Comet Tsuchinshan–ATLAS — the evening-sky great comet
Past perihelion on 27 September and closest to Earth (0.47 AU) on 12 October, it blazed in the western sky after sunset with a long tail and a rare anti-tail.
The moment of Comet Tsuchinshan–ATLAS — the evening-sky great comet: https://nowsky.org/e/tsuchinshan-2024 — opens at the right place and time, replayable, no login.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
C/2023 A3 Tsuchinshan–ATLAS carries two names because it was found independently in early 2023 by the Purple Mountain (Tsuchinshan) Observatory in China and the ATLAS telescope in South Africa. At discovery it was more than 7 AU from the Sun — beyond Jupiter's orbit — and being visible from that far meant a large nucleus.
It survived perihelion on 27 September 2024 at 0.39 AU from the Sun. Around 9 October, nearly in line with the Sun, forward scattering by its dust grains briefly pushed it to magnitude −4, and the SOHO solar observatory caught its long tail on camera.
After passing 0.47 AU from Earth on 12 October, its long tail was visible to the naked eye in the western sky just after sunset through the middle of the month. As Earth crossed the comet's orbital plane, a thin, straight anti-tail pointing toward the Sun appeared as well — an uncommon sight.
It then faded quickly and was a telescope object by late November. Its orbit is nearly hyperbolic, so it will not return. Set the sky view to Seoul at 19:00 on 15 October 2024 for the comet low in the west, and use the orbit view's event replay for the moment of closest approach to Earth.
Why it matters
Tsuchinshan–ATLAS was the first naked-eye great comet for the northern hemisphere since NEOWISE in 2020. Its simultaneous discovery on two continents is symbolic too — a near-Earth-object survey (ATLAS) and a traditional observatory (Purple Mountain) sweeping the same sky.
The forward-scattering surge to magnitude −4 on 9 October showed how much a comet's brightness depends on geometry. With the comet between Sun and Earth, its dust bounces sunlight toward the observer and the comet looks tens of times brighter than it is. The effect is predictable, and it happened as predicted.
An anti-tail is a projection effect seen only when Earth crosses the comet's orbital plane: dust lagging behind along the orbit, viewed edge-on, looks like a spike pointing at the Sun. This comet's anti-tail was among the most clearly recorded in recent years.
How to watch
Tsuchinshan–ATLAS is now receding into the outer solar system, a target for large telescopes only, and its near-hyperbolic orbit means it will not return.
Choose the comet in the sky view's event list to replay the evening of 15 October 2024 — low in the west just after sunset, above Venus, tail pointing upward. Move the time to 9 October for the arrangement when it was briefly visible in daylight beside the Sun.
The orbit view's event replay draws the comet rounding the Sun at 0.39 AU and passing near Earth's orbit; the near-alignment of Sun, comet and Earth around 9 October is the geometry that produced the forward-scattering surge.
See this for yourself — replay the moment in the orbit view
Background
In spring 2024 some observations — stalled brightening and a changing tail — suggested the comet was disintegrating, and there were predictions of a disappointing comet. Over the summer the brightness recovered and it passed perihelion intact.
The southern hemisphere saw it first, in the eastern sky before dawn in late September; after perihelion it moved into the northern evening sky. Observers in South Africa and Australia sent the first naked-eye photographs.
Late in October 2024 another comet, C/2024 S1 ATLAS, broke up while grazing the Sun. The contrast between Tsuchinshan–ATLAS surviving and S1 perishing repeated that of NEOWISE and ATLAS in 2020.
The discovery story is a snapshot of the modern survey system. Purple Mountain Observatory first observed the comet on 9 January 2023 but follow-up observations did not come and the report was delayed; on 22 February the ATLAS telescope in South Africa found and reported it independently, and the two observations were linked. Earlier images from December 2022 then turned up in the Minor Planet Center’s archives and the orbit was quickly fixed. It was a faint 18th-magnitude point at discovery, but once the orbit showed a perihelion of 0.39 AU it drew attention as a great-comet candidate.
FAQ
Why does it have two names?
It was discovered independently by China's Purple Mountain (Tsuchinshan) Observatory and the ATLAS telescope in South Africa.
When was it brightest?
Around 9 October, when forward scattering briefly pushed it to magnitude −4; the comfortable naked-eye views came in mid-October evenings.
What is an anti-tail?
A projection effect seen when Earth crosses the comet's orbital plane: dust lagging along the orbit looks like a spike pointing at the Sun.
Will it come back?
No — its orbit is nearly hyperbolic and it is leaving the solar system.
Related
- Comet NEOWISE — the great comet of 2020
- Comet Hale–Bopp perihelion
- Halley's Comet returns (2061)
- The Sun — object index
- Venus — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.