No two comets are alike.
Of course they can appear similar. In fact, right now our sky is decorated with two quite bright comets that have an external resemblance: C/2025 A6 (Limmon) And C/2025 R2 (SWAN). They are both in orbits that fly through the inner solar system and then return to deep space, avoiding the orbit of Neptune. Lemmon's period (time to orbit the Sun) is about 1,300 years, while SWAN's is about 650 years. None of them come particularly close to Earth.
While SWAN's orbit is very well aligned with the plane of the planets orbiting the Sun, Lemmon's orbit is highly tilted, tilted by more than 140 degrees, meaning the two comets have very different histories: SWAN may have interacted with some planets in the past, especially Jupiter, which would have shortened its orbit over time.
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Lemmon is somewhat brighter than SWAN and is an excellent object to view with binoculars, but neither is particularly dazzling compared to other objects from recent history. For example, in mid-2020 a beautiful comet C/2020 F3 NEOWISE was bright enough to be easily seen by the eye. AND C/1995 O1 (Hale-Bopp) it was so bright in 1997 that I saw this from inside my house looking out the window in a well-lit room!
All these variations raise an obvious question: Why do some comets glow spectacularly while others seem to fade away?
The most obvious reason is proximity. As a comet approaches Earth, it will typically appear brighter.
The best example of this is perhaps the most famous comet of all: 1/P Halliealso called Halley's Comet (or more correctly Halley's Comet). It became extremely bright in 1910 when it came within 25 million kilometers of Earth, but the viewing geometry at its next appearance in 1986 was much poorer, so it appeared much dimmer. I remember standing in a huge line to look at it through the telescope at the University of Michigan Observatory, where I was studying, but seeing it only as a somewhat faint, fuzzy dot. It wasn't the best view for my first real comet, but being a super nerd even then, my enthusiasm for astronomy was not diminished by the experience.
However, distance from Earth is not the only factor. The closer a comet gets to the Sun, the hotter it gets and the more likely it is that the comet will emit volatile material and become brighter. But even in this case, it is difficult to say in advance how this or that comet will behave. Two comets may have favorable orbits, but one may be so bright that it makes headlines, and the other may never become bright enough to be seen without a telescope. As fellow astronomer (and renowned comet hunter) David Levy likes to say, “Comets are like cats: they have tails, and they do exactly what they want.” This almost cat-like variability is largely due to the structure of the comet itself, which can change over time.
It's hard to even determine which comets are; astronomers don't really have a formal, officially accepted definition for them. But for our discussion here, we can generalize and say that comets are bodies of ice and rock, typically a few to a few tens of kilometers wide, that orbit the Sun. They are divided into two large groups: short-period comets orbit the Sun for less than 200 years, and long-period comets orbit the Sun longer. Short-period comets do not travel that far from the Sun. For example, Halley's Comet barely crosses the orbit of Neptune before falling back into the inner solar system. Long-period comets can travel so far that the Sun can barely contain them with gravity. C/2023 A3 (Zuchinshan-ATLAS)a bright comet that was so bright it could be seen during the day in October 2024 passed into the blackness 10,000 times farther than Halley!
Long-period comets also tend to be brighter than comets with shorter orbits. This is because they are so rarely found in the inner solar system, meaning they are usually more pristine. As a comet approaches the Sun, various ices on or below its surface heat up and can turn directly into gas. This gas leaks into space and carries with it dust—tiny grains of rock. Together, the gas and dust form a fuzzy, enveloping coma (from the Latin for “hair”) around the comet's solid nucleus. Although the core may be several kilometers in diameter, the coma may be tens of thousands of kilometers wide, wider than some planets. This ejected material reflects a lot of sunlight, making the comet appear much brighter.
Because gas and dust are pushed away by the solar wind and the pressure of sunlight, they may form a long tail (or sometimes separate tails)which can extend for millions of kilometers, which makes the comet even more attractive. In 2007, an incredible comet C/2006 P1 (McNaught) grew a tail 75 million kilometers long, which is half the distance from the Earth to 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 gas and dust. Consequently, they typically don't emit as much reflective material into orbit, so they don't become as bright.
Again, like cats, comets are notorious for breaking such basic rules. Consider Comet C/1973 E1 (crane)which astronomers discovered in 1973, when it was still relatively far away in its multimillion-year orbit around the Sun. Despite its distance, it was already quite bright, raising hopes that it would become even brighter and become one of the most impressive comets ever seen. However, for unknown reasons, it did not perform as well as expected and was generally considered a disappointment.
It is believed that the comet experienced some kind of flare shortly before its discovery; an ice pocket may have burst and released an unusual amount of gas. This would make the comet appear brighter than it would otherwise be at that distance, raising high hopes for what would ultimately turn out to be a dim image.
This could also work in the opposite direction: at the end of 2007. Comet 17P/Holmesusually a faint, short-period object, visible only through a telescope, that suddenly becomes several times brighter. million, becomes a visible object to the naked eye. The culprit could be a gas release or a collision with a small asteroid – no one knows for sure. But the resulting expanding cloud of material became so huge that it looked like a disk to the naked eye, although at the time the comet was about 240 million kilometers from Earth – further than the average distance to the planet Mars.
The main lesson of observing comets is simple: you never know what they will do next. A boring one can suddenly and brilliantly turn into a spectacle, while another that looks promising at first can instead turn into obscurity. This reinforces an even simpler astronomy lesson: watch the sky! It will pay off in the end.
