Why Aren’t All Comets as Bright as Comet Lemmon?

No two comets are the same.

Of course they can appear similar. In fact, two fairly bright comets that share a superficial resemblance currently grace our skies: C/2025 A6 (Lemmon) and C/2025 R2 (SWAN). They are both in orbits that pass through the inner solar system and then return to deep space, beyond the orbit of Neptune. Lemmon has a period (the time it takes to orbit the sun) of about 1,300 years, while SWAN’s is about 650 years. Neither comes particularly close to Earth.

While SWAN’s orbit is very well aligned with the plane of the planets as they orbit the sun, Lemmon’s orbit is very tilted, tilted by more than 140 degrees, implying that the two comets have very different histories: in the past, SWAN may have interacted with some planets, notably Jupiter, which would have shortened its orbit over time.

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Lemmon is a bit brighter than SWAN and makes an excellent object for binocular viewing, but neither is particularly dazzling compared to other objects in recent history. In mid-2020, for example, the magnificent comet C/2020 F3 NEOWISE was bright enough to be easily visible to the naked eye. And C/1995 O1 (Hale-Bopp) was so brilliant in 1997 that I saw it from inside my house looking out the window in a well-lit room!

All of these variations raise an obvious question: Why do some comets shine spectacularly while others seem to fade?

The most obvious reason is proximity. If a comet gets closer to Earth, it will generally appear brighter.

Perhaps the best example is the most famous comet of all: 1/P Halley, also called Halley’s Comet (or more correctly, Comet Halley). In 1910, it became extremely bright as it approached within 25 million kilometers of Earth, but the observational geometry of its next appearance in 1986 was much worse, so it appeared much dimmer. I remember waiting in line to observe it through a telescope at the University of Michigan Observatory, where I was a student, but only seeing it as a somewhat boring fuzzy dot. It wasn’t the best view for my first real comet, but – already being a super-nerd – my enthusiasm for astronomy was not diminished by this experience.

Distance from Earth is not the only factor, however. The closer a comet gets to the sun, the warmer it gets and the more likely it is to release volatile matter and light up. But even then, it is difficult to predict in advance how a given comet will perform. Two comets may have favorable orbits, but one may be so radiant that it makes headlines, while the other may never become bright enough to see without a telescope. As my fellow astronomer (and famous comet hunter) David Levy likes to say: “Comets are like cats: they have tails and they do precisely what they want. » This almost feline inconstancy is mainly linked to the structure of the comet itself, which can change over time.

It is difficult to delineate which comets, even are; astronomers don’t actually have a formal, officially accepted definition for them. But for our discussion here, we can generalize by saying that comets are bodies made of ice and rock, usually a few to a few tens of kilometers wide, that orbit the sun. They are classified into two broad groups: short-period comets have orbits of less than 200 years around the sun, and long-period comets have longer orbits. Short-period comets do not travel very far from the sun. For example, comet Halley barely crosses Neptune’s orbit before falling back into the inner solar system. Long-period comets can travel so far away that the sun can barely hold them gravitationally. C/2023 A3 (Tsuchinshan-ATLAS), a bright comet so bright it was visible during the day in October 2024, goes 10,000 times farther than Halley in the dark!

Long-period comets also tend to be intrinsically brighter than those with smaller orbits. That’s because they fall so rarely in the inner solar system, meaning they’re generally more pristine. When a comet gets close enough to the sun, various ices on or just below its surface heat up and can turn directly into gas. This gas escapes into space and carries with it dust, tiny grains of rock. Together, the gas and dust form a fuzzy, enveloping coma (from the Latin “hair”) around the comet’s solid core. While the core can be a few kilometers across, the coma can be tens of thousands of kilometers wide, which is larger than some planets. This ejected material reflects a lot of sunlight, making the comet much brighter.

As the gas and dust are pushed back by the solar wind and the pressure of sunlight, they can form a long tail (or sometimes separate tails), which can extend for millions of kilometers, making the comet even more eye-catching. In 2007, the incredible comet C/2006 P1 (McNaught) grew a tail of 75 million kilometers, half the distance between the Earth and the Sun!

Short-period comets approach the sun much more often than their long-period counterparts, and each time they do so, they exhaust more of their gases and dust. Therefore, they generally do not emit as much reflective material per orbit and therefore do not become as bright.

Again, just like cats, comets are known to break these fundamental rules. Consider comet C/1973 E1 (Kohoutek), which astronomers discovered in 1973, when it was still relatively far away in its millions-year orbit around the sun. Despite the distance, it was already quite bright, giving hope that it would brighten further to become one of the most spectacular comets ever observed. For unknown reasons, the brightness was not as bright as expected and was generally considered a disappointment.

The comet is believed to have had an explosion shortly before its discovery; perhaps an ice pocket burst and released an unusual amount of gas. This would have made the comet brighter than it otherwise would be at that distance, creating high expectations for what would ultimately be a dull display.

It can also work the other way: in late 2007, comet 17P/Holmes, normally a faint, short-period object that is only visible through a telescope, suddenly brightened by a factor of a million, becoming an object visible to the naked eye. The culprit could have been a gas explosion or a collision with a small asteroid – no one knows for sure. But the resulting cloud of expanding material became so enormous that it appeared to the naked eye as a disk, even though the comet was about 240 million kilometers from Earth at the time, farther than the average distance from the planet Mars.

The overarching lesson of comet observation is simple: you never really know what they will do next. A boring film can suddenly and flamboyantly become a spectacle, while one that initially seems promising may instead descend into obscurity. This reinforces an even simpler lesson in astronomy: keep your eyes on the sky! Eventually, it will pay off.