Space and Astronomy News for the 25th of September 2025

There are a few comets in the sky at the moment. As this isn't a common event, it's probably a good time to chat again about these wonderful and often surprising visitors to the inner parts of the Solar System.

What are comets?

Comets are sometimes referred to as "dirty snowballs". They are a complex collection of dust, rocks, gases, water and other materials all held together under their own gravity.  They often have long, looping orbits and as they approach the Sun, energy received melts water ices and turns frozen CO2 into gas. These materials are then thrown away from the comet, lit up by the Sun and they become the spectacular astro objects we're so fond of. 

Where do comets come from?

There's a number of groups of comets in the Solar System. Some comets such as the famous Halley's Comet are referred to as periodic comets that are on a shorter and sometimes known orbit around the Sun. There's also a vast region on the outskirt of the Solar System called the Oort Cloud that could contain hundreds of billions if not trillions of comets. These were originally thought to have formed with the Solar System, but astronomers are also now considering that it might have captured bodies from other star systems.**

Occasionally, passing stars, interaction with planets or the galaxy itself will disturb one of these icy comets and send it towards the inner Solar System from the Oort Cloud.

What comets are above our heads at the moment?

The brightest of the bunch is probably Comet C/2025 R2 (SWAN). This was discovered less than two weeks ago from images taken by the SWAN spacecraft by Vladimir Bezugly.

Tonight, 25th of September 2025, it will visible very low on the horizon between the thin, crescent Moon and Mars.

Where to find Comet C/2025 R2 (SWAN) from Sydney tonight using the phone version of Stellarium. 

Using TheSkyLive for comet tracking.

C/2025 K1 (ATLAS) is a fainter comet, which will be also low in the sky tonight.

A screenshot of the Stellarium app showing where the faint Comet C/2025 K1 (ATLAS) will be tonight not long after sunset as seen from Sydney. 

This comet is visible in the sky tonight but will require binoculars and a clear view to the western horizon. 

Comet C/2025 A6 (Lemmon)

Another comet visible in our skies just as it's getting light. It will very, very hard to spot!

A screenshot of the Stellarium phone app showing where the faint Comet C/2025 A6 (Lemmon) will be just in the northeastern sky before sunrise as seen from Sydney. 

Is there any significance to this many comets being in the sky at the moment?

No, it's simply a coincidence.  There's always a lot of comets in the Solar System and even the odd visitor or two from other parts of the Milky Way.  While it's pretty rare for amateur astronomers to be able to capture images showing two comets in the same picture, there's nothing unique that's occurred to allow this happen.

Comet C/2025 R1 (SWAN) and Comet C/2025 K1 (ATLAS) capture nears Mars by Glenn Pickford on the 21st of September and posted to the BINTEL Society Facebook group. Check out his post here.

How do I track where and when to see theses comets from my location?

The easiest way is to use any one of a wide variety of apps or websites to make a personal viewing chart for your location.  Stellarium is an astronomy website as well as an Apple and Android app that we're quite fond of.  

Another option is TheSkyLive which is possibly a more complete guide to the night sky with "What's up tonight" sections for planets and special objects like these comets.  Again, this can be used both in a browser and as apps. There

Why do comets sometimes have more than one tail?

You might spot comets with two or more tail, emerging from it in different directions. These are from different stream of dust and other small particles being pushed away from the body of the comet that reflect sunlight and from gas streaming away which is ionised and lit up by energy received from the Sun. Tails can even become detached from the comet's body!

Do comets shoot across the sky?

No.

Comets move from night to night but so slowly you don't really notice them. If you see something in the night sky that suddenly appears and streaks across the sky, even if it does have a "tail", it's not a comet. What you are seeing is a meteor* which is a small piece of space rock or even a fragment of a spacecraft or rocket that's fallen to Earth and burned up in the atmosphere. Larger chunks can be quite bright and even make a sound as they move across the sky. These are sometimes called "fireballs" and will spark a search by keen meteorite hunters to recover a fallen piece of space rock.

Why are some comets green?

You might see some photos of comets that show their central core glowing a pale, ghostly green. Here's photos taken of Comet C/2025 R2 (SWAN) taken with a Seestar S50 Smart Telescope that shows this.

Photo of Comet C2025 R2 (SWAN) showing the green nucleus taken by Mike Kane with a ZWO Seestar S50 from his observatory in Port Macquarie. You can check out his full post here.

What was even more intriguing was that while the central nucleus of some comets glow green, their tails don't Despite many decades of head scratching, his has been something that astronomers hadn't been able to fully explain until a recent discovery by researchers at the University of NSW about a certain type of carbon molecule that's created and then quicky destroyed before it can migrate to the comet's tail.  You read about  green comets and their non-green tails at our BINTEL article from a few years ago here.  One thing to note is that while the green-ish colours are fairly easy to photograph, comets will only appear as shades of black, white and grey to your eyes through a telescope or binoculars.

Do other stars other than our Sun have comets?

Absolutely! We've already had at least two comets visit the Solar System from elsewhere in the Milky Way that have arrived in recent years. There's likely been an untold number in the past. With new, more powerful observatories coming online, astronomers expect to find many more. 

We know these comets originate from outside the Solar System because of their high speed. They're travelling too fast to be captured by the Sun's gravity and will continue through the Solar System and head into deep space. 

Comet 2I/Borisov was discovered in 2019. ("2I" meant it was the second interstellar interloper and the "Barisov" of the name it from its discoverer, Gennadiy Borisov.) It seemed to very similar in composition to other comets that have arrived from the Solar System's Oort Cloud.  Comet 2I/Borisov went through a large outburst as it made a close approach to the Sun and largely broke up.

Comet 3I/ATLAS is in the news at the moment. (Again, "3I" means it's the third interstellar visitor and "ATLAS" is the instrument that captured the images on which it was first found.) This new arrival might sneak up to be bright enough to see with your eyes along, however grab a pair of binoculars to make viewing it easier. 

Where to see Comet 3I/ATLAS, a comet from another star, tonight from Sydney in the western sky not long after dark. You'll need binoculars or a telescope to spot it!

It's also very old, possibly between 7.6 and even 14 billion years old. In comparison, the Sun, the planets around it and our Earth are about 4.6 billion years. It's certainly older than any comet we've seen before and more than likely the oldest object we've had this close to us. Due to its complex path and interaction with other stars over a truly vast length of time, we have no real chance of identifying of identifying the star it is originally from. 

Could comets bring life from elsewhere in the galaxy?

This is something that astronomers have wondered about for generations. With NASA announcing recently that there's a very high chance of life on Mars in ancient times that produced similar biosignatures to those found on Earth - more on that discovery at our blog here -  we could be facing a range of scenarios that involve life originating from outside of Earth and not only being transported to here but also to other planets. Comets could be one way that happened.  It's certainly something to ponder about when the clouds keep you away from your telescope eyepiece. 

Can we send a spacecraft out to meet Comet 3I/ATLAS to get a closer look before it leaves the Solar System?

No. 

Studies carried out in July 2025 showed that any spacecraft launched from Earth would need to travel at over 86,400 km/h to catch up with Comet 3I/ATLAS.  This significantly faster than any current rocket system humanity has managed to build. 

Saturn and Neptune in the same field of view for binoculars

It not only was it the opposition of Saturn a few days ago, but it was also the opposition of Neptune this week. We didn't make a big thing about it as outer planets such as Uranus and Neptune are hard to spot and their great distances means the variations seen at opposition aren't much.  This year Neptune and Saturn appear close together in the same part of the sky, making our outermost planet much easier to spot. 

Neptune will be visible all night in the constellation of Pisces and is only a couple of degrees away from the much brighter Saturn.

A chart showing where Neptune is located in the sky compared to Saturn in the coming weeks.  If you can find Saturn in binoculars or a telescope a couple of hours after sunset, use this guide to help locate Neptune. It appears as a pale blue star or small "dot" depending on the size of your telescope. The outer red circle marking is 4 degrees across, or less than the field of view of most binoculars. 

Sunrise at the South Pole: Spring Equinox 

You might have read in last week's BINTEL Newsletter that we had spring equinox on the 23rd of September 2025. This is the part of our path around the Sun each year when the Earth is aligned "upright" and all parts of our planet receives an equal length of day and night. It's also an extended "sunrise" after a long night at the South Pole and "sunset" at the North Pole.

Check out the post here.

Cheers,

Earl White 

BINTEL

25th September 2025

*A meteor is a piece of rock or dust that burns up in the Earth's atmosphere. A meteorite is a piece of space rock that is large or tough enough to survive this fiery journey and hit the Earth. You find meteors up in the sky and meteorites on the ground. And before you ask, I have no idea what you call a piece of space rock that hits the Earth and then bounces back up into the air.....

**Learning about comet formation and the outer Solar System is a complex and developing area of astronomy. I've only just touched on this topic here.