No two comets are precisely alike.
In fact, they will seem comparable. Two decently brilliant comets that share a superficial resemblance are gracing our skies proper now, in actual fact: C/2025 A6 (Lemmon) and C/2025 R2 (SWAN). They each are on orbits that swoop by the internal photo voltaic system after which again out into deep area, previous the orbit of Neptune. Lemmon has a interval (the time it takes to orbit the solar) of about 1,300 years, whereas SWAN’s is about 650 years. Neither will get significantly near Earth.
Whereas SWAN’s orbit could be very effectively aligned with the airplane of the planets as they orbit the solar, Lemmon’s orbit is extremely inclined, tipped by greater than 140 levels, implying that the 2 comets have very completely different histories: up to now, SWAN might have interacted with among the planets, notably Jupiter, which might’ve shortened its orbit over time.
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Lemmon is considerably brighter than SWAN and a fantastic object for binocular-based viewing, however neither is especially dazzling in comparison with others from current historical past. In mid-2020, for example, the attractive comet C/2020 F3 NEOWISE was brilliant sufficient to see simply by eye. And C/1995 O1 (Hale-Bopp) was so brilliant in 1997 that I noticed it from inside my home whereas searching a window in a well-lit room!
All this variation raises an apparent query: Why do some comets glow spectacularly whereas others appear to fizzle out?
The obvious purpose is proximity. If a comet will get nearer to Earth, it should usually look brighter.
The perfect instance of that is arguably from essentially the most well-known comet of all of them: 1/P Halley, additionally known as Halley’s Comet (or extra accurately, Comet Halley). In 1910 it turned extraordinarily brilliant because it approached inside 25 million kilometers of Earth, however the viewing geometry for its subsequent apparition in 1986 was a lot worse, so it appeared a lot dimmer. I keep in mind standing in an enormous line to take a look at it by a telescope on the College of Michigan’s observatory, the place I used to be an undergrad, however solely seeing it as a considerably boring fuzzy dot. That wasn’t one of the best view for my very first comet, however—being a supernerd even then—my enthusiasm for astronomy wasn’t diminished by the expertise.
Distance from Earth isn’t the one issue, nevertheless. The nearer a comet will get to the solar, the extra the previous is heated up, and the extra possible it’s that the comet will launch unstable materials and brighten. However even then, it’s tough to say prematurely how any given comet will carry out. Two comets might need favorable orbits, however one may be so radiant that it makes headlines, whereas the opposite might by no means get brilliant sufficient to see and not using a telescope. As my fellow astronomer (and famend comet hunter) David Levy likes to say, “Comets are like cats: they’ve tails, they usually do exactly what they need.” That nearly feline fickleness largely has to do with the construction of the comet itself, which might change over time.
It’s arduous to demarcate what comets even are; astronomers don’t even have a proper, formally accepted definition for them. However for our dialogue right here, we are able to generalize to say that comets are our bodies product of ice and rock, often a couple of to some dozen kilometers broad, that orbit the solar. They’re categorized into two broad teams: short-period comets have orbits of lower than 200 years across the solar, and long-period comets have orbits that take longer. Brief-period comets don’t get all that far out from the solar. For instance, Comet Halley barely crosses the orbit of Neptune earlier than falling again into the internal photo voltaic system. Lengthy-period comets can get so distant that the solar can barely maintain on to them gravitationally. C/2023 A3 (Tsuchinshan-ATLAS), an excellent comet that was so brilliant that it was seen in the course of the day in October 2024, will get 10,000 instances so far as Halley does out into the black!
Lengthy-period comets additionally are usually intrinsically brighter than ones with smaller orbits. That’s as a result of they so hardly ever drop into the internal photo voltaic system, which implies they’re often extra pristine. When a comet will get shut sufficient to the solar, numerous ices on or simply beneath its floor heat up and may flip straight into gasoline. This gasoline leaks into area and carries mud—tiny grains of rock—together with it. Collectively the gasoline and mud kind a fuzzy, enveloping coma (from the Latin for “hair”) across the strong nucleus of the comet. Whereas the nucleus could also be a couple of kilometers throughout, the coma could be tens of hundreds of kilometers broad, which is larger than some planets. This ejected materials displays lots of daylight, making the comet seem a lot brighter.
Because the gasoline and mud are pushed away by the photo voltaic wind and the strain of daylight, they will kind a protracted tail (or generally separate tails), which might stretch for hundreds of thousands of kilometers, making the comet much more eye-catching. In 2007 the unbelievable comet C/2006 P1 (McNaught) sprouted a 75-million-km tail, half the space from Earth to the solar!
Brief-period comets get near the solar rather more usually than their long-period kin, and each time they do, they deplete extra of their gasoline and mud. Consequently, they don’t usually emit as a lot reflective materials per orbit, in order that they don’t get as brilliant.
Then once more, very similar to cats, comets are infamous for breaking such fundamental guidelines. Think about Comet C/1973 E1 (Kohoutek), which astronomers found in 1973, when it was nonetheless comparatively far out in its multimillion-year orbit across the solar. Regardless of the space, it was already pretty brilliant, elevating hopes that it could brighten extra to develop into probably the most spectacular comets ever seen. For causes unknown, it didn’t brighten as a lot as anticipated, nevertheless, and was usually thought of a disappointment.
It’s thought that the comet had an outburst of some sort shortly earlier than its discovery; maybe a pocket of ice erupted and blasted out an uncommon quantity of gasoline. This might have made the comet appear brighter than it could in any other case be at that distance, setting nice expectations for what would in the end be a lackluster show.
This may work the opposite approach, too: in late 2007 Comet 17P/Holmes, usually a faint short-period object that’s solely seen by a telescope, out of the blue brightened by an element of 1,000,000, turning into a conspicuous naked-eye object. The perpetrator may have been an outburst of gasoline or a collision with a small asteroid—nobody is aware of for certain. However the ensuing increasing cloud of fabric grew so enormous that it was obvious to the bare eye as a disk, although the comet was about 240 million km from Earth on the time—farther away than the common distance of the planet Mars.
The overwhelming lesson in observing comets is straightforward: you by no means actually know what they’ll do subsequent. A boring one might out of the blue and decoratively develop into a spectacle, whereas one other that originally seems to be promising might as an alternative dwindle to obscurity. This reinforces an excellent less complicated lesson from astronomy: Hold your eyes on the sky! Ultimately, it’ll repay.
