A bright ‘Sun-skirting’ comet will grace southern skies this week. Here’s how to see it
- Written by Jonti Horner, Professor (Astrophysics), University of Southern Queensland
Hot on the heels of a spectacular comet late last year, another celestial visitor is set to put on a show. And Southern Hemisphere observers have a ringside seat.
Comet C/2024 G3 (ATLAS) has just experienced a very close encounter with the Sun, and will become visible low in the western sky after sunset in the coming days. With luck, it will prove to be a spectacular sight.
It is often said that comets are like cats – their behaviour is notoriously hard to predict. So while astronomers have been awaiting the arrival of C/2024 G3 (ATLAS) for several months, they have been cautious of raising expectations too high.
Over the past couple of days, the comet has swung past the Sun, and survived its close encounter intact. The time has come for it to emerge to our evening skies.
A Sun-skirting snowball
At the heart of comet C/2024 G3 (ATLAS) is a dirty snowball or “cometary nucleus” only a few hundred metres to a couple of kilometres across. When it was discovered, in April 2024, it was still more than 600 million kilometres from Earth and the Sun.
In the days following the comet’s discovery, astronomers worked out its orbit – and it turned out to be a “Sun-skirter” that would pass less than 14 million kilometres from our star on January 13.
As a comet approaches the Sun, it begins to heat up. The ice on and near the surface “sublimes”, causing jets of gas and dust to erupt from the nucleus’ surface. This shrouds the nucleus in a vast cloud of gas and dust, which is then blown away from the Sun, creating the comet’s tails.
Comets that get particularly close to the Sun can become spectacular. This is what happened with comet C/2006 P1 (McNaught), which put on an incredible show for observers in the Southern Hemisphere in early 2007.
For that reason, astronomers often get excited when a new comet is found that will pass particularly close to the Sun. However, comets are also notoriously fragile and friable. Many small comets on Sun-skirting orbits simply fall apart in a puff of dust and disappointment.
When comet C/2024 G3 (ATLAS) was discovered it was very faint, which suggested its nucleus is relatively small. As a result, it seemed likely to fall apart during its close approach to the Sun.
One piece of evidence gave astronomers hope. The comet’s orbit indicated it had passed this way at least once before – and survived.
Comet C/2024 G3 (ATLAS) putting on a show as it passes through the field of view of NASA’s SOHO spacecraft’s LASCO/C3 camera.
NASA/SOHO
Now the time has come – as I write this, the comet has passed its closest approach to the Sun intact. It has already put on an incredible show as the third brightest comet ever observed by NASA’s space-based solar observatory SOHO, and even been imaged by keen astrophotographers in broad daylight. Over the coming days, it will move into the evening sky as it moves away from the Sun.
A bright comet, low in the west after sunset
Comet C/2024 G3 (ATLAS) will begin to appear in the evening sky, low to the west-southwest. For observers in the Northern Hemisphere, it will remain lost in the Sun’s glare. But for those south of the equator, it has the potential to be easily visible for the next few evenings, as the sky darkens after the Sun sets.
Unfortunately, the comet’s orbit is also taking it away from Earth, so it will fade relatively quickly. At its closest to the Sun, the comet was almost as bright as the planet Venus (currently a spectacular sight in the western evening sky). Within a couple of weeks, the comet will cease to be visible to the naked eye.
What does that mean? If you want to get the best possible view, you need to seize your chance in the next few days. Each evening, the comet will set later, and be farther from the Sun in the sky. But it will also be fainter from one night to the next.
So when, and where, should I look?
If you want to see comet C/2024 G3 (ATLAS), you’re going to need to find somewhere with a clear, unobstructed view of the horizon slightly to the south of due west. Using a planetarium app or website like Stellarium will allow you to work out what time the Sun and the comet will set from your location, so you can plan your observations.
The videos below show the comet’s location a little after sunset at three different latitudes (around Cairns, Brisbane and Melbourne/Auckland) over the evenings from January 16 to 23.
Note the position of the comet in these videos is just the location of the comet’s head. The tail will rise upwards from the comet into the sky, leaning a bit to the right. The videos also don’t give a real feel for how bright the comet will be – you’ll just have to go out and see.
The location of comet C/2024 G3 (ATLAS) 35 minutes after sunset for observers at a latitude of 17 degrees S (equivalent to Cairns, Australia). The location of comet C/2024 G3 (ATLAS) 35 minutes after sunset for observers at a latitude of 27 degrees S (equivalent to Brisbane, Australia). The location of comet C/2024 G3 (ATLAS) 40 minutes after sunset for observers at a latitude of 37 degrees S (equivalent to Melbourne, Australia or Auckland, Aotearoa New Zealand).Get to your chosen viewing location around sunset, to maximise your chances of spotting the comet. As soon as the Sun is below the horizon, you can start scanning for the comet with binoculars, your camera, or the naked eye.
At first, the sky will be too bright, but the comet should eventually reveal itself, low to the horizon, before it sets.
From my own experience with comet C/2023 A3 (Tsuchinshan-ATLAS) last year, seeing a comet low in the twilight sky can be challenging. I found it was easy to spot the comet through my camera. Once found, I could find it with the unaided eye.
The time-lapse video I shot, below, shows that comet rising in the morning sky in early October 2024. You can see how the comet gets easier to see the darker the sky becomes. For the new comet C/2024 G3 (ATLAS), imagine the video running backwards, with the comet setting as the sky becomes darker, to get a feel for how the comet might look.
Comet C/2023 A3 (Tsuchinshan-ATLAS) rising in the east from south-east Queensland, in October 2024.Of course it is hard to say just how bright and easy to spot comet C/2024 G3 (ATLAS) will be.
With each night that passes, the comet will get higher in the sky, and so in theory be easier to spot. It will be dimming, however, so the best view will be had during the next week.
Authors: Jonti Horner, Professor (Astrophysics), University of Southern Queensland
