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Comet Holmes and the case of the Disappearing Tail

But what's a Comet, anyway?

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The Web is abuzz about Comet Holmes.

Comet 17P/Holmes had been expected to make just another routine ultra low visibility departure from perihelion passage this year. But in a record for a comet, it unexpectedly brightened by a factor of around a million on October 24, making it more reminiscent of a stellar nova explosion.

"Amateur astronomers the world over have been stunned and amazed by the weirdest new object to appear in the sky in memory," wrote Sky and Telescope .

"The comet shocked skywatchers as it went from a dim 17th magnitude then suddenly to 3 magnitude" wrote Theo from the Pacific Northwest.*

Holmes brightened from below the threshold of binoculars to being clearly visible with the unaided eye. The outburst presented as a circular fuzzy patch in the constellation of Perseus, and has proceeded to grow in size. The comet was then reported to have spawned a tail.

More drama was to come.

Spaceweather currently reports that "last Friday, astronomers watched in amazement as the comet's tail broke off!".

And Comet Holmes may yet make further news, should this be a prelude to the full-on fissioning of the comet's core into several pieces. The comet is expected to remain visible to the naked eye for some weeks.

Why did Comet Holmes flare up?

Yet Comet Holmes has brightened up unexpectedly on a previous occasion: that of November 1892 led to its discovery by E. Holmes in England. It was subsequently determined to be a periodic comet, with a revolutionary period of about every seven years. You can see its orbit in relation to other planets courtesy of Larry's animation here.

Much less, though, has been made of the fact of where it was when the current flare-up occurred. The Armagh Observatory's website puts the comet at some 230m miles from the Sun - a distance from, and quite a long time after, its closest point to the Sun, which occurred on May 4, 2007.

In the accepted view, comets are thought to comprise rocky and icy solids in a mixture termed "rocky snowballs" by some and "icy rockballs" by others. They get brighter and sprout tails as the "icy" component is thought to sublimate from solid to gaseous state under the influence of solar heating, which typically begins affecting them at a point just inside the orbit of Jupiter. This gives rise initially to an expanding light-reflecting aura known as the coma, which is then "blown away" in the opposite direction from the sun by the solar "wind".

Multiple repeated thermally-induced mechanical stresses are thought to give rise to a cometary aging process, whereby a large single core disintegrates after multiple closer solar encounters into much smaller fragments. When such orbiting debris intersects the Earth's orbit, it can give rise to meteor showers.

Professionals, however, are well aware of exceptions.

Comet Wirtanen fragmented in 1957 when it was just inside the orbit of Saturn. Conversely, in December 1680, the eponymous comet studied by Halley and Newton passed intact within 100,000km of the Sun.

Things took a more serious turn, as reported in 1990 by New Scientist.

The object named Chiron, discovered in 1977 as an asteroid orbiting between Jupiter and Neptune, had begun to "look like a gigantic comet".

Forget for a moment the stuff comets are made of - here, the very categories of astronomy were under attack.

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