Waiting for a cosmic show: The elusive brilliance of comets

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 obvious. These comets are often heralded as events of the year or decade, and online photos might suggest that's indeed the case. However, these events have often been disappointing for someone hoping to spot the comet from a city balcony rather than seeking out a dark sky and clear horizon.

There are billions of celestial bodies in the Solar System that can become comets, and estimating the exact number is difficult. Most lie far beyond Neptune's orbit in the Kuiper Belt and in the even more distant Oort Cloud for long-period comets. Only a few venture close enough to the Sun due to gravitational perturbations, forming a coma and tails. Some remain in the inner system for extended periods, becoming short-period comets that return to the Sun every few to several decades.

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

In recent years, a few comets visible in the sky have been observed, primarily through online photos. In 2006, comet McNaught (C/2006 P1) was visible to the naked eye for observers in the southern hemisphere. Comet NEOWISE (C/2020 F3) also presented attractively, albeit briefly and far from city lights.

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

In 1997, everyday photography relied on analog, film-based point-and-shoot cameras. They had even fewer settings than today's simplest smartphone's Auto mode. Few of those cameras allowed for a tripod attachment or long exposure settings, making capturing good photos of a comet challenging.

Today, most smartphones use image processing techniques that enhance night sky visibility. Enthusiasts also use stacking software, simplifying long exposures without a driven tripod. A smartphone photo can even be a way to locate a faintly visible comet before attempting to spot it with the naked eye. Comets visible today, thanks to smartphone photos, even in cities, might have gone unnoticed in the past.

When far from the Sun, a comet resembles an icy asteroid. When it gets closer to Jupiter's orbit, material from its surface begins to sublimate, creating a sparse envelope known as the coma, referred to as the comet's head. Its size can reach hundreds of thousands or even millions of miles, making comets visible to the naked eye.

Due to solar radiation and the solar wind, tails, known as comas, also form, sometimes stretching for millions of miles. The dust tail, directed away from the Sun, curves 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, visible in photos of C/2023 A3 (Tsuchinshan-ATLAS).

The sizes of cometary nuclei are too small to be seen with the naked eye but influence the material amount forming the comet's head and tails. Hale-Bopp's nucleus was estimated at about 37 miles, while C/2023 A3 (Tsuchinshan-ATLAS) is at most 2 miles in diameter. Most comets are 6 miles or smaller. The largest tracked is comet Bernardinelli-Bernstein (C/2014 UN271), with a diameter of about 75 miles, though it will only approach the Sun to Saturn's orbit and won't be visible unaided.

Hale-Bopp's coma, at perihelion, was about 1.9 million miles in size, larger than the Sun's diameter. This is why it was so bright, although at perihelion it was about 83 million miles from the Sun, and the closest it came to Earth was 122 million miles. C/2023 A3 (Tsuchinshan-ATLAS) came more than twice as close to the Sun and thrice as close to Earth as Hale-Bopp. However, its coma was several times smaller, contributing to its poorer visibility compared to the Great Comet of 1997.

Comet Hale-Bopp reached a brightness of -0.8 magnitude, but comet Ikeya-Seki (C/1965 S1) was significantly brighter at -10 magnitude. Despite its small nucleus, it traveled just 280,000 miles from the Sun.

Size is not the only criterion for a comet's visibility. Comets travel on various orbits whose shape and size determine how long they stay near the Sun. The comet's position relative to Earth and the Sun during its approach, including the inclination to the ecliptic, influences how well-placed it is relative to the horizon at night, affecting brightness.

Comet C/2023 A3 (Tsuchinshan-ATLAS) was most attractive near the Sun, situated low on the horizon, making it hard 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 had a similarly favorable position, visible well throughout the night in early 1996, and was considered more interesting by many than Hale-Bopp.

The millennial generation knows how much skies have brightened over cities since the turn of the century. Decades ago, the night sky was more attractive, even in large metropolitan areas. Today, where there were once dark clearings, there are housing developments and bright lights.

It's challenging to accurately predict a comet's brightness evolution right after discovery, leading often to much hype with little to show. Comets that get very close to the Sun can be torn apart by its gravity or vaporize before reaching Earth's vicinity.

Many astronomers believe the approaching comet C/2024 S1 (ATLAS), initially promising, will be a big disappointment. Even if it meets expectations, it still wouldn’t be as spectacular as Hale-Bopp. We're still waiting for its successor.