☄ Tsuchinshan-ATLAS C/2023 A3

How to follow comet Tsuchinshan-ATLAS live

The panel above recomputes the position of Tsuchinshan-ATLAS every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination). It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where Tsuchinshan-ATLAS is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of Tsuchinshan-ATLAS with no math at all.

Comet fact sheet

About Tsuchinshan-ATLAS

C/2023 A3 (Tsuchinshan-ATLAS), named after the two observatories that detected it independently, was the brightest comet most observers in both hemispheres had a chance to see in nearly three decades. In mid-October 2024 it reached an apparent magnitude close to -4.9 according to observer reports in the COBS (Comet Observation Database), making its tail visible even from light-polluted urban skies just after sunset, with extensions up to 50 degrees at the best geometry.

The comet emerged from perihelion in September 2024 without breaking apart, which was not guaranteed: dynamically new comets on their first visit to the inner Solar System are more vulnerable to intense solar heating. The survival of the nucleus, estimated at a few kilometres across, and the favourable October geometry turned the apparition into an event accessible to the general public with two weeks of naked-eye visibility and a long tail.

History and discovery

The first detection was made on 9 January 2023 by the Purple Mountain Observatory (Zijinshan), in Nanjing, China, which gives the "Tsuchinshan" part of the name (a pinyin romanisation of the Chinese observatory name). Several days later, the ATLAS network (Asteroid Terrestrial-impact Last Alert System), operated by the University of Hawaii with telescopes in Hawaii, Chile, South Africa and Namibia, independently confirmed and refined the orbit.

The double name Tsuchinshan-ATLAS follows the standard International Astronomical Union (IAU) practice: when two independent observatories find the same object within a few days, both are credited in the official designation. Objects found by the ATLAS system receive the ATLAS code in the name.

At discovery in January 2023, the object was more than 7 AU from the Sun, already considerably brighter than expected at that distance for a nucleus of its estimated size. That excess activity hinted at a potentially spectacular inner-Solar-System passage, though the fragility of dynamically new comets made any prediction uncertain.

Orbit and nature

C/2023 A3 has an orbit with eccentricity slightly above 1 (a hyperbola), meaning, unlike periodic comets such as Halley, it will never return to the inner Solar System. It arrived from somewhere in the Oort Cloud, the spherical reservoir of icy bodies surrounding the Solar System at distances of thousands to tens of thousands of astronomical units.

Perihelion occurred on 27 September 2024 at 0.39 AU from the Sun, inside Mercury's orbit. Closest Earth approach was on 12 October 2024 at roughly 0.47 AU (about 70 million km). The nucleus, estimated at a few kilometres across, survived the intense solar heating without breaking apart.

Nucleus, coma and tail

The Tsuchinshan-ATLAS nucleus was estimated at a few kilometres across based on infrared observations and activity modelling. Surviving the intense perihelion heating at 0.39 AU, closer to the Sun than Mercury, was one of the encouraging developments for astronomers: dynamically new comets on their first inner-Solar-System visit frequently disintegrate or lose most of their activity under intense solar heating.

The dust tail was the most spectacular element of the apparition. On 9 October 2024, during solar conjunction, estimates indicated the dust tail may have reached 30 degrees in the morning sky before the comet emerged to public visibility. On 10 October, as the comet cleared the evening twilight, the dust tail was estimated at up to 50 degrees.

A rare phenomenon was observed in mid-October: the anti-tail. As Earth passed through the comet's orbital plane, the geometric perspective made the dust tail appear to extend both behind and in front of the nucleus, creating a silhouette astronomers described as a unicorn. The anti-tail is a perspective effect: larger, slower dust particles are spread along the comet's entire orbit, and when Earth crosses the orbital plane they appear as a spike projecting "forward" from the nucleus.

The sky spectacle

The best viewing window for the general public was 12 to 25 October 2024, with the comet visible on the western horizon for 1 to 2 hours after sunset, gradually moving to higher positions in the sky over the following nights. The visibility peak coincided with maximum brightness and the longest tail, making the first two weeks of October 2024 the ideal observation period.

Media coverage was intense. Professional and space observatories, including the ESA/NASA Solar and Heliospheric Observatory (SOHO), tracked the passage in ultraviolet and visible light. Photographs of the comet with the Milky Way, against illuminated city skylines, and in diverse night landscapes circulated widely on social media during the second week of October 2024, recreating for the 2024 generation the excitement that Hale-Bopp generated in 1997.

Science and observations

Tsuchinshan-ATLAS was one of the best-instrumented comets in recent history, thanks to the reach of space telescopes and global amateur observation networks.

• SOHO: the LASCO coronagraph on the Solar and Heliospheric Observatory tracked the comet during and after perihelion, providing data on activity evolution close to the Sun.
• Anti-tail: the anti-tail visible for several days in mid-October was documented in detail by astrophotographers and studied as a geometric perspective phenomenon, resulting from Earth crossing the comet's orbital plane.
• Greenish coma: long-exposure photographs showed the characteristic green coma of comets with high C2 (diatomic carbon) production, excited by solar ultraviolet radiation.
• Orbital period: C/2023 A3 is estimated to take more than 80,000 years to complete one orbit, meaning its previous inner-Solar-System passage occurred before Homo sapiens had reached Europe.
• Composition: pre-perihelion spectroscopic campaigns identified CN, OH, C2 and C3 lines in the coma, a typical first-passage Oort Cloud comet profile. The CO/H2O ratio was monitored to assess nucleus thermal history.
• Perihelion survival: fragmentation or dissipation at perihelion was a plausible scenario given the intense heating at 0.39 AU. Intact nucleus survival was confirmed by post-perihelion observations showing continued activity.

Facts worth knowing

• The double name follows the standard IAU practice: when two observatories independently find the same object within a few days, both are credited in the official designation.
• Before perihelion, some estimates speculated it might reach magnitude -5, which would have made the tail visible in daylight. The actual brightness fell short of that, but it was still the brightest comet since McNaught in 2007 and the brightest for the northern hemisphere since Hale-Bopp in 1997.
• The dust tail reached up to 50 degrees on 10 October 2024, equivalent to 100 Full Moons lined up across the sky.
• The anti-tail, visible for several days in mid-October, created a unicorn-horn silhouette that went viral on astronomical photography platforms.
• C/2023 A3 is estimated to take more than 80,000 years to complete one orbit. Its previous inner-Solar-System passage occurred before Homo sapiens had reached Europe.
• The ATLAS network (Asteroid Terrestrial-impact Last Alert System) was created primarily to detect asteroids days before a possible Earth impact; comet discoveries are a byproduct of its continuous all-sky survey.
• The greenish tint visible in long-exposure photographs comes from diatomic carbon (C2) in the coma, excited by solar ultraviolet radiation.
• Perihelion survival at 0.39 AU was considered far from guaranteed by astronomers: first-visit Oort Cloud comets frequently break apart or lose most activity under intense solar heating.

Other comets

See the full comet catalogue.