PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "NULL" OBJECT = TARGET TARGET_NAME = "SL9" OBJECT = TARGET_INFORMATION TARGET_TYPE = "COMET" PRIMARY_BODY_NAME = "SL9" ORBIT_DIRECTION = "UNK" ROTATION_DIRECTION = "UNK" TARGET_DESC = " Significance ============ The impact of comet Shoemaker-Levy 9 onto Jupiter represented 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 delivered 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 came close to Jupiter in 1994 it actually impacted 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 23 identified fragments 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 deg from the vertical. The times of the impacts were known to within a few hours but observations in early 1994 significantly improved the precision of the predictions. The impacts happened on the back side of Jupiter as seen from Earth in an area that was also in darkness. This area was close to the limb of Jupiter and was 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 impacted 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 passed 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 were multiple. At least 23 major fragments were identified. No observations are capable of resolving the individual fragments to show the solid nuclei. Images with the Hubble Space Telescope suggested that there were discrete, solid nuclei in each of the largest fragments which, although not spatially resolved, produce a single, bright pixel that stood out above the surrounding coma of grains. Reasonable assumptions about the spatial distribution of the grains and about the reflectivity of the nuclei implied sizes of 2 to 4 km (diameter) for each of the 11 brightest nuclei. Because of the uncertainties in these assumptions, the actual sizes were very uncertain. No outgassing was detected from the comet. The dust distribution suggests that the material ahead of and behind the major fragments in the orbit were 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. Summary of impact times, impact locations, and impact geometries ---------------------------------------------------------------- Published estimates of the impact times and locations of the fragments of SL9 are given below (Table 5 in [CHODAS&YEOMANS1996]). Impact was defined to occur at the 100mbar level of Jupiter's atmosphere. The impact for all fragments except J and M are based on independent orbit solutions given by [CHODAS&YEOMANS1996]. The estimates for the 'lost' fragments J and M were obtained by applying the tidal disruption model to the orbit for fragment Ql and matching the astrometry of these two fragments relative to Ql. The third column of the table contains the final pre-impact prediction for each of the fragments as distributed electronically by the UMd e-mail exploder. The fourth column lists the final best estimates, which were inferred directly from impact phenomena for 16 fragments, and computed from the orbit solutions for the rest. All times are as viewed from the Earth, and therefore include the light travel time. The impact time uncertainties are rough estimates which indicate a confidence level in the accepted time; they are not formal 1-sigma uncertainties. The impact latitude is jovicentric, while the longitude is System III, measured westwards on the planet. The meridian angle is the jovicentric longitude of the impact point measured from the midnight meridian towards the morning terminator. At the latitude of the impacts, the limb as viewed from the Earth was at meridian angle 76 deg, and the terminator was at meridian angle 87 deg. The final column gives the angular distance of the impacts behind the limb. Event Impact Time (UTC) Impact Location Merid. Ang. Dist. ----------------------------- --------------- Angle Behind Limb Date Predicted Accepted +/- Lat. Lon. (July) h m s h m s (s) (deg) (deg) (deg) (deg) A 16 19:59:40 20:10:40 60 -43.35 184 65.40 7.7 B 17 02:54:13 02:50:00 180 -43.22 67 63.92 8.8 C 17 07:02:14 07:10:50 60 -43.47 222 66.14 7.1 D 17 11:47:00 11:52:30 60 -43.53 33 66.16 7.1 E 17 15:05:31 15:11:40 120 -43.54 153 66.40 6.9 F 18 00:29:21 00:35:45 300 -43.68 135 65.30 7.7 G 18 07:28:32 07:33:33 3 -43.66 26 67.09 6.4 H 18 19:25:53 19:31:59 1 -43.79 99 67.47 6.1 J 19 02:40 01:35 3600 -43.75 ~316 68.05 ~6 K 19 10:18:32 10:24:17 2 -43.86 278 68.32 5.5 L 19 22:08:53 22:16:49 1 -43.96 348 68.86 5.1 M 20 05:45 06:00 600 -43.93 ~264 69.25 ~5 N 20 10:20:02 10:29:20 2 -44.31 71 68.68 5.1 P2 20 15:16:20 15:21:11 300 -44.69 249 67.58 5.8 P1 20 16:30 16:32:35 800 -45.02 ~293 65.96 6.9 Q2 20 19:47:11 19:44:00 60 -44.32 46 69.26 4.7 Q1 20 20:04:09 20:13:53 1 -44.00 63 69.85 4.3 R 21 05:28:50 05:34:57 10 -44.10 42 70.21 4.1 S 21 15:12:49 15:16:30 60 -44.22 33 70.34 4.0 T 21 18:03:45 18:09:56 300 -45.01 141 67.73 5.7 U 21 21:48:30 22:00:02 300 -44.48 278 69.54 4.5 V 22 04:16:53 04:23:20 60 -44.47 149 69.96 4.2 W 22 17:59:45 08:06:16 1 -44.13 283 71.19 3.4" END_OBJECT = TARGET_INFORMATION OBJECT = TARGET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "CHODAS&YEOMANS1996" END_OBJECT = TARGET_REFERENCE_INFORMATION END_OBJECT = TARGET END