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Big ‘Christmas comet’ on verge of destruction

AFP-JIJI

A comet that sky-gazers hope will provide one of the greatest celestial shows of the century is at risk of fizzling out.

So say astronomers tracking the eagerly awaited Comet ISON as it races toward its searing encounter with the sun.

Formally known as C/2012 S1 (ISON), the comet was spotted by a pair of hard-working amateur Russian astronomers, Vitaly Nevski and Artyom Novichonok, on Sept. 21, 2012.

It is called ISON because they used a telescope called the International Scientific Optical Network near Kislovodsk, in the northern Caucasus.

After the discovery was validated by the International Astronomical Union, interest in the enigmatic wanderer grew.

Calculations showed that after looping around the sun on America’s Thanksgiving Day, the comet would erupt in a blaze in glory toward the end of the year — earning it the tabloid titles of “Christmas comet” and “comet of the century.”

But fears are multiplying that the great show will be a no-show. Light signatures from ISON, which has just streaked past Mars, indicate it is about to break up, said Ignacio Ferrin, an astrophysicist at the University of Antioquia in Medellin, Colombia. “This disintegration will take place before it reaches perihelion,” where an orbit comes closest to the sun or a planet, Ferrin said. ISON is supposed to reach perihelion Nov. 28.

Comets brighten as they get closer to the sun and their icy surfaces evaporate into tails of gas and dust.

But Ferrin said the light curve from ISON has slowed and is now practically constant, meaning it is showing no signs of getting brighter as it progresses.

This is a signature that matches four previous comets that ended in catastrophic breakups, he said.

“Comets in general appear to be quite fragile, and are observed to fragment or split,” said Duncan Steel, a visiting astronomer at Armagh Observatory in Northern Ireland. “It has always been a good bet that ISON would do this.”

Comets are huge clusters of primeval dust and ice, including organic molecules that, say some, delivered the building blocks of life to the young Earth.

Doomed to orbit the sun in periods that can range from years to millenniums, comets undergo thermal stresses as they near the star.

Veterans that make short flybys, such as Halley’s Comet, have a crust of silicates and “tar” composed of carbon molecules that insulate them from the heat.

But rare visitors, such as ISON, have no such protection. Internal gases start to expand in the heat, stressing the crumbly “dirty snowball” structure.

Comets can also be torn apart by gravitational forces if they cross the path of a planet. This famously happened with Comet Shoemaker-Levy 9, whose fate was dictated by Jupiter. Fragments of the comet smashed into the giant planet in 1994.

As a sun-grazing comet at perihelion, ISON will be less than 1.2 million km from the sun’s surface — just three times the distance between the Earth and the moon — subjecting it to temperatures of 2,800 degrees C.

According to preliminary estimates by the Lowell Observatory and Southwest Research Institute in Arizona, ISON has a good chance of surviving its solar ordeal and the gravitational rip at perihelion. Comets smaller than 200 meters across are almost always destroyed when passing at such a distance, but ISON appears to be between 1,000 and 4,000 meters across.

What, though, will be left of ISON after its brush with the sun? Will enough remain for it to be a real comet? Or will it be just a sad, shriveled lump?

“We have absolutely no idea,” said Patrick Rocher of the Institute of Celestial Mechanics at the Paris Observatory.