Comet ISON vanishes as it circles the sun

The large block of ice and rock had been expected to skim just 730,000 miles (1.17 million kilometers) above the sun's surface around 1830 GMT.

It was estimated that ISON would undergo temperatures of 4,900 degrees Fahrenheit (2,700 Celsius) and lose three million tonnes of its mass per second as it made its journey around the sun.

Most astronomers had predicted that ISON would not survive the trip.

Several solar observatories watched the comet during its closest approach to the sun, known as perihelion.

And the comet became faint while still within view of NASA's Solar Terrestrial Relations Observatory, and the joint European Space Agency and NASA's Solar and Heliospheric Observatory.

NASA's Solar Dynamics Observatory, however, could not see the comet.

"It does seem that comet ISON probably has not survived its journey," Naval Research Laboratory comet scientist Karl Battams said after looking at space images.

"I am not seeing anything that emerges from behind the solar disk and that I think could be the nail in the coffin," he told a roundtable organized by the US space agency NASA.

Phil Plait of the Bad Astronomy blog agreed, saying he had a "strong suspicion that ISON may be an ex-comet."

Carey Lisse, a senior research scientist at Johns Hopkins Applied Physics Laboratory, compared the comet to a "loose snow ball," a rather weak and easily broken up with half or a third of its mass coming from water.

ISON is also about half the size of an average comet, with an estimated maximum diameter of 0.75 miles (1.2 kilometers).

ISON has galvanized astronomers since its discovery by a Russian team in September 2012 because it traces its origins to the start of the solar system some 4.5 billion years ago.

Several million years ago, ISON escaped from the Oort cloud, a grouping of debris halfway between the sun and the next closest star.

"This is a very dynamic situation. We have never seen a comet like this coming from the Oort cloud and going in the sun grazing orbit," said Battams.

Lisse called it a "dinosaur bone" of the solar system's formation, stressing the key role of comets in building planets.

"This particular comet is of extreme interest to us," NASA planetary science director James Green said, explaining that it is rare to find comets that come from so far away in the solar system.

Because ISON was detected from far in the solar system, astronomers had time to observe it. They found that the core of the comet was surrounded by a cloud of carbon dioxide.

"It is looking like carbon dioxide may be a very fundamental molecule in that early solar system rather than carbon monoxide," said Lisse.

As ISON approached the sun, the fluctuations of its tail showed usually invisible solar wind patterns made of particles constantly ejected by the sun.