 C/2025 A6 (Lemmon) on 28 October 2025 | |
| Discovery[2] | |
|---|---|
| Discovered by | D. Carson Fuls[1] |
| Discovery site | Mount Lemmon Survey |
| Discovery date | 3 January 2025 |
| Orbital characteristics[3][5] | |
| Epoch | 11 September 2025 (JD 2460928.5) |
| Observation arc | 368 days (1.01 years) |
| Earliestprecovery date | 12 November 2024[2] |
| Number of observations | 1538 |
| Aphelion | 243.3 AU (inbound)[3] 215.5 AU (outbound) |
| Perihelion | 0.5299 AU[4] |
| Semi-major axis | 122 AU (inbound)[3] 108 AU (outbound) |
| Eccentricity | 0.9957 (inbound)[3] 0.9951 (outbound) |
| Orbital period | ≈1350 years (inbound)[3] ≈1120 years (outbound) |
| Max.orbital speed | 57.8 km/s at perihelion[4] |
| Inclination | 143.66° |
| 108.10° | |
| Argument of periapsis | 132.97° |
| Mean anomaly | 359.96° |
| Last perihelion | 8 November 2025[4] |
| TJupiter | –0.682 |
| EarthMOID | 0.308 AU |
| JupiterMOID | 0.571 AU |
| Physical characteristics | |
| Comet total magnitude (M1) | 10.5 |
| 3.5 (2025-10-24) | |
C/2025 A6 (Lemmon) is anon-periodic comet discovered by theMount Lemmon Survey in images obtained on 3 January 2025. It made its closest approach to Earth on 21 October 2025 and is visible to thenaked eye. As of 12 November 2025[update], the comet is aboutapparent magnitude 5,[6] and is visible to the naked eye and inbinoculars in theconstellation ofOphiuchus but is less than 20 degrees from the Sun.
C/2025 A6 (Lemmon) was discovered as an asteroidal object with an apparent magnitude of about 21.5 by theMount Lemmon Survey in images obtained on 3 January 2025 when it was 4.5 AU from the Sun. Consequently,precovery images byPanSTARRS dating from 12 November 2024 were found.[2] The object was found to have a very condensedcoma, 2.2 arcseconds across. A short tail, two arcseconds long was found in images from 21 February 2025.[7]
When first discovered the comet was expected to only brighten toapparent magnitude 10, but brightened toapparent magnitude of about 3.5, which would make it about 400 times brighter than original expectations.[8]
_on_17_September_2025.jpg)
After being hidden in the Sun's glare during June and July as it came tosolar conjunction on 2 July 2025,[9] the comet was spotted again in the morning sky, significantly brighter than predicted. On 12 August the comet had a coma about one arcminute across and anapparent magnitude of 13.9. By August 19, when the comet was 1.7 AU from the Sun, its total magnitude (nucleus+coma) was estimated to be 11, but the comet did not show an obvious tail.[10] A short spiky tail was visible in images from 25 August.[11] The future brightness of the comet is unknown, but the comet is not expected to have been in an outburst when it exitedsolar conjunction.
The comet in August was located in the constellation ofGemini and on 6-7 September was briefly inCancer before entering the constellation ofLynx.[11] On September 21 the ion tail was active, with some of twirls and knots, as it interacts with thesolar wind and the comet becoming more active as it approaches the Sun.[12] A blob of gas was visible moving away from the head along the ion tail on September 23.[13] By late September it had brightened to 8th to 9th magnitude, being visible with binoculars and small telescopes.[14] On 30 September 2025 the comet had brightened to magnitude 6.6 and photographically the ion tail was about 3 degrees long and there was also a dust tail visible too.[15] The comet experienced a disconnection event on 2 October 2025 as the solar wind stripped away a section of its tail.[16]
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In early October it enteredLeo Minor and after thatUrsa Major[11] and by 10 October 2025 wascircumpolar for northernlatitudes above 48°N.[17] On 10 October the comet had an apparent magnitude of 5.6 and its coma was 10 arcminutes across. Photographically, its ion tail was 12 degrees long on October 12.[18] The comet was first spotted withnaked eye on 15 October, with an estimated magnitude of 4.8, while 50mm binoculars revealed a tail 1.6 degrees long.[6] On 16 October it passed less than a degree fromCor Caroli and moved towards the southeast at a rate of 4 degrees per day.[11]
C/2025 A6 (Lemmon) approached Earth at a distance of 0.60 AU (90 million km; 56 million mi) on 21 October 2025.[10][19] Around the same time another comet was also visible,C/2025 R2 (SWAN), which made its closest approach to Earth one day earlier.[20] It reached anapparent magnitude of 3.5[8] to 4.4[1][6] according to different estimates, indicating that it could be visible to thenaked eye from sufficiently dark skies[21][11]. During its closest approach the comet was visible in the sunset sky with asolar elongation of 42 degrees.[22][19] It crossed thecelestial equator on 2 November 2025.[23]
Between 15 September and 12 October 2025, observations from theTeide Observatory reveal two large spiral jets emanating from the comet, which were likely producing its dust tail as it continues to approach perihelion.[24]
Spectra of the central part of the coma of C/2025 A6 (Lemmon) were taken with the Échelle spectrograph FLECHAS at the University Observatory Jena on 13 and 18 October 2025. In the wavelength range between 4500 and 7000 Å several emission features in the spectra of C/2025 A6 were detected, with those ofC2, [OI],NH2, as well asNa being the most prominent ones. The equivalent width of the Na D1 and D2 emission lines increased significantly between the two observation epochs, while the comet's heliocentric distance decreased from 0.79 to 0.71 AU.[25]
The comet has an inboundorbital period of about 1,350 years, indicating a previous perihelion in the second half of the 7th century. On 8 November 2025 occurred the perihelion of the comet when it was 0.53 AU (79 million km; 49 million mi) from the Sun. This perihelion passage reduced the orbital period to about 1,120 years.[3] During the 2025 apparition, the comet approached Earth at a distance 0.596 AU (89.2 million km; 55.4 million mi), on 21 October 2025.[19] It also approached Jupiter at a distance of 2.33 AU on 16 April 2025.[18]
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