H. A. Weaver,^1* Z. Sekanina,² I. Toth,³ C. E. Delahodde,⁴ O. R. Hainaut,⁴ P. L. Lamy,⁵ J. M. Bauer,⁶ M. F. A'Hearn,⁷ C. Arpigny,⁸ M. R. Combi,⁹ J. K. Davies,¹⁰ P. D. Feldman,¹ M. C. Festou,¹¹ R. Hook,¹² L. Jorda,⁵ M. S. W. Keesey,² C. M. Lisse,¹³ B. G. Marsden,¹⁴ K. J. Meech,⁶ G. P. Tozzi,¹⁵ R. West¹²

At least 16 fragments were detected in images of comet C/1999 S4 (LINEAR) taken on 5 August 2000 with the Hubble Space Telescope (HST) and on 6 August with the Very Large Telescope (VLT). Photometric analysis of the fragments indicates that the largest ones have effective spherical diameters of about 100 meters, which implies that the total mass in the observed fragments was about 2 × 10⁹ kilograms. The comet's dust tail, which was the most prominent optical feature in August, was produced during a major fragmentation event, whose activity peaked on UT 22.8 ± 0.2 July 2000. The mass of small particles (diameters less than about 230 micrometers) in the tail was about 4 × 10⁸ kilograms, which is comparable to the mass contained in a large fragment and to the total mass lost from water sublimation after 21 July 2000 (about 3 × 10⁸ kilograms). HST spectroscopic observations during 5 and 6 July 2000 demonstrate that the nucleus contained little carbon monoxide ice (ratio of carbon monoxide to water is less than or equal to 0.4%), which suggests that this volatile species did not play a role in the fragmentation of C/1999 S4 (LINEAR).

¹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, USA.
² Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
³ Konkoly Observatory, Post Office Box 67, Budapest H-1525, Hungary.
⁴ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile.
⁵ Laboratoire d'Astronomie Spatiale du CNRS, BP 8, 13376 Marseille, Cedex 12, France.
⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA.
⁷ Department of Astronomy, University of Maryland, College Park, MD 20742, USA.
⁸ Institut d'Astrophysique, Université de Liège, B-4000 Liège, Belgium.
⁹ Space Physics Research Laboratory, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA.
¹⁰ Joint Astronomy Centre, Hilo, HI 96720, USA.
¹¹ Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, F-31400, Toulouse, France.
¹² European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany.
¹³ Space Telescope Science Institute, 3900 San Martin Drive, Baltimore, MD 21218, USA.
¹⁴ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA.
¹⁵ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125, Florence, Italy.
^*   To whom correspondence should be addressed.

   Present address: Laboratoire d'Astronomie Spatiale du CNRS, BP 8, 13376 Marseille, Cedex 12, France.

   Present address: Department of Astronomy, University of Maryland, College Park, MD 20742, USA.