Breakup of a comet.

Last September, something strange happened to a comet called Schwassmann-Wachmann 3. It became 1,000 times brighter. By October, skywatchers could even have detected it with the naked eye.

Simultaneously observing the icy body with two telescopes on the same Chilean mountaintop, astronomers have now found evidence of even more dramatic activity. The comet appears to have broken into three or four pieces. In infrared images taken with the European Southern Observatory's 3.6-meter telescope in La Silla, Hans U. Kaufl discerned three separate blobs. Five hundred meters away, Hermann Boehnhardt monitored the body in visible light with the observatory's New Technology Telescope. He also saw three blobs, two of which coincide with those seen in the infrared.

Astronomers Boehnhard of the Ludwig Maximilian University in Munich, and Kaufl, of the European Southern Observatory in nearby Garching, Germany, report their findings in a Dec. 13, 1995, circular of the International Astronomical Union. They made the observations in mid-December and estimate that the fragments were moving apart at the rate of 250 kilometers a day.

On Dec. 27, using the Spacewatch telescope atop Kitt Peak near Tucson, James V. Scotti of the University of Arizona in Tucson confirmed that the comet had split. On Dec. 28 and 29, astronomers in Modria, Slovakia, observed the fragments.

If Schwassmann-Wachmann 3 broke apart as early as September, it would account for the comet's brighter appearance, notes Brian G. Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass. Sunlight striking the exposed surfaces of a recently fragmented comet causes it to expel large amounts of gas and dust. The more dust a comet spews, the more light it reflects and the brighter it appears. Indeed, notes Marsden, if another comet, Shoemaker-Levy 9, hadn't fragmented into some 20 pieces, it might not have been visible when it smashed into Jupiter in July 1994 (SN: 12/17/94, p. 412).

Schwassmann-Wachmann 3, however, may not have shattered until after the September brightening, says Kaufl. Images taken in early December by other astronomers at La Silla do not show fragments; instead, they indicate an elongated nucleus. The images suggest that the nucleus of the comet split up no earlier than mid-November.

Kaufl speculates that although the breakup occurred after the comet brightened, the two events have a common explanation. Cracks and rifts on the icy nucleus may have widened in September, when the comet passed closest to the sun. The widening cracks would have not only released greater amounts of gas and dust, making the comet brighter, but also set the stage for later fragmentation, he suggests.

Regardless of when the fragmentation took place, it attests to the fragility of comets, notes Marsden. Over the years, astronomers have detected about 30 comets whose nuclei have split, leading some planetary scientists to theorize that each of these icy bodies consists of a loosely held assembly of house-sized snowballs (SN: 5/7/94, p. 298).