C/2025 A6 (Lemmon)
A non-periodic comet that brightened unexpectedly to naked-eye visibility.
C/2025 A6 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey on 3 January 2025. It became notable for brightening far beyond initial predictions, reaching naked-eye visibility during its close approach to Earth in October 2025.
The comet was first spotted as an asteroidal object at magnitude 21.5, about 4.5 AU from the Sun. Precovery images from PanSTARRS, taken on 12 November 2024, were later found. By 21 February 2025, it showed a very condensed coma 2.2 arcseconds across and a short tail two arcseconds long. Initial expectations placed its peak brightness at magnitude 10, but it ultimately reached about magnitude 3.5—roughly 400 times brighter than predicted.
After solar conjunction on 2 July 2025, the comet reappeared in the morning sky significantly brighter than forecast. On 12 August, its coma spanned about one arcminute at magnitude 13.9. By 19 August, at 1.7 AU from the Sun, its total magnitude was estimated at 11, though no obvious tail was visible. A short spiky tail appeared in images from 25 August. The comet moved through Gemini in August, briefly entered Cancer on 6–7 September, then passed into Lynx. Its ion tail became active by 21 September, showing twirls and knots from solar wind interactions. A gas blob moved away from the head along the tail on 23 September. By late September, it had brightened to magnitude 8–9, visible in binoculars and small telescopes. On 30 September, at magnitude 6.6, the ion tail stretched about 3 degrees photographically, alongside a visible dust tail. A disconnection event occurred on 2 October, when the solar wind stripped away a section of the tail.
In early October, the comet entered Leo Minor, then Ursa Major, becoming circumpolar for northern latitudes above 48°N by 10 October. On that date, its apparent magnitude was 5.6, with a coma 10 arcminutes across; the ion tail measured 12 degrees long photographically on 12 October. It was first spotted with the naked eye on 15 October at magnitude 4.8, while 50mm binoculars revealed a tail 1.6 degrees long. On 16 October, it passed less than a degree from Cor Caroli, moving southeast at 4 degrees per day.
The comet made its closest approach to Earth on 21 October 2025, at a distance of 0.60 AU (90 million km). Around the same time, another comet, C/2025 R2 (SWAN), also approached Earth one day earlier.
Quick Facts
- Discovery Ref
- MPEC_2025-D55
- Discoverer
- D. Carson FulsYoshida
- Discovery Site
- Mount Lemmon Survey
- Discovery Date
- 3 January 2025
- Orbit Ref
- barycenter · jpldata
- Epoch
- 23 September 2025 / (JD 2460941.5)
- Observation Arc
- 488 days (1.34 years)
- Earliest Precovery Date
- 12 November 2024MPEC_2025-D55
- Obs
- 1,883MPC
- Perihelion
- 0.5299 AUperihelion
- Aphelion
- 242 AU (inbound)barycenter / 222 AU (outbound)
- Semimajor
- 121 AU (inbound)barycenter / 110 AU (outbound)
Facts from the source article.
Lore & Background
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. Precovery images by PanSTARRS dating from 12 November 2024 were later found. The object initially showed a very condensed coma 2.2 arcseconds across, and a short tail two arcseconds long was detected in images from 21 February 2025. Originally expected to brighten only to apparent magnitude 10, the comet instead reached about magnitude 3.5, roughly 400 times brighter than original expectations.
After solar conjunction on 2 July 2025, the comet reappeared in the morning sky significantly brighter than predicted. By late September it had brightened to 8th to 9th magnitude, becoming visible with binoculars and small telescopes. On 30 September 2025 the comet had brightened to magnitude 6.6, with an ion tail about 3 degrees long and a dust tail also visible. It experienced a disconnection event on 2 October 2025 as the solar wind stripped away a section of its tail. The comet was first spotted with the naked eye on 15 October, with an estimated magnitude of 4.8. It made its closest approach to Earth on 21 October 2025 at a distance of 0.60 AU, reaching an apparent magnitude of 3.5 to 4.4 according to different estimates, and was visible in the sunset sky with a solar elongation of 42 degrees.
Spectra taken on 13 and 18 October 2025 at the University Observatory Jena detected emission features of C2, [OI], NH2, and Na in the coma. The equivalent width of the sodium D1 and D2 emission lines increased significantly between the two observation epochs as the comet's heliocentric distance decreased from 0.79 to 0.71 AU. By November 2025, OH-line spectra were detected from its coma through the Five-hundred-meter Aperture Spherical Telescope (FAST). The comet has an inbound orbital period of about 1,340 years, indicating a previous perihelion in the second half of the 7th century; its perihelion passage reduced the orbital period to about 1,170 years.
Reader's Guide
C/2025 A6 (Lemmon) is significant as a comet that dramatically exceeded brightness predictions, becoming a naked-eye object visible from dark skies during its October 2025 apparition. Its discovery by the Mount Lemmon Survey and subsequent precovery by PanSTARRS illustrate the role of survey telescopes in detecting distant comets. The comet's brightening from an expected magnitude 10 to about magnitude 3.5—a factor of 400—made it a notable target for both amateur and professional observers. Its close approach to Earth on 21 October 2025, coinciding with the visibility of another comet, C/2025 R2 (SWAN), provided a rare opportunity for comparative observation. The comet's ion tail exhibited dynamic activity, including a disconnection event on 2 October 2025, and its dust tail was linked to two large spiral jets observed from the Teide Observatory. Spectroscopic studies revealed emissions of C2, [OI], NH2, and sodium, with the sodium lines strengthening as the comet neared the Sun. The comet's orbital period of about 1,340 years inbound, reduced to about 1,170 years after perihelion, places it among long-period comets with a history of previous apparitions centuries ago. Its legacy includes multiple NASA Astronomy Picture of the Day features, documenting its changing tail, brightness, and visibility against the night sky.
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