en C 2024 G3 (ATLAS)

C/2024 G3 (ATLAS)
	Comet ATLAS photographed by Donald Pettit aboard the ISS on 11 January 2025
Discovery
Discovered by	Tyler Linder
Discovery site	ATLAS–CHL (W68)
Discovery date	5 April 2024
Orbital characteristics
Epoch	Present: 16 June 2024 (JD 2460477.5); Inbound: 1 January 1800 (JD 2378496.5); Outbound: 1 January 2200 (JD 2524593.5)
Observation arc	267 days
Number of; observations	294
Aphelion	6,391 AU (Inbound); 14,234 AU (Outbound)
Perihelion	0.093 AU
Semi-major axis	3,196 AU (Inbound); 7,117 AU (Outbound)
Eccentricity	0.999970 (Inbound); 1.000013 (Present); 0.999987 (Outbound)
Orbital period	≈180,000 years (Inbound); ≈600,000 years (Outbound)
Inclination	116.851°
Longitude of; ascending node	220.331°
Argument of; periapsis	108.125°
Mean anomaly	–0.0003°
Last perihelion	13 January 2025
Earth MOID	0.483 AU
Jupiter MOID	3.837 AU
Physical characteristics
Comet total; magnitude (M1)	7.6
Comet nuclear; magnitude (M2)	12.4
Apparent magnitude	–3.8; (2025 apparition)

C/2024 G3 (ATLAS) is a non-periodic comet, which reached perihelion on 13 January 2025, at a distance of 0.09 AU (13 million km) from the Sun. Dubbed the Great Comet of 2025, it is currently the brightest comet of 2025,^[6] with an apparent magnitude reaching −3.8 on the day of its perihelion.^[5] The comet is visible in the southern hemisphere before and after perihelion. It can only be observed in the daytime sky around perihelion in the northern hemisphere.^[7]

Observational history

The comet was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey on 5 April 2024, in images obtained with a 0.5-m reflector telescope located in Río Hurtado, Chile. The comet at the time was a magnitude 19 object about 4.38 AU (655 million km) from Earth. Further observations indicated it had a diffuse coma about 4.5 arcseconds across and a straight tail.^[1]

By 30 October 2024, the comet had an apparent magnitude of 11.9 per reported observations at the Comet Observation Database (COBS),^[5] visible with large telescopes. By mid December 2024 it had brightened to 8th magnitude and was located in Scorpius, visible at dawn in the southern and equatorial regions.^[8] By the end of the month, its magnitude was reported to be between 5 and 5.5, having a coma about two arcminutes across and a tail up to 18 arcminutes long.^[5]

On 2 January 2025, Terry Lovejoy reported that the comet experienced an outburst,^[9] estimating its magnitude to be 3.7 photographically and 3.2 visually. On 3 January the comet had brightened to a magnitude of 2–2.4. The comet was reported to have a nuclear shadow, a dark lane in the tail, and was marginally visible with naked eye on that day.^[10] On 7 January the comet was reported to be of first magnitude, with a tail about 20 arcminutes long.^[5] The comet was photographed by cosmonaut Ivan Vagner onboard the International Space Station on 10 January.^[11] The comet was also observed aboard the station by astronaut Donald Pettit on the following day.^[12]^[13]

The comet entered the field of view of the LASCO C3 coronograph on SOHO on 11 January and was visible until 15 January.^[14] In case of this comet, it is very difficult to predict peak brightness (spread more than 10 mag). G. van Buitenen predicts −4 magnitude, accounting for forward scattering, however it will be just 5 degrees from Sun at peak magnitude.^[15] The comet became bright enough to photograph in broad daylight,^[16]^[17] and was reported to be visible with naked eye.^[18] After perihelion the comet headed southwards, while in mid northern latitudes was low, being 2 degrees above the horizon at the end of the civil twilight.^[8]

After perihelion the comet reappeared in the southern hemisphere. It became clearly visible with naked eye and on 18 January its tail was reported to be 4 degrees long, while its magnitude was estimated to be −0.9.^[5] However, on 19 January, Hungarian astrophotographer Lionel Majzik reported that a bright streamer appeared within the tail while its head became less prominent, indicating that the comet could have disintegrated.^[19]^[20] Reports of an earlier fragmentation event were recorded as early as 12 January,^[21] and while the exact cause of the comet's breakup were currently unknown, it is likely that intense solar heating during perihelion had caused jetting and outgassing from the nucleus, leading to its eventual fragmentation.^[20] Despite this, the comet retained the shape of its tail, now resulting in a "headless" comet similar to Comet Lovejoy in 2011.^[20]

John Bortle considers the comet due to its brightness and night time appearance as the Great Comet of 2025.^[22]^[23]

Orbit

At the time of discovery, it was assumed that this was a new comet from the Oort cloud, and with an absolute magnitude (H=9), there was very little to no chance that it would survive perihelion.^[24]^[25] But as the orbit was refined, it was found to be likely a dynamically old comet, having made close approaches to the Sun before.^[26]

Some media report C/2024 G3 (ATLAS) approaches the Sun once every 160,000 years.^[12]^[27]^[28] However, according to long-term orbital calculations by the JPL Horizons On-Line Ephemeris System, after approach to the Sun in 2025, aphelion distance of the comet will be more than twice as far away as before, and its orbital period will be about 600,000 years.^[3]