Like many astronomers around the world, Michael Mattiazzo is anxiously watching the sky right now.
The veteran comet chaser is monitoring the progress of Comet Leonard as it streaks towards the Sun.
The small comet is not yet visible in the southern hemisphere, but if it survives the next seven days, it could put on a nice show in Australian skies for Christmas.
But as the saying goes, comets are like cats: they both have tails and they do precisely what they want.
“[Comets] are difficult to predict and that’s what makes them so interesting,” said Mr Mattiazzo, an amateur astronomer from Victoria who runs the Southern Comet Homepage.
“They can sometimes surprise you and they can often disappoint you.”
And right now, C/2021 AI Comet Leonard is threatening to break apart.
“It’s hanging by a thread,” said Mr Mattiazzo, who has been observing comets since 1986 and has discovered several of these astronomical objects.
“Given my observing experience, I know that this comet is slightly in trouble because it’s at the borderline of whether it will survive its approach to the Sun or not.
“It’s a day-to-day wait.”
Where did Comet Leonard come from?
Comet Leonard was discovered on January 3 this year by US astronomer Greg Leonard.
At about just 1 kilometre across, the dirty snowball – or snowy dirtball – hails from way out in the icy reaches of our Solar System, astronomer Jonti Horner said.
“It’s a lump of ice and rubble that is left over from the formation of the Solar System 4.5 billion years ago.’
The comet moves on a highly elliptical orbit that takes it out about 3,700 times further from the Sun than we are.
Whizzing along at speeds of up to 70 kilometres per second, it has probably taken tens of thousands of years to reach us.
“The last time it came through the inner Solar System was probably 80,000 years ago,” Professor Horner said.
The comet is predicted to swing around the Sun on January 3, 2022 – if it makes it that far.
“One good thing is the Earth approach happens before the Sun approach,” Mr Mattiazzo said.
As a comet gets closer to the Sun, it heats up and can break apart.
“If it ends up [breaking apart] after its Sun approach, that’s fine.”
When can we see it?
If the comet hangs together, we’ll see it in the skies above Australia from December 17 onwards.
The comet is currently visible to the naked eye in the early morning from dark-sky sites in the northern hemisphere.
Images Mr Mattiazzo has remotely taken using a US telescope show the comet has a halo of blue gas and a straight dust tail.
“When you see a flattened head on a comet, you know it’s a small object, so it’s going to battle to win the solar encounter,” Mr Mattiazzo said.
It will sweep past Earth on December 12.
As it comes closer – flying past about 35 million kilometres away – it will get brighter but drop from view in the northern hemisphere.
“The northerners will lose out, they will only see it up to about December 11,” Mr Mattiazzo said.
Then the southern hemisphere will get the prime view.
“The fun will start when it is visible from December 17, that’s when I’ll be out every night trying to capture it with a telescope and a camera,” Mr Mattiazzo said.
On December 18, the comet has a close encounter with Venus, coming to within just 4 million kilometres of our closest planetary neighbour.
That will make it the closest approach of a comet to Venus in recorded history, Professor Horner said.
“That means from the point of view of Venus, it would be really, really spectacular,” he said.
It also means we will be able to use Venus as a guide to find it in the sky.
You’ll see comet to the left of Venus low above the western horizon on December 17 and 18, once the sky gets dark enough after twilight.
Then each night after that it gets higher and higher in the sky.
By Christmas night it should be relatively high, Mr Mattiazzo said.
“It should be 25 degrees above the horizon and also there will be a beautiful alignment of the planets with Venus, Saturn and Jupiter.”
What will it look like?
We don’t know yet, but as far as comets go, it’s not predicted to be as spectacular as Comet Lovejoy, the great Christmas comet of 2011.
“For a comet to reach great status, it needs to be very bright. We usually get one of those every 10 years,” Mr Mattiazzo said.
But if it stays together it could reach a magnitude that will look like a fuzzy blob to the naked eye, or clearer in binoculars.
“Even though it will be potentially naked-eye visible, I would strongly recommend people use a pair of binoculars to observe it and a camera to photograph it,” Mr Mattiazzo said.
If we are lucky we may see a bright tail as sunlight reflects off the dust trail stretching away from the Sun towards us, in what is known as forward scattering.
“It’s like an aeroplane contrail at sunset,” Mr Mattiazzo said.
And the comet could put on a good show even if it does start to break up.
“All the dust in the comet will be in that forward-scatter position, so it would be very well lit up by the Sun,” Mr Mattiazzo said.
“Even if it does break up, it may be a fascinating sight.”
But first, it has to survive the next week.
abc.net, 9 December 2021