http://www.ifa.hawaii.edu/~yan/57p.html<br /><br /> The Unstable Lives of Comets:<br /> The Fragmentation of Comet 57P/du Toit-Neujmin-Delporte<br /><br /> Yanga R. Fernández, Scott S. Sheppard, David C. Jewitt<br /> University of Hawai`i<br /><br /> (Click here for a more technical description.)<br /> http://www.ifa.hawaii.edu/~yan/57ptech.html<br /><br />Summary: Recent observations from Mauna Kea with the University of Hawai`i's<br />2.2-meter telescope have revealed a zoo of tiny mini-comets strung out in a<br />line trailing behind the comet 57P/du Toit-Neujmin-Delporte. At some point<br />in the recent past this comet has apparently suffered a significant<br />catastrophe, violent enough to break off many pieces of its nucleus. The<br />event was probably triggered by thermal stresses within the nucleus due to<br />it being warmed by sunlight. While it is not uncommon for one or two<br />companions to be seen near a comet that has fragmented, these recent<br />observations have revealed at least 19 companions, a rare finding. A similar<br />situation was found with observations a few years ago of comet LINEAR C/1999<br />S4, taken by the Hubble Space Telescope. In that case the fragments were<br />very short-lived, and they could be followed for only a few days. Here we<br />should be able to observe the fragments for several weeks and watch how the<br />fragments move and evolve, which should give us insight into the<br />constitution and fragility of this comet and hopefully comets in general.<br /><br />Details:<br /><br />Motivated by the initial report of a single, previously-unknown companion<br />associated with Comet 57P/du Toit-Neujmin-Delporte, we obtained deep imaging<br />of the comet's environs to take the census of any population of fragments<br />that might exist near the comet. We used the Univ. of Hawaii 2.2-m telescope<br />on Mauna Kea and a charge-coupled device (CCD) to make a digital map of the<br />sky around the comet. The observations were performed on the nights of July<br />17/18 and July 18/19, 2002 (Hawaii Standard Time).<br /><br />We were delighted to find a zoo of fragments strung out in a line extending<br />almost 30 minutes of arc away from the comet itself. For comparison, the<br />diameter of the full Moon also covers 30 minutes of arc. So far we have<br />confirmed the existence of 18 fragments, which brings the total number of<br />companions to 19. The discovery has been announced by the Central Bureau for<br />Astronomical Telegrams, the internationally-recognized official<br />clearinghouse for reporting cometary discoveries. We identified fragments by<br />taking successive images of a field and detecting their motion against the<br />background stars. A mosaic of the relevant mapped region is shown in Figure<br />1, with the location of the fragments circled. The comet's head is at left.<br />At the distance of the comet, the mosaic spreads over about 1,000,000<br />kilometers (about 620,000 miles).<br /><br /> [mosaic of 57P]<br /><br />Figure 1. Mosaic of the region near comet 57P, the head of which is at left,<br />and the discovered fragments (circled and labelled with official<br />designations). North is up and East is to the left, the standard orientation<br />in the sky. The comet was observed while it was almost 180 degrees away from<br />the Sun, so the Sun, Earth, and the comet are almost along a straight line<br />in space. As projected onto the sky, the comet's motion with respect to the<br />Sun is almost due East; the fragments all trail along behind the comet and<br />have virtually the same motion.<br /><br />What are the brightnesses of these fragments? One way to express this is<br />relative to the faintest object that most well-sighted people can see with<br />the naked eye. Component "B", the first fragment found and the brightest, is<br />about 250,000 fainter than this, and the dimmest components, "K" and "L",<br />are fully 10,000,000 times fainter than this.<br /><br />The fragments show a wide variety of activity levels and condensations. We<br />cannot yet be sure what the sizes of the fragments are but the brightest<br />ones are probably less than a few hundred meters (few hundred yards) across.<br />For comparison, Diamond Head crater is several hundred meters across. The<br />smallest fragments are probably no more than a few tens of meters across,<br />roughly the size of a house. A gallery of the 18 objects that we found is<br />shown in Fig. 2.<br /><br /> [montage of fragments]<br /><br />Figure 2. A close-up of each of the 18 new fragments discovered by us. The<br />labeling system is the same as in Figure 1. There is great diversity among<br />the fragments' physical characteristics.<br /><br />What could have caused this fragmentation? The cometary nucleus is basically<br />a conglomerate of water ice and rocky material. When the comet is within<br />roughly 400,000,000 kilometers (250,000,000 miles) of the Sun, the sunlight<br />is strong enough to start evaporating the ice in large quantities. (For<br />comparison, Earth is 150,000,000 km (93,000,000 miles) from the Sun.) Since<br />the ice and rock are intimately mixed, the warming and evaporating ice<br />produces great thermal and physical stresses on the body of the nucleus.<br />Under normal circumstances, only vapor and tiny dust grains are all that fly<br />off the surface of the nucleus -- and here on Earth we see a comet with a<br />long tail, for example as widely seen in the late 1990s with comets<br />Hyakutake and Hale-Bopp. Occasionally though the stress is great enough that<br />entire chunks of the nucleus are also forced off. Our observations shown in<br />the figures show the end state of this event. Now while the basic idea is<br />thought to be understood, the details are still uncertain, basically because<br />we do not know many fundamental structural properties of cometary nuclei. In<br />the case of this comet we cannot yet determine even when the fragmentation<br />took place; further observations are necessary. With sufficient data<br />fragmenting comets can provide a laboratory for us to witness major<br />evolutionary events and can help us understand a comet's basic constitution.<br /><br />More Trivia/Background: Comet 57P/du Toit-Neujmin-Delporte was discovered in<br />1941 by the 3 people for whom it is named. The "57P" just means it is the<br />57th comet in the list of comets that have been seen on two of their<br />passages around the Sun. (The first comet in this list, "1P", is the famous<br />Halley's Comet.)<br /><br />----------------------------------------------------------------------------<br />Acknowledgement: We thank the CBAT/MPC for allowing a link to their pages in<br />accordance with their WWW policy.<br />----------------------------------------------------------------------------