ESO SL9 NEWS BULLETIN ===================== Issue : 5 Date : Thursday, July 14, 1994, 12:00 UT (14:00 CEST; 08:00 Chilean time) Items : 5-A: News from La Silla 5-B: ESO images now also available on FTP 5-C: Observations of SL9 from South Africa 5-D: Not with a Bang but a Whimper ? 5-E: Who reads this News Bulletin ? 5-A. NEWS FROM LA SILLA It is obvious that the staff and visitors at La Silla are getting more and more enthusiastic about this event and all its scientific and human aspects. The tension is mounting and there is a rush to get ready in time. There is a widespread feeling of expectation and curiosity and of somehow moving into the centre of attention of other scientists and the public. Alas, the weather at La Silla was rather unstable last night, sometimes completely clear, but most of the time with many high-altitude cirrus clouds. At 21:00 local time, just after an image of the comet had been obtained with the NTT, the sky clouded over for a while. Nevertheless, whenever it was possible to observe, the seeing was good. The moon is now getting older, and moving closer and closer to Jupiter in the sky. It will only be a few degrees away from Jupiter when the first impacts happen, just over two days from now. The astronomers are not happy about the strong moonlight which lights up the entire sky and makes it more difficult to observe faint objects. One thing is better though: it is easier to move around on the mountain during the night and to see the road ! Heinz Barwig and Otto Baernbantner (Universitaets-Sternwarte Muenchen, Germany) arrived at La Silla yesterday. They immediately checked the boxes with their special, multi-channel high-speed instrument, which will be used at the ESO 1-metre telescope to observe the possible reflections of impact light flashes from Jupiter's moons. Everything was in order and today they will start to mount it at the telescope. The observations continued at the NTT and the Bochum 60 cm telescope. During the exposure of the comet with the NTT, a software crash occurred, but fortunately, the telescope was clever enough to continue the exposure and also to track correctly until the end. When the image was read from the CCD, some pale astronomer faces were seen at the computer consoles, but they became much brighter when the image of the comet appeared on the screens and was found to be in order ! But there is now another difficulty. The comet is so close to Jupiter that the planet's light is becoming a real problem: it is so bright that even some image processing can hardly make the fainter fragments visible anymore. Part of the SL9 image obtained with the NTT last night (at July 13.0 UT) will be available as ESO Press Photo SL9J/94-08 later this afternoon and will then also be displayed at the ESO WWW Portal. It shows the "early" fragments A, B, etc., around 4 million kilometres from Jupiter. It cannot be excluded that this will be the last image to show the first impacting fragments this clearly. 5-B. ESO IMAGES NOW ALSO AVAILABLE ON FTP Following request from various sides, an anonymous ftp account has been set up to make recent ESO SL-9 images available also in this way. It is at: "ecf.hq.eso.org" in directory "pub/sl9-eso-images" Here the recent GIF files will be found together with a copy of the HTML file, which also has links to the images in the ESO WWW Portal. We shall try to keep this directory up to date, with copies from the WWW area, over the next few days. 5-C. OBSERVATIONS OF SL9 FROM SOUTH AFRICA David Laney at the South African Astronomical Observatory (SAAO) has recently provided the following details of their observation plans. SAAO is particularly well placed to observe the first collision, fragment A on Saturday evening. Contact will be made with David before and after the event from the International Press Conference taking place at the ESO Garching Headquarters (see Bulletin 4 for more details). As with other observatories around the globe, SAAO is making a considerable effort to observe Jupiter and its environment during the period (July 16-22) when the 21 major fragments of Comet Shoemaker-Levy 9 are scheduled to strike. Seven fragments will strike while Jupiter is above the horizon in a dark sky as seen from South Africa: fragments A, H, L, Q2, Q1, T, and U. Images will be obtained at SAAO using the 1.0-m telescope and optical CCD (Matt Senay, University of Hawaii, methane and sodium filters - comet fragments approaching Jupiter, Jupiter after impact, fireball plumes, if any, extending beyond the edge of Jupiter itself) and the 0.75 m telescope with PtSi infrared camera (Kaz Sekiguchi, SAAO, infrared images of Jupiter and moons). These will be accessible, in GIF format, by anonymous ftp at the following Internet addresses: host: ftp.ru.ac.za (146.231.128.1), directory: /pub/saao and host: kudu.ru.ac.za (146.231.128.5), directory: /pub/saao. In /pub/saao at either address are two subdirectories. The subdirectory "images" will have infrared and optical comet/Jupiter images obtained at SAAO. These should begin to appear within 1-2 hours of each impact time, if all goes as planned. All images will be in GIF format. Text files will describe each image. Information on detection of fireballs in the infrared and at optical wavelengths may also appear. 5-D. NOT WITH A BANG BUT A WHIMPER ? The weekly scientific journal Nature is publishing a letter and an article today (14th July) which discuss a radically different model for Comet Shoemaker-Levy 9 and the likely collision events. The letter is from Willy Benz (University of Arizona, Steward Observatory) and Erik Asphaug (NASA Ames Research Center). The article is by Paul Weissman (JPL) who has, with others, proposed similar models in the past. The fundamental suggestion by these authors is that SL9 fragments consist not of large single solid bodies but of very losely bound small "dirty snowballs" each having a diameter of typically only 50m. They assume that the comet was a "primordial rubbish pile" of hundreds of thousands of such small bodies held together only by their own self- gravity. Weissman summarises the model for the breakup of SL9 as follows: "Asphaug and Benz's dynamical simulations show the nucleus of tightly packed snowballs being torn apart by Jupiter's gravity during the close approach, the hundreds or thousands of snowballs stretching into a long column in space. But as the column lengthens and moves away from Jupiter, the individual snowballs begin to clump together due to their own self-gravity. The truly amazing result is that the number of clumps formed appears to be a function of the density of the individual snowballs. At a density less than 0.4 g/cm3, no clumping occurs; at a density of 2.4 g/cm3 all the snowballs come back together to form a single body. But at intermediate values, in particular between 0.4 and 0.9 g/cm3, the snowballs form 15 to 20 clumps. SL9 consisted of 21 individual nuclei when it was discovered last year." "Results are modified if the original comet nucleus was rotating. Asphaug and Benz's simulations rule out a retrograde rotation, because the snowballs then form a large central clump and smaller outlying clumps; this was not observed for SL9. But if the comet had a prograde rotation, one obtains 15-20 clumps if the density of the snowballs is higher, perhaps 1.3 g/cm3. Asphaug and Benz's results also suggest that the original comet nucleus was fairly small, at most 1.5 km in diameter, in agreement with work by Scotti and Melosh" He ends by commenting on the likely behaviour of the fragments during the collisions: "As the clumps approach Jupiter for their final plunge into the atmosphere at 60 km sec-1, Jupiter's gravity will again pull them apart. Rather than hitting as a single solid body, they will likely come in as an elongated shotgun blast of smaller pellets. Because of Jupiter's rapid rotation, the impact sites will be spread in longitude, like machine gun bullets lacing into a moving target. Each snowball will individually ablate and burn up like a meteor in Jupiter's upper atmosphere. Lacking the momentum and the structural integrity of a single solid body, they will likely not penetrate deeper into the atmosphere where they might explode with multi-thousands of megatons of energy. Thus the giant impacts will produce a spectacular meteor shower of bright bolides, but not the massive fireball explosions that have been predicted by some researchers." The observations of dramatic events when the fragments hit Jupiter would certainly give problems for these theories, while negative observations will unfortunately not prove things one way or the other (the fragments may just be small). However, it is not excluded that the upcoming observations will indeed shed light on the internal structure of comets in unexpected ways. 5-E. WHO READS THIS NEWS BULLETIN ? As indicated below, this bulletin is distributed by fax and over the computer networks. Since its birth last Sunday, we have received quite a few requests from News Agencies, TV stations, so the number of faxes leaving ESO every afternoon towards all ESO member states and some other European countries is now about 30. Moreover, since this bulletin was started there have been a total of 318 accesses over the World-Wide-Web. These have come from a total of 19 countries on all five continents. ---------- 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