The next great 'daytime' comet?

Mar. 29—Newly discovered comet C/2023 A3 (Tsuchinshan-ATLAS) is on its way into the inner solar system and already grabbing headlines as the next possible "Daytime Comet."

Early estimates say it may outshine the brightest stars in the sky and become visible to the naked eye during daylight hours for a brief time in October 2024.

Here is everything you need to know about the incoming comet, including when and where to look for it!

How Bright Will It Be?

That's the million-dollar question. Gideon van Buitenen, who keeps a website detailing the magnitudes of all visual comets, predicts that Tsuchinshan-ATLAS will be much brighter than the Green Comet (C/2022 E3 ZTF) and rival the brightest stars.

Tsuchinshan-ATLAS should peak at around magnitude 0 to 1 in the first half of October 2024. However, there's a chance that it will become even brighter, due to a unique effect called forward-scattering. Forward-scattering is a trick of perspective. For a brief window of time, the comet will be backlit, causing sunlight to reflect off its particles and toward Earth. This forward-scattering could boost the brightness to around magnitude -5. That would make it even brighter than Venus and visible in daylight — hence the term "Daytime Comet."

But, of course, it's much too soon to know if these grand predictions will come true. And that's if it even survives its closest approach to the Sun. However, scientists believe the coma (the cloud of gas that surrounds the head) is large, so odds are in our favor that it makes it past the Sun intact.

Keep in mind, though, the light of a comet is more spread out than a star's bright point. Also, the predicted light curve (how fast it becomes bright and then dims again) for Tsuchinshan-ATLAS is rather sharp, so our window to see it at its brightest is limited. By the end of October, it will already have dropped to magnitude 4.

Will You Need A Telescope?

Because we don't know yet how well the celestial object will perform, it's hard to know whether or not you'll need a telescope to see it. Current estimates, which are more than a year out, predict that all we'll need to see it is our eyes.

But if you want to follow it into late October 2024 and throughout November as it dims, a telescope will come in handy. Of course, the first step is always binoculars, which are easy to use and open up a bigger range of the sky to you. But if you've explored the sky with binoculars already, a simple Dobsonian reflector is a good choice. And the bigger the telescope (for example, a 6-inch versus a 3-inch), the more you can see, but also the higher the price.

In the meantime, you can brush up on your observing skills with the Moon and the planets as we wait for Tsuchinshan-ATLAS to arrive.

Where And When To Look

Casual observers may get their first good look in mid-September 2024. At that time, Tsuchinshan-ATLAS will be in the morning sky before sunrise and possibly around magnitude 3 or 4. Those south of the equator have the better view, as the comet will be in the constellation Sextans the Sextant.

For northerners, the head of the comet will be below the horizon, but the tail alone could be bright enough to spot. Some of you may remember Comet McNaught from 2007, which was so bright that — even though its head was below the horizon — viewers could still see the tail stretching above the horizon. Of course, it's much too soon to know if Tsuchinshan-ATLAS will put on a show as good as that.

By September 27, northerners might be able to catch the entire comet before sunrise. It passes into the constellation Leo the Lion on October 1, 2024. Around October 2, it will again be too close to the Sun to spot. That's because as the comet speeds out of the inner solar system, it will pass almost directly between Earth and the Sun.

But, never fear, the comet will return to visibility, this time in the evening sky for northern observers on October evenings. October 12 is a good date to start looking for it again, but this time in evening skies. Comets move fastest when closest to the Sun, so now Tsuchinshan-ATLAS has raced into Virgo. You'll also be able to spot Venus near the horizon to the comet's south. The comet moves from Virgo into Serpens Caput by October 18, and then into Ophiuchus by October 22. It will begin to dim rapidly by November.

Where Is The Comet Now?

At the time of this article's publishing (March 24, 2023) the comet is about seven astronomical units (AU) from the Sun. One AU equals the distance from Earth to the Sun. That means that — for the moment — it's somewhere between the orbits of Jupiter and Saturn. At its closest approach, it will be less than half an AU from the Sun.

Comet Tsuchinshan-ATLAS has been making its way toward the inner solar system for thousands of years. It's a long-period comet that takes approximately 80,000 Earth years to orbit the Sun once.

The comet's brisk speed of 180,610 miles per hour (290,664 km/h) will bring it to perihelion — its closest point to the Sun — on September 28, 2024. So, despite its zippy pace, we have more than a year yet before we'll get a good look at the comet.

Other Comets

Currently, we know of no other comets that will be within the range of naked-eye observing before Tsuchinshan-ATLAS's appearance in September 2024. But at any point, a new visitor could approach Earth and become a popular observing target.

By the way, the Green Comet of early 2023 was a bit of a curious nickname, because most comets, if they show any color at all, appear green. However, the green color is usually visible in photos and not so much with your eye alone. So, there's a good chance Tsuchinshan-ATLAS will also look like green as it gets closer to us.

Discovery And Naming

Tsuchinshan-ATLAS (C/2023 A3), was discovered independently by two different observatories. Observers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in South Africa discovered the comet on February 22, 2023. Then, observers at the Purple Mountain Observatory in China realized they had independently discovered the comet on images from January 9. Tsuchinshan is a translation of "Purple Mountain," thus the comet's official name of Tsuchinshan-ATLAS comes from the two observatories.

Neither of these observatories are new to making discoveries. Purple Mountain Observatory discovered at least six previous comets, not to mention 149 minor planets and more than 600 near-Earth objects. The ATLAS project has four telescopes, two in Hawaii, one in Chile, and one in South Africa, and collectively they have discovered 80 comets that bear the ATLAS name. ATLAS has also discovered 875 near-Earth objects and 14,372 supernovae.

Previous Daytime Comets

If Tsuchinshan-ATLAS does meet the wildest expectations and become bright enough to see in daylight, it will join an elite group. Over the last three-hundred-some years, only nine comets have been visible in daytime skies. The Great Comet of 1744 was bright enough to be visible in the daytime, and it sported six brilliant tails. The gigantic September comet of 1882 became nearly as bright as the sun's limb at perihelion before breaking into four pieces. And, more recently, for a brief time in 1965, observers of Comet Ikeya-Seki called it ten times brighter than the full Moon. However, at its brightest, the comet was right next to the sun.

Of course, it could also be like 2013's Comet ISON, which was hyped up to be as bright as the full Moon, but instead broke up near the sun and was never even visible to the unaided eye. Let's hope for a better future for Tsuchinshan-ATLAS!