After the disappointing Green Comet, astronomers have just discovered a new comet that has the potential to be next year's big story: C/2023 A3 (Tsuchinshan-ATLAS).
Although it is still more than 18 months away from its closest approach to Earth and the Sun, Comet Tsuchinshan-ATLAS is already buzzing on social media, with optimistic articles about how it could be a spectacular sight.
What's the full story on this new icy wanderer?
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Introduction of Comet C/2023 A3 (Tsuchinshan-ATLAS)
Every year, several dozen new comets are discovered: dirty snowballs that move on very elongated paths around the sun. The vast majority are far too faint to see with the naked eye. Perhaps one comet per year will approach the edge of naked-eye visibility.
However, every now and then a much brighter comet will pass by. Because comets are things of ephemeral and transient beauty, the discovery of a comet with potential always leads to excitement.
Comet C/2023 A3 (Tsuchinshan-ATLAS) certainly meets this requirement. The comet, discovered independently by astronomers from China's Purple Mountain Observatory and the Asteroid Terrestrial-impact Last Alert System, ATLAS, is currently between the orbits of Jupiter and Saturn, a billion kilometers from Earth. It falls inwards and moves into an orbit that will take it within 59 million kilometers of the Sun in September 2024.
The fact that the comet was found while so far away is one of the reasons for astronomers' excitement. Although the comet is currently about 60,000 times too faint to see with the naked eye is bright for something so far from the sun. And observations suggest it's on a trajectory that could make it truly spectacular.
A recipe for comet greatness
It all comes down to a combination of the comet's path through the solar system and the potential size of its nucleus - the solid center.
As comets get closer to the Sun, they become warmer and their surface ice becomes sublime (changes from a solid to a gas). This gas erupts from the comet's surface and carries dust with it, forcing the nucleus into what is called a coma - a giant cloud of gas and dust. The coma is then pushed away from the Sun by the solar wind, leaving a tail (or tails) pointing directly away from the Sun.
The closer a comet gets to the sun, the hotter its surface becomes and the more active it becomes. Historically, the vast majority of the brightest, most spectacular comets have followed paths that brought them closer to the Sun than Earth's orbit. The closer the better, and Tsuchinshan-ATLAS certainly meets those requirements.
In fact, this new comet seems to tick all the boxes. It appears to have a large core, making it brighter (bright enough to be discovered this far from the Sun). It is destined to have a very close encounter with our star. More importantly, it will then pass almost directly between Earth and the Sun, approaching within 45 million miles of us, just two weeks after perihelion (its closest approach to the Sun). The closer a comet comes to Earth, the brighter it will appear to us.
Put that together and you have a recipe for a comet that can shine as brightly as the brightest stars. Some forecasts are even more bullish, suggesting it could be a hundred times brighter!
The curse of prophecy
Predicting how newly discovered comets will behave is a dangerous game. Some can be spectacular, while others fizzle.
Take, for example, Comet Kohoutek in 1973. Like Tsuchinshan-ATLAS, Kohoutek was discovered unusually far from the Sun, orbiting close to our star. Pay attention to the hype. Astronomers promised the public "the comet of the century" and predicted that Kohoutek could become bright enough to be seen in broad daylight.
But comets are like cats. Kohoutek brightened as it turned toward the sun, but more slowly than expected. Instead of being visible in broad daylight, it was only as bright as the brightest stars, and faded quickly after perihelion. It was still a good show, but far from the comet of the century. Due to the hype, many called Kohoutek a spectacular disappointment.
It turns out that Kohoutek transited the inner solar system for the very first time. It had never come this close to the Sun, so its surface was rich in highly volatile ice that began sublimating while the comet was still far away. At that great distance, the comet was much brighter than other, more experienced comets - and that brightness suggested that the comet would be truly spectacular.
As the comet got closer to the Sun, these volatiles were depleted and the comet's final activity was less than initially predicted, making it weaker.
There is a very real chance that Tsuchinshan-ATLAS, like Comet Kohoutek, will make its first approach to the inner solar system. We don't know for sure yet, but if so, it could also be less spectacular than predicted.
Where everything falls apart
But it could be worse. Comets are prone to disaster. They fragment, disintegrate, and disintegrate surprisingly often. Those entering the inner solar system for the first time are especially vulnerable.
A recent example of such fragmentation was comet C/2020 F8 (SWAN). When SWAN was discovered, things looked promising; it would likely become a naked-eye object in May 2020. But as it approached the sun, it suddenly brightened, then became blurry and began to fade. By the time it should have been at its brightest, it had all but disappeared and disintegrated before our eyes.
On the other hand, fragmentation events can sometimes turn a good comet into a great one. Three years after Kohoutek came comet C/1975 V1 (West), and he used to be truly spectacular.
It got even closer to the Sun than Tsuchinshan-ATLAS will - and was already dazzling when its core broke into four pieces at perihelion. That fragmentation released a huge amount of gas and dust, and the comet brightened considerably, becoming visible even in broad daylight.
Will Tsuchinshan-ATLAS be worth the anticipation?
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We will only know for sure whether Comet Tsuchinshan-ATLAS will produce a spectacle. It may fall apart and become less clear, or it may surprise us.
It could light up more than expected - making for an amazing view of the morning sky in late September and early October 2024, and an even better view of the evening sky in mid-October 2024.
We just don't know. But we'll get our first hints in the coming months. By watching the comet brighten as it glides towards the sun, we'll get our first clues about its true fate - so keep your fingers crossed.
