Although comet LINEAR held promise of greatness when discovered last September, astronomers' hopes faded as it failed to brighten as much as anticipated. But now, LINEAR has come through with a spectacular display. In what may be its death throes, astronomers say the "dirty snowball" at the heart of LINEAR seems to have suddenly fallen apart. If true, it would be another reminder of how fragile the solar system's icy building blocks really are.
In recent weeks, there's been "a lot of evidence something was going awry with this comet," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who runs the international notification service for astronomers. As LINEAR approached the sun and got warmer, observations showed that jets of water vapor and other gases unleashed from the ice were pushing it off course. That, Marsden says, implied LINEAR was a relative lightweight--perhaps a kilometer or so across--and thus particularly vulnerable to the erosive heat of the sun. And at least twice--on 5 and 20 July--LINEAR suffered outbursts in which it threw off large parts of itself, including a big chunk seen drifting down the tail a few days later.
So Marsden was not surprised when astronomer Mark Kidger of the Instituto de Astrofísica de Canarias on La Palma, in the Canary Islands, sent word that he had witnessed what seemed to be the demise of LINEAR. Last Sunday, using a 1-meter telescope, Kidger still saw a typical-looking comet--a bright, slightly flattened ball of dust with a tail streaming away. The dirty snowball or nucleus was shrouded in the brightest part of the head. Over the next 4 nights, Kidger saw the comet's head become more elongated and fainter, until it resembled the famous "squashed comet" Shoemaker-Levy, which on close inspection consisted of a string of fragments. By yesterday, the comet seemed to have disintegrated completely; Kidger couldn't even see surviving fragments.
Astronomers are divided on LINEAR's fate. "As Mark Twain put it, I have a feeling LINEAR's demise has been exaggerated," says cometary astronomer Harold Weaver of Johns Hopkins University, who detected the first outburst using the Hubble Space Telescope. Weaver thinks Kidger may just have witnessed another, larger outburst that still left the comet's nucleus intact. But Marsden thinks LINEAR may be gone for good; whatever is left of the comet is probably going to disperse over the next week, he believes. "We've had other comets like this, such as comet Bennett of 1974, that got more and more diffuse over a couple of days," says Marsden. Astronomers should know within a few days whether LINEAR, too, has fallen apart.