2024 has already treated people to awe-inspiring celestial events, including the spectacular Northern Lights and the “Great North American Solar Eclipse” in April. Yet, the year is far from over, and skywatchers have another cosmic spectacle to look forward to: a bright naked-eye comet. Comet C/2023 A3 (Tsuchinshan–ATLAS), discovered last year, promises to light up the autumn evening sky, offering a dazzling display for stargazers.
Discovery of Tsuchinshan–ATLAS
The journey of Comet Tsuchinshan–ATLAS began when it was first spotted by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in South Africa on February 22, 2023. Initially mistaken for an asteroid, further observations revealed its true nature as a comet. The Purple Mountain Observatory in China confirmed this identification, having photographed the same object six weeks earlier. When first observed, the comet was located beyond Jupiter’s orbit, approximately 680 million miles from the Sun.
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Promising Encounter
Comet Tsuchinshan–ATLAS is expected to make its closest approach to the Sun on September 27, 2024, coming within 36 million miles, a distance comparable to Mercury’s orbit. Shortly thereafter, on October 12, it will pass within 44 million miles of Earth. These proximities suggest the comet could brighten significantly, potentially becoming as radiant as some of the brightest stars or even as brilliant as Venus around October 8, 2024.
Past Comet Spectacles
In recent years, two comets have captured public attention. The “Great Green Comet” (C/2022 E3) passed near Earth in February 2023, and Comet 12P/Pons-Brooks, nicknamed the “Devil’s Comet” due to its horn-like gaseous appendages, shone brightly last month. However, both required dark, non-light polluted skies and were faintly visible even with binoculars or small telescopes. Tsuchinshan–ATLAS, in contrast, holds the potential to be a much more accessible spectacle.
Will It Deliver?
There is cautious optimism about Tsuchinshan–ATLAS. While it might brighten to a stunning first magnitude and develop a notable tail, there is a caveat. Tsuchinshan–ATLAS is a “first-timer” from the Oort Cloud, a distant reservoir of icy bodies. Comets from the Oort Cloud often experience initial surges in brightness due to volatile materials vaporizing far from the Sun. However, their brightening can slow or stop as they approach the Sun, leading to potential underperformance.
Role of Forward Scattering
One phenomenon that might enhance the comet’s display is “forward scattering of sunlight.” This occurs when the geometry of the comet relative to the Sun and Earth causes sunlight to scatter forward, significantly brightening the comet if it is particularly dusty. Historic comets like Comet Skjellerup–Maristany (C/1927 X1) and Comet McNaught (C/2006 P1) have benefitted from this effect, dramatically increasing in brightness and becoming visible even in daylight.
Waiting Game
Skywatchers in the Northern Hemisphere will rely on reports from the Southern Hemisphere to track the comet’s progress during the summer, as it will be too far south to be visible. By mid-October, if all goes well, Tsuchinshan–ATLAS should be a striking sight in the western evening sky, potentially accompanied by a vivid tail.
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Despite promising indicators, comets are notoriously unpredictable. Past comets have both dazzled and disappointed, making it challenging to predict their performance with certainty. As comet expert Dr. Fred Whipple famously advised, “If you must bet, bet on a horse, not a comet!”