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C/2025 A6

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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 was visible to the naked eye.
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. 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.
When first discovered the comet was expected to only brighten to apparent magnitude 10, but brightened to apparent magnitude of about 3.5, which would make it about 400 times brighter than original expectations.

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.
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. By November 2025, the OH-line spectra were detected originating 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. 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 1,170 years. 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. It also approached Jupiter at a distance of 2.33 AU on 16 April 2025.