ESO SL9 NEWS BULLETIN ===================== Issue : 7 Date : Saturday, July 16, 1994, 16:00 UT (18:00 CEST; 12:00 Chilean time) Items : 7-A: News from La Silla 7-B: Chronology of SL9 news 7-C: Some fragments have faded 7-D: How large are the fragments ? 7-A. NEWS FROM LA SILLA The weather was perfect, with good seeing, good transparency, and not a single cloud or even thin cirrus ! It was a very good night for astronomical observations and the bright moon in the sky was a beautiful sight. As feared (and expected), it turned out to be impossible to obtain any images of SL9 last night with the NTT or the Danish 1.5 metre telescope. Even if it would have been possible to keep Jupiter, now just a few arcminutes away, out of the camera field its very strong light would still have entered by multiple reflections in the telescope optics. It would most probably have damaged the detectors and the observers decided that it was too risky to try. Exactly for this reason, the spider which carries the f/35 IR secondary mirror at the top of the 3.6-metre telescope was wrapped with black velvet to minimize the diffraction of the light and the spikes on the image. The TV camera of that telescope was also changed to a more suitable one, since the standard one is optimized for very faint objects. The special infrared instrument TIMMI was mounted and is working fine and last night it was already observing Jupiter in all the filters which will be used during the next days. A fine occultation of the bright star Spica (Alpha Virginis) by the Moon was also monitored in thermal IR by TIMMI, as well as by a group of enthusiastic astronomers from the cat-walk! When the Moon moves in front of such a bright star, you can really see how fast is our satellite is moving in the sky. The observations at the Dutch 90-cm have now really started. It will work in two modes: 1) taking images of the planet and 2) trailing the satellites on the CCD to record possible brightness variations. Both modes have now been tested, and give satisfactory results. The Munich special high-speed photometer is also in good shape. Among the SL9/Jupiter observers, Klaus Jockers and Keith Horne arrived on July 14th; Rita Schulz, Joachim A. Stuewe, Therese Encrenaz, Laurent Jorda yesterday, the 15th; the teams are now almost complete. Nick Thomas, Pierre Colom, Dominique Bockelee-Morvan, Didier Despois and Anne-Marie Lagrange will arrive today, since they start later. Two Chilean teams have been to La Silla. The Canal 13 team was there on the 14th and expects to present daily updates on the collision; they filmed a lot of things and should have enough material. Last night, the TV news of Canal 7 showed what they filmed on the 13th at La Silla. A Chilean friend of one of the ESO astronomers, quite satisfied with the fine presentation of the work at La Silla, but also having seen the impressive beards of some of the observers in very close-up, called the mountain and remarked on "the wild astronomers of ESO" ! 7-B. CHRONOLOGY OF SL9 NEWS The discovery and subsequent investigations of comet Shoemaker-Levy 9 have been documented in the Circulars of the International Astronomical Union (IAUC). Below is a condensed overview from this source of the major steps forward in our knowledge and understanding of this very peculiar object: Date IAUC Information 1993 Mar 26 5725 Discovery by Eugene and Carolyn Shomaker and David Levy at Palomar (24 Mar); total magnitude about 14; unusual 1 arcminute long string. Image (26 Mar) by Jim Scotti (Spacewatch, Arizona) shows at least five nuclei. Brian Marsden (IAU Telegram Bureau) finds that the comet is probably near Jupiter. 1993 Mar 27 5726 Brian Marsden calculates elliptical orbit, according to which the comet is now near Jupiter; probably with a 6 million km approach around 1992 July 28. 1993 Mar 29 5730 Image (27 Mar) by David Jewitt and Jane Luu (Hawaii) shows at least 17 nuclei. 1993 Mar 30 5732 Annette Cochran (Texas) observes no gas in comet. 1993 Apr 4 5744 Brian Marsden finds that the comet must at least temporarily be in orbit around Jupiter. 1993 Apr 6 5752 Infrared K observations by MacLeod and McLeod (Steward Observatory, Arizona); K-magnitude 12. 1993 May 22 5800 Calculations by Brian Marsden and Syuichi Nakano (Japan) show close encounter with Jupiter in early July 1992 and probably again towards the end of July 1994 at a distance of only 45,000 km, i.e. a collision will take place. Andrea Carusi (Rome) finds that some of the nuclei may then be captured into orbits around Jupiter. 1993 May 22 5801 Brian Marsden finds that more than half of the train will most likely collide with Jupiter in July 1994. 1993 Nov 22 5892 Based on many accurate observations by Jim Scotti, Marsden determines moment of closest approach as 1992 July 8.0, at 20.000 km above clouds. 1993 Nov 22 5893 Brian Marsden also determines that all nuclei will collide with Jupiter on 1994 July 18 and during the following days. 1993 Dec 14 5906 First observations after five months pause (comet behind the Sun as seen from the Earth) by Jim Scotti and Tom Gehrels. Brian Marsden now find that the close encounter took place on 1992 July 7.8 and that collisions will begin on 1994 July 17.6. 1993 Dec 17 5909 Don Yeomans and Paul Chodas (JPL, Pasadena) computes that the impacts will take place on the rear side, but close to the limb, and at latitudes -47 - -49 degrees. 1994 Jan 20 5924 Jewitt and Luu report that basic structure is unchanged, but there are significant changes in relative brightnesses of some components and less dust around. 1994 Feb 18 5936 Hubble Space Telescope images (on 1993 July 1 and 1994 Jan 24 - 27, by Harold Weaver) show that some nuclei are double and that two have disappeared in the meantime. A total of 21 nuclei are visible. 1994 Mar 10 5947 New HST images are described by Weaver. Only upper limits can be placed on the sizes of the nuclei. The largest is perhaps 4 km in diameter. More changes in relative brightnesses. 1994 Apr 12 5973 More HST observations from 1994 Mar 29 - 30. Weaver provides detailed description of individual features. No OH (gas) seen in UV spectrum. 1994 Jun 1 5999 Jewitt and Trentham (Hawaii) find that nucleus G is double. 1994 Jun 24 6010 New observations with the HST on May 17 confirm splitting of G and documents further changes in the brightnesses 1994 Jul 9 6017 High-accuracy astrometry from La Silla. New accurate impact timings by Syuichi Nakano. 7-C SOME FRAGMENTS HAVE FADED On IAUC 6019 (July 14, 1994), T. Rettig, J. Hahn, S. Tegler and G. Sobczak, University of Notre Dame; and M. Mumma and M. DiSanti, NASA Goddard Space Flight Center, communicate that preliminary analysis of images of the cometary fragments (excluding fragments T = 4 and P1 = 8a) obtained with the Hubble Space Telescope wide-field camera between January and July 1994 shows that the inner 0.3 arcsec regions have remained relatively constant or have slowly decreased in brightness over the seven-month period. Similar results are noted with larger diaphragms. The fragments show minimal brightness activity through the May observations. However, for six fragments observed during June and July, there is a rather dramatic brightness decrease from the May observations. In a 1.1 arcsec diaphragm, five fragments (W, V, S, R and H) show a brightness decrease by about 50 percent, and fragment U by about 30 percent. Also in the June and July images, the brightness profiles of fragment P2 show a very much more distributed source of dust within the inner 1 arsec. These profiles indicate that the central source may have been disrupted and may suggest initial tidal dispersal of the swarm material. We note that between May and June most of the fragment images have become elongated in the direction of Jupiter." 7-D HOW LARGE ARE THE FRAGMENTS ? Zdenek Sekanina (JPL, Pasadena) writes in IAUC 6020 (July 15) that preliminary results of his analysis of CCD photometry obtained with the HST Planetary Camera in late January, late March, and early July 1994 suggest the presence of a major point source in the center of most of the condensations and offer consistent values for the dimensions of the largest fragments. Under a reasonable assumption about the reflectivity of the surface of the nuclei (4 percent of the infalling light in the red spectral region), Sekanina finds the following that the largest effective diameters (km) in Jan., Mar., and July, are: F: 2.3, -, -; G: 4.2, 3.6, -; H: 3.2, -, -; P1: 1.3, <1, -; P2: 2.4, -, -; Q1: 3.9, 3.0, 3.7; Q2: 3.2, 1.6, 2.4; R: 2.6, 2.7, 2.2; S: 3.6, -, -; T: 4, 1.4, -; U: 1.3, -, -. The peak of the dust cloud is found to be displaced with respect to the primary fragment in the general direction away from the Sun. Sekanina remarks that although the evidence points to an apparently continuing disintegration of the large fragments in numerous discrete events, objects a few km across still seem to have been present in early July, and the temporal variations in the effective diameters are likely to be primarily a rotational effect of strongly irregular shape. ---------- This daily news bulletin is prepared for the media by the ESO Information Service on the occasion of the July 1994 collision between comet Shoemaker-Levy 9 and Jupiter. It is available in computer readable form over the ESO WWW Portal (URL: http://http.hq.eso.org/eso-homepage.html) and by fax to the media (on request only). News items contained therein may be copied and published freely, provided ESO is mentioned as the source. ESO Information Service European Southern Observatory Karl-Schwarzschild-Strasse 2 D-85748 Garching bei Muenchen Germany Tel.: +49-89-32006276 Fax.: +49-89-3202362