An ancient celestial traveler, designated C/2023 A3 Tsuchinshan-ATLAS, is set to make its first close pass by Earth in mid-October 2023. This Oort Cloud comet, discovered in 2023, is approaching the inner solar system on a highly elliptical orbit, marking a significant event in documented human history. It was identified by astronomers at China’s Tsuchinshan Observatory, also known as “Purple Mountain,” and an ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope in South Africa. The comet has been named to honor both observatories, but don’t miss this rare opportunity— it won’t return for another 80,000 years.
The comet successfully made its closest approach to the Sun on September 27, defying expectations that it might break apart due to its volatile and icy composition. Instead, it emerged largely intact and is now set to come within approximately 44 million miles of Earth on October 12. This close encounter offers a unique opportunity for observation and study as it continues its journey through the solar system.
 |
| Comets traveling through the inner solar system are not uncommon, but many do not survive a close encounter with the Sun. For instance, the icy comet ISON, photographed on November 19, 2013, reached solar perihelion later that month, only to disintegrate under the intense heat and gravitational forces near Earth’s parent star. Such dramatic fates highlight the challenges comets face as they approach the Sun. |
Comets are more fragile than people may realize, thanks to the effects of passing close to the Sun on their internal water ice and volatiles such as carbon monoxide and carbon dioxide, said NASA astronomer Bill Cooke, who leads the Meteoroid Environment Office at NASA’s Marshall Space Flight Center. He noted that Comet Kohoutek, which reached the inner solar system in 1973, disintegrated during its close pass to the Sun, and Comet ISON met a similar fate in 2013.
While Comet Tsuchinshan-ATLAS will be ideally positioned for viewing from the Southern Hemisphere, observers in the Northern Hemisphere should also have a good chance to see it. Peak visibility is expected on October 9-10, as the half-moon moves away. Cooke suggests finding a dark location just after nightfall, looking to the southwest about 10 degrees above the horizon, where the constellations Sagittarius and Scorpio can be identified. The comet should be visible between them. By October 14, it may also be seen midway between the bright star Arcturus and the planet Venus.
“And savor the view,” Cooke advised, as this ancient traveler will be gone again by early November, not to return for another 800 centuries.
Comet light, comet bright: it’s unlikely that Tsuchinshan-ATLAS will be visible during daylight hours, except possibly at twilight, according to Cooke. In the past 300 years, only nine comets have been bright enough to be seen during the day, with the most recent being Comet West in 1976 and, under ideal conditions, Comet Hale-Bopp in 1997.
The brightness of comets is measured using the same scale as stars, which dates back to around 150 B.C. when it was developed by the ancient scholar Hipparchus and later refined by Ptolemy. Stellar magnitude operates on a logarithmic scale, meaning a magnitude 1 star is 100 times brighter than a magnitude 6 star. The lower the number, the brighter the object, enhancing its visibility to both telescopes and the naked eye.
 |
| Comets with long, elliptical orbits, like Lovejoy (C/2014 Q2), which reached perihelion in 2015 and won’t return until 2633, are rarely seen more than once in a lifetime. Comet Tsuchinshan-ATLAS, approaching Earth on October 12, won’t re-enter the inner solar system for another 80,000 years. |
Typically, a comet would have to reach a magnitude of –6 to –10 to be seen in daylight, said Cooke. “That’s extremely rare.” Current estimates place Tsuchinshan-ATLAS’s brightness between 2 and 4. For context, the brightest visible star in the night sky, Sirius, has a magnitude of –1.46, while Venus can reach –4, and the International Space Station may achieve a brightness of –6.
Cooke emphasized that comets can be unpredictable due to their extended nature, with brightness often spread out and dimmer than their magnitude suggests. However, they can benefit from “forward scattering,” a phenomenon that causes sunlight to reflect more intensely off the gas and debris in the comet’s tail and coma, creating a brighter appearance for observers.
If there is a lot of forward scattering, the comet could be as bright as magnitude –1,Cooke said, potentially making it visible to the unaided eye and truly spectacular through binoculars or a small telescope.
As for Comet Tsuchinshan-ATLAS, Cooke noted that while it’s not expected to approach the gas giants of our solar system closely, it could eventually be ejected from the solar system due to the gravitational influences of other planets and its tenuous connection with the Sun. However, this resilient traveler likely still has a long journey ahead. “I learned a long time ago not to gamble on comets,” Cooke laughed. “We’ll have to wait and see.”