Waiting for the next great comet: Why Hale-Bopp stands unrivaled

Have you seen comet C/2023 A3 (Tsuchinshan-ATLAS)? Surely, with so many pictures of it online. But have you seen it with the naked eye? That is not so certain. Such comets are often hailed as events of the year or decade, and online photos might suggest they are a spectacle. However, for someone hoping to see the comet with the naked eye from a city balcony, rather than seeking out a dark sky and clear horizon, these have long been disappointing.

There are billions of celestial bodies in the Solar System that can become comets, and estimating the exact number is challenging. Most of them lie far beyond Neptune's orbit in the Kuiper Belt, and in the case of long-period comets, in an even more distant spherical structure called the Oort cloud. Only a few venture close enough to the Sun due to gravitational influences to form a coma and tails. Some remain in the inner Solar System for extended periods, becoming short-period comets that revisit the Sun every few to several decades.

The latest data from the JPL Solar System Dynamics group indicates there are about 4,000 discovered comets and their fragments, with nearly 600 numbered. Around 100 comets were discovered last year. However, even this relatively large number doesn't mean comets are easy to see.

In recent years, there have been a few comets visible in the sky that most people will remember primarily from online pictures. In 2006, comet McNaught (C/2006 P1) was quite visible to the naked eye, but observers in the Southern Hemisphere were the lucky ones. Comet NEOWISE (C/2020 F3) also appeared attractively, though briefly and far from urban centres.

There are more photos of comet C/2023 A3 (Tsuchinshan-ATLAS) on the internet now than there are of comet C/1995 O1 Hale-Bopp from nearly 30 years ago. However, that comet, the Great Comet of 1997, was visible for 18 months and is considered the last such spectacular sight. We are still waiting for an equally impressive comet that everyone can easily see.

In 1997, analog, film-based point-and-shoot compact cameras served as the everyday camera for most people. They had even fewer settings than today's simplest smartphone's Auto mode. Few of those compacts allowed for tripod attachment and long exposure setting. The chances of capturing a good photo of a comet with a compact camera were slim.

Today, nearly every smartphone uses image processing techniques that improve the visibility of the night sky. Not to mention enthusiasts using stacking software, which simplifies taking long exposures without a driven tripod. A smartphone photo is sometimes a method to locate a faintly visible comet before attempting to find it with the naked eye. Comets visible today thanks to smartphone photos, even in cities, might have previously gone unnoticed.

A comet, when it's far from the Sun, resembles an icy asteroid. When it gets closer than Jupiter's orbit distance, material from the surface begins to sublimate, creating a very sparse envelope known as the coma, commonly referred to as the comet's head. Its size can reach hundreds of thousands or even millions of kilometres. This is why comets eventually become visible even to the naked eye.

Due to solar radiation and solar wind, tails—commonly referred to as comas—also form, sometimes stretching for millions of kilometres. The dust tail, directed away from the Sun, is curved due to the comet's orbital motion. The ion tail, aligned with the solar wind, consists of charged particles. Occasionally, an anti-tail directed towards the Sun appears, which can be seen in photos of C/2023 A3 (Tsuchinshan-ATLAS).

The sizes of the cometary nuclei are too small to be seen with the naked eye, but they influence the amount of material forming the comet's head and tails. The size of Hale-Bopp's nucleus was estimated at about 60 kilometres. Meanwhile, C/2023 A3 (Tsuchinshan-ATLAS) is at most 3 kilometres in diameter. Most comets are 10 kilometres or smaller. The largest tracked is comet Bernardinelli-Bernstein (C/2014 UN271) with a diameter of about 120 kilometres. It will approach the Sun only to the distance of Saturn's orbit and won't be visible to the naked eye.

Hale-Bopp's coma, at perihelion, was about 3 million kilometres in size, larger than the Sun's diameter. This is why it was such a bright comet, even though at perihelion, it was about 134 million kilometres from the Sun, and the minimum distance from Earth was as much as 196 million kilometres. C/2023 A3 (Tsuchinshan-ATLAS) came more than twice as close to the Sun and three times closer to Earth than Hale-Bopp. However, its coma grew to several times smaller in size. Something else contributed to its poorer visibility than the Great Comet of 1997.

Comet Hale-Bopp reached a brightness maximum of -0.8 mag, but comet Ikeya-Seki (C/1965 S1) was significantly brighter with a magnitude of -10 mag. Its nucleus is only a few kilometres away, but the perihelion of its orbit was merely 450,000 kilometres from the Sun.

Size is not the only criterion for a comet's good visibility. Comets travel on various orbits, whose shape and size determine how long they will stay near the Sun. The comet's location relative to Earth and the Sun during its approach, including the inclination of its orbit to the ecliptic, decides how favourably it will be positioned relative to the horizon during the night and, for most comets, how bright it will be.

Comet C/2023 A3 (Tsuchinshan-ATLAS) appeared most attractive when it was close to the Sun. It was situated low on the horizon, making it very difficult to spot in cities. Thanks to its broad orbit, almost perpendicular to the ecliptic, comet Hale-Bopp could be observed high in the already dark sky. Comet Hyakutake also had a favourable position, visible well throughout the night in early 1996. Many astronomy enthusiasts consider it more interesting than Hale-Bopp.

The millennial generation knows how much the skies around cities have brightened over the turn of the century. Several decades ago, the night sky looked quite attractive even in large metropolitan areas. Today, where there used to be a dark clearing, we have housing developments and bright lights around each.

It's hard to accurately predict a comet's brightness evolution right after its discovery, which is why we often experience a lot of hype with little to show. Comets that venture very close to the Sun can be torn apart by solar gravity or completely vaporize before they reach Earth's vicinity.

Many astronomers believe that the approaching comet C/2024 S1 (ATLAS), initially very promising, will turn out to be a big disappointment. But even if it met expectations, it still wouldn't qualify as a comet as spectacular as Hale-Bopp. We are still waiting for its successor.