The ‘Christmas Comet’ (46p/Wirtanen) photographed when 15,800,000 km distant. Picture Flickr/Pepe Manteca
The ‘Christmas Comet’ (46p/Wirtanen) photographed when 15,800,000 km distant. Picture Flickr/Pepe Manteca

Here comes a Christmas comet spectacular

WE'RE in for a heavenly fireworks display this Christmas. First, we're going to get lit-up by a hail of meteors. Then a frosty green comet will cross our skies.

And Australia's in the box-seat for both.

The Geminid meteor shower, which peaks this Friday, is being billed as one of the most spectacular of the year.

And Comet 46P/Wirtanen is just emerging from its dive past the Sun on a path that will bring it to its closest point to the Earth in more than 70 years. On December 17 it will pass by at some 30 times the distance of the Moon.

THE CHRISTMAS COMET

You can already see Comet 46P/Wirtanen with a pair of binoculars. And it is expected to be visible to the naked eye at its closest approach - though you will have to get away from any city glare.

It's going to be a fuzzy green blob, as its tail will be pointing away from Earth for most of its pass. Except for Friday this week.

"There is a possibility of observing a dust tail or trail around December 14 when the Earth crosses the orbital plane of the comet and we see the enhanced dust tail edge-on," Australian comet-watch website Southern Comet reports.

It's green because the coma - the particle cloud of material ejected from the nucleus by the Sun's warmth - contains poisonous cyanogen (CN) and diatomic carbon (C2). When ionised by ultraviolet light from the Sun, these both fluoresce.

Historically, green comets are seen as good omens - which bucks the general trend of them being feared as messengers of ill fortune.

Comet Wirtanen orbits once every 5.4 years. But it only rarely passes close enough to Earth to be seen. This is its closest approach for some 70 years.

It was discovered in 1948 by US astronomer Carl Wirtanen. And, given its close pass, it's exciting astronomers once again.

It's a rare opportunity for ground and space-based telescopes to get a good, clear look at a comet's solid nucleus. Normally the primeval bundle of ice and rubble at the heart of comets are too far away and obscured by their halo.

The Hubble Space Telescope has been drafted for this look, as the Chandra X-ray Observatory.

"As for close comet encounters, there is nothing as good as Wirtanen predicted until 2061," says Auburn University astrophysicist Dennis Bodewits, who is leading the Hubble observation. He hopes to investigate the chemical processes which affect the comet's gas emissions.

'ROCK COMET' FIREWORKS

Most meteor showers are caused by the debris trailing behind a passing comet. But not the Geminids. It's actually the result of a disintegrating asteroid.

And that makes the Geminid meteor shower different. The meteors are denser than usual, causing them to be slowed more as they hit the atmosphere. They also tend to flare with brighter colours because of their mineral makeup. Some fragment into meteor streams.

They're called the Geminids as they appear to radiate out of the constellation Gemini. They will linger up until about December 17.

Astronomers are expecting some 100 to 120 meteors to flash across the night sky every hour at its peak Friday (though how many you actually see will depend on how much light pollution surrounds you).

Asteroid 3200 Phaethon appears to be only a loose assembly of rocks, dust and stuff. It's about 5km wide and some 4.6 billion years old.

Some astronomers believe it's actually the leftovers of a burnt-out comet, though this is yet to be confirmed. Generally, however, it's regarded to be a 'rock comet'.

The asteroid takes a beating as it passes very close to the Sun in its eccentric orbit. Some 750C of raw heat fractures its boulders and loosens clumps of dust. And these 'fall off' Phaethon only to follow it on its path as a tail.

This is what Earth is about to pass through.