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Comet brightens mysteriously by a factor of a million

A comet usually too faint to be seen with the naked eye has brightened by a factor of a million since Tuesday, suggesting its surface may have cracked open and expelled clouds of dust and gas. Astronomers are scrambling to observe the strange object, which is likely to fade in the coming days and weeks.

Comet 17P/Holmes, which orbits the Sun every seven years on a path that takes it from the distance of Jupiter's orbit to about twice that of Earth's, is usually 25,000 times too dim to be seen with the naked eye. But since 23 October, it has brightened by a million times and now resembles a bright yellow star.

"This is equivalent to the planet Saturn suddenly becoming as bright as the Full Moon," David Morrison, senior scientist at the NASA Astrobiology Institute in Moffett Field, California, US, wrote in an email newsletter.

Comets do sometimes show extreme changes in brightness. They are thought to occur when the Sun's heat vaporises newly exposed ice on the comet, blasting dust off its surface. Sunlight reflecting off the dust increases the comet's brightness.

Fresh ice

So it is possible that a crack has opened up on the comet's nucleus, which is less than about 3 kilometres across, exposing fresh ice to the Sun.

This is not the first time the comet has been caught brightening. In fact, it had brightened to become faintly visible to the naked eye when it was discovered by an observer named Edwin Holmes in 1892. It faded, then brightened again the following year before returning to obscurity.

Astronomers observed it during its close passes with the Sun in 1899 and 1906, then lost it again until 1964, when Brian Marsden, former director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US, correctly predicted its position.

"Since then, it's been behaving well - until now," says Marsden.

Comet Holmes can be found in the constellation Perseus.



Comet 17P/Holmes brightened by a factor of a million in about 36 hours (Image: Richard Hill/Loudon Observatory)


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