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Published09/25/2024, by Rod Nerdahl
A comet is headed for our vicinity in the solar system this fall and it’s a celestial visitor with the potential to put on a show you will want to know about! You haven’t heard about the comet? Well, neither had astronomers until 21 months ago, when it was discovered halfway between the orbits of Jupiter and Saturn; hurtling toward the Sun at 33,000 miles per hour. This visitor from the outer limits of the solar system is Comet Tsuchinshan-ATLAS (pronounced Choo-cheen-SHAHN), although astronomers also refer to it as Comet C/2023 A3.
The Discovery of Comet Tsuchinshan-ATLAS
In early January 2023, astronomers monitoring the heavens at the Tsuchinshan (“Purple Mountain”) Observatory in Nanjing, China, discovered a very faint object. Six weeks later, observations made at the ATLAS (“Asteroid Terrestrial-impact Last Alert System”) Observatory in Sutherland, South Africa, revealed that the object had a tail, establishing its identity as a comet. Following the astronomical convention of naming comets for their discoverers (in this case, the Chinese and South African observatories), the object was dubbed “Comet Tsuchinshan-ATLAS.”
Follow-up observations revealed that Comet Tsuchinshan-ATLAS would pass within 36 million miles of the Sun on September 27, 2024 and 44 million miles of Earth two weeks later. These are close encounters astronomically speaking. Further study revealed that the comet came from the Oort Cloud. It is a very distant realm, filled with billions of icy planetesimals, most of which are leftovers from the formation of the solar system. Post-discovery observations also showed that the far point of Comet Tsuchinshan-ATLAS’ orbit is one-third the distance to Proxima Centauri, the nearest star to Earth (other than the Sun).
These details suggest that Comet Tsuchinshan-ATLAS has the potential to be a bright, naked-eye comet—if everything turns out right. But comets can be unpredictable, as accomplished comet hunter and author, David Levy, noted in one of his books: “Comets are like cats: they have tails, and they do precisely what they want.”
Comet Tsuchinshan-ATLAS: Considering the Possibilities
Comet NEOWISE as seen looking toward the Clubhouse at the Hazeltine National Golf Club in Chaska, MN on July 10, 2020. This image approximates the unaided eye view of Comet Tsuchinchan-ATLAS when it comes into view during mid-October this year. (Image credit: Rod Nerdahl)
Although astronomers do not define categories for evaluating a comet’s behavior, the author of this article has created three generic scenarios, based on maximum brightness, tail length, etc., that can be used to approximate what Comet Tsuchinshan-ATLAS may look like during its appearance this fall:
• Great Comet: Initial observations of Comet Tsuchinshan- ATLAS suggested that it might become a “great comet”—one of the rare examples of a comet that receives worldwide attention. Some early predictions suggested that Comet C/2023 A3 might reach a maximum magnitude of -3 (slightly less than that of planet Venus). If Comet Tsuchinshan- ATLAS becomes this bright, it might be visible during daytime under favorable conditions! Great comets have magnitudes with negative number values—they are very bright. In addition, it was also suggested that Comet Tsuchinshan-ATLAS’ tail could reach 20 degrees (the distance between the end of the Big Dipper’s handle and the far side of its bowl) or more. Most experts regard this scenario as an unlikely possibility.
• Naked Eye Comet: This scenario charts a course midway between the “razzle-dazzle” of a Great Comet and the more numerous, less bright and more challenging comets. “Naked eye” comets have maximum magnitudes between 0 and +3. They can be seen with unaided eyes by observers living in city and suburban locations. Advocates of this scenario think Comet Tsuchinshan-Atlas could achieve a maximum brightness of +0.2 and develop a tail 6 degrees long. There’s good reason to believe this model is reasonable. Comet Tsuchinshan-Atlas behavior thus far, closely resembles that of Comet NEOWISE, an Oort Cloud comet that visited the inner solar system four years ago. Side-by-
side comparisons of the orbital parameters of these two comets strongly suggest that Comet Tsuchinshan-Atlas’ performance will mirror that of Comet NEOWISE. If accurate, Comet C/2023 A3 will provide a memorable experience!
• Dark Sky Comet: Several comet experts, erring on the side of caution, think that Comet Tsuchinshan-Atlas’ maximum brightness will be about magnitude +3 and its tail 3 degrees long. If this scenario is accurate, people living in city and suburban locations will not be able to see the comet with unaided eyes. Binoculars would permit the comet to be observed, although pinpointing its exact location would be a challenge. Serious amateur comet hunters would relish the opportunity to observe the comet despite these circumstances. Casual sky observers, however, might decide to forego travel to a dark sky site to view a faint comet.
Comet Tsuchinshan-ATLAS: The View from the Ground
This sky chart provides a simulated view of the sky looking toward the western horizon. It shows the changing positions of Comet Tsuchinshan-ATLAS and planet, Venus, from October 11 to November 1, 2024. (Image credit: Rod Nerdahl)
The accompanying illustration assumes that Comet Tsuchinshan-Atlas’ behavior will follow the “Naked Eye Comet” scenario. The illustration shows the comet’s position and its predicted appearance during “mid-evening twilight (45 minutes after sunset). Sunset times change a lot near the September equinox, averaging about a minute-and-a-half per day during the last half of October. Sunset on October 12 (the first day Comet Tsuchinshan-ATLAS is shown on the illustration) will be at 6:34 pm. Sunset on October 30 (the last day the comet is depicted on the illustration) will be at 6:05 pm.
Compass direction marks have been added to the illustration, showing that Comet Tsuchinshan-ATLAS will be visible in the western sky. The only other object that will be visible in the sky then, will be the planet, Venus. It will be easy to find 45 minutes after sunset, sitting a little over five degrees above the southwestern horizon. It will not be dark enough to see stars at that time, but you will be able to use dazzling Venus to help locate the comet.
The changing positions of Comet Tsuchinshan-ATLAS and Venus are plotted on the illustration at three-day intervals. Notice that the comet’s position will change rapidly at first, as its distance from the Sun increases. Although the comet is expected to be bright on evenings close to October 12, it may be difficult to see then, masked by the lingering glare of sunset. As the comet’s height above the horizon increases in the days that follow, visibility will improve. Forward scattering of sunlight by dust in the comet’s tail is expected to enhance the comet’s brightness—this effect being most pronounced around October 18 and 19. Thereafter, Comet Tsuchinshan-ATLAS will begin fading rapidly (binoculars will likely be needed to locate the comet during the last third of the month). Once you find the comet with binoculars, shift to viewing with eyes alone—the comet will be easier to pinpoint and observe that way.
Dark-colored, dashed lines have been added to the illustration showing the paths Comet Tsuchinshan-Atlas and Venus will follow. Dots have been added to the path the comet will follow, showing the comet’s location on days in between those for which the comet’s physical appearance is labeled.
Comet Tsuchinshan-ATLAS: The Bottom Line
There is still some uncertainty about how the comet is doing at present, it has been too close to the Sun to observe safely since mid-August. This circumstance will continue through the final days of September, when the gap between the comet and the Sun will widen, permitting safe observation with professional instruments again. Until then, we need to wait. Some unanswered questions about the comet remain. Will the comet survive its close encounter with the Sun on September 27? Will it be an eye-popping “Great Comet,” one that gives an amazing account of itself, a satisfying “Naked Eye Comet” or one that appeals most to amateur and professional comet hunters? And finally, does the comet contain enough dust to brighten appreciably when it is higher in the sky around October 18 and 19? Stay tuned, we live in a universe where anything can happen and often does. Remember, comets, like cats, do what they want—and occasionally both please us!
