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A comet coming in 2024 could outshine the stars

But before focusing on our latest comet discovery, I first want to mention that this month marks the 50th anniversary of the discovery of Comet Kohoutek. If you are of a certain age, you might cringe a little with the mere mention of that particular celestial object.

As has often been said, the only thing predictable about comets is their unpredictability. When Comet Kohoutek was discovered when still remarkably far from the sun — out near the orbit of Jupiter (though nowhere near the planet itself) — the inference was that it was a giant among comets that would become extremely brilliant. Brightness predictions ranged up to magnitude -10 (as bright as a first or last quarter moon). Dr. Brian Marsden at the Central Bureau for Astronomical Telegrams announced that this could be the "comet of the century." The mainstream media took him at his word and ballyhooed the approach of a comet so bright that it might even be visible in broad daylight. The world was prepared to witness a blazing celestial light show.

But Kohoutek turned out to be very ordinary as naked-eye comets go and far dimmer than predictions had originally suggested. Most people missed it entirely, partly because of light pollution and also because it was rather low to the horizon. The recriminations were nasty with astronomers and news media blaming each other and the public blaming both.

I decided to start off with this sad saga, because half a century later on social media, I'm already beginning to see announcements about a new comet that some are promoting that "could be bright in 2024." Well ... maybe it will, maybe it won't.

First identified as an asteroid

The comet in question is C/2023 A3 (Tsuchinshan–ATLAS), discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in South Africa on February 22nd. ATLAS is a robotic early warning system developed specifically for detecting near-Earth asteroids a few weeks to days before they might impact Earth.

Originally though to be an asteroid, it was later determined that the same object was photographed six weeks earlier by the Purple Mountain Observatory (Tsuchinshan) in the east of Nanjing, China. Later, images of it were captured on Dec. 22, by the Zwicky Transient Facility (ZTF) at Palomar Observatory. These images also revealed a very condensed coma and short tail which indicated that it was not an asteroid, but a comet.

Will it sizzle or fizzle?

When first discovered in the constellation Serpens, Comet Tsuchinshan–ATLAS was an exceedingly faint (18th-magnitude) object some 680 million miles (1.09 billion km) from the sun. But when it reaches its closest point to the sun (perihelion) on Sept. 27, 2024, that distance will have shrunk to 36 million miles (near the orbit of Mercury). Such an enormous change in solar distance would typically cause a comet to increase its intrinsic luminosity by 17 magnitudes. Furthermore, Tsuchinshan–ATLAS will pass moderately close to Earth about two weeks after perihelion.

But will this comet truly become a celestial showpiece or will it end up as a dud like Kohoutek was in 1973-74? The unpredictability of how a comet will appear or how bright it will be is no surprise to those who study these enigmatic objects. What we will ultimately see depends on many variables — the comet's orbit, the relative locations of the comet, Earth and sun, and of course the size and composition of that icy clumping of solar system rubble that forms the comet's nucleus, usually only a few kilometers across. Its dusty, rocky material and frozen gases are not unlike what comprises the rings of Saturn.

Potentially bright future

Astronomers have developed general formulas and models for measuring the brightness of comets based on the observed behaviors of literally hundreds of them for more than a century. But comets, like people, have their individual quirks.

If Comet Tsuchinshan–ATLAS lives up to its most optimistic expectations, it should put on its best show in 2024, between Oct. 12 — when it will pass closest to Earth at a distance of 44 million miles (71 million km) — and Oct. 19, when it will appear low in the west-southwest evening sky from 1 to 3 hours after sunset. During that time frame it could possibly appear as bright as a first or second magnitude star and might also show a significant tail in spite of low altitude and bright moonlight (full moon unfortunately occurs on Oct. 17).

So let me stress here that there is a lot of "potential" that Tsuchinshan–ATLAS can evolve into a bright comet. However, there is a major stumbling block in order for that to happen.

More likely a dud

Unfortunately, it would seem more likely that Tsuchinshan–ATLAS, like Kohoutek and other comets with a similar lineage (Cunningham in 1940-41, Austin in 1990 and ISON in 2013), could ultimately fizzle, because it is a "new" comet coming out of the Oort cloud, the spherical shell surrounding the rest of the solar system that consists of pieces of icy space debris that be as large as mountains. The Oort Cloud could contain billions or even trillions of objects.

The latest orbital computations show that Tsuchinshan–ATLAS has an eccentricity of 1.0002280, which means it is traveling in a parabolic orbit, coming directly out of the Oort cloud. So, it has never passed near the sun before. And that's bad news.

If the characteristics shown by other "Oort comets" also hold for Tsuchinshan–ATLAS, then its surface is likely coated with very volatile materials such as frozen nitrogen, carbon monoxide, and carbon dioxide. Such ices vaporize far from the sun, giving a distant comet a short-lived surge in brightness that in turn, raises unrealistic expectations.

Already been "around the block"

If, on the other hand, the latest calculations showed that Tsuchinshan–ATLAS was traveling in an elliptical orbit and returning to the sun from the distant past, its coating of highly volatile materials would have already been shed, and what we are now seeing is the true underlying level of activity. When Comet Hale-Bopp was still very far from the sun, orbital data showed that it was traveling in an elliptical orbit with a period of roughly 4200 years.

In other words, it had been here before, perhaps more than a few times. Which is why many astronomers confidently expected, nearly two years in advance, that Hale-Bopp was eventually going to evolve into a bright and spectacular object.

Sadly, we cannot be so confident about Tsuchinshan–ATLAS.

Exceptions to the rule

Fortunately, the "Oort cloud rule" is not absolute. There have been exceptions. In the spring of 1957, Comet Arend-Roland was a first-timer that turned out to be quite spectacular, becoming as bright as first magnitude and shedding a tail as long as 30 degrees and in addition, also producing a tail directed toward the sun measuring 15 degrees in length.

Another comet that travelled from the Oort cloud and put on a spectacular show was Comet McNaught in January 2007. It will long be remembered not only for unfurling a magnificently huge tail, but also becoming so bright as to be briefly visible during the daytime next to the sun.

So maybe there is still hope for Comet Tsuchinshan–ATLAS.

Late July 2024: "The wall"

For now, astronomers can only sit back and quietly monitor the comet's progress as it slowly approaches the sun and Earth. But when will we know with certainty that it will become a showpiece or just another faint, fuzzy comet?

The phrase "hit the wall" means encountering an obstacle that inhibits progress. In the past, "Oort comets" that have showed encouraging progress in terms of brightening, have seen their steady brightening suddenly slow after crossing the orbit of Mars. Like a celestial marathon runner, it's like the comet hitting a wall.

So far as Comet Tsuchinshan–ATLAS is concerned, it will not get to that point in its orbit until late July of 2024. If the comet has brightened in accordance to the formulas and models up to that point in time, it should then be shining at around 8th magnitude and readily visible in good binoculars and small telescopes.

If it then continues to steadily brighten beyond that time, then there's a good chance it will evolve into an eye-catching sight. But if its brightening trend suddenly slows, or even comes to halt, all bets for a good show are off.

So, that's the story. Until then, all we can do is wait and watch.

If you want to check out Comet Tsuchinshan–ATLAS next year, our guides for the best telescopes and best binoculars are a great place to start. And if you're looking to take stellar photos of the night sky, check out our guides on how to photograph the moon, or ones recommending the best cameras for astrophotography and best lenses for astrophotography.

Follow Pegasus Aerospace System on Twitter @systemaerospace. Follow us on Twitter @systemaerospace or Facebook, Linkedin and Instagram @pegasusaerospace.

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