Comet Linear, the brightest comet of the year and the target of astronomers around the world this week as it made its closest approach to the sun, appears to have disintegrated before their eyes.
Astronomer Mark Kidger of the Instituto de Astrofisica in the Canary Islands said the comet's glowing central coma began to grow larger and fainter Monday. By Thursday night, Linear no longer had any bright central nucleus, and its coma, a gaseous cloud around the nucleus, and its tail were fast disappearing.
Later observations seem to confirm that the comet is dying, said Brian Marsden, director of the Central Bureau for Astronomical Telegrams of the International Astronomical Union.
The bureau reported Kidger's finding to the scientific community late Thursday.
"You can still see it," Marsden said, "but it's been taken off life support, so it will not last very long. Give it a week."
Linear showed no signs that it had broken into a train of large fragments, as comet Shoemaker-Levy did in 1992. Those fragments went on to smash into Jupiter in 1994.
Instead, Linear looked as though it has blown apart in a cloud of space dust.
Astronomers around the world are zeroing in on Linear this weekend to see whether there's anything left of its nucleus.
If it's really gone, it would be a sharp disappointment to Hal Weaver, an astronomer and research scientist at the Johns Hopkins University.
Weaver observed Linear with the Hubble Space Telescope from July 5 to 7 and published photos of the comet as it briefly brightened and threw off a large piece.
He is scheduled to fly to Hawaii next week to observe the comet again, using the NASA Infrared Telescope Facility on Mauna Kea.
"A lot of other people have plans to observe the comet as well," he said. "I'm very anxious to hear what the results will be in the next couple of nights."
If it's really gone, he said, "I guess I'll just go down to the beach."
But "I'm skeptical the comet has completely dissolved away," he said. Although Linear and other comets have been seen to break up or throw off hunks of themselves, he said, "I don't remember a case where a nucleus dissolved away to nothing but dust."
If Linear has done that, Marsden said, it would suggest that it is an unusually small and fragile comet.
Comets are composed mostly of dust particles loosely held together by water and carbon dioxide ices. As they approach the sun, the ices turn to gas and fly into space, releasing some of the dust particles.
Some of the liberated gas and dust becomes visible as a halo or coma. The rest is swept away into a long tail by the solar wind.
Marsden said observations have suggested that Linear was on its first visit to the inner solar system. Its early brightness and odd trajectory suggest that it was spewing a lot of its ices into space in the form of gas jets.
Eventually, as it neared the sun and encountered higher heat and gravitational forces, he said, "it would lose all its ice and remaining dust until there was nothing to hold it together anymore."
Astronomers had high hopes for Linear when it was discovered in September by scientists at the Lincoln Near Earth Asteroid Research (Linear) project at the Massachusetts Institute of Technology.
It seemed fairly bright then, considering that it was farther from the sun than the orbit of Jupiter. There was even talk that it might become a naked-eye comet this week, as it came within 35 million miles of Earth and 72 million miles of the sun.
It never did, although observers with binoculars and small telescopes could see it in recent weeks as it slowly passed by the North Star and the Big Dipper in the northern sky.