Star-gazers focus on sun-grazer

Comet Ison is moving at about 220,000 km/h. Photo / NASA

A visiting comet could disintegrate when it runs a scorching gauntlet past the sun, treating star-gazers to a flashy spectacle.

Astronomers are waiting to see whether Comet Ison will stay intact as it comes within potentially fatal distance of the sun for the first time in coming days.

At its closest point, next Friday, the so-called "sun-grazer" will be just 1.165 million km above the solar surface - a distance less than the sun's own diameter and "pretty darn close" in space terms, according to Stardome astronomer Dr Grant Christie.

The comet has travelled from a formation of cometary bodies nearly a light year away to make its first visit to the solar system's central region.

With no history to go by, scientists can't say whether it will be able to withstand the sun's blistering temperatures of up to 2800C.

If it does break up - a demise expected to be a gradual disintegration rather than a sudden explosion - the results could make for spectacular viewing.

Even if not, the comet has the potential to release material over the next few weeks as it heats up, brightening its profile in the sky.

Whether it endures the fly-by and continues on its orbit depends on the comet's size.

Its frozen nucleus is estimated to be between 400 and 1200 metres across which means that it should survive the close encounter.

But if it measured less than 400m across, it would be most likely to disintegrate.

Dr Christie said the power of the sun could first result in the comet's internal materials jetting out through cracks in its surface.

"If that process gets stronger, then what could happen is the whole object could slowly break apart and spread out.

"If you had a much faster release of material from the comet, you could get quite a good display ..."

Observations by the Hubble Space Telescope had also suggested the comet's nucleus was rotating, with one pole pointing at the sun.

If so, the pole facing away from the sun had not yet been subjected to much heating and its store of volatile material, such as frozen water and carbon dioxide, remained intact.

As the comet grew nearer to the sun, the release of a lot more volatile material could produce a burst in brightness.

Unfortunately, New Zealand was not well positioned to see the comet, which was getting closer to the top of the rising sun each day, Dr Christie said.

As it drew closer to the sun over the next week, it would become harder to see in the dawn light unless it underwent a jump in brightness.

"In order to see the comet, you'd have to be up at maybe 5.30am and have a really clear eastern horizon - preferably over the ocean."

If the comet developed a spectacular tail, it could also be visible on the western horizon soon after sunset.

Comet Ison

Approaching the sun and will reach its closest distance - just 1.165 million km above the solar surface - next Friday.

Its orbit shows it comes from the Oort Cloud, a reservoir of cometary bodies that orbit nearly a light year from the sun - between 30,000 and 60,000 times further than the Earth from the sun.

It will be the first time this comet has entered the central region of the solar system since the system was formed 4.5 billion years ago.

Comet Ison is moving at about 220,000 km/h.