Suzy B Software 2

home *** CD-ROM | disk | FTP | other *** search

/ Suzy B Software 2 / Suzy B Software CD-ROM 2 (1994).iso / nasa / faq1993e / comet.faq < prev

Wrap

Text File | 1995-05-02 | 16.8 KB | 290 lines

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Comet/Jupiter Collision FAQ * * * * The following is a list of frequently asked questions concerning the * * collision of comet Shoemaker-Levy 9 with Jupiter. Thanks to all those * * who have 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. * * * * Last updated 28-Dec-1993 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 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 of each comet fragment? 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: How can I observe the collisions? Q2.6: Will Hubble, Galileo or Voyager be able to image the collision? 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 70 degrees east (toward the sunrise terminator) from the midnight meridian. 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? Some GIF images can be obtained via anonymous ftp from pdssbn.astro.umd.edu in the /ftp/pub/images directory. The GIF images here are named SL9*.GIF. Also there are a some 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? Kevin Zahnle from JPL predicts that each comet fragment will explode into a fireball about 200 km below the cloudtops of Jupiter. The fireball will initially be hidden underneath the cloudtops but will expand rapidly, with the plume material rising to about 1.02 to 1.25 Jupiter radii above the cloudtops. The temperature of the fireball may reach up to 30,000 degrees C with a luminosity of 4x to 25x the brightness of Jupiter. The energy of collision is said to be equivalent to between 200,000 and 100,000,000 megatons of TNT. The lower number is for 1 km diameter fragments; current nuclei diameter estimates range from 1/2 km to as high as 3 or 4 km. The fragments will be traveling with a speed of 60 km/sec relative to Jupiter [9]. The explosions could cause ammonia ice to well up to the top layers of Jupiter's atmosphere forming high cirrus clouds that would create an abnormally bright equatorial cloud zone [8]. The clouds may also partially obscure atmospheric activity such as the Red Spot much like in the case of Uranus' atmosphere whose Voyager flyby images revealed a near featureless disk. The dust "wings" of the comet will start interacting with the planet more than a month ahead of the large fragments, and will continue to interact for more than a month afterward. The long-term monitoring of Jupiter is therefore even more important. There is a technical paper on the consequences of the explosions available via anonymous ftp from oddjob.uchicago.edu in the /pub/jupiter directory. The paper and figures are available in Postscript format; a couple of the computational figures are also available in TIFF format. SPECIFICS Q2.1: What are the impact times of each comet fragment? The table below is part of John Spencer's post and shows the estimated impact times and the Galilean satellite reflector availability. See the file named Spencer-impact.times in the /pub/predictions/entry directory of pdssbn.astro.umd.edu for more information. =============================================================================== Nucleus UT date of Designation impact (July) Satellite Orbital Longitudes ----------------- ------------- (degrees past superior conj.) Jewitt Sekanina Fitted IAUC ------------------------------------------- DPS abs Preprint Time 5906 Amalthea Io Europa Ganymede Callisto =============================================================================== 21 A 16.81 194 340+ 103+ 75+ 35+ 20 B 17.08 25* 34+ 130 89+ 41+ 19 C 17.27 168 74+ 150 99+ 45+ 18 D 17.46 304 112 169 108 49+ 17 E 17.61 17.6 49+ 142 183 115 52+ 16 F 17.98 320+ 218 221 134 60+ 15 G 18.29 18.3 184 281 253 150 67+ 14 H 18.79 18.8 183 22+ 303+ 175 78+ 13 J 19.08 37+ 83+ 333+ 190 84+ 12 K 19.41 19.4 272 149 6* 206 91+ 11 L 19.91 19.9 271 250 57+ 231 102+ 10 M 20.24 152 318+ 90+ 248 109+ 9 N 20.40 265 350+ 106 256 112 8 P 20.61 58+ 33+ 128 266 117 7 Q 20.82 20.8 207 75+ 149 277 121 6 R 21.21 21.3 135 156 189 297+ 130 5 S 21.63 21.6 78+ 241 231 318+ 139 4 T 21.73 150 261 241 323+ 141 3 U 21.91 281 298 260 332+ 145 2 V 22.19 117 353o 287 345+ 151 1 W 22.33 22.3 219 22+ 302+ 353+ 154 ------------------------------------------------------------------------------ Uncertainties: 0.1 0.1 72 20 10 5 2 ============================================================================== (posted 17 Dec 1993) Key : "+" means that the impact will be visible from the satellite "*" means the satellite will be visible in eclipse "o" means the satellite will be occulted by Jupiter See the file called nucl_impact.931217 at pdssbn.astro.umd.edu /pub/predictions/geometry for impact locations and impact times by Sekanina, Chodas, Yeomans, and Scotti. With continuous astrometry starting in January 1994, the following are for 3-sigma (uncertainty) predictions for the fragment impact times: at minus 2 months - 55 minutes at minus 1 month - 40 minutes at minus 1 week - 21 minutes at minus 6 hours - 9 minutes The time between impacts is thought to be know 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? Shoemaker-Levy 9 is thought to have made its closest approach to Jupiter on July 8, 1992 at a distance of about 1.53 Jupiter radii from Jupiter's center [8]. The comet is thought to have reached apojove (farthest from Jupiter) on July 14, 1993 at a distance of about 0.33 Astronomical Units from Jupiter's center. The orbit looks somewhat elliptical. See postscript files on pdssbn.astro.umd.edu in the /ftp/pub/images directory for a visual representation. The right ascension and declination of the comet along with some orbital elements can be obtained via anonymous ftp at pdssbn.astro.umd.edu in the /pub/ephemeris directory. The filenames are elements.28, iauc5893 and iauc5892. 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 8, 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 and how long is the fragment train? Images taken with the Hubble Space Telescope suggest 3-5 km diameter fragments. Models of Shoemaker-Levy's breakup in 1992 suggest that the original intact comet may have been only 2 km across. If this is the case, the largest fragments could be no more that 500 meters across [1]. The angular length of the train was about 51 arcseconds in March 1993 [2]. The length of the train then was about one half the Earth-Moon distance. In the day just prior to impact, the fragment train will stretch across 20 arcminutes of the sky, more that half the Moon's angular diameter. The translates to a physical length of about 4 million kilometers. Q2.5: How can I observe the collisions? One can monitor the atmospheric changes on Jupiter using the naked eye, photography, or CCD imaging. It is important, however, to observe Jupiter for several months in advance in order to know which features are due to comet impacts and which are naturally occurring. A video camera may be able to record real time atmospheric changes if your telescope is big enough. The Red Spot and other features have been recorded using a video camera and a 14" telescope with eyepiece projection (2" eyepiece works best). With WWV ticking away in the background one can obtain an excellent record of this unusual event. One may be able to witness the collisions indirectly by monitoring the brightness of the Galilean moons that may be behind Jupiter as seen from Earth. Some suggest that moons may brighten by as much as 0.2 to 2 magnitudes (ALPO conference, August 1993). The MSDOS program GALSAT will calculate and display the locations of the Galilean satellites for a given day and time and can be obtained via ftp from oak.oakland.edu in the /pub/msdos/astronomy directory. One could monitor the moons using a photometer, a CCD, or a video camera pointed directly into the eyepiece of a telescope. If you do video you can get photometric information by frame grabbing and treating these like CCD frames (applying darks, bias', and flats). Radio emissions due to the impacts may be strong enough to be detected by small radio telescopes. Some suggest to listen in on 15-30 MHz during the comet impact, but to avoid 27 MHz because this frequency is used for CB communications (ALPO conference, August 1993). So it appears that one could use the same antenna for both the Jupiter/Io phenomenon and the Jupiter/comet impact. There is an article in Sky & Telescope which explains how to built a simple antenna for observing the Jupiter/Io interaction [4]. Q2.6: Will Hubble, Galileo or Voyager be able to image the collision? The Hubble Space Telescope, like earthlings, will not be able to see the collisions but will be able to monitor atmospheric changes on Jupiter. The comet collisions are visible from both Voyager 1 and Voyager 2. Voyager 1 will be 52 AU from Jupiter and will have a near-limb observation viewpoint. Voyager 2 will be in a better position to view the collision from a perspective of looking directly down on the impacts, and it is also closer at 41 AU. Jupiter will appear as 2.5 pixels from Voyager 2's viewpoint and 2.0 pixels for Voyager 1. If there is any imaging to be done by Voyager, it will only be by Voyager 2. The impacts will occur on the limb of Jupiter as seen from the Galileo Space Probe. The Ida data playback is scheduled to end at the end of June, so there should be no tape recorder conflicts with observing the comet fragments colliding with Jupiter. The problem is how to get the most data played back when Galileo will only be transmitting at 10 bps. One solution is to have both Voyager 2 and Galileo record the event and and store the data on their respective tape recorders. The Voyager 2 data will be played back first at something like 3500 bps. The images will be small, but at least the time of each comet fragment impact can be determined. Using this information, data can be selectively played back from Galileo's tape recorder. From Galileo's perspective, Jupiter will be 60 pixels wide and the impacts would only show up at about 1 pixel, but valuable science data can still collected in the visible and IR spectrum along with radio wave emissions from the impacts. The imaging of the comet impacts by Voyager 2 and Galileo has not been officially approved yet, but it is very likely to happen. 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). ACKNOWLEDGMENTS Thanks to Ross Smith for starting a FAQ and to all those who have contributed : Mordecai-Mark Mac Low, Phil Stooke, Rik Hill, Robb Linenschmidt Elizabeth Roettger, Ben Zellner, Kevin Zahnle, Ron Baalke, David H. Levy, Richard A. Schumacher, Louis A. D'Amario, John McDonald, Michael Moroney, Byron Han, Wayne Hayes, and David Tholen. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * . ' |||||||||||||||||||| . || Dan Bruton || . . . || Texas A & M || `. . || astro@tamu.edu || `. ` : : ` |||||||||||||||||||| .