The Great Comet of 2024?

Article October 15, 2024

Everyone is getting psyched for Comet C3/2023 Tsuchinshan-ATLAS as it makes its presence known in the October evening skies. I’m one of those folks getting psyched. Great comets don’t come along very frequently, after all. What do I mean by a “great” comet? What the heck is a comet anyway? And how can you spot this one before it’s gone? 

C/2023 A3 Tsuchinshan-ATLAS as seen on September 30th from Namibia through a 12” telescope.
C/2023 A3 Tsuchinshan-ATLAS as seen on September 30th from Namibia through a 12” telescope. Credit: Gerald Rehamann, Michael Jäger, Denis Möller

Let’s take a bit of a deep dive into the objects that astronomers like to call the cats of the solar system—they have long tails and trying to predict how they’re going to behave is a mug’s game.

 

Bad Omens?

For a long time, the appearance of a comet in the night sky was considered a Very Bad Thing. It probably meant someone important was about to kick it (apparently average folks don’t get celestial signposts when their ends are nigh) or some major disaster was about to occur.

Actually comets are small objects redolent with ices that originate in the outer solar system—some of them in the very outermost part of the solar system that there is. Way, way out there, beyond even where our telescopes can make out, is the Oort Cloud. This is a…well, a cloud of objects surrounding us in every direction. It’s unknown just how far out the Oort Cloud would extend, but guesses range anywhere from 1-3 light years out from the Sun. And yet objects in the Oort Cloud are still orbiting the Sun, controlled by its gravity. That makes them part of our solar system, even if it is the outermost boonies.

A diagram of the Oort Cloud.
A diagram of the Oort Cloud. Credit: NASA

These Oort Cloud Objects are essentially comets waiting to happen. Occasionally something—collisions, gravitational interactions, teenage aliens from Proxima Centauri pulling a prank (okay fine, I made that last one up)—will knock an Oort Cloud Object out of its extremely distant orbit and the gravity of the Sun will start to make it fall in towards the inner solar system. 

As you might guess from something that formed that far away from anything warm, these Oort Cloud Objects are icy. There’s rock and dust in there as well, but a lot of their makeup is frozen stuff. Water, yes, but also things like carbon dioxide, methane, and ammonia. As they fall in towards the Sun, though, they start to warm up.

When ices get warm enough in the void of space, they don’t turn to liquid the way ice melting on Earth does. You need air pressure for liquid to stay a liquid, something the vacuum of space is, by definition, lacking. Instead these ices undergo a process called “sublimation”, which means they turn directly from a solid to a gas. This can happen gently and gradually or quickly and  explosively, depending on the nature of the ice deposit, and the sublimating ices often blow dust off the surface of the Oort Cloud Object. 

The incoming Oort Cloud Object, therefore, starts leaving a trail of gas and dust behind it in its wake, like a long tail. This gets more and more pronounced as it comes farther and farther in towards the Sun. The sunlight begins to catch this tail and make it shine brightly. The Oort Cloud Object has become a comet.

Fun fact for you: comets are the reason we know the Oort Cloud is there at all. It’s so distant from its nearest light source, the Sun, that even our most powerful telescopes are incapable of resolving the presence of the Oort Cloud. But we know those comets are coming from somewhere.

It should also be noted that not all comets originate in the Oort Cloud. Some of them start out as Kuiper Belt objects like Pluto. In fact, one formation theory for Pluto is that it’s essentially a whole bunch of potential comets that decided to unionize.

 

Anatomy of a Comet 

The parts of a typical comet.
The parts of a typical comet. Credit: wisegeek

At the heart of the comet is the nucleus. This is the former Oort Cloud (or Kuiper Belt) Object. The streams of material being released from its surface sometimes gather around it, forming what might be considered a sort of thin, sad atmosphere. This is known as the comet’s coma. As long as the nucleus still has a steady supply of ices, this object will remain capable of producing a coma and a comet tail.

Diagram showing the orientation of a comet’s tails relative to the Sun.
Diagram showing the orientation of a comet’s tails relative to the Sun. Credit: NASA

The tail itself is complicated. For one thing, it’s usually more than one tail. The brightest one, the one most likely to be visible with the naked eye (if a comet gets that bright) is the dust tail. This is all those particles of material getting shot off the nucleus by the sublimating gases. This tail catches the Sun and looks white, or sometimes greenish white. It gets pushed by the solar wind and will always point roughly away from the Sun, but it’s also highly affected by the comet’s movement, so it frequently has a curve to it.

The sublimating gases will also form their own tail. This one is fainter, and usually looks blueish in color. These gases are ionized, so this tail is sometimes called the gas tail or the ion tail. Being ionized means the gas molecules have a charge to them, which means they’re very affected by the solar wind. The ion tail will always point directly away from the Sun. With a really good comet, one that gets nice and bright and visible in Earth’s skies, both tails may be visible. 

 

Okay, back to Tsuchinshan-ATLAS

The comet that has everyone’s attention was first discovered by Purple Mountain Observatory in China in January 2023 (Tsuchinshan is an Anglicization of the Chinese name for this site), though at the time nobody recognized it as a comet. It was next seen by the Asteroid Terrestrial-impact Last Alert System, which sounds somewhat alarming but mostly just means somebody really wanted the nickname ATLAS, in February 2023. When it became apparent that this was the same thing seen by Purple Mountain a month earlier, the comet got its official name.

It’s been inbound from the Oort Cloud for a long time. Like…many tens of thousands of years at least. Maybe millions. But then, that’s not necessarily unusual for something from the Oort Cloud. After all, that’s a lot of ground to cover.

Where Tsuchinshan-ATLAS can be found in the sky during October.
Where Tsuchinshan-ATLAS can be found in the sky during October. Credit: Sky & Telescope

Tsuchinshan-ATLAS made its closest approach to the Sun, known as perihelion, on September 27th. That’s a major milestone for a comet to get past. As you might imagine for something made largely of ice, getting close to a star can be risky. Many comets disintegrate at this point in their orbits. In fact, it was looking for a while there like we might get another potentially good comet this fall in C/2024 S1 ATLAS (yes, ATLAS again) but alas, it currently appears that the Sun is going to destroy this one before it has a chance to become great.

Not Tsuchinshan-ATLAS though. It survived perihelion and is now visible in the evening sky. As the days go on, it will move farther and farther from the Sun in Earth’s sky, making it easier to spot later. Of course, at the same time, the comet itself is getting farther and farther from the Sun (and from Earth—it made its closest approach to Earth on October 12th when it came within 44 million miles of us), so it will also be getting fainter. 

Basically, look south-southwest after sunset (the clearer your horizon, the better), and you’ll have your best shot at seeing our distant visitor. If you can manage it when the Moon isn’t in the sky, even better. And this might just be humanity’s only shot. Its outward path isn’t fully mapped yet, but it’s possible that Tsuchinshan-ATLAS’s path was altered enough by the gravity of the Sun to throw it out of the solar system entirely.

 

“Great” Comets

Some comets of the past have had the word “great” built into their very names, as long as they appeared before modern scientific naming conventions were in place. The last to receive this title was the Great Comet of 1910, though there have definitely been some worthy of the word since then. What makes a comet great is really a matter of opinion, but one thing is certain: it has to put on a good show.

I may be dating myself here, but I still remember the one-two punch of the great comets Hyakutake and Hale-Bopp in 1996 and 1997 (I was young enough at the time that I just assumed this meant great comets were a frequent occurrence). I was able to get some observations in of 2020’s Comet NEOWISE, which might be considered a great comet. But it’s always hard to predict when the next great comet will come along.

Will Tsuchinshan-ATLAS go down in history as a great comet? We can’t say for sure until it’s finished putting on its showcase, but all signs certainly look good! It will definitely be worth a trip outside after sunset in the next couple of weeks. Look to the west. You just might see something great.