 -43.45 149 65.78 97.71 E33 61+ 146+ 187 117+ 5.6 .08 3 .46 .33 3 1 0 0 F = 16 18 00:26:39 -43.52 130 64.40 98.67 F24 343o 226 226 136+ 6.0 .12 4 .57 .41 3 1 0 0 G = 15 18 07:27:36 -43.58 23 66.58 97.10 G31 194t 286 255 151+ 4.8 .07 3 .38 .27 2 1 0 0 H = 14 18 19:25:55 -43.72 96 66.85 96.88 H30 194t 27+ 305 176 4.7 .07 3 .38 .27 2 1 0 0 K = 12 19 10:17:50 -43.79 275 67.72 96.24 K32 282 152+ 8+e 207 4.8 .07 3 .40 .28 2 1 0 0 L = 11 19 22:07:07 -43.90 343 68.09 95.95 L32 278 253 59+ 232 5.1 .07 3 .39 .28 3 1 0 0 N = 9 20 10:21:15 -44.27 67 67.87 96.03 N21 286 357o 112+ 257 6.7 .12 4 .68 .48 3 1 0 0 P2= 8b 20 15:09:51 -44.59 243 66.57 96.88 P19 71+ 37+ 132+ 268 6.6 .09 4 .59 .41 3 1 0 0 Q2= 7b 20 19:32:02 -44.34 39 68.83 95.33 202t 74+ 150+ 277 8 2 1 1 Q1= 7a 20 19:59:29 -44.04 55 69.25 95.08 Q35 216 78+ 152+ 278 5.4 .07 3 .41 .29 3 1 0 0 R = 6 21 05:24:17 -44.07 36 69.35 95.00 R30 139 157 191 297 6.2 .08 4 .47 .33 3 1 0 0 S = 5 21 15:09:53 -44.16 30 69.78 94.69 S41 74+ 241 232 318 5.9 .08 4 .42 .29 3 1 0 0 T = 4 21 18:05:50 -45.00 139 67.44 96.18 T14 161t 266 244 324 13.2 .16 8 1.00 .70 7 2 1 0 U = 3 21 21:52:39 -44.45 274 68.98 95.20 U15 276 298 260 332 14.5 .19 9 1.13 .79 7 2 1 1 V = 2 22 04:14:43 -44.41 145 69.32 94.96 V16 107+ 352o 286 345 10.3 .16 6 1.00 .71 5 1 1 0 W = 1 22 07:56:53 -44.16 278 70.38 94.25 W32 218 23+ 302 353 7.2 .10 4 .53 .37 4 1 1 0 Satellite Codes: + impact is visible from satellite o satellite is occulted by Jupiter at impact e satellite is eclipsed but not occulted at impact t satellite is in transit across Jupiter ------------------------------------------------------------------------------- Notes: 1. Fragments J=13, M=10, and P1=8a are omitted because they have faded from view. The March'94 HST images show that P2=8b and G=15 have split; we do not have sufficient data to obtain independent predictions for the sub-components. 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. The impact time uncertainty is a 1-sigma value in minutes. 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 System III, measured westwards on the planet. The large uncertainty in impact longitudes is due to Jupiter's fast rotation. 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. At the latitude of the impacts, the Earth limb is at meridian angle 76 deg and the terminator is at meridian angle 87 deg. 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 will be closer to the limb. 7. Satellite longitudes are given for Amalthea, Io, Europa, and Ganymede. The longitudes are measured east from superior conjunction (the anti-Earth direction). Longitude uncertainties listed as "0" are simply less than 0.5 deg. 8. According to these predictions, the only impact certain to occur during a satellite eclipse is K=12 with Europa eclipsed. Since the discovery of Comet Shoemaker-Levy 9, some of the fragments of have disappeared from view. These were probably smaller fragments to begin with, and they have probably disintegrated further, but some sizable pieces may remain amongst the debris at these locations in the train. We have computed approximate impact times for three of these missing fragments, based on the few available positional measurements, and using our tidal disruption model: Fragment Impact Last Seen Date/Time July (UTC) ----------------h--m----------------- J = 13 19 02:40 Dec. 1993 M = 10 20 05:45 July 1993 P1= 8a 20 16:30 Mar. 1994 The uncertainties on these impact times are large: probably about 60 min., 1-sigma. The impact locations are consistent with those of the other fragments, i.e., at jovicentric latitude -44 and well behind the limb of Jupiter as seen from the Earth. [Courtesy of Paul Chodas, JPL] ============================================================================== 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. 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. Relative Likelihood of Fireball Visibility for Fragments of Shoemaker-Levy 9 ----------------------------------------------------------------------------- Mark Boslough and David Crawford (Sandia National Labs, Albuquerque, NM) -------------------------------------------------------------------------- Fragment Best Locations for Viewing Likelihood of Jupiter at Impact Time Visible Fireball (*) -------------------------------------------------------------------------- A Africa (except W. Africa), :-( Middle East, Eastern Europe B Eastern N. America, Mexico, :-( Western S. America C New Zealand, Hawaii :-( D Australia, New Zealand, :-( Japan E India, Southern China, :-| S.E. Asia, Western Australia F S. America :-| G New Zealand, Hawaii :-) H Africa (except W. Africa), :-) Middle East, Eastern Europe K Australia, New Zealand :-) :-) L Brazil, W. Africa, Spain :-) :-) N Australia, New Zealand :-| P2 India, Southern China, :-| S.E. Asia, Western Australia Q1 Africa (except W. Africa), :-) :-) :-) Q2 Middle East, Eastern Europe :-) R Hawaii, West coast N. America :-) :-) S India, Southern China, :-) :-) :-) S.E. Asia, Western Australia T Africa (except W. Africa), :-( Middle East, Eastern Europe U Brazil, W. Africa, Spain :-| V Western U.S., Mexico :-| W New Zealand, Hawaii, :-) :-) :-) Eastern Australia Key: :-) :-) :-) Best group (Most likely to be visible) :-) :-) Second best group :-) Third best group :-| Not very good :-( Worst (Least likely to be visible) (*) Based on the following sources: 1) Ballistic fireball trajectories of M.B. Boslough, D.A. Crawford, A.C. Robinson and T.G. Trucano (Sandia National Laboratories) "Watching for Fireballs on Jupiter", EOS, July 5, 1994. 2) Relative brightnesses of fragments from Karen Horrocks, May, 1994. 3) Predicted impact locations in distance beyond Jupiter's limb by Chodas et al. (above).
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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 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 .
Society of Amateur and Radio Astronomers (SARA) say that amateur radio astronomers may have to wait approximately three hours after impact for the impact sites to rotate to the central meridian of Jupiter before anything unusual is detected. This wait is typical due to the Jovian decametric synchrotron emissions being emitted as a beam of radiation. Due to the large time differential from impact to radio observations any disturbance may have settled and not be detected. SARA suggest that the radio observer begin the watch approximately 30 minutes before the fragments hit to four hour after.
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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. However,
current calculations suggest that the brightenings may be as little as 0.05%
of the sunlit brightness of the moon . 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. See also Q2.1 of this FAQ for satellite locations.
The following files contain information concerning the reflection of light by Jupiter's moons and are available at SEDS.LPL.Arizona.EDU :
galsat53.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
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 .
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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
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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 . Astronomers
believe that in 1886 Comet Brooks 2 was ripped apart by tidal forces near
Jupiter . 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 . 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).
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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 .
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  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.
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Q2.7: How long is the fragment train?
The angular length of the train was about 51 arcseconds in March 1993
. 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 | +=============================================+
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Q2.8: Will Hubble, Galileo, etc. be able to observe the
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
impact points are 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 was 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 down on the impacts, and it is also closer at 41 AU.
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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 (email@example.com.U) or fax (UK  333840). The Society of Amateur and Radio Astronomers (SARA) is collecting radio observations of the events. Observations can be sent to the chairman of the SARA Comet Watch Committee: Tom Crowley, 3912 Whittington Drive, Altanta, GA 30342, EMAIL : firstname.lastname@example.org.
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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 ftp.tamu.edu (188.8.131.52) in the
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".
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