Fact Sheet: Comet Shoemaker-Levy 9 and Jupiter ---------------------------------------------- Updated, January 1994 ************************************************************************ * * * A formatted, printed copy of this fact sheet may be obtained from * * the press offices of the American Astronomical Society or the * * Division of Planetary Sciences, or from the Astronomy Deparmtment * * at the University of Maryland, College Park, MD (301-405-3001). * * * ************************************************************************ Comet Shoemaker-Levy 9 and Jupiter A comet, already split into many pieces, will strike the planet Jupiter in the third week of July of 1994. It is an event of tremendous scientific interest but, unfortunately, one which is likely to be unobservable by the general public. Nevertheless, it is a unique phenomenon and secondary effects of the impacts will be sought after by both amateur and professional astronomers. Significance The impact of comet Shoemaker-Levy 9 onto Jupiter represents the first time in human history that people have discovered a body in the sky and been able to predict its impact on a planet more than seconds in advance. The impact will deliver more energy to Jupiter than the largest nuclear warheads ever built, and up to a significant percentage of the energy delivered by the impact which is generally thought to have caused the extinction of the dinosaurs on Earth, roughly 65 million years ago. History The comet, the ninth short-period comet discovered by Gene and Carolyn Shoemaker and David Levy, was first identified on a photograph taken on the night of 24 March 1993 with the 0.4-meter Schmidt telescope at Mt. Palomar. On the original image it appeared 'squashed'. Subsequent photographs at a larger scale taken by Jim Scotti with the Spacewatch telescope on Kitt Peak showed that the comet was split into many separate fragments. Before the end of March it was realized that the comet had made a very close approach to Jupiter in mid-1992 and at the beginning of April, 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 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. Cometary Orbit According to the most recent computations, the comet passed less than 1/3 of a Jovian radius above the clouds of Jupiter late on 7 July 1992 (UT). The individual fragments separated from each other 1 1/2 hours after closest approach to Jupiter and they are all in orbit around Jupiter with an orbital period of about two years. Calculations of the orbit prior to 7 July 1992 are very uncertain but it seems very likely that the comet was previously in orbit around Jupiter for two decades or more. Because the orbit takes the comet nearly 1/3 of an astronomical unit (30 million miles) from Jupiter, the sun causes significant changes in the orbit. Thus, when the comet again comes close to Jupiter in 1994 it will actually impact the planet, moving almost due northward at 60 km/sec aimed at a point only halfway from the center of Jupiter to the visible clouds. The 21 identified fragments will all hit Jupiter in the southern hemisphere, at latitudes near 45°S, between 16 and 22 July 1994, approaching the atmosphere at an angle roughly 45° from the vertical. The times of the impacts are now known to within a few hours but observations in early 1994 will significantly improve the precision of the predictions. The impacts will occur on the back side of Jupiter as seen from Earth in an area that is also in darkness. This area will be close to the limb of Jupiter and will be carried by Jupiter's rotation to the front, illuminated side less than half an hour after the impact. The grains ahead of and behind the comet will impact Jupiter over a period of four months, centered on the time of the impacts of the major fragments. The grains in the tail of the comet will pass behind Jupiter and remain in orbit around the planet. The Nature of the Comet The exact number of large fragments is not certain since the best images show hints that some of the larger fragments may be multiple. At least 21 major fragments have been identified. No observations are capable of resolving the individual fragments to show the solid nuclei. Images with the Hubble Space Telescope suggest that there are discrete, solid nuclei in each of the largest fragments which, although not spatially resolved, produce a single, bright pixel that stands out above the surrounding coma of grains. Reasonable assumptions about the spatial distribution of the grains and about the reflectivity of the nuclei imply sizes of 2 to 4 km (diameter) for each of the 11 brightest nuclei. Because of the uncertainties in these assumptions, the actual sizes are very uncertain and there is a small but not negligible possibility that the peak in the brightness at each fragment is due not to a nucleus but to a dense cloud of grains. No outgassing has been detected from the comet but calculations of the expected amount of outgassing suggest that more sensitive observations are needed because most ices vaporize so slowly at Jupiter's distance from the sun. The spatial distribution of dust suggests that the material ahead of and behind the major fragments in the orbit are likely large particles from the size of sand up to boulders. The particles in the tail are very small, not much larger than the wavelength of light. The brightnesses of the major fragments were observed to change by factors up to 1.7 between March and July 1993, although some became brighter while others became fainter. This suggests intermittent release of gas and grains from the nuclei. Studies of the dynamics of the breakup suggest that the structural strength of the parent body was very low and that the parent body had a diameter of order 5 km. This is somewhat smaller than one would expect from putting all the observed fragments back together but the uncertainties in both estimates are large enough that there is no inconsistency. The Impact into Jupiter The predicted outcomes of the impacts with Jupiter span a large range. This is due in part to the uncertainty in the size of the impacting bodies but even for a fixed size there is a wide range of predictions, largely because planetary scientists have never observed a collision of this magnitude. If the cometary nuclei have the sizes estimated from the observations with the Hubble Space Telescope and if they have the density of ice, each fragment will have a kinetic energy equivalent to roughly ten million megatons of TNT (1029 to 1030 ergs). The predictions of the effects differ in how they model the physical processes and there are significant uncertainties about which processes will dominate the interaction. If ablation (melting and vaporization) and fragmentation dominate, the energy can be dissipated high in the atmosphere with very little material penetrating far beneath the visible clouds. If the shock wave in front of the fragment also confines the sides and causes the fragment to behave like a fluid, then nuclei could penetrate far below the visible clouds. Even in this case, there are disagreements about the depth to which the material will penetrate, with the largest estimates being several hundred kilometers below the cloudtops. In any case, there will be an optical flash lasting a few seconds as each nucleus passes through the stratosphere. The brightness of this flash will depend critically on the fraction of the energy which is released at these altitudes. If a large fragment penetrates below the cloudtops and releases much of its energy at large depths, then the initial optical flash will be faint but a buoyant hot plume will rise in the atmosphere like the fireball after a nuclear explosion, producing a second, longer flash lasting a minute or more and radiating most strongly in the infrared. Although the impacts will occur on the far side of Jupiter, estimates show that the flashes may be bright enough to be observed from Earth in reflection off the inner satellites of Jupiter, particularly Io, if a satellite happens to be on the far side of Jupiter but still visible as seen from Earth. The flashes will also be directly visible from the Galileo spacecraft. The shock waves produced by the impact onto Jupiter are predicted to penetrate into the interior of Jupiter, where they will be bent, much as the seismic waves from earthquakes are bent in passing through the interior of Earth. These may lead to a prompt (within an hour or so) enhancement of the thermal emission over a very large circle centered on the impact. Waves reflected from the density-discontinuities in the interior of Jupiter might also be visible on the front side within an hour or two of the impact. Finally, the shock waves may initiate natural oscillations of Jupiter, similar to the ringing of a bell, although the predictions disagree on whether these oscillations will be strong enough to observe with the instrumentation currently available. Observation of any of these phenomena can provide a unique probe of the interior structure of Jupiter, for which we now have only theoretical models with almost no observational data. The plume of material that would be brought up from Jupiter's troposphere (below the clouds) will bring up much material from the comet as well as material from the atmosphere itself. Much of the material will be dissociated and even ionized but the composition of this material can give us clues to the chemical composition of the atmosphere below the clouds. It is also widely thought that as the material recombines, some species, notably water, will condense and form clouds in the stratosphere. The spreading of these clouds in latitude and longitude can tell us about the circulation in the stratosphere and the altitude at which the clouds form can tell us about the composition of the material brought up from below. The grains of the comet which impact Jupiter over a period of several months may form a thin haze which will also circulate through the atmosphere. Enough clouds might form high in the stratosphere to obscure the clouds at lower altitudes that are normally seen from Earth. Interactions of cometary material with Jupiter's magnetic field have been predicted to lead to observable effects on Jupiter's radio emission, injection of material into Jupiter's auroral zone, and disruption of the ring of grains that now encircles Jupiter. Somewhat less certainly the material may cause observable changes in the torus of plasma that circles Jupiter in association with the orbit of Io or may release gas in the outer magnetosphere of Jupiter. It has also been predicted that the cometary material may, after ten years, form a new ring about Jupiter although there are some doubts whether this will happen. Plans to Study the Event It is generally expected that nearly every observatory in the world will be observing events associated with the impact. These observatories will include several Earth-orbiting telescopes (Hubble Space Telescope, International Ultraviolet Explorer, Extreme Ultraviolet Explorer) and several interplanetary spacecraft (Galileo, Clementine, and possibly others). Most observatories are setting aside time and resources but delaying detailed planning until the last possible minute in order to optimize their observations based on the latest theoretical predictions and the latest observations of the cometary properties. Jupiter and the comet have now reappeared after several months behind the sun and the first observations in early December yielded accurate positions which changed and improved the predictions of the point of impact. Observations of the physical properties of the comet are also beginning in December. Several ground-based telescopes will carry out programs to study the long- term changes in brightness of the fragments, the changes in the spatial distribution of the grains, the spectral properties of the comet to constrain the composition of the grains, and to estimate better the size of the nuclear fragments which will be more easily separated from the grains with the just repaired HST. Support for the studies of the event will be coming from many sources. Within the United States, support is being supplied by NASA, by NSF, by DoD, and by many observatories which operate under private or state-university budgets. Around the world, studies are being supported by national governments, by universities, by research societies, and by various international organizations such as the International Science Foundation, the European Space Agency, and the European Southern Observatory. Further Information People wanting further information about opportunities for the general public should contact the Planetary Society. Address inquiries to The Planetary Society, 85 North Catalina Ave., Pasadena CA 91106, telephone (818) 793 5100. Updated information will be published regularly in the monthly magazine Sky and Telescope and the monthly magazine Astronomy. These can be obtained at newsstands or libraries as well as by subscription. Accredited press reporters can contact the Press Officer of the American Astronomical Society (Dr. Stephen Maran, AAS Executive Office (202) 328 2010) or the Press Officer of the Division for Planetary Sciences of the AAS (Dr. Nadine Barlow, Lunar and Planetary Institute (713) 280 9021). This fact sheet will be updated intermittently, perhaps once per month, as new developments dictate. It is distributed through the American Astronomical Society and other distribution mechanisms. Michael F. A'Hearn Lucy-Ann A. McFadden Department of Astronomy University of Maryland