Comet McNaught in the skies over Norway. The Comet should be visible in Australian skies from tonight.

Credit: NASA/Roger Johansen

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SYDNEY: The brightest comet seen in Australian skies in over 40 years should be visible from tonight, according to astronomers.

Comet McNaught is named after astronomer Robert McNaught, who discovered the comet in August 2006 at Australia's Siding Spring Observatory near Coonabarabran in northwestern New South Wales. It should be visible to observers at the latitude of Sydney, low on the western horizon after sunset - at around 8:30pm.

The comet reached its closest approach to the Sun on Friday, and has begun heading back out into deep space. It approached the Sun from above Earth's orbital plane, and was visible in the skies over the Northern Hemisphere until November last year, when it neared the Sun and became lost in the glare to naked-eye observers.

The comet will be visible again tonight on the other side of the Sun. But according to James Briggs of the Perth Observatory in Western Australia, "it won't appear as bright as it actually is because it appears close to the Sun in bright twilight." However, he added, "that it appears bright in these conditions means that it is very bright."

A guide to finding Comet McNaught in the skies over the southern states of Australia throughout this week. Tonight, January 15th, the comet will be visible just above Mercury. Image: Steve Quirk

According to scientists, comets are balls of ice and rock made of debris left over from the formation of the outer planets some 4.6 billion years ago. When deep in space, they are dark-coloured and nearly invisible, but, as they fall sunward, solar radiation heats them and causes jets of vapourised ice to shoot out from below the surface.

The solar wind, a stream of charged particles constantly emanating from our Sun, pushes the vapourised matter into a long tail behind the comet. As a comet moves closer to the Sun, more ice is vapourised, and the comet's tail becomes longer and more pronounced. No matter what direction the comet is moving, the solar wind orients the tail directly away from the Sun.

Most comets do not get close enough to the Sun for them to be seen without telescopes; only once every few years can a comet be seen with the unaided eye. Astronomers originally thought that Comet McNaught would be visible only to keen amateur stargazers with telescopes, but said Briggs, "it's about 200 times brighter than we thought it would be."

Scientists divide comets into two types: 'short period' and 'long period'. According to the U.S. space agency NASA, short period comets originate in the Kupier Belt, a region beyond the orbit of Neptune, and orbit the Sun more often than once every 200 years. Perhaps the best known short period comet is Halley's Comet, which visits the Sun once every 76 years, and was last visible in 1986.

Long period comets such as McNaught, according to NASA, are much more unpredictable, taking up to 30 million years to complete one orbit of the Sun. They originate in the Oort Cloud, a vast region which stretches out 100,000 times the distance from the Sun to the Earth. Scientists think that up to a trillion comets may be lurking in the Oort Cloud, where the gravitational influence of passing stars occasionally causes one to fall sunwards.

According to Briggs, McNaught is significantly brighter than either Halley or Hale-Bopp, and at its height will outshine Venus by two or three times. Australians have about one week to view the comet with the naked eye, he said. After that, it should still be visible with a small telecope for a further two weeks.