Comet LINEAR: Breaking up isn't hard to do.

It took some 4 billion years for Comet LINEAR-S4 to leave its icy home far beyond Pluto and journey to the inner solar system. Alas, that trip, likely the comet's maiden voyage, appears to have become its last.

Astronomers had high hopes for the comet, discovered in September 1999 by the Lincoln Near-Earth Asteroid Research program in Socorro, N.M. At that time, the comet resided near the orbit of Jupiter and its brightness suggested that it would rank as a naked-eye spectacle when it made its closest approach to Earth late last month. The comit never became that bright. Moreover, Hubble Space Telescope images taken Aug. 5 reveal that it has broken into at least 10 pieces.

This is the first time that astronomers have viewed a cometary breakup in such detail, notes Harold A. Weaver of Johns Hopkins University in Baltimore.

A comet is a fragile amalgam of ice, dust, and rock. When it passes near the sun, radiation explosively vaporizes some of the ices. For LINEAR, that activity had dire consequences, says Weaver.

The comet's saga began heating up on July 5 when Hubble images caught the body in a sudden outburst, growing 1.5 times brighter in just 4 hours. The outburst blew off a piece of the comet's icy crust, like a cork popping off a champagne bottle. Hubble pictures recorded 2 days later show at least one house-sized fragment trailing the comet's nucleus. In capturing such images, "we lucked out completely," says Weaver.

In retrospect, he says, the outburst marked the beginning of LINEAR's end.

As telescopes around the world tracked the comet, astronomer Mark Kidger of the Instituto de Astrofisica de Canarias in the Canary Islands, Spain, first noticed that LINEAR had dimmed and its showy tail had all but disappeared. On July 25, while observing the comet with the Jacobus Kapteyn Telescope in the Canary Islands, he found that the inner shroud of dust was no longer teardrop-shaped. Instead, it resembled a fat cigar.

A day later, the comet was even more elongated. Because no discernible fragments were visible, Kidger began to suspect that the fading comet had explosively disintegrated. He reported his observations in a July 27 circular of the International Astronomical Union (IAU).

Intrigued, Weaver reprogrammed Hubble to take a second look. Had the comet vented all its ice, leaving nothing but a pile of dust? Hubble's sharp eye revealed that the comet indeed had broken apart into an armada of frozen fragments. Images taken Aug. 6 with the European Southern Observatory's Very Large Telescope in Cerro Paranal, Chile, have confirmed the finding.

LINEAR's fragility may be evidence that it is small, no more than 1 kilometer across, Weaver says. Venting gases shatter small comets more easily than large ones.

In a July 30 IAU circular, Zdenek Sekanina of NASA's Jet Propulsion Laboratory in Pasadena, Calif., suggests another reason why LINEAR broke apart so easily. It may be a fragment of a much larger comet, and such fragments tend to suddenly disintegrate.

Astronomers continue to monitor the comet's fate. At press time, new Hubble observations were set for Aug. 10.