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C/2025 A6 (Lemmon)

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

C/2025 A6 (Lemmon)
C/2025 A6 (Lemmon) on 23 October 2025 with a 135mm telephoto lens
Discovery[2]
Discovered byD. Carson Fuls[1]
Discovery siteMount Lemmon Survey
Discovery date3 January 2025
Orbital characteristics[3][5]
Epoch1 September 2025 (JD 2460919.5)
Observation arc349 days
Earliest precovery date12 November 2024[2]
Number of
observations		1279
Aphelion243.9 AU (inbound)[3]
219.7 AU (outbound)
Perihelion0.5299 AU[4]
Semi-major axis122 AU (inbound)[3]
110 AU (outbound)
Eccentricity0.9957 (inbound)[3]
0.9952 (outbound)
Orbital period≈1350 years (inbound)[3]
≈1155 years (outbound)
Max. orbital speed57.8 km/s at perihelion[4]
Inclination143.66°
108.10°
Argument of
periapsis		132.97°
Mean anomaly359.95°
Next perihelion8 November 2025 12:59 UT[4]
TJupiter–0.684
Earth MOID0.308 AU
Jupiter MOID0.571 AU
Comet total
magnitude (M1)		10.6

C/2025 A6 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey in images obtained on 3 January 2025. It made its closest approach to Earth on 21 October 2025 and is visible to the naked eye. As of 3 November 2025, the comet is about apparent magnitude 4.3,[6] and is visible to the naked eye and in binoculars in the constellation of Ophiuchus near the globular cluster Messier 12.

Observational history

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Discovery

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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.[2] 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]

Approaching perihelion

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The comet on 17 September 2025

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.

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] The comet experienced a disconnection event on 2 October 2025 as the solar wind stripped away a section of its tail.[16]

Time-lapse of C/2025 A6 (Lemmon) captured in October 2025 showing 55 minutes of movement. The tail can be seen pointing away from the Sun.

In early October it entered Leo Minor and after that Ursa Major[11] and by 10 October 2025 was circumpolar for northern latitudes 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 with naked 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 from Cor 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 an apparent magnitude of 3.5[8] to 4.4[1][6] according to different estimates, indicating that it could be visible to the naked eye from sufficiently dark skies[21][11]. During its closest approach the comet was visible in the sunset sky with a solar elongation of 42 degrees.[22][19] It will cross the celestial equator on 2 November 2025.[23]

Physical characteristics

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Near its brightest, on October 30, 2025

Between 15 September and 12 October 2025, observations from the Teide 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 of C2, [OI], NH2, as well as Na 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]

Orbit

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

The comet has an inbound orbital period of about 1,350 years, indicating a previous perihelion in the second half of the 7th century. On 8 November 2025 occurs the perihelion of the comet 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 1,155 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]

See also

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References

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  1. ^ a b S. Yoshida. "C/2025 A6 (Lemmon)". aerith.net. Retrieved 10 September 2025.
  2. ^ a b c G. Paek; K. Chambers; Y. Ramanjooloo; R. Wainscoat; I. Smith; et al. (2025). "Comet C/2025 A6 (Lemmon)". Minor Planet Electronic Circulars. 2025-D55. Bibcode:2025MPEC....D...55P. doi:10.48377/MPEC/2025-D55.
  3. ^ a b c d e f Horizons output. "Barycentric Osculating Orbital Elements for Comet Lemmon (C/2025 A6)". Retrieved 9 October 2025. (Solution using the Solar System's barycenter (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0)
    Inbound period (PR) = 4.931E+05 / 365.25 days = 1350 years
    Outbound period (PR) = 4.217E+05 / 365.25 days = 1155 years
  4. ^ a b c "Perihelion on 8 November 2025". JPL Horizons. Retrieved 29 October 2025. (when rdot = 0)
  5. ^ "C/2025 A6 (Lemmon) – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Jet Propulsion Laboratory. Retrieved 10 September 2025.
  6. ^ a b c "COBS Observation list: C/2025 A6". COBS – Comet OBServation database. Crni Vrh Observatory. Retrieved 16 October 2025. (C/2025 A6 lightcurve)
  7. ^ D. C. Fuls; R. Weryk; J. Fairlamb (22 February 2025). D. W. Green (ed.). "Comet C/2025 A6 (Lemmon)". Central Bureau for Astronomical Telegrams. 5508. Bibcode:2025CBET.5508....1F.
  8. ^ a b G. van Buitenen. "C/2025 A6 (Lemmon)". astro.vanbuitenen.nl. Archived from the original on 21 March 2025. Retrieved 10 October 2025. (Difference in brightness: 2.512(10 − 3.5) = 400)
  9. ^ "Solar conjunction on 2 July 2025". JPL Horizons. Retrieved 4 September 2025.
  10. ^ a b H. Sato; M. Mattiazzo; K. Kadota; et al. (20 August 2025). D. W. Green (ed.). "Comet C/2025 A6 (Lemmon)". Central Bureau for Astronomical Telegrams. 5594. Bibcode:2025CBET.5594....1S.
  11. ^ a b c d e B. King (3 September 2025). "Sweet Prospects for Comet Lemmon (C/2025 A6)". Sky & Telescope. Retrieved 4 September 2025.
  12. ^ T. Phillips. "Spaceweather.com Time Machine: Thursday, Sep. 25, 2025". www.spaceweather.com. Retrieved 26 September 2025.
  13. ^ N. James (23 September 2025). "Observation by Nick James: C/2025 A6 (Lemmon) tail motion - BAA Image Library". britastro.org. British Astronomical Association. Retrieved 26 September 2025.
  14. ^ S. Atkinson (26 September 2025). "How to see comet C/2025 A6 (Lemmon) this autumn". BBC Sky at Night Magazine. Retrieved 27 September 2025.
  15. ^ "Comet Lemmon brightens". Astronomy Magazine. Retrieved 3 October 2025.
  16. ^ A. Wood (14 October 2025). "Solar wind tears a chunk from Comet Lemmon's tail in incredible new astrophotography images". Space.com. Retrieved 16 October 2025.
  17. ^ "Circumpolar at declination +42". JPL Horizons. Retrieved 6 September 2025.
  18. ^ a b V. Gonano; S. Nakano; C. Hergenrother; et al. (13 October 2025). D. W. Green (ed.). "Comet C/2025 A6 (Lemmon)". Central Bureau for Astronomical Telegrams. 5621.
  19. ^ a b c "Earth approach on 21 October 2025". JPL Horizons. Retrieved 1 September 2025.
  20. ^ D. Dobrijevic (19 October 2025). "Watch comets Lemmon and SWAN make their closest approach to Earth on Oct. 24 (video)". Space.com. Retrieved 28 October 2025.
  21. ^ "Cometary Activity: Mission A6". Unistellar. Archived from the original on 30 August 2025. Retrieved 31 August 2025.
  22. ^ E. Irizarry (21 August 2025). "5 bright comets approaching Earth. Charts here!". EarthSky.org. Retrieved 24 August 2025.
  23. ^ "Crossing the celestial equator at declination 0". JPL Horizons. Retrieved 30 August 2025.
  24. ^ M. Serra-Ricart; J. Licandro; M. R. Alarcon (21 October 2025). "Strong jet activity observed on comet C/2025 A6 (Lemmon) with the Two-Meter Twin Telescope". The Astronomer's Telegram. 17450. Bibcode:2025ATel17450....1S.
  25. ^ M. Mugrauer; K. U. Michel; B. Dincel; E. Loesch (27 October 2025). "Follow-Up Imaging and Spectroscopic Observations of Comet C/2025 A6". The Astronomer's Telegram. 17456.
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