The Comet C:/2025 A6 (Lemmon)

 

Comet C:/2025 A6 (Lemmon) imaged by Kurt Thrust at the JPO
on the 21st October 2025
using a fixed tripod mounted Canon 600d DSLR
 with a 135mm F2 Samyang lens. Stacks of 10x5 sec subs at ISO 1600.

" Kurt was very lucky to capture some short exposure subs of the Comet C/2025 A6 (Lemmon) from the approach road to the Jodrell Plank Observatory, on the 21st October 2025. In the early evening twilight and amongst the clouds and light pollution, the comet was extremely difficult to see with the naked eye. It was located very low in the north west and to the west of the bright star Arcturus, which Kurt used as a guide. In such circumstances it is very useful to have the Stellarium App on your smartphone! 

Bearing in mind the shortness of the sub exposures a reasonable amount of detail can be seen in Kurt's two stacked images. Close inspection reveals: a small green coma around the comet nucleus, separate ion and dust clouds and some disturbance in the fainter part of the ion tail created by the current strong solar wind". - Joel Cairo CEO of the Jodrell Plank Observatory.

"Comet C/2025 A6 (Lemmon) is a long-period comet discovered on 3 January 2025 by the Mount Lemmon Survey at Mount Lemmon, Arizona. 

 It is notable for its striking greenish coma and its relatively rare visit to the inner Solar System, offering a “once-in-a-millennium” viewing opportunity.

Orbital and Physical Characteristics:

Perihelion distance (q): ~0.5299 AU from the Sun.

Closest approach to Earth: ~0.596 AU (~89 million km) on 21 October 2025. 

Orbital eccentricity: approximately 0.9957 (inbound) indicating a very elongated trajectory. 

Inclination: ~143.66° relative to the ecliptic. 

Estimated orbital period before perturbations: ~1,350 years. 

The comet’s coma displays a green hue, attributed to diatomic carbon (C₂) fluorescing under solar ultraviolet radiation. 

Observational History and Brightness Evolution:

Upon discovery, the comet had a magnitude of ~21.6 while at ~4.5 AU from the Sun. 

 As it moved inward, its brightness increased more rapidly than initially predicted, making it an excellent target for astrometric monitoring and public sky-watching.

By late September and early October 2025, the comet’s magnitude had improved to around +8 to +6.6, with a visible ion tail several degrees long in photographs. 

 Projections suggested it might reach magnitudes around +3.5 to +4 at closest approach — potentially visible to the naked eye under dark skies. 

Coma and Tail Morphology:

The nucleus of the comet is surrounded by a diffuse coma of sublimated ices and entrained dust. As the comet approached perihelion, solar heating drove the sublimation of volatile ices, producing jets and releasing dust particles. These created a dual-tail structure:

An ion tail, composed of ionised gas interacting with the solar wind, giving a straight, bluish or greenish stream pointing away from the Sun. 

A dust tail, composed of larger dust grains lagging behind the nucleus, giving a broader, more curved appearance. 

On 2 October 2025, a tail‐disconnection event was reported — a phenomenon where part of the ion tail is severed by a magnetic shock in the solar wind. 

Significance and Future Return:

Because of its long period and highly elongated orbit, Comet Lemmon is not expected to return for ~1,000 years or more — making this passage effectively a once-in-a-human-lifetime event. 

 Moreover, its green coma offers both aesthetic and scientific interest: the chemistry of C₂ and other radicals in cometary comae is a window into the pristine ices preserved since the Solar System’s formation.

Viewing Circumstances (for Northern Hemisphere):

From mid- to late October 2025, the comet became progressively favourably placed in the evening sky. It moved through constellations such as Boötes and Serpens, and at its peak was visible at solar elongations of ~30-40°. 

 Observers are advised to look from a dark site, away from light pollution, and employ binoculars or small telescopes to first locate the object; naked-eye spotting is possible under very good conditions. 

Scientific Importance:

Comets such as C/2025 A6 (Lemmon) are fragments from the outer Solar System (likely the Oort Cloud) that are only occasionally perturbed into the inner region. Studying their composition, dust and gas production rates, and response to solar heating helps refine models of Solar System evolution, volatile delivery to the terrestrial planets, and the dynamics of cometary orbits. Moreover, morphological changes (such as tail disconnection events) provide in situ diagnostics of the solar wind–comet interaction.

Summary:

Comet C/2025 A6 (Lemmon) presents a rare observational opportunity: a long‐period comet with a greenish coma, brightening unexpectedly to visibility, and approaching within ~0.6 AU of Earth in October 2025, with perihelion at ~0.53 AU in November. Its highly elongated orbit, infrequent return, and the dual‐tail morphology make it both a compelling target for amateur and professional astronomers, and a valuable subject for cometary science". Professor G.P.T. Chat visiting astrophysicist at the Jodrell Plank Observatory.