C/2023 A3 (Tsuchinshan–ATLAS)

During the search performed by 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 (1.09 billion km; 680 million mi) from the Sun.[9] After the first orbit calculations it was noticed that it is 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 waiting confirmation list but after no follow up observations were reported, it was removed in 30 January 2023 and was considered lost.[9] Based on the naming conventions of comets, the comet received the name of both observatories.[9]

The object was subsequently detected in images taken by 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 is a comet.[1] More evidence of cometary activity were reported by Hidetaka Sato, M. Mattiazzo, and Cristóvão Jacques.[9]

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.[9] It will become better visible from Earth about three weeks after perihelion, after mid October, when it is estimated to be of fourth magnitude.[9] Gideon van Buitenen estimated that the comet will reach a magnitude of 0.9 during perihelion and −0.2 at the time of the closest approach to Earth, assuming H = 5.2 and 2.5n = 10, and will benefit from the effects of forward scattering.[10][11] On 9 October 2024 the comet will be 3.5 degrees from the Sun.[12]

The comet has a retrograde orbit, lying at an inclination of 139°. Τhe comet has its perihelion at a distance of 0.391 AU and the 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.[9] The orbit is weakly bound to the Sun before entering the planetary region of the Solar System.[3] 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.[3] 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.[13]

• Positions of the comet C/2023 A3 in autumn 2024
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  The position of comet C/2023 A3 in August and September 2024 with the expected apparent magnitudes. The comet is located in the constellation Leo (Leo) between the two stars 55 and 57 Leonis about six degrees south of the ecliptic at the beginning of August and then moves towards the constellation Sextans. With increasing apparent brightness, it turns back toward the constellation Leo in the second half of September at maximum southern ecliptic latitude (just under 14 degrees of arc).
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  The position of comet C/2023 A3 in October 2024. The comet is located in the southernmost tip of the constellation Leo about ten degrees south of the ecliptic and moves in the first half of the month with decreasing apparent brightness across the constellation Virgo. It then moves into the western head of the constellation Serpens Caput, and then moves across the constellation Ophiuchus. By the end of the month, the comet reaches a northern ecliptic latitude of just over 27 degrees of arc. Therefore, in the second half of October the comet should be well visible on the western horizon after sunset.

• C/2023 A3 at the JPL Small-Body Database