Comet Tsuchinshan–ATLAS — the evening-sky great comet
This comet rounded the Sun on 27 September 2024 and came closest to Earth on 12 October. In mid-October it showed a long tail and a rare anti-tail in the western sky after sunset.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
Comet C/2023 A3 Tsuchinshan–ATLAS carries two names. China's Purple Mountain (Tsuchinshan) Observatory photographed it first in January 2023, and the ATLAS telescope in South Africa found it separately in February. At discovery the comet was about 7.3 AU from the Sun, farther out than Jupiter's orbit. Being visible from so far away meant it was already fairly active.
On 27 September 2024 the comet passed 0.39 AU from the Sun, about the distance of Mercury's orbit. In the spring some astronomers had predicted that it was breaking apart, but it came safely around the Sun.
Around 9 October the comet lay almost exactly between the Sun and Earth. Its dust then scattered sunlight forward toward Earth, and it brightened to magnitude −4.9, brighter than Venus. It was so close to the Sun in the sky, however, that it was mainly recorded by the camera of the SOHO solar observatory.
After passing 0.47 AU from Earth on 12 October, the comet's long tail could be seen with the naked eye in the western sky just after sunset in mid-October. Around 14 October Earth crossed the plane of the comet's orbit, and a thin, straight anti-tail pointing toward the Sun appeared as well. The comet then faded quickly.
When and where to see it
Comet Tsuchinshan–ATLAS is now heading out of the solar system and can only be followed with large telescopes. The pull of the planets bent its outgoing path into a hyperbola, so it will leave the solar system and never return.
Choose this comet in the sky view's event list to replay the western sky just after sunset in mid-October 2024. The comet sits low above the horizon with its tail pointing upward. Move the date to 9 October to see the arrangement in which the comet sat right beside the Sun.
In the orbit view's event replay, the comet swings around the Sun at 0.39 AU and passes near Earth's orbit. Around 9 October the Sun, the comet and Earth line up almost straight; that is the arrangement that made the comet so bright.
See this for yourself — replay the moment in the orbit view
Past returns and history
In spring 2024 the comet's brightness hardly increased for a while. Astronomer Zdenek Sekanina suggested that it had begun to break apart and published a paper saying it might disintegrate before its closest approach to the Sun. Over the summer, however, its brightness rose again, and the comet passed the Sun intact.
The discovery came from several observations joined together. Purple Mountain Observatory first photographed it on 9 January 2023, but follow-up observations did not come in time and the report was delayed. On 22 February the ATLAS telescope found and reported it independently, and the two sets of observations were linked. Later, the comet also turned up in images taken in December 2022 by the ZTF telescope at Palomar Observatory in the United States, and its orbit was quickly worked out.
Just before it rounded the Sun, in late September, the comet was first seen well in the eastern sky before dawn from the southern hemisphere, where observers took the first naked-eye photographs. It then moved into the evening sky of the northern hemisphere. Late in October that year another comet, C/2024 S1 ATLAS, broke apart as it neared the Sun. A similar contrast between a surviving comet and a vanishing one had also appeared in 2020.
Why it matters
Tsuchinshan–ATLAS was the first comet in four years, since NEOWISE in 2020, to give the northern hemisphere a big naked-eye show. Its discovery also reflects how the sky is watched today. An automatic telescope built to find asteroids near Earth (ATLAS) and a long-established observatory (Purple Mountain) were scanning the same sky and found the same comet.
The brightness on 9 October showed how much a comet's brightness depends on where it sits. When a comet lies between the Sun and Earth, its dust scatters sunlight forward, so it looks much brighter than when seen from the side. This effect could be calculated ahead of time, and it happened as predicted.
An anti-tail is created by the direction from which we look. Heavier dust lags behind along the path the comet has travelled. When Earth sees that sheet of dust edge-on, it looks like a spike pointing toward the Sun. That is why an anti-tail is visible only for the short time when Earth passes through the plane of the comet's orbit.
FAQ
Why does it have two names?
Because it was discovered separately by China's Purple Mountain (Tsuchinshan) Observatory and the ATLAS telescope in South Africa.
When was it brightest?
Around 9 October, when it lay between the Sun and Earth and reached magnitude −4.9. The easiest naked-eye views came in mid-October evenings.
What is an anti-tail?
When Earth views the dust left along a comet's path edge-on, the dust looks like a spike pointing toward the Sun. That is an anti-tail.
Will it come back?
No. Its outgoing path is a hyperbola, so 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.