C∕2025 A6 (Lemmon) Comet

Polish version is here

Comet C/2025 A6 (Lemmon) belongs to the group of long-period objects originating from the outer regions of the Solar System, most likely from the Oort Cloud. It was discovered on January 3, 2025, as part of the Mount Lemmon Survey using a 1.5-meter-aperture telescope. At the time of detection, it was located 4.5 astronomical units from the Sun, with an apparent magnitude of about 21.5^m. Shortly after the announcement of the discovery, earlier images of the object were identified in Pan-STARRS survey data collected in November 2024, allowing for a more precise determination of its orbit.

Initial observations revealed a condensed coma approximately 2.2 arcseconds across and a short dust tail about 2 arcseconds in length. Early predictions suggested that the comet would reach a peak brightness of around 10^m, but as it approached the Sun it exhibited far greater activity, eventually reaching a magnitude between 3.5^m and 4.0^m, nearly 400 times brighter than originally expected.

The orbit of comet C/2025 A6 (Lemmon) is highly eccentric (e ≈ 0.9957) and inclined by about 143.7° relative to the ecliptic plane. Before its current passage, the comet’s orbital period around the Sun was approximately 1,350 years, indicating that its previous perihelion occurred in the second half of the 7th century. During its current passage, the comet reached perihelion on November 8, 2025, at its closest point to the Sun, a distance of 0.53 AU. The minimum distance from Earth was 0.60 AU (about 89 million km) on October 21, 2025.

In April 2025, the comet passed within 2.33 AU of Jupiter, an encounter that may have caused slight perturbations in the orientation of its orbit and in the rotation rate of its nucleus. As a result of these gravitational interactions, the comet’s orbital period is expected to shorten to about 1,160 years during its next revolution. At its peak brightness in October 2025, the solar elongation reached around 42°, allowing the comet to be observed in the evening sky even with the naked eye from areas with low light pollution.

Between October 1 and 2, part of the comet’s tail was detached by the solar wind following a sudden increase in solar magnetic activity. From late September to early October, the comet’s brightness reached about magnitude 6.6^m, and by mid-October it exceeded 5^m, making it visible to the unaided eye under favorable observing conditions.

In the second half of October, the comet’s apparent motion against the background stars reached 4° per day. Around mid-month, it passed close to the star Cor Caroli in the constellation Canes Venatici. During the same period, another comet, C/2025 R2 (SWAN), was also visible, making the autumn of 2025 a particularly memorable season for comet observers.

Observations

October 25, 2025 – Jaworzno (Poland)

urban area, high light pollution

That evening, shortly after sunset, the comet was clearly visible within the boundaries of the constellation Serpens, particularly in the region known as Serpens Caput, the Serpent’s Head. The comet appeared truly magnificent in the photograph taken that evening (Photo 1).

Photo 1 – without labels

Photo 1 – with labels

Spectroscopic observations conducted in October 2025 at observatories in Jena and Tenerife revealed a rich emission spectrum of the comet spanning the 4500–7000 Å range. Characteristic bands of diatomic carbon molecules were identified, along with emission lines of excited oxygen, amines, and sodium (Na D₁ and Na D₂). The intensity of these features increased significantly between October 13 and 18, 2025, as the comet approached the Sun.

Observations made during the same period at the Teide Observatory (IAC) revealed two spiral gas-and-dust jets rotating together with the motion of the nucleus. Analysis of changes in the coma’s morphology allowed the nucleus rotation period to be estimated at approximately nine hours. These jets, which constitute the main source of dust in the comet’s tail, indicate the presence of several active sublimation regions, probably dominated by volatile carbon-bearing species such as CO and CO₂.

Comet C/2025 A6 (Lemmon) represents a valuable research target for studies of the dynamical evolution of long-period comets. The presence of spiral jets further indicates a complex rotation and surface heterogeneity, making this comet an excellent object for comparison with other long-period comets such as C/1996 B2 (Hyakutake) and C/2020 F3 (NEOWISE).

Photo 1 Parameters:

Canon EOS 60D
Total exposure time: 3 minutes (stack of 12 RAW frames at 15s each)
ISO: 1000
Lens: zoom
Aperture: f/4

Weird Science

C∕2025 A6 (Lemmon) Comet

Obse­rva­tions

Observations