Comet Brightens Mysteriously By A Factor Of A Million

Comet brightens mysteriously by a factor of a million 22:04 25 October 2007
NewScientist.com news service
Maggie McKee


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. "I have never before seen anything like it."

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.
[link to space.newscientist.com]