Newly discovered Comet C/2023 A3 might reach naked-eye brightness when it flies past Earth in 2024. Check out our forecast of what to expect to see in the months ahead.
Recent Comet ZTF (C/2022 E3) captured the public's interest in part because of social media. While many may have been disappointed at not seeing a bright green apparition in the heavens, the comet met or exceeded expectations in amateur astronomers' eyes. With multiple tails, a peak magnitude of 5 and scenic pairings with Capella and Mars, it inspired us to "jacket up" and brave cold winter nights so we could see it for ourselves. Anytime there's excitement about a celestial object, no matter its magnitude or the hype that frequently surrounds it these days, amateurs can use the moment as an opportunity to educate and connect people to the sky.
Barring new discoveries or exceptional outbursts from predicted comet arrivals this year, Comet ZTF will likely stand as brightest we'll see until Comet 12P/Pons-Brooks graces the sky next spring at around magnitude 5. But it's a newly discovered comet — Comet Tsuchinshan-ATLAS (C/2023 A3) — that's been getting the most buzz in recent weeks. It has the potential to become a naked-eye object when it passes near Earth in October 2024. Estimates of its peak brightness range from magnitude 1 (or better depending how dusty it is — more on that in a moment) to magnitude 4, similar to Comet ZTF.
Predicting cometary behavior is always fraught with uncertainty. Of course, that's what makes comets so compelling to watch. Amateur and professional astronomers alike are carefully monitoring C/2023 A3 to better forecast its future behavior.
Astronomers at China's Purple Mountain Observatory initially reported the object as a 19th magnitude asteroid on January 9th. After no follow-up observations were logged that month, it was removed from the objects-awaiting-confirmation list and considered lost. Then on Feb. 22, the Asteroid Terrestrial-impact Last Alert System (ATLAS) robotic survey independently rediscovered the object. Orbital calculations confirmed a match to the Chinese asteroid. Later, prediscovery images by the Zwicky Transient Facility from December 2022 revealed a strongly condensed coma and 10-arcsecond tail, branding it a comet.
Astronomers use a standard power-law formula and accumulated observations to predict a comet's future brightness. Right now, in mid-March, C/2023 A3 is a faint, 18th-magnitude pip in the constellation Serpens, located 973 million kilometers (6.5 a.u.) from Earth. Based on what we know up until now, amateurs may get their first peek of the Oort Cloud visitor by this time next year. At an expecteed magnitude of 13.5 to 14, it will be inching westward across northern Libra in the morning sky in early March. From late March through mid-June, it will travel through galaxy-rich Virgo, with a predicted brightening to about magnitude 10. Around the time of the summer solstice, a 6-inch scope under dark skies should pin it down.
By mid-July 2024, the comet (now around magnitude 9) will fade from sight in the glow of evening twilight from mid-northern latitudes. Southern hemisphere observers will hold onto the apparition for another month before dusk claims it in mid-August. Like a jack-in-the-box toy, the visitor pops back into the morning sky (southern observers only) from mid to late September before tumbling back sunward into the dawn light. Closest approach to Earth occurs on October 12, 2024, at a distance of 70.7 million kilometers (43.9 million miles). Several days after, C/2023 A3 swings back into the evening sky.
Meanwhile, Northern Hemisphere skywatchers lose sight of the comet for more than two months (mid-July to late September in 2024) except for a brief, low-altitude appearance at dawn at September's end. Then it turns back into the evening sky around the date of closest approach and quickly frees itself from the solar glare. By October 14, 2024, we'll see it in a dark sky at nightfall. Well, almost dark. The waxing gibbous-to-full Moon will compromise the view until about October 19th.
You can see the problem here. C/2023 A3 will be at very small solar elongations for two weeks on either side of perihelion, the very time it reaches a peak brightness of around magnitude 1. The double whammy of low altitude and twilight will test the mettle of even experienced observers.
But all is not lost. Although our fuzzy friend will be fading by mid-October 2024 — likely hovering around magnitude 3 at that time — it rapidly ascends the southwestern sky. Binoculars and telescopes should provide wonderful views of the object as it speeds across Serpens Caput into Ophiuchus.
What other factors might affect the comet's brightness? Dynamically new comets — those arriving for the first time from the Oort Cloud — often display lots of activity at great distances. Even without significant heating from the Sun, subsurface carbon monoxide ice can vaporize and release dust to form a coma. Such comets often flare up and raise expectations only to falter and fade as they approach perihelion. Comet C/2023 A3 appears to be a dynamically new comet but contrary to expectations has exhibited little activity — so far. That doesn't mean it won't do so, but for the moment it's something of a head-scratcher.
On the other hand, there's potentially good news that could transform C/2023 A3 into something special.
"If this comet is dusty and develops a significant dust tail — which tends to be best after perihelion — then we could have some rather significant forward-scattering enhancement around that time," says comet expert Alan Hale. "On October 9th (according to the most recent orbit), the comet’s phase angle will be as high as 173°."
That would place it nearly in line with the Sun and provide just the back-lighting needed to pump up its magnitude. To understand forward scattering, imagine yourself driving behind a truck down a dusty road in the direction of a low Sun. Clouds of dust kicked up by the truck's tires scatter so much sunlight into your eyes, it's nearly impossible to see the road ahead. Forward scattering played a major role in the spectacular appearance of Comet McNaught (C/2006 P1) and remains a tantalizing possibility with Comet Tsuchinshan-ATLAS.
There's still much to learn about the C/2023 A3's orbit, activity level and dust content before we can sharpen our predictions. Even then I suspect the comet will do exactly as it pleases.