Comet Chasing in May

Comet chasing is the visual observation of telescopic comets.  Jump to:  Observing synopses    Summary data    Buy me a Coffee 

We've been doing this page for 20 years, and have never seen so few comets available in amateur telescopes. For most observers, at least an 18-inch telescope will be required to see a comet this month. Some part of that is due to changes in our algorithms, but its primarily due to a true lack of observable comets. Remember, what sets this page apart from others is that we claculate whether or not a comet is actually visible in a telescope, rather than just report its magnitude. See the Observing Synopses for which comets are visible from your latitude, the best dates and the time of night.

• C/2025 F2 (SWAN) - disintegrated between April 20-25. As of the start of May it could still be imaged in long exposures as a very faint smudge but is otherwise unobservable.Beware that other sources may continue to report it as being observable. This comet was discovered in SWAN images. It brightened rapidly and was reported at magnitude 9.5 on April 4. After April 12 it began to fade, which marked the beginning of its disintegration. 

• C/2021 G2 (Atlas) is past perihelion, which occurred in early September 2024. It reached maximum brightness of magnitude 14 in late August.

• C/2023 A3 (Tsuchinshan-ATLAS) was very impressive in October. Although fading, it remains a telescopic comet throughout the month.

• 29P/Schwassmann-Wachmann has frequent outbursts, typically resulting in a brightening of 1 - 4.0 magnitudes. It varies in its distance from the Sun from 5.8 AU (at perihelion) to 6.3 AU (at aphelion), which is an unusually small variation for a comet, and remains quite far from the sun at all times. This means that it can be observed more or less continuously. See Mission 29P from the BAA for up to date information. 

• C/2025 D1 (Groeller) was disovered on February 20, 2025. It is an intrinsically bright (H 1.2 to 2.5) comet that is active at distances greater than 15 AU. It has a very distant perihelion of 14.1 AU. This comet will not be observable in amateur instruments except via imaging. 

• C/2014 UN271 (Bernardinelli-Bernstein) is an interesting Oort cloud comet that was reported as a result of the Dark Energy Survey. Soon after it was made public, images showed a cometary coma. Discovery credit goes to Pedro Bernardinelli and Gary Bernstein. This is a remarkable comet that was at 29 AU in 2014, with a perihelion of 10.9 AU in 2031. Its orbit extends out to 40,000 AU! It was unusually bright for its distance. HST observations that isolate the nucleus estimate the diameter to be between 120 and 140 km, making this possibly the largest comet nucleus yet measured. Because of its distant perihelion, this comet is not expected to become bright enough to be visually observed except in large instruments, and not until the end of the decade, but it is likely going to be the subject of intense scientific scrutiny. 

Comets that have apparently disintegrated: C/2025 F2, C/2024 G3, C/2024 S1 (ATLAS), C/2022 S3 (PANSTARRS), C/2021 P4 (ATLAS), C/2020 Q1 (Borisov), C/2020 P1 (NEOWISE), and C/2019 Y4 (ATLAS). Beware that various sources often fail to keep track of whether or not a comet still exists!

Comet Visibility in the Eyepiece

When you read below that a particular aperture is required to see a comet you can have a reasonable degree of confidence that the comet can in fact be seen in the eyepiece. 

This page uses code developed for SkyTools to predict the visibility of a comet.  Predicting how much aperture is required to see a comet is a very complex task. The magnitude alone is a very poor predictor.  SkyTools considers the magnitude, coma diameter, and degree of concentration of the comet, based on recent observations.The predictions are made for a Country/Suburban site, which is approximately Bortle 5, unless indicated otherwise. But always remember, comets are like cats. They both have tails and do what they want, and not always what we expect. This is one of the things that makes comet chasing interesting! 

Observing Synopses for May