Comet C/2023 A3 (Tsuchinshan-ATLAS): Photographing a Once-in-a-Lifetime Visitor from Assam

Photographs: © Pranabesh Barman

In October 2024, a comet appeared low in the western sky after sunset. NASA described it as an ancient comet from the Oort Cloud, making its first documented pass by Earth. Its official name is C/2023 A3 (Tsuchinshan-ATLAS). For a few weeks, people across India could see it with their own eyes or easily photograph it with an ordinary camera.

The two photographs in this article were taken from Assam. One shows the comet’s bright head and long, faint tail among scattered clouds. The other, taken on the evening of 19 October 2024, shows the comet fading into a sky brightened by a rising Moon. Together they record a visitor that may never come back.

What the photographs show

The first photograph shows a broad patch of sky with clouds. Near the centre is a small bright point with a straight, pale tail stretching up and to the left. The bright point is the comet’s coma, the cloud of gas and dust around its solid nucleus. The tail always points away from the sun. Since the sun had set below the horizon, the tail points upwards, away from it.

The second photograph has the comet labelled. Its camera data records the conditions exactly:

Setting	Value
Camera	Nikon Z6 II
Lens	50 mm f/1.8G
Aperture	f/1.8
Shutter speed	3 seconds
ISO	2500
Date and time	19 October 2024, 6:12 pm IST

The comet’s head looks faint and slightly reddish here, with only a short stub of tail. The reason is in the sky. At that moment the sun was about 19° below the horizon, so full darkness had only just begun. The Moon was 94% lit, two days after full moon, and was rising in the east at that very minute. Moonlight brightens the whole sky and drowns out faint objects like a comet’s tail. That is why the tail is so much weaker here than in the first frame.

The background stars appear as tiny rings rather than sharp points. This usually means the focus was very slightly off, which is easy to do at night with a lens wide open at f/1.8. It doesn’t affect the scientific record, but it is a useful lesson for anyone photographing the next comet.

Where the comet came from

Comet Tsuchinshan-ATLAS came from the Oort Cloud, a vast, distant shell of icy bodies thought to surround the solar system far beyond the planets. Comets from there have very long orbits, and many are seen only once in recorded history.

Its name records its two discoveries:

• 9 January 2023: Purple Mountain Observatory in China (Tsuchinshan means “purple mountain”) first detected it. With no follow-up observations, it was dropped from the list of objects awaiting confirmation.
• 22 February 2023: the ATLAS survey (Asteroid Terrestrial-impact Last Alert System) independently found it from its telescope in South Africa. At the time the comet was about 7.3 astronomical units from the sun, far beyond the orbit of Jupiter.

Earlier images later showed it was already active in December 2022, releasing gas while still almost 8 astronomical units from the sun.

“C/2023 A3” is its formal designation. “C/” marks a comet that is not periodic, meaning it won’t return on any predictable, short timescale. “2023 A3” means it was the third comet discovered in the first half of January 2023.

Its journey past the sun and Earth

Event	Date	Distance
Closest to the sun (perihelion)	27 September 2024	About 0.39 AU (roughly 58 million km), inside Mercury’s orbit
Closest to Earth	12 October 2024	About 71 million km
Earth crosses the comet’s orbital plane (anti-tail)	14 October 2024	—

One astronomical unit (AU) is the average distance from Earth to the sun, about 150 million km.

Why it became so bright

Comets are notoriously hard to predict. Some fade or break apart, others brighten suddenly. In 2024 a published analysis suggested this comet had begun to break up and might disintegrate before reaching the sun. It didn’t. It survived perihelion intact and became one of the brightest comets in years.

Part of the reason was an effect called forward scattering. Around 9 October 2024, the comet passed almost between Earth and the sun. Dust particles scatter much more light forwards, towards the observer, when the sun is behind them. It works the same way as dusty car windscreens or haze looking much brighter when you face the sun.

At its peak the comet reached magnitude around −4.9, according to the Comet Observation Database. That is as bright as Venus at its brightest. Most people, however, saw it in twilight, close to the sun’s glare, where it looked fainter than that number suggests. The SOHO spacecraft’s coronagraph, which blocks out the sun to study its surroundings, recorded the comet from 7 to 11 October. It was the second brightest comet SOHO had seen since the spacecraft launched in 1995.

The tails, and the “anti-tail”

A comet usually has two tails. The dust tail is broad, pale and slightly curved. It is made of dust grains pushed away by sunlight. The ion (gas) tail is narrower, straight, often bluish, and is driven by the solar wind. In a normal camera exposure, the dust tail is usually the more visible one, as in the first photograph.

In mid-October 2024, observers also saw an anti-tail: a thin spike that seems to point towards the sun, opposite the main tail. The comet hasn’t reversed its tail. On 14 October Earth passed through the plane of the comet’s orbit. From that angle, dust spread along the orbit behind the comet appears in perspective on the sunward side.

Photographs from mid to late October showed the tail stretching more than 15–20 degrees across the sky. That is longer than the width of two outstretched hands at arm’s length.

Will it come back?

Probably not within any timescale that matters to us. Before entering the inner solar system, the comet was only loosely bound to the sun. The gravity of the planets has changed its orbit, and calculations show its outbound path is now hyperbolic: open, not closed. NASA has noted it may leave the solar system entirely. Either way, these 2024 photographs are effectively the only time humans have recorded it.

How to photograph a comet

The second photograph shows a practical setup that works well: a full-frame camera, a 50 mm lens, a wide aperture and a short exposure. Here is a fuller approach for future comets.

Plan the date and time. Bright comets are often close to the sun, so they show low in evening or morning twilight. Check their position each day with a planetarium app such as Stellarium. Above all, check the Moon. A bright Moon, as on 19 October 2024, can hide most of the tail.

Use a stable tripod. Even a 3-second exposure will blur if you hold the camera.

Pick the right lens.

• 24–35 mm: the comet with the landscape below it
• 50–85 mm: the tail fills more of the frame
• 135 mm and longer: detail in the head and tail, but stars streak quickly unless you track the sky

Keep the exposure short. The longer the lens, the shorter the exposure needs to be, because Earth’s rotation makes stars streak. At 50 mm, 2–5 seconds works well. The 3 seconds used here kept the stars from trailing.

Open the aperture and raise the ISO. f/1.8–f/2.8 with ISO 1600–3200 is a good starting point. Check the histogram on the camera screen so the sky isn’t washed out.

Focus carefully. This is the hardest part. Switch to manual focus. Zoom in on a bright star or planet in live view and turn the ring until the star is a tight point, not a ring. Many photographers stop down very slightly (from f/1.8 to about f/2.2) to sharpen stars at the edges of the frame.

Take many frames. Stacking 20–50 short exposures in software such as Sequator or DeepSkyStacker greatly reduces noise and brings out faint tail structure.

Shoot RAW. It gives far more control over the twilight glow and the comet’s colours.

Conclusion

Comet C/2023 A3 (Tsuchinshan-ATLAS) was discovered in early 2023, came within about 0.39 AU of the sun, and passed about 71 million km from Earth in October 2024. Forward scattering made it unusually bright, and in mid-October it showed an anti-tail. It is now heading back out, and its orbit may carry it out of the solar system for good. The two photographs from Assam show it in two very different skies: a clear, dark one where the tail stretched across the frame, and one brightened by a rising Moon on 19 October, recorded at f/1.8, 3 seconds, ISO 2500 with a 50 mm lens.