No two comets are exactly alike.
Of course, they can appear similar. Two decently bright comets that share a superficial resemblance are gracing our skies right now, in fact: C/2025 A6 (Lemmon) and C/2025 R2 (SWAN). They both are on orbits that swoop through the inner solar system and then back out into deep space, past 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 gets 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 highly inclined, tipped by more than 140 degrees, implying that the two comets have very different histories: in the past, SWAN may have interacted with some of the planets, notably Jupiter, which would’ve shortened its orbit over time.
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Lemmon is somewhat brighter than SWAN and a fine object for binocular-based viewing, but neither is particularly dazzling when compared with others from recent history. In mid-2020, for instance, the beautiful comet C/2020 F3 NEOWISE was bright enough to see easily by eye. And C/1995 O1 (Hale-Bopp) was so bright in 1997 that I saw it from inside my house while looking out a window in a well-lit room!
All this variation raises an obvious question: Why do some comets glow spectacularly while others seem to fizzle out?
The most obvious reason is proximity. If a comet gets closer to Earth, it will generally look brighter.
The best example of this is arguably from the most famous comet of them 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 viewing geometry for its next apparition in 1986 was much worse, so it appeared much dimmer. I remember standing in a huge line to look at it through a telescope at the University of Michigan’s observatory, where I was an undergrad, but only seeing it as a somewhat dull fuzzy dot. That wasn’t the best view for my first actual comet, but—being a supernerd even then—my enthusiasm for astronomy wasn’t diminished by the experience.
Distance from Earth isn’t the only factor, however. The closer a comet gets to the sun, the more the former is heated up, and the more likely it is that the comet will release volatile material and brighten. But even then, it’s difficult to say in advance how any given comet will perform. Two comets might have favorable orbits, but one might be so radiant that it makes headlines, while the other may never get bright enough to see without a telescope. As my fellow astronomer (and renowned comet hunter) David Levy likes to say, “Comets are like cats: they have tails, and they do precisely what they want.” That almost feline fickleness mostly has to do with the structure of the comet itself, which can change over time.
It’s hard to demarcate what comets even are; astronomers don’t actually have a formal, officially accepted definition for them. But for our discussion here, we can generalize to say that comets are bodies made of ice and rock, usually a few to a few dozen kilometers wide, that orbit the sun. They’re classified into two broad groups: short-period comets have orbits of less than 200 years around the sun, and long-period comets have orbits that take longer. Short-period comets don’t get all that far out from the sun. For example, Comet Halley barely crosses the orbit of Neptune before falling back into the inner solar system. Long-period comets can get so far away that the sun can barely hold on to them gravitationally. C/2023 A3 (Tsuchinshan-ATLAS), a brilliant comet that was so bright that it was visible during the day in October 2024, gets 10,000 times as far as Halley does out into the black!
Long-period comets also tend to be intrinsically brighter than ones with smaller orbits. That’s because they so rarely drop into the inner solar system, which means they’re usually more pristine. When a comet gets close enough to the sun, various ices on or just beneath its surface warm up and can turn directly into gas. This gas leaks into space and carries dust—tiny grains of rock—along with it. Together the gas and dust form a fuzzy, enveloping coma (from the Latin for “hair”) around the solid nucleus of the comet. While the nucleus may be a few kilometers across, the coma can be tens of thousands of kilometers wide, which is bigger than some planets. This ejected material reflects a lot of sunlight, making the comet appear much brighter.
As the gas and dust are pushed away by the solar wind and the pressure of sunlight, they can form a long tail (or sometimes separate tails), which can stretch for millions of kilometers, making the comet even more eye-catching. In 2007 the incredible comet C/2006 P1 (McNaught) sprouted a 75-million-km tail, half the distance from Earth to the sun!
Short-period comets get close to the sun much more often than their long-period kin, and every time they do, they deplete more of their gas and dust. Consequently, they don’t typically emit as much reflective material per orbit, so they don’t get as bright.
Then again, much like cats, comets are notorious for breaking such basic rules. Consider Comet C/1973 E1 (Kohoutek), which astronomers discovered in 1973, when it was still relatively far out in its multimillion-year orbit around the sun. Despite the distance, it was already fairly bright, raising hopes that it would brighten more to become one of the most spectacular comets ever seen. For reasons unknown, it didn’t brighten as much as expected, however, and was generally considered a disappointment.
It’s thought that the comet had an outburst of some kind shortly before its discovery; perhaps a pocket of ice erupted and blasted out an unusual amount of gas. This would have made the comet seem brighter than it would otherwise be at that distance, setting great expectations for what would ultimately be a lackluster display.
This can work the other way, too: 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 a conspicuous naked-eye object. The culprit could have been an outburst of gas or a collision with a small asteroid—no one knows for sure. But the resulting expanding cloud of material grew so huge that it was apparent to the naked eye as a disk, even though the comet was about 240 million km from Earth at the time—farther away than the average distance of the planet Mars.
The overwhelming lesson in observing comets is simple: you never really know what they’ll do next. A dull one may suddenly and flamboyantly become a spectacle, while another that initially looks promising may instead dwindle to obscurity. This reinforces an even simpler lesson from astronomy: Keep your eyes on the sky! Eventually, it’ll pay off.
