C/2025 A6 (Lemmon)

For other comets discovered by the Mount Lemmon Survey, see Comet Lemmon.

C/2025 A6 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey in images obtained on 3 January 2025. It has an inbound orbital period of about 1,350 years and will pass perihelion on 8 November 2025 when it will be 0.53 AU (79 million km; 49 million mi) from the Sun. This perihelion passage will reduce the orbital period to about 1,150 years. It makes its closest approach to Earth on 21 October 2025 and may be visible to the naked eye 42 degrees from the Sun after sunset. As of 13 October 2025^[update], the comet is about apparent magnitude 5.4,^[6] and is visible in binoculars about 20 degrees (approximately two fists) below the handle of the Big Dipper.

C/2025 A6 (Lemmon)

C/2025 A6 (Lemmon) on 2 October 2025 from a 70 mm (3 in) telescope
Discovery[1]
Discovered by	D. Carson Fuls[2]
Discovery site	Mount Lemmon Survey
Discovery date	3 January 2025
Orbital characteristics[3][4]
Epoch	10 August 2025 (JD 2460897.5)
Observation arc	325 days
Earliest precovery date	12 November 2024
Number of observations	758
Aphelion	243.5 AU (inbound)[3] 219.3 AU (outbound)
Perihelion	0.5299 AU[5]
Semi-major axis	122 AU (inbound)[3] 110 AU (outbound)
Eccentricity	0.9957 (inbound)[3] 0.9952 (outbound)
Orbital period	≈1350 years (inbound)[3] ≈1150 years (outbound)
Max. orbital speed	57.8 km/s at perihelion[5]
Inclination	143.66°
Longitude of ascending node	108.10°
Argument of periapsis	132.97°
Mean anomaly	359.93°
Next perihelion	8 November 2025 12:58 UT[5]
TJupiter	–0.684
Earth MOID	0.308 AU
Jupiter MOID	0.571 AU
Comet total magnitude (M1)	9.6
Comet nuclear magnitude (M2)	11.3

Observational history

C/2025 A6 (Lemmon) was discovered as an asteroidal object with an apparent magnitude of about 21.5 by the Mount Lemmon Survey in images obtained on 3 January 2025 when it was 4.5 AU from the Sun. Consequently, precovery images by PanSTARRS dating from 12 November 2024 were found.^[1] The object was found to have a very condensed coma, 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 to apparent magnitude 10, but is now expected to brighten to apparent magnitude of about 3.5, which would make it about 400 times brighter than original expectations.^[8]

After being hidden in the Sun's glare during June and July as it came to solar 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 an apparent 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 exited solar conjunction.

Time-lapse of C/2025 A6 (Lemmon) captured in September 2025 showing 69 minutes of movement. The tail can be seen moving away from the core.

The comet in August was located in the constellation of Gemini and on 6-7 September was briefly in Cancer before entering the constellation of Lynx.^[11] On September 21 the ion tail was active, with some of twirls and knots, as it interacts with the solar 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] In early October it will enter Leo Minor and after that Ursa Major^[11] and by 10 October 2025 will be circumpolar for northern latitudes above 48N.^[16] On 16 October it will pass less than a degree from Cor Caroli and will move towards the southeast at a rate of 4 degrees per day.^[11]

Animation of C/2025 A6 around Sun - 2025 close approach
   C/2025 A6 ·    Earth ·   Mercury ·    Venus ·    Mars

C/2025 A6 (Lemmon) will approach Earth at a distance of 0.60 AU (90 million km; 56 million mi) on 21 October 2025.^[10][17] It is expected to reach an apparent magnitude of 3.5^[8] to 4.4^[2][6] according to different estimates, with the more optimistic predictions indicating that it could be visible to the naked eye from sufficiently dark skies.^[18][11] During its closest approach the comet will be visible in the sunset sky with a solar elongation of 42 degrees.^[19][17] It will cross the celestial equator on 2 November 2025.^[20]