Comet C/2023 A3
A comet is coming to our sky this October.
And it might – just might – be bright enough to see without a telescope.
Comet C/2023 A3 Tsuchinshan- ATLAS is on its way!
Comet: NASA
A Comet
A comet is a kind of space iceberg.
In astronomical terms, they are quite small, around 10 km across
Comet Halley nucleus: ESA, Giotto
Astronomer Fred Whipple called comets ‘dirty snowballs’.
And so they are.
They are made of ice containing rocks and covered in dust.
When comets drift towards the sun, the ice begins to sublime.
It changes straight from solid to gas.
The Sun’s radiation pushes the gas away to make the comet’s tail.
At the same time, dust pours from the comet surface.
This heavier dust forms a separate tail of particles.
So a comet passing the Sun often has two distinct tails.
Comet Hale-Bopp, showing gas and dust tails.
ESO, E Slawick
Oort Cloud
Comet C/ 2023 A3 has come from the Oort Cloud.
The Oort Cloud is a huge spherical area surrounding our Solar System.
Oort Cloud: ESA
It spreads from half a light year to two light years from the Sun.
So it stretches to halfway to the nearest star.
The cloud represents the outer limits of our Solar System.
At this huge distance, the gravitational hold of the Sun is feeble.
Comets can easily be dislodged by other gravitational influences.
They can sometimes be pulled inward towards the Sun.
The cloud contains millions of comets.
October’s comet may have taken a million years to travel from the Oort Cloud to us.
Comet Names
Comet C/2023 A3 Tsuchinshan-ATLAS is not a catchy name.
So how do comets get their names?
The nice thing is that comets are named after their discoverer.
Comet Hale-Bopp, which was bright in 1997, was named after two co-discoverers.
They were Americans Alan Hale and Thomas Bopp.
Many comets are now discovered by teams of astronomers.
Then the comet bears the name of the organisation they work for.
So Comet IRAS-Araki-Alcott was discovered independently by three observers.
They were Genichi Araki, George Alcott and the team working with the Infra Red Astronomy Satellite, IRAS.
Comet IRAS-Araki-Alcott
Comets also have a designation that shows the date of discovery.
So Comet C/2023 A3 was discovered in January, 2023.
The C/ prefix indicates that it will not return in the forseeable future.
Periodic comets, which return regularly, have the prefix P/.
So the name Comet C/2023 A3 Tsuchinsan-ATLAS tells a story.
It was discovered in January 2023.
It will not be seen again for hundreds of years.
The comet was discovered at the Chinese Tsuchinshan Observatory.
At the same time it was found by the scarily-named Asteroid Terrestrial-impact Last Alert System (ATLAS).
ATLAS is based in South Africa.
An exception to these naming rules is the most famous comet of all.
Halley’s Comet was not discovered by Edmond Halley.
In the 18th Century, Halley realised that three comets seen in the past were actually the same comet returning periodically.
He predicted the comet would return. It did.
Halley’s Comet, 1986
But sadly Halley did not live to see his prediction come true.
Comet Halley reappeared in 1758 but Halley died in 1742.
How Bright?
So how bright will C/2023 be?
The brightness of comets as they pass by Earth is notoriously difficult to predict.
Back in 1973, Comet Kohoutek was previewed as ‘The comet of the century’.
Some predictions gave it a brightness surpassing any star or planet.
In reality Kohoutek failed to live up to its billing.
Ever since, astronomers have been reluctant to label any coming comet as extraordinary.
We saw the last notable comet in 2020
Comet NEOWISE was discovered by the (Near Earth Object) Wide-Angle Infrared Survey Explorer.
When NEOWISE emerged from the Sun’s glare, we saw it glowing above the north-east horizon.
Comet NEOWISE
It could be that Comet C/2003 A3 puts on a similar show.
It may even be brighter – or not!
We’ll have to wait, hope and see.
Comet C/2023 A3, 70 minute time lapse in May 2024.
Cpayoub, CC0, via Wikimedia Commons
In May, C/2003 A3 was photographed approaching the Sun.
The image shows it moving against the starry background.
Trajectory of C/2023
On September 27th the comet rounded the Sun at a distance of 60 million km.
We’ll be able to see it as it moves away from the Sun during October.
At it’s closest to Earth, on October 12, the comet will be around 70 million km away.
Where and when to look
Basically, Comet C/2003 will be best seen in second half of October.
The likely observing window is from October 14th to the end of the month.
During this fortnight the comet will climb higher in the sky.
Unfortunately it will also become dimmer as it moves away from us.
The time to look is after sunset.
Look to the west, in the direction of sunset.
As the sky darkens, you will see Venus shining brightly low down.
To the right is the orange star Arcturus.
Comet C/2023, October 14th after sunset.
Stellarium / Dennis Ashton
On October 14th, Comet C/2023 will lie between Venus and Arcturus.
You will need a low western horizon, the comet is low in the evening sky.
Comet C/2023, October 22nd after sunset.
Stellarium / Dennis Ashton
By 22nd October the comet will be higher in the sky above Venus.
How long or bright the tail will be we don’t yet know.
Latest News
Comet C2023 A3, New Zealand
As I write this blog, there is good news from the Southern hemisphere.
The comet is already putting in an appearance in Australia, New Zealand and South America.
Comet 2023 A3, through telescope
The tail is growing and becoming brighter as it approaches the Sun.
Let’s hope that continues and that a cosmic comet show is on the way!
The author: Dennis Ashton is a Fellow of the Royal Astronomical Society and a Wonderdome presenter.
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