It might be called the curse of comets – over and over again, hopes have been raised of a dazzling curve of light across the sky, only for nothing to be visible to the naked eye. The pattern runs so deep that when bright comets do appear, some people miss them because they can’t believe it’s finally real. 

The cycle happened twice last year with 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS). As we wait to see if C/2024 G3 (ATLAS) reinforces the pattern or breaks it, why does this happen so often?

What’s A Comet?

Comets are commonly described as “dirty snowballs”, a mix of ice and dust. Most have quite elongated orbits. Being too small to have an atmosphere, there is no pressure to sustain liquids at the surface, so ice turns directly to gas as it warms up, known as subliming. As the gas escapes it carries dust away with it, which gets lit up by sunlight, creating a coma and sometimes tail that can extend for millions of kilometers.

Unlike Earthly snowballs, however, comets are made from a mix of ices – familiar water ice, dry ice (carbon dioxide), carbon monoxide, and frozen methane being the most common. Each ice sublimes at a different temperature. 

A Short History Of Comet Hype

By far the most famous comet is Halley’s. With a 76-year orbit, it has made many recorded visits to the inner Solar System, including the portrayal of its 1066 visit in the Bayeux Tapestry. Many of these have been spectacularly bright. However, each time Halley approaches the Sun, the Earth is in a different position. Naturally, if we’re on the other side of the Sun when Halley is melting fastest, the view will be nothing like as good as when we’re better placed.

In 1985-86, the media was filled with excitement about Halley’s approach. Some astronomers warned that this would be the worst alignment in two thousand years, and not to expect to see anything without binoculars. That doesn’t make for great headlines, however, so stories of the past – where the tail could sometimes be seen stretching halfway across the sky – got far more publicity than the fact it wouldn’t be like that this time. The fact the 1910 visit had been one of the best just made things worse.

There was a surge in interest in small telescopes and binoculars, and marketing departments were none too keen to spread the message these would produce little more than a faint fuzzy blob this time. 

The come-down was hard, and millions of people still remember the event with disappointment. 

Since then, every few years another comet has attracted a round of hype as it approached the Sun. Some have delivered spectacular images for photographers with talent and advanced equipment, but only Hale-Bopp in 1996 and McNaught in 2007 have been bright enough to inspire awe among those working with eyes alone. Even these were only good in one hemisphere and away from city lights.

The trail of disappointment is so long that when McNaught lit up Southern Hemisphere skies, those who saw it had a hard time convincing others it was real, and worth getting out of the city to see. With only a few days before it faded, most people missed their chance. Ironically, McNaught was a surprise package, attracting little attention beforehand.

Why Does This Keep Happening?

Although plenty of comets make their closest approach each year, few get very bright. There are plenty of short-period comets buzzing around the inner Solar System, but they shed some of their outer layer of ice on every visit. Even those that started off quite large have shrunk over the years, and no longer put on much of a show.

Meanwhile, visitors from the outer Solar System, or even beyond, usually only get bright when they closely approach the Sun and gas and dust come flying off. At that point, they may make for superb photographs for those with good equipment – but to the naked eye, they are hidden in the solar glare. 

As a result, catching most comets is a trade-off between having enough distance between the target and the Sun to see it under reasonably dark skies, and losing a lot of the brightness.

Halley’s Comet is so famous because it’s larger than most comets (and must have been larger still thousands of years ago when it was first sighted). That size means that more ice turns to gas, carrying off more dust in the process, and all this extra material means it can produce a mighty tail without getting into such hard-to-see territory as sungrazers like Comet Lovejoy and C/2024 G3. 

Unfortunately, in 1986, even when Halley wasn’t all that close to the Sun physically, our relative locations put it in the solar glare in our skies, and comets its size are rare.

Reporting of sungrazers can be confusing. Reports that C/2023 A3 (Tsuchinshan-ATLAS) might get as bright as Venus were not wrong – it did indeed nearly get to that point. However, people reading that may think of how bright Venus looks at the moment when it is seen against a dark sky, not at the times when it is just as bright, but can barely be spotted because it sets not long after the Sun.

Photographs taken using filters or long exposures can also reinforce inaccurate expectations.

On top of that, without knowing the mix of ices on a comet’s surface, predictions of what material will be released and at what points are vague. A comet with a lot of dry ice will become active earlier than one that is almost entirely water ice. Sometimes a comet rich in ices with very low freezing points will get quite bright while still far from the Sun, raising hopes. However, once all these ices near the surface have escaped, what is left is mostly water ice, which proves much more resistant to heating. In these cases, brightness will usually continue to increase as it approaches the Sun, but far more slowly than expected.

Comets like Halley, which have been around before, are more predictable – but even there you never quite know the mix of layers that will be exposed.

That means there is always a lot of uncertainty about how bright a comet will get. In the modern media environment, stories that refer to the top of the range are more likely to go viral than those that explain the possibilities. Although they aren’t being lied to, people get the impression that an outcome that might be a one-in-ten chance is astronomers’ best prediction.

Don’t Despair

We’re now closer to Halley’s next closest approach (in 2061) than the last one. Social media groups expressing the intent to view the next visit may be a joke, but for those still around to see it, the event should be a much better one than 1986. (Those with great faith in life-extending medical advances might be excited the 2134 pass will be better still).

It probably won’t be necessary to wait that long either. Superb events such as Comet McNaught, described by some observers as looking like “a fountain in the sky”, can happen with little warning. Dazzling as Halley’s performance in 1910 was, four months earlier an even brighter comet was seen, with almost no warning.

How many disappointments are worth putting up with to see one event like that is a matter of opinion, but don’t write off witnessing a display for the ages, just because many haven’t delivered.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.