Extract from ABC News
C/2023 A3 (Tsuchinshan–ATLAS) is set to become brighter in the early morning sky over the next few days. (Supplied: John Drummond)
In short:
A brighter-than-expected comet discovered back in 2023 is due to hit its closest point to Earth this week.
The comet can currently be seen just before sunrise with the naked eye as a small blob near the eastern horizon.
What's next?
Comets are extremely hard to predict, which means the comet may brighten or fizzle in the coming days and weeks.
A comet which has brightened southern skies more than expected in recent weeks might be the most impressive one this year, according to astronomers.
The comet, called C/2023 A3 Tsuchinshan-ATLAS (or Tsuchinshan-ATLAS for short) will make its closest flyby of the Sun on Friday, getting closer to the centre of the Solar System than the orbit of Venus.
The comet is already visible to the naked eye as a small blob near the eastern horizon just before sunrise, and has been regularly snapped by Southern Hemisphere astrophotographers over the past few weeks.
Plus, while comets are known to be fickle, astronomers say it could get even brighter. If it stays on its current course and doesn't break up, it is shaping up to be as bright as Jupiter or Mars during early mornings — and later twilight — over the next month.
Comet Tsuchinshan-ATLAS started from somewhere inside the Oort cloud, a ring of icy debris on the edge of the Solar System.
Discovered early in 2023, the comet has been slowly making its way into the inner Solar System, says University of Queensland astronomer Jonti Horner.
"It was found unusually far from the Sun," Professor Horner says.
"So that was an indication this was a promising comet."
How to see it
In Australia, comet Tsuchinshan-ATLAS is currently only visible just before sunrise in the morning sky.
To see it over the next few days, look east and low on the horizon at around 4:30am local time.
A star viewer showing the location of Comet ATLAS at 4.30AM September 27 from Sydney (Credit: Stellarium/ABC Science)
In the southern hemisphere, it will most likely continue getting brighter and higher in the morning sky until September 27.
At this point it will reach perihelion — the closest point to the Sun — where it will gradually become obscured by the Sun and much harder to see.
Being close to the Sun is both a blessing and a curse for those trying to catch a glimpse of the comet.
An orbit that gets relatively close to the Sun is important for comets to have a bright nucleus (the main bulk of the comet) and large tail. But it will be very tricky to see because it is close where the Sun appears in our sky.
If it survives its trip around the Sun it will become easier to see in the evening sky from October 14 as it travels away from us.
To see it then, look low towards the horizon in the west, immediately after sunset.
A star viewer showing the location of Comet ATLAS at 5.40PM October 14 from Sydney. (Credit: Stellarium/ABC Science)
How bright will it get?
At this stage, astronomers are estimating the comet will reach a brightness of around +2.5 magnitude.
Magnitude is a logarithmic measurement of brightness used by astronomers, and +2.5 is about as bright as the third brightest star in the Southern Cross.
However, the comet might be even brighter due to a phenomenon called "forward scattering", where light is amplified through dust and ice crystals "scattering" the light towards the viewer.
This is the same phenomena that causes a halo of light around the Sun after a rainstorm, or highlights the faint outer rings of Saturn, as seen in the incredible photo below.
An eclipse of Saturn, taken by the Cassini orbiter showing the forward scattering effect. (NASA/JPL/Space Science Institute)
This forward scattering effect could brighten the comet up from around +2.5 to -2.7 at it's peak, which is as bright as Jupiter or Mars.
"If the alignment is just right, and the comet is giving off lots of dust, then this forward scattering can make the comet brighten between 10 and 100 times," Professor Horner says.
The scattering effect made comets like McNaught — an extremely bright comet that peaked in 2007 — visible during the day.
And there may be a chance of catching comet Tsuchinshan-ATLAS during the day around October 9.
But if you try, it's a best to use a star tracker app (so you know where to look in the sky) and do not look directly at the Sun.
To get a glimpse of it, Professor Horner recommends placing a building (or another object) in front of your view to block the Sun.
History of comets
Astronomers are usually reluctant to get publicly excited about a comet before it arrives.
John Drummond, an astronomer who researches historical comets at the University of Southern Queensland, says this is because comets can change so quickly, sometimes breaking up or failing to become as bright as expected.
When they're over hyped, it can lead to disappointment, says Mr Drummond, a Royal Astronomical Society fellow.
"Everyone gets out, and it just turns out to be a fizzer."
Will C/2023 A3 (Tsuchinshan–ATLAS) survive its trip around the Sun or break up as it gets closer? (Credit: Juan lacruz/Wikimedia/CC BY 4.0)
This can even lead to other highly promising comets not being talked about before they happen. Similar to auroras, comet activity is something that can only be properly understood at short notice.
Professor Horner agrees there has been plenty of other comets that have ultimately disappointed stargazers or fizzled out.
"Maybe the best example in the last decade is comet C/2012 S1 (ISON)," he says.
"At one point, people were predicting it could be the comet of the century, brighter than the full Moon.
"But it never really got bright enough to easily spot with the naked eye … breaking up as it got closer to the Sun."
This unpredictability is what makes comets so exciting for both professional and amateur astronomers, and has been for more than 100 years.
The promise of seeing something awe-inspiring has led to moments of wonder and mishap, says Mr Drummond, including on the morning of Halley's comet in 1910.
"People were getting up before five o'clock in the morning, going outside in the cold to have a look," he says.
"They later realised they'd locked themselves out of their house and they were banging on the door and waking up all the neighbours."
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