Changes from last Posting: The latest predictions of times and locations of fragment impac

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Changes from last Posting: The latest predictions of times and locations of fragment impacts are given below. Answers to all questions except Q1.2 and Q1.3 have been updated. To subscribe to the "Comet/Jupiter Collision FAQ" mailing list, send a message to astro@tamu.edu. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Comet/Jupiter Collision FAQ * * * * Last Updated 18-Jan-1993 * * * * 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 contributed. Contact Dan Bruton (astro@tamu.edu) or John Harper * * (jharper@tamu.edu) with comments, additions, corrections, etc. The * * Postscript version and updates of this FAQ are available via anonymous * * ftp to tamsun.tamu.edu (128.194.15.32) in the /pub/comet directory or to * * seds.lpl.arizona.edu in the /pub/astro/SL9 directory. This FAQ is also * * available in hypertext format: * * http://info.cv.nrao.edu/staff/pmurphy/jove-comet-wham-2.html * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * GENERAL QUESTIONS Q1.1: Is it true that a comet will collide with Jupiter in July 1994? Q1.2: Who are Shoemaker and Levy? Q1.3: Where can I find a GIF image of this comet? Q1.4: What will be the effect of the collision? SPECIFICS Q2.1: What are the impact times and impact locations of the comet fragments? Q2.2: What are the orbital parameters of the comet? Q2.3: Why did the comet break apart? Q2.4: What are the sizes of the fragments and how long is the fragment train? Q2.5: Will Hubble, Galileo, Ulysses, or Voyager be able to image the collisions? Q2.6: How can I observe the collisions? Q2.7: To whom do I report my observations? GENERAL QUESTIONS Q1.1: Is it true that a comet will collide with Jupiter in July 1994? Yes, the shattered comet Shoemaker-Levy 9 (1993e) is expected to collide with Jupiter over a 5.6 day period in July 1994. The first of 21 comet fragments is expected to hit Jupiter on July 16, 1994 and the last on July 22, 1994. All components of the comet will hit on the dark farside of Jupiter, out of sight from Earth. The impact of the center of the comet train is predicted to occur at about -44 degrees Jupiter latitude at a point about 67 degrees east (toward the sunrise terminator) from the midnight meridian. 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 12 - 24 minutes after each hit, the impact points will rotate into view as seen from Earth. For the impact locations mentioned above, anything ejected higher than 500-1000 km above the cloud tops during the impacts will be visible 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? Some GIF images can be obtained via anonymous ftp from tamsun.tamu.edu in the /pub/comet directory. The GIF images here are named SL9*.GIF. Also there are a some Hubble Space Telescope images at ftp.cicb.fr in the /pub/Images/ASTRO/hst directory. 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 oddjob.uchicago.edu in the pub/jupiter directory. 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 and impact locations of the comet fragments? The new solutions below incorporate astrometric data taken after December 9, when the comet was first seen after it emerged from solar conjunction. The new predictions are much more accurate than the old ones because they are based on measurements over a longer time span. The new impact point estimates 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. The following table gives the latest impact predictions. A letter designation for the cometary fragments has become standard. The 21 major fragments are denoted A through W in order of impact, with letters I and O not used. Fragment A is the closest to Jupiter and will impact first; fragment W is the farthest and will impact last; fragment Q is the brightest (and therefore presumably biggest), with G, H, K, L, and S also quite bright. +====================================================================+ | Fragment UT Date/Hour Jovicentric Meridian Angle Behind | | of Impact Latitude (deg) Angle (deg) Limb (deg) | | d hr | |====================================================================| | A July 16 19 -43.3 64 9 | | B July 17 02 -43.3 65 9 | | C July 17 06 -43.4 65 8 | | D July 17 11 -43.4 65 8 | | E July 17 14 -43.7 63 8 | | F July 17 23 -43.5 66 8 | | G July 18 07 -43.8 66 8 | | H July 18 19 -43.9 66 7 | | J July 19 01 -43.7 67 7 | | K July 19 10 -44.0 67 7 | | L July 19 22 -44.3 69 7 | | M July 20 05 -43.9 68 6 | | N July 20 09 -44.0 68 6 | | P July 20 14 -44.0 68 6 | | Q July 20 19 -44.3 69 6 | | R July 21 05 -44.1 69 5 | | S July 21 15 -44.4 69 5 | | T July 21 17 -44.1 70 5 | | U July 21 21 -44.1 70 5 | | V July 22 04 -44.1 70 5 | | W July 22 07 -44.2 70 5 | +====================================================================+ The impact times have been quoted to the nearest hour, and do not include the light time to the Earth (about 40 min); the uncertainty in these times is about 2 hr. The impact latitudes are all around -44 deg, well south of the Great Red Spot. The meridian angle is the Jovicentric longitude of impact measured from the midnight meridian (the line of longitude pointed away from the Sun) to the morning terminator. The impact points are thus 20-25 deg to the nightside of the morning terminator. The longitudes of the impacts are not well known yet because the times are uncertain and Jupiter rotates quickly; we can only say that the impact points will be scattered around the planet at the -44 deg latitude line. The rightmost column gives the angular distance of the impact point behind Jupiter's limb as seen from the Earth. The later impacts will be closest to the limb. These predictions indicate that the impact of the largest fragment Q will be visible during evening in Europe and Asia, but will occur in daylight hours for North America. See the Postscript file impact.ps at tamsun.tamu.edu in the /pub/comet directory for a graph with impact times and Galilean moon eclipses. 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, 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 [14] for a visual representation of the orbit. The right ascension and declination of the comet along with some orbital elements can be obtained via ftp at tamsun.tamu.edu in the /pub/comet directory as elements.nuc. 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 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]. Furthermore, images of Callisto and Ganymede show crater chains which may have resulted from the impact of a comet similar to Shoemaker-Levy 9 [3,17]. 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. 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 and how long is the fragment train? 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 (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 observations. 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. Here 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 | +=============================================+ Q2.5: Will Hubble, Galileo, Ulysses, or Voyager be able to image 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. Q2.6: How can I observe 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 comet 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. 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 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/astrnomy 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, biases, and flats). 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, 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 magazine which explains how to build a simple antenna for observing the Jupiter/Io interaction [4]. Q2.7: To whom do I report my observations? Observation forms by Steve Lucas are available via ftp at tamsun.tamu.edu in the /pub/comet directory. These forms also contain addresses of "Jupiter Watch Program" section leaders. JUPCOM.ZIP contains Microsoft Write files. For other addresses see page 44 of the January 1994 issue of Sky & Telescope magazine [14]. 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] Burnham, Robert "Jove's Hammer", Astronomy, October 1993, page 38-39. [10] IAU Circulars : 5800, 5801, 5807, 5892, and 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] Scotti, J. V. and H. J. Melosh, "Estimate of the size of comet Shoemaker-Levy 9 from a tidal breakup model" Nature 365, 733-735 (1993). [14] Beatty, Kelly and Levy, David H., "Awaiting the Crash" Sky & Telescope, January 1994, page 40-44. [15] Jewitt et al., Bull. Am. Astron. Soc. 25, 1042, (1993). [16] "AstroNews", Astronomy, January 1994, page 19. [17] "AstroNews", Astronomy, February 1994, page 16. [18] Zahnle, Kevin and Mac Low, Mordecai-Mark, "The Collisions of Jupiter and Comet Shoemaker Levy 9", submitted to Icarus October 29,1993. [19] Mac Low, Mordecai-Mark and Zahnle, Kevin "Explosion of Comet Shoemaker-Levy 9 on Entry into the Jovian Atmosphere", submitted to Nature January 7,1994. [20] Sekanina, Z., Chodas, P.W., and Yeomans, D.K, "Tidal Disruption and the Appearance of Periodic Comet Shoemaker-Levy 9", submitted to Astronomical Journal, November 1993. ACKNOWLEDGMENTS Thanks to Ross Smith for starting a FAQ and to all those who have contributed : Robb Linenschmidt, Mordecai-Mark Mac Low, Phil Stooke, Rik Hill, Elizabeth Roettger, Ben Zellner, Kevin Zahnle, Ron Baalke, David H. Levy, Eugene and Carolyn Shoemaker, Jim Scotti, Richard A. Schumacher, Louis A. D'Amario, John McDonald, Michael Moroney, Byron Han, Wayne Hayes, David Tholen, Patrick P. Murphy, Greg F Walz Chojnacki, Jeffrey A Foust, Paul Martz, Kathy Rages, Paul Chodas, Zdenek Sekanina, Don Yeomans, and Richard Schmude. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

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