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Tuesday, 1 August, 2000, 16:08 GMT 17:08 UK
Comet Linear fizzes away
Comet Linear Mark Kidger
Comet Linear: Its icy core is falling apart (Mark Kidger)
By BBC News Online science editor Dr David Whitehouse

Astronomers around the world are monitoring the break-up of comet Linear, which was recently a fine sight in binoculars.

It is believed that intense solar heating when the comet rounded the Sun triggered a massive disruption of the comet's fragile icy core.

Cometary splittings rarely ever lead to the rapid disappearance of a comet like this

Dr Mark Kidger
It is still bright enough to see through small telescopes, but experts think that the comet might disappear completely in a few days.

The break up of a bright comet is unusual but not unprecedented. In 1994, comet Shoemaker-Levy 9 broke up before it struck Jupiter.

"We have observed a few comets in the process of breaking up - comet West in 1976, comet Ikeya-Seki in 1965 and others - but never with so much detail as we're seeing in comet Linear," said Mark Kidger, an astronomer at the Instituto de Astrofisica de Canarias.

He added: "Cometary splittings rarely ever lead to the rapid disappearance of a comet like this - in fact, I don't know of another case."

Closest approach

Dr Kidger noticed that the comet was breaking up as he monitored a cloud of gas (called the coma) surrounding the comet's core using the Jacobus Kapteyn Telescope in the Canary Islands.

Comet Linear was discovered last September by an automated sky survey telescope and it made its closest approach (perihelion) to the Sun on 26 July.

"At perihelion, there are very rapid aspect changes as regions of the nucleus previously in shadow are suddenly subjected to intense heating," said Dr Kidger.

Shoemaker-Levy 9 HST
Shoemaker-Levy 9 broke up before it struck Jupiter
"This causes strong thermal stresses that may have been a primary cause of the comet's break-up."

But even on the day before perihelion, it was clear that the comet was in trouble as it approached within 114 million km (70 million miles) of the Sun.

"The very first images on July 25th were enough to show me that something odd was going on," said Dr Kidger. "The comet's inner coma was no longer teardrop-shaped. It had a shape like a short, fat cigar. My first thought was Shoemaker-Levy. It looked just like those first images of Comet Shoemaker-Levy 9 after it was discovered."

Meteor shower

Alerts were sent around the world to astronomers to look at the fragmenting Comet Linear.

But unlike Comet Shoemaker-Levy 9, which broke into many well-defined bright fragments, Linear seems to be dissolving into an amorphous haze of gas and dust.

"The small size of comet Linear and its exposure to solar radiation is causing a more complete and rapid dissolution than we saw in Shoemaker-Levy 9," said Brian Marsden of Harvard's Minor Planet Center in the US. "Comet Linear will probably have completely dispersed in a week or so."

Dr Kidger added: "The surface brightness of the innermost coma is fading fast."

Many well-known annual meteor showers, including the Perseids, are caused by dusty debris from comets burning up in the atmosphere of Earth.

However, because of this comet's orbit, there will be no "Linearid" meteor shower. When it finally disappears in a few days, this visitor from beyond the orbit of Neptune will be gone forever.

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17 Jul 00 | Sci/Tech
Comet visible in binoculars
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