C/2023 A3 (Tsuchinshan–ATLAS)

C/2023 A3 (Tsuchinshan–ATLAS) is a comet from the Oort cloud discovered by the Purple Mountain Observatory on 9 January 2023 and independently found by ATLAS South Africa on 22 February 2023. The comet will pass perihelion at a distance of 0.39 AU on 27 September 2024, when it could become visible to the naked eye.

Discovery
During the search performed by the Asteroid Terrestrial-impact Last Alert System using the 0.5-m f/2 Schmidt reflector at the Sutherland Observatory in South Africa an asteroidal object with an estimated magnitude of 18.1 was detected in images taken on 22 February 2023, when the comet was about 7.3 AU from the Sun. After the first orbit calculations it was noticed that it was the same as an 18.7 magnitude object reported to the Minor Planet Center by the Purple Mountain Observatory that was detected in images taken on 9 January 2023. It was entered in the objects awaiting confirmation list but after no follow up observations were reported, it was removed in 30 January 2023 and was considered lost. Based on the naming conventions of comets, the comet received the name of both observatories.

The object was subsequently detected in images taken by the Zwicky Transient Facility (ZTF) in Palomar Observatory on 22 December 2022, when it had a magnitude of 19.2–19.6. These images also revealed it had a very condensed coma and a small straight tail 10" in length, indicating it was a comet. More evidence of cometary activity was reported by Hidetaka Sato, M. Mattiazzo, and Cristóvão Jacques.

Towards perihelion
The comet by January 2024 had brightened to an apparent magnitude of 13.6 and according to Bob King was visible through 15-inch telescopes at x142 magnification. The comet was then moving through the constellations of Libra and Virgo. By the end of April it had brightened to about magnitude 10 and could be observed through small telescopes, showing a short tail. The spectrum of the comet on 31 May 2024, when the comet was 2.33 AU from the Sun, indicated strong cyanide emission and that comet is carbon depleted. The comet had a large dust to gas ratio.

In May and June the brightening rate of the comet slowed, with the comet staying between magnitudes 10 and 11, while a dusty tail measuring 5 to 15 arcminutes long was observed visually to extend eastwards. Czech-American astronomer Zdenek Sekanina suggested that this indicates that the comet nucleus has been fragmenting, with the fragmentation starting in late March, as indicated by an increase in the brightening rate and the subsequent decrease in dust production and the narrow teardrop-shaped dust tail. He predicted that the comet would disintergrate before perihelion. Observations of the comet with TRAPPIST robotic telescope indicate that dust production reached a minimum in May, when the comet was near a phase angle of zero, and started to increase again one month later, while gas production rates increased slowly throughout that period. In mid June the comet entered the constellation of Leo, in the evening sky, and after that it will be lost in the Sun's glare until late September. In early July a faint ion tail measuring about one and half degree in length was observed photographically.

During the last week of September it will be seen in the dawn sky, better visible from the Southern Hemisphere, and it is predicted to have brightened to second magnitude. Perihelion takes place on 27 September. After that it will move again in conjunction with the Sun. On 9 October 2024 the comet will be 3.5 degrees from the Sun. It will appear in the evening sky in mid October.

Brightness predictions
Upon discovery announcement the comet was estimated to reach a total magnitude of +3 during perihelion, assuming an absolute magnitude (H) of 7 and 2.5n = 8, when it will be in low solar elongation. Maximum brightness may occur about three weeks after perihelion, in mid October, when it is estimated to be of fourth magnitude. Gideon van Buitenen estimated that the comet will reach a magnitude of 0.9 during perihelion and −0.2 at the time of closest approach to Earth, assuming H = 5.2 and 2.5n = 10, and will benefit from the effects of forward scattering.

Revised data from June 2024 suggest the comet is going to brighten to an apparent magnitude of 2.2, assuming H = 6 and 2.5n = 7.5, which is the average brightening rate of long period comets in the inner solar system. However, the comet is expected to be at least one magnitude brighter due to the effects of forward scattering, which could boost the brightness by several magnitudes around the peak of the effect on 9.8 October 2024.

Orbit
The comet has a retrograde orbit, lying at an inclination of 139°. Τhe comet has its perihelion on 27 September 2024, at a distance of 0.391 AU. Τhe closest approach to Earth will be on 12 October 2024, at a distance of 0.47 AU. The comet doesn't approach close to the giant planets of the Solar System. The orbit is weakly bound to the Sun before entering the planetary region of the Solar System. Due to planetary perturbations, the outbound orbit will have a greater eccentricity than the inbound orbit and is not bound to the Sun as it is weakly hyperbolic. The weakly hyperbolic trajectory may or may not result in the comet being ejected from the Solar System. It is expected to be 200 AU from the Sun in the year 2237.