C/2024 S1 (ATLAS)

The comet was discovered during the ATLAS survey, using a 0.5-m reflector telescope located at Haleakala, Hawaii. It is the second Kreutz comet discovered from the ground since C/2011 W3 (Lovejoy), and is likely a fragment of the same parent body that also produced C/1882 R1 and C/1965 S1 (Ikeya–Seki).^[5] The comet then was estimated to have an apparent magnitude of 15.3 and featured a coma about a half arcminute across. Further observations reported the coma was as large as three arcminutes across and the tail up to 156 arcseconds long.^[1] The comet had upon discovery a greenish color, probably due to the presence of diatomic carbon.^[6] The apparent magnitude of the comet was variously reported to range from magnitude 16.9 up to 11.5.^[1] The presence of the green coma indicated that it was closer to 11.5. A visual observation was reported with a 30-cm reflector telescope, with a reported magnitude of 11.7.^[6] The comet upon discovery was located in the constellation of Hydra.^[7] The comet kept to a magnitude of 12–13 throughout the next week.^[8]

On 8 October the comet was found to be elongated and to lack a central condensation, and to be overall fainter than it was on 3 October, indicating that the comet possibly disintegrated, similarly to C/2019 Y4 (ATLAS) in 2020. The comet on 8 October was located 0.802 AU from the Sun, which although not too close, could be enough for gases to have vaporized and in combination with tidal or rotational forces may have led to the fragmentation of the nucleus.^[9] However, later observations from October 15 onwards consistently showed a well-condensed nucleus, and the comet underwent a 2-magnitude outburst between October 15 and 16.^[10] On 19 October the coma was reported to be triangular in shape and 2.4 arcminutes across while a tail 24 arcminutes long was observed. On 20 October the comet was reported to have an apparent magnitude of 8.2 but the next day it was reported it had faded to 10, while the coma and tail decreased in size, indicating another outburst had taken place.^[11] On 23 October, with the comet being 4.5 degrees above the horizon, the coma was one arcminute across, had a total magnitude of 10.8 and the tail was 17 arcminutes long.^[11]

On 26 October 2024, Hanjie Tan, a PhD student from Guangzhou, reported the discovery of a fragment ahead of C/2024 S1 in LASCO C3 images.^[12]

Just a few hours before reaching perihelion the comet evaporated within two hours which could be seen on images from SOHO LASCO C2 around 28 October 07:00 UTC.^[13][14] The comet was predicted to be visible in the morning sky before perihelion, with better visibility from the southern hemisphere. During perihelion, if the comet had not disintegrated, it could have brightened to a magnitude of -5 to -7, which is brighter than the planet Venus.^[15] If it had survived perihelion it would be visible again before dawn, more favorably located for the southern hemisphere,^[7] and could have looked like C/2011 W3 (Lovejoy), another Kreutz sungrazer.^[16]

It is theorized that C/2024 S1 (ATLAS) had disintegrated due to the combined effects of rotational instability and sublimation loss shortly before it reached perihelion.^[4]

• C/2023 A3 (Tsuchinshan–ATLAS), a comet that became visible to the naked eye in early October 2024
• C/1945 X1 (du Toit), another Kreutz sungrazer that was detected and disintergrated before perihelion