We’ll know more in two weeks. That’s the headline news about Comet Tsuchinshan-ATLAS, which could become a naked-eye object in late September and mid-October as seen from the northern hemisphere.
The comet, also called C/2023 A3—which comes in the wake of April’s comet 12P/Pons–Brooks, 2023’s “green comet” and 2020’s comet NEOWISE—is currently lost in the glare of the sun as seen from Earth. With some scientists doubting whether it will hold together, dedicated sky-watchers were getting nervous about whether the comet will, in practice, become visible at all.
The news is good.
NASA Recovers Comet
As reported by SpaceWeather.com, the comet was recently detected in images by NASA’s STEREO-A spacecraft. More importantly, it appears to be intact. That more or less ensures that amateur astronomers will be able to study it as it gets closer to the sun and becomes visible from Earth. It’s currently shining at magnitude 7, which is just outside the threshold for seeing an object with the unaided eye.
This makes mid-September the time when we’ll know much more about whether the comet will be bright enough to become visible to the naked eye. Then, the comet will re-emerge from the sun’s glare and be seen by an army of astronomers in the southern hemisphere.
It’s expected to be visible from the northern hemisphere in the east just before sunrise between September 27-October 2 and in the west just after sunset from October 12-31.
Notoriously Erratic
Comets are notoriously erratic, and there’s no guarantee that this one will brighten enough to become a naked-eye object. A study published in July predicted the “imminent collapse” of Comet Tsuchinshan-ATLAS before it reaches perihelion—its closest to the sun—on September 27. That appears not to have happened—thus far—with its nucleus so far intact. It even seems to have a bright tail in the STEREO-A images.
Talking to SpaceWeather.com, astronomer Qicheng Zhang at Lowell Observatory, Flagstaff, predicts that the comet will brighten to magnitude +4 as it gets closest to the sun on September 27 and may reach an even brighter magnitude -3 on October 9. The latter scenario would make it potentially visible in the day, albeit to those with binoculars and/or a telescope.
The Comet’s Tail
A comet has a nucleus—a solid core composed of rock, dust and frozen gas—and two tails as it gets close to the sun. One is a bluish plasma tail of electrons and ions ionized by the sun’s heat. The other comprises ice and dust caused by the sun heating the comet, evaporating its gas and ice. The latter is much more open and wispy.
The comet’s orientation, it becomes visible from the northern hemisphere, means it’s possible that its dust tail—which will stream away from the sun, sculpted and buffeted by the solar wind—will likely reflect light toward Earth, making it appear brighter. Astronomers call this forward scattering.
Zhang also said the comet could have a dust tail spanning 20 degrees. That’s more than the span between your index finger and pink finger when you hold your hand at arm’s length against the sky.
Oort Cloud Origins
A long-period comet that comes into the inner solar system to loop around the sun once every 80,000 years, Comet Tsuchinshan-ATLAS comes from the Oort Cloud, a sphere around our solar system home to millions of comets. It’s also very large, with a nucleus around 12-25 miles (20-40km) across.
Comet Tsuchinshan-ATLAS was first discovered by astronomers at China’s Purple Mountain Observatory in January 2023 and later by the Asteroid Terrestrial Impact Last Alert System (ATLAS) search station in Sutherland, South Africa, in February 2023.
Wishing you clear skies and wide eyes.
