In the next two weeks, a recently discovered comet will almost certainly become bright enough to see without optical aid; just your eyes and a dark site will suffice. It might even briefly brighten so much that you’ll be able to see it during the day.

Or it might not. Comets are irritating that way. As Canadian comet hunter David Levy once quipped, “Comets are like cats: they have tails, and they do precisely what they want.”

The object in question is called C/2023 A3 (Tsuchinshan-ATLAS)—or just 2023 A3, to save typing. Astronomers discovered it in January 2023 as an asteroidlike dot in images of the sky taken at the Purple Mountain Observatory in China. It was very faint and in subsequent weeks wasn’t seen again, so it was presumed lost.

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But the following month other astronomers spotted it again, this time in images from the automated Asteroid Terrestrial-Impact Last Alert System (ATLAS), a NASA-funded collection of telescopes that scan the sky looking for uncataloged celestial objects that change brightness or position. In those images, the target could be seen to have a faint tail—the formal hallmark of a comet. With its cometary status confirmed, the object was given its official name, giving credit to both observatories. (Comets discovered by the Purple Mountain Observatory are given the traditional name Tsuchinshan.)

As soon as astronomers gauged its orbit, they could see that 2023 A3 was unusual. The shape of an object’s orbit is specified by a characteristic called its eccentricity, or deviation from a circle. Something on a circular orbit has an eccentricity of 0. An object with an eccentricity greater than 0 but less than 1 has an elliptical orbit. The higher the value, the more elongated the ellipse. If the object is moving too rapidly to be bound to the sun by gravity, it can escape the solar system; that gives it an eccentricity of 1 (an orbital shape of a parabola) or higher (a hyperbola). C/2023 A3 has an eccentricity of 1.000137, which is just barely hyperbolic.

This most likely means 2023 A3 got its start in the deep darkness well beyond Neptune’s orbit, in a region called the Oort Cloud. This roughly spherical region has a huge volume, and it may contain trillions (yes, trillions with a “t”) of small bodies made of rock and ice. These objects are old, having condensed from the cold material far out from the sun as the solar system itself formed billions of years ago. And not much else seems to have happened out there ever since, making them pristine time capsules from that long-gone era.

These Oort Cloud objects are on incredibly long orbits, taking tens of thousands of years for a single pass around the sun. They may have an eccentricity just under 1, but as they travel sunward, the gravity of the outer gas giant planets can tweak them just enough to give them a hyperbolic orbit.

That’s likely the case for 2023 A3. The solid part of a comet, called the nucleus, is a mix of rock, dust and ices (usually frozen water, carbon dioxide and other molecules). As it approaches the inner solar system, though, those ices turn to gas, creating a fuzzy atmosphere around the nucleus called the coma. The effects from solar wind and pressure from sunlight increase as it gets closer to the sun, and that material is blown back to create its long dust tail and ion tail—voilà, a bona fide comet!

Comets that we see coming in from the Oort Cloud are usually on their first passage through the inner solar system and can be very fragile. They can calve (split apart) or even disintegrate entirely, like the comet C/2019 Y4 (ATLAS) did in 2020. When that happens, the breakup usually releases a copious amount of dust. That appeared to happen with 2023 A3, which was ejecting more dust than gas early in 2024, but the dust production has since declined—a happy sign that the comet is still managing to hold itself together.

The real question is how bright 2023 A3 will get—and when. The comet reaches perihelion, its closest approach to the sun, today, September 27, when it will be just 60 million kilometers out. That’s roughly the same distance from our star as Mercury, so the comet will be getting a big blast of heat and light. Because of our viewing geometry, it will be in Earth’s early-morning sky at that time, albeit very low on the horizon.

Over the next week, as it rounds the sun, the comet will change its position in the sky, eventually becoming an evening object around October 9 or 10. It reaches perigee, or its closest approach to Earth, a couple of days later on October 12, when it will be 71 million kilometers away from us.

Here’s the fun part, with a big caveat: Dust in the comet’s coma and tail tends to forward scatter light, meaning it sends light that hits it ahead at a shallow angle, like a rock skipping on water. That amplifies the comet’s brightness when it’s between us and the sun. Some estimates put it at a magnitude of –5.0, which is brighter than Venus at that time!

By this point in its orbit, however, 2023 A3 will be so close to the sun in the sky that it will be only up during the day. That will make it tough to see, of course: the glare of the sun is mighty and can drown out even extraordinarily bright objects. Moreover, it’s very dangerous to look near the sun.

Rather than risk severe retinal burns, I’d recommend waiting just a few days. As the comet moves out in its orbit and starts heading back into deep space, it should still be visible to the unaided eye for many days in the west after sunset. It may even become the most visually spectacular comet since 2020’s C/2020 F3 (NEOWISE).

But that’s very iffy. How bright 2023 A3 will be is anyone’s guess. Some projections put its maximum brightness at magnitude 2.0 to 3.0, or about as bright as the stars in the Big Dipper. If it has an outburst caused by fragmentation, however, it could become much brighter. Or it might sputter out and halt its production of dust and gas, which would make it fainter than expected. Such fizzles happen from time to time, as in the case of Comet Kahoutek in the 1970s.

Betting on bright comets is an optimist’s game. While we can measure their positions with great accuracy, how bright they’ll get is a crapshoot. The best thing you can do is go out and look for yourself! If you do—and try to look for 2023 A3 when it’s up after sunset—binoculars are your best friend. You can find maps on many websites, including Sky & Telescope, Stellarium and The Sky Live.

Mind you, the hyperbolic nature of 2023 A3’s orbit means that once it rounds the sun, it will never return. This is literally your only chance to see it, so I hope you have clear skies and a good view. It may be some time before we get another comet so promising.