2025 August 25

C/2025 A6 (Lemmon)

Nick James

C/2025 A6 (Lemmon) was discovered as an apparently asteroidal object on 2025 January 3.342 in images taken by David Fuls using the 1.5-m reflector of the Mt Lemmon Survey. It was originally placed on the PCCP as CCNG6P2. Follow-up images showed the object to be cometary and it was announced as C/2025 A6 (Lemmon) on CBET 5508.

The comet comes to a perihelion of 0.53 au on 2025 November 8 and the orbit is periodic with a period of a few thousand years. It has recently reappeared after solar conjunction and it is rather brighter than expected although we don’t yet have enough data to predict how bright it will get in early November. It might reach magnitude 5 or brighter if the current brightening is sustained. Please observe it if you can and make magnitude estimates to submit to COBS.

The comet is currently a morning object but it will move into the evening sky in mid-October. Those of us at higher northern latitudes are quite favoured as shown in the plots below. These show the (very uncertain) predicted magnitude based on H=7.9, G=3.2 and the (very accurate) altitude of the comet above the western (evening) and eastern (morning) horizons when the Sun is 15 degrees down.

The Moon is new on October 21 and full on November 5th so conditions are good for when the comet will be at its brightest and furthest above the horizon.

Current images are in the section archive here.

Update: 2025-09-07 – The comet continues to brighten, both in terms of total magnitude and near-nucleus magnitude. This gradual brightening is encouraging although it is still too early to tell how bright it will be in late October.

Update: 2025-09-14 – The comet continues to brighten smoothly and it is now looking reasonably likely that we will have a naked eye comet in the evening sky at the end of October. These two plots show the near nucleus magnitude (measured in a 9 arcsec radius) and the total magnitude (limited to observations made using Comphot on the green channel of a camera with a 72mm refractor). The scatter is small in both cases and, as would be expected, the total magnitude is rising faster than the nuclear magnitude.