Comet ISON is coming

Scientists predict the comet ISON will come within 1.2 million kilometers (730,000 miles) of Earth's light source on November 28 - giving us a once in a lifetime opportunity to see such an event.

"That's less than the diameter of the sun, so it's really close," says scientist Michael Khan of the European Space Agency (ESA).

While most sun-grazing comets are rather small, measuring only about 10 meters (32 feet) in diameter, ISON is more than a kilometer wide. This may make it hardy enough to escape the fragmentation that often occurs when smaller comets pass so close to the sun.

The "Great Comet" Ikeya-Seki had a similar size to ISON when it flew close to the sun in 1965 and survived.

Ikeya-Seki's larger size shielded its interior from the sun's extreme temperature.

If ISON survives without disintegrating, it will be exposed to the sun's surface temperature of around 2,000 degrees Celsuis, says Dr Werner Curdt, a senior scientist at the Max Planck Institute for Solar System Research in Germany.

Space researchers are hopeful that ISON will pass by unscathed.

Comet of the century

ISON was discovered by astronomers Vitali Neyski and Artyom Novichonok in September 2012. It was named after the telescope they used at the International Scientific Optical Network (ISON) - and dubbed the "comet of the century" for a good reason.

Unlike Halley's Comet, which is viewable from Earth every 75 to 76 years, hyperbolic comets like ISON will only ever be seen once.

ISON is not on an orbit. Khan says this means it will never come close to Earth again - instead it will shoot off into the greater realms of space.

The brief appearance of ISON within Earth's orbit, coupled with its close encounter with the sun, means researchers have just this one opportunity to gather as much scientific data from it as they can.

"We are going to have the Rosetta spacecraft observe it from close quarters," says Khan.

There are plans for a landing craft to be deployed to the Comet 67P/Churyumov-Gerasimenko - but not, it seems, for ISON.

Researchers want to learn more about the lifecycle of comets - about which, says Khan, we still know very little.

What scientists do know, however, is that comets are made up of a combination of material left over from the formation of our solar system 4.6 billion years ago, says Curdt.

"Everyone wants to know the composition of the material that formed the solar system," Curdt says. "Comets, like ISON, these new comets, are the remainder of material that was present when the solar system was formed."

ISON, it's thought, originated from the Oort Cloud - about one light-year from the sun.

And because comets spend much of their existence in cold conditions, they remain well preserved - giving scientists the opportunity to study what conditions were like at the time of the Big Bang.

It's a waiting game

It will pass the sun at an incredible speed, heating up as it goes.

But what exactly will happen "is anyone's guess," says Curdt.

"As it gets closer to the sun," says Curdt, "it's going to get brighter. The comet will change, it will lose a lot of material, a lot of water. But no one really knows what will happen to it because it's a new comet and we don't have very much experience with new comets, especially ones going so close to the sun."

Whether it gets burnt up because of thermal load, the intense heat, or it's drawn into the sun through gravitational pull, will be part of the waiting game for scientists, researchers and amateur astronomers.

Even how bright ISON will glow is guess work.

"There are guesses that say it may be brighter than Venus," says Curdt, "I've seen guesses that say it might be as bright as the moon. There are guesses that say it might disintegrate and not be visible to the naked eye at all."

If ISON survives its journey past the sun, scientists say it's possible that the tail of the comet will head back into space, giving avid space watchers - especially in the Northern Hemisphere - one final chance to see it as it disappears.

Either way, says Khan, it's going to be a cosmic display.

"It will be less spectacular after the encounter with the sun, but, if it stays intact, it will continue to be very bright and we will be able to see it in the early weeks of December."