The top panel is a composite of two NASA/ESA Hubble Space Telescope (HST) images of Comet C/1999 S4 (LINEAR) taken on 5 August 2000 and subtends a region 58,000 km x 28,000 km at the comet. The bright dust tail extending to the left was created primarily during the complete destruction of part of the comet's nucleus on or about 22 July 2000. The rest of the nucleus broke up into larger fragments, which resemble mini-comets and are seen clustered near the western tip of the tail (celestial east is the left and north is to the top). The bottom panel shows an image of the comet taken ~35 hrs after the HST images from the European Southern Observatory's Very Large Telescope (VLT) and subtends a region 103,000 km x 58,000 km at the comet. The VLT image was processed using unsharp masking, which suppresses light from the tail and enhances the visibility of the fragments. The long, nearly vertical streaks are the tracks of stars passing through the field near the comet. (Higher resolution PDF and PS versions of this image can be downloaded by clicking on the icons below.) Credit: H. Weaver (Johns Hopkins University), C. Delahodde, O. Hainaut, R. Hook (European Southern Observatory), Z. Levay (Space Telescope Science Institute), and the HST/VLT observing team (see below), NASA/ESA, ESO (342 KB) (1262 KB)

Comet LINEAR (C/1999 S4)

Comet C/LINEAR, whose official name is C/1999 S4, was discovered on 27 September 1999 by the Lincoln Near Earth Asteroid Research (LINEAR) program when it was 4.3 astronomical units from the Sun (AU; 1 AU is the average Earth-Sun distance = 150 million km = 93 million miles). At that time, the comet was unusually bright, given that it was still very far from the Sun (for comparison, Jupiter is about 5 AU from the Sun), which fueled expectations that C/LINEAR might become visible to the naked eye during the summer of 2000. Although C/LINEAR fell far short of the original brightness prediction, the comet still provided plenty of ``fireworks'' for astronomers. Starting in early June 2000, the comet's brightness showed unusually large temporal variations, and its nucleus completely disrupted into its component parts during late-July 2000. This website presents some images and discussion of the breakup of C/LINEAR. A series of six articles on C/LINEAR is being published in the 18 May 2001 issue of Science magazine. A listing of those Science papers is provided for your convenience. The breakup of C/LINEAR raises a host of major scientific questions, including the missing mass problem. We first observed the C/LINEAR with the Hubble Space Telescope (HST) near the end of the first week of July 2000. Serendipitously, we caught the comet in the midst of a major outburst. The series of images below documents the dramatic changes in the comet during that time. (Click on the image below to see the larger version, or click on the icons below to get the PDF and PS versions.) Caption: The three images above were taken with NASA's Hubble Space Telescope and chronicle a violent outburst in the life of comet LINEAR, also known as C/1999 S4. The orbiting observatory's Space Telescope Imaging Spectrograph (STIS) tracked the comet over 2 days (July 5.94 to 7.96, capturing a dramatic leap in the brightness of the core region around the nucleus [left image]; watching the dust created during the outburst flowing outwards into the coma, or atmosphere of the comet [middle image]; and culminating in the discovery of a castoff chunk of material from the nucleus sailing along its tail [the bright dot trailing behind the comet in the picture at right]. The white region represents the brightest part of the coma. The nucleus cannot be seen in these images because its light is overwhelmed by the light from the dust particles. Credit: NASA, H. Weaver and P. Feldman (Johns Hopkins University), M. A'Hearn (University of Maryland), C. Arpigny (Liege University), M. Combi (University of Michigan), M. Festou (Observatoire Midi-Pyrenees), and G.-P. Tozzi (Arcetri Observatory) (97 KB) (177 KB) Caption: The above frame is a magnified view of the last frame of the previous image (the 3-frame mosaic). The chunk of material seen in the tail is roughly 290 miles (470 km) from the nucleus and appears to be moving down the tail at about 6 miles per hour. The fragment's size is unknown but is probably house-sized, or smaller. The fragment may be a pancake-shaped piece of the surface of the nucleus that was lifted off by vigorous ice sublimation. The + symbol shows the position of the nucleus. Credit: NASA, H. Weaver and P. Feldman (Johns Hopkins University), M. A'Hearn (University of Maryland), C. Arpigny (Liege University), M. Combi (University of Michigan), M. Festou (Observatoire Midi-Pyrenees), and G.-P. Tozzi (Arcetri Observatory) (12 KB) (29 KB) The principal results from the HST and VLT investigations of C/LINEAR appear in the 18 May 2001 issue of Science magazine. A few of the figures from our article are given below. Please note that these figures are copyrighted by Science and cannot be used without explicit permission from that journal! Fig. 1: HST WFPC2 images of C/LINEAR. The top panel is a mosaic of two images taken in wide field mode (WFC) on 5.38 August and on 5.45 August 2000 and demonstrates that the bright tail dominated the comet's optical emission. However, near the western tip of the tail (within the outlined box; celestial north is straight up and east is to the left) one can see clear evidence for individual fragments. This region is magnified in the middle panel, which shows that the fragments resembled mini-comets with their own comae and tails. Our highest resolution image of the tip region, taken in planetary camera (PC) mode on 5.18 August 2000, is displayed in the bottom panel. The fragments are labeled with their letter names in the middle and bottom panels. The diamond gives the predicted position of the original nucleus using the JPL-87 orbit solution, while the box shows the predicted position of the nucleus using the JPL-95 orbit solution. Their separation, 19.3 arcsec on this date, serves as a scalebar for each of the displayed images. The nearly vertical streaks are trails from stars passing through the field that were not completely removed during image processing. Copyright © 2001 by the American Association for the Advancement of Science (231 KB) (613 KB) Fig. 2: VLT image of C/LINEAR taken on 6.99 August 2000. Three individual images were combined and then processed using both unsharp masking and wavelet filtering. At least 16 fragments were detected, and they are labeled with their letter names. Celestial north is straight up and east is to the left. The diamond gives the predicted position of the original nucleus using the JPL-87 orbit solution, while the box shows the predicted position of the nucleus using the JPL-95 orbit solution. Their separation was 20.3 arcsec on this date. The nearly vertical streaks are trails from stars passing through the field that were not completely removed during image processing. Copyright © 2001 by the American Association for the Advancement of Science (26 KB) (141 KB) Fig. 4: Individual fragment and its spatial brightness profile. The top panel shows the HST PC image of fragment K, with the estimated position of the nucleus marked (x). The spatial brightness profile for the region between the dashed lines (30 deg acute angle) is plotted in the bottom panel. This region is centered on the sunward direction and was selected to avoid tailward moving debris. The compass shows the directions of celestial north and east, and the width of the entire image is 1.87 arcsec. The data and their error bars (1 sigma) are plotted along with our best fit model for the nucleus contribution (thin solid line), the coma contribution (dashed line), and the sum of the two (thick solid line). The nucleus contributes about half of the observed signal in the peak pixel, and its corresponding R-band magnitude is about 26. Copyright © 2001 by the American Association for the Advancement of Science (8 KB) (17 KB)

HST/VLT C/LINEAR Team: H. Weaver and P. Feldman (Johns Hopkins University), Z. Sekanina and M. Keesey (Jet Propulsion Laboratory), I. Toth (Konkoly Observatory), M. A'Hearn and C. Lisse (University of Maryland), C. Arpigny (Liege University), J. Bauer and K. Meech (University of Hawaii), M. Combi (University of Michigan), J. Davies (Joint Astronomy Centre), M. Festou (Observatoire Midi-Pyrenees), O. Hainaut, R. Hook, and R. West (European Southern Observatory), P. Lamy and C. Delahodde (Laboratoire d'Astronomie Spatiale), B. Marsden (Smithsonian Astrophysical Observatory), and G.-P. Tozzi (Arcetri Observatory) Go to the top of this webpage Direct questions to: Hal Weaver Last updated: 21 May 2001