Great Comets and Great Disappointments, by Nick James

For our sixth main lecture of the 2025/26 season, Nick James, Head of the Comet Section of the British Astronomical Association, took us on a whistlestop tour of both great and disappointing comets.

Before this, however, we had two short talks from FAS members.

Firstly, Mike Meynell launched the 2026 solar viewing season. Solar viewing has been running since 2003 and is a joint enterprise between FAS and the Royal Observatory Greenwich (ROG). Last year was hugely successful, with around 5,000 visitors across 13 events supported by 35 FAS volunteers. The sessions begin in April and run through to the end of August, typically from around 11am to 4pm. We use both white-light and hydrogen-alpha solar telescopes to enable the public to see the photosphere and chromosphere respectively, and we have handy visual guides to help people learn about the Sun and understand what they may see.

If any FAS members are interested in volunteering, the dates are on the website, and buddying and support from existing volunteers ensure that everyone feels confident to get involved. There is nothing needed except enthusiasm, so please do sign up if you are interested!

Secondly, Paul May gave an introduction to the Cherenkov telescopes on La Palma. Cherenkov telescopes look at high-energy gamma rays. They don’t detect gamma rays directly. Instead, they capture the brief flashes of blue light produced when high-energy gamma rays hit Earth’s atmosphere. Cherenkov radiation was first predicted by Oliver Heaviside in the 1880s, but it took until the 1930s for it to be confirmed experimentally by Pavel Cherenkov.

La Palma has three Cherenkov telescopes. Two are a pair and part of the MAGIC system (Major Atmospheric Gamma Imaging Cherenkov). The third is part of the Cherenkov Telescope Array Observatory (CTAO) northern hemisphere array, complementing the larger southern array in Chile.

The Cherenkov telescopes are visually arresting, comprising a honeycomb mirror system, and can respond to any point in the sky in 25 seconds, catching gamma rays before they fade. They have a sampling rate of 2 billion frames per second. Cherenkov telescopes let us study the most extreme objects in the universe - supermassive black holes, pulsars, supernova remnants. They are currently the most powerful ground-based tools for high-energy astrophysics.

Following this, Nick James took us to the opposite extreme – exploring some of the coldest, slowest, and oldest objects in the Solar System - comets. Comets are fascinating remnants of planetary formation, and their discovery generates considerable excitement across the globe.

Comets are icy bodies left over from the formation of the Solar System, often described as “dirty snowballs” of dust, rock, and frozen gases. They fall into two main groups: short-period comets, which originate in the Kuiper Belt and return in under 200 years, and long-period comets, which come from the distant Oort Cloud and can take thousands of years to reappear. As a comet approaches the Sun, its ices vaporise, forming a glowing coma and two distinct tails - one of dust, one of ionised gas - revealing clues about early planetary materials. Despite increasing numbers of discoveries, comets remain surprisingly hard to understand, because the nucleus - the solid “head” - is tiny, dark, and wrapped in a bright, glowing coma that overwhelms most cameras. Even powerful telescopes struggle to see through that haze. Additionally, comets change rapidly as they heat up, so the very moment we try to observe them, they’re already evolving.

And what makes a great comet? Not just the size of the nucleus, but also how the comet moves in the Solar System in relation to us and the Sun. Some can be very bright but hard to see as they sail so close to the Sun, and comets need to be brighter now due to the increased light pollution we experience on Earth.

Nick highlighted some impressive comets that have been visible in recent decades, including:

• Halley’s Comet (1P/Halley): The celebrity of the comet world. It returns every 76 years (with the last visit in 1986) and has been recorded for over two millennia. It is represented in the Bayeux Tapestry when it came to within 0.1 AU of Earth on 24.04.1066.

• Comet Hale–Bopp (C/1995 O1): One of the brightest comets of the 20th century, visible for a record 18 months to the naked eye. It was so bright it could be seen even from light-polluted cities.

• Comet Hyakutake (C/1996 B2): Passed extremely close to Earth and produced a spectacularly long tail stretching across the sky. A dramatic, sudden visitor.

• Comet NEOWISE (C/2020 F3): A modern favourite. It became a stunning naked-eye object in the northern hemisphere, giving many people their first real comet experience.

• Comet McNaught (C/2006 P1): The “Great Comet of the Southern Hemisphere,” famous for its enormous, fan-shaped tail.

But not all comets are easy to see - in fact, most aren’t. Some of the most disappointing comets are those overhyped by the media but hard to see in reality, including:

• Comet Kohoutek (C/1973 E1): Hyped as the “Comet of the Century,” expected to blaze across the sky… and instead appeared faint, diffuse, and underwhelming. It’s still the gold standard for comet disappointment.

• Comet ISON (C/2012 S1): Touted as a potential “once-in-a-lifetime daylight comet.” Then it swung too close to the Sun, disintegrated, and left only a ghostly smear. A heartbreak for astronomers and skywatchers.

• Comet 3I/ATLAS (C/2025 N1): An interstellar comet which remained faint and scientifically interesting, but never became a prominent object for observers.

Nick rounded off our evening by answering questions around where comets come from (potentially a never-ending supply in the Kuiper Belt or the Oort Cloud), that comet tails are not in fact behind the comet but pushed away from the Sun no matter what direction the comet is heading, and that spectral analysis is carried out to help determine composition.

A huge thanks to Nick for his talk and enthusiasm for comets which, despite all our attention and interest, remain some of the most unpredictable objects we study.

Pictures from the evening (by Mike Meynell):