Subscribers are the nucleus inside my fuzzy head 

A team of astronomers wanted to use Hubble Space Telescope to observe a comet. They got time on the ‘scope, but when they wanted to get their observations it turns out there were some constraints that made viewing it with the orbital observatory impossible. So they submitted a new target, the comet C/2025 K1 (ATLAS) — [note: not the interstellar comet, but a different one discovered by the ATLAS sky survey], and when they got their observations they also got a surprise: the comet had broken up (technically called calving, or in this case fragmenting since it’s more than one piece) into at least five pieces!

[Click the images here and below to get much larger versions.]

The images were taken one day apart each on November 8 – 10, 2025. The astronomers were expecting to see a single nucleus — the solid part of the comet — but instead were amazed to see it had broken apart into several pieces. The difference from day to day is pretty obvious, too. There were four pieces the first day, but the brightest split by the second observation, while another had faded by the third [link to journal paper].

So what’s going on? Comets are basically big piles of rock, gravel, and dust held together by ices (like frozen water, carbon dioxide, and more). When they get near the sun, the ice in the solid nucleus turns to gas (called sublimation), creating the fuzzy head surrounding the nucleus, and the long streaming tail.

But that ice is the glue holding the nucleus together, so as it sublimates pieces of rocks and such get loose. Sometimes, when a comet gets too close to the sun, enough ice goes away that really big chunks are freed, essentially shattering the comet. On occasion the entire nucleus disrupts into zillions of little pieces, and we say the comet disintegrated.

This usually happens at or around perihelion, when the comet is closest to the sun, and in this case K1 was about a month past that point, which fits. It got about as close to the sun as Mercury’s orbit, so the heat it faced was pretty intense.

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Interestingly, this is a hyperbolic comet, too. That means its orbit is a hyperbola — it’s moving too rapidly for the sun to hold onto it, so it will fly away into interstellar space. It’s likely been orbiting the sun for billions of years, but got a kick from something that added enough speed to push it past escape velocity. The orbit of the comet is tilted with respect to the planets, so it never got near, for example, Jupiter, making me wonder if a passing star millennia ago, or even tides from the galaxy itself, is what gave it that little bit of extra oomph. The fragmentation doesn’t surprise me, since comets like this visiting the early solar system for the first time tend to be more fragile than ones on shorter orbits. The likely explanation is that comets closer to the sun get all that sort of activity out of their systems early on, and what we see now is tougher material less prone to fragmentation.

This serendipitous observation is pretty helpful. Usually, Hubble sees fragmenting comets well after the event, since it takes a while to get the ‘scope pointed to a new target. In this case the comet had just happened to fall apart days before, so the timing was perfect. In fact, it presented a mystery: there were bright outbursts of light seen from the comet, typically associated with fragmenting — brighter ice under the surface is exposed, plus the sudden release of lots dust reflects more sunlight — but the brightening seen was well after the breakup. The astronomers think that it might take time after the breakup for the dust to be released into space efficiently enough to increase the brightness.

Also, the comet is a bit weird; it has far less carbon in it than most solar system comets. That’s interesting indeed, since that low carbon content is usually seen in interstellar comets! Given this comet has a hyperbolic orbit, it might possibly have started out orbiting another star, got ejected, and has been wandering the space between the stars ever since. While possible, this is still odd, because the comet’s orbit isn’t super hyperbolic, which is what you’d expect for something moving at high speed through the galaxy before encountering the sun. Still, we’ve only positively IDed three such interstellar visitors, so we don’t have much data on how they actually behave.

 All in all, these observations wound up being pretty important. Seeing a comet after an outburst and fragmentation reveals what it’s interior is like, so that’s cool, plus, who knows? If this truly is an interstellar comet, the data become that much more precious.

My friend Maria D’Souza is a lot of fun. A skeptic (back when it was still cool) and a dork, when she says she wants me to be on her podcast the only answer is “yes”. Especially when the podcast is EnterpriseSplaining, where she, Jamie Bernstein, and Bill Stiteler watch an episode of “Star Trek: Enterprise” and explain it. Well, one of them watches it and tries to explain it to the other two. Then they do a rewatch later and go over their notes (they call this “Second Contact”).

They also have guests on, and they asked me ages ago; I said hold off until you get to the episode “Regeneration”, for reasons. They did, so I did, and now it’s live! There are lots of options to listen, including YouTube.

I reveal why I wanted to be on this episode specifically in the podcast; it’s because many years ago my pal Andre Bormanis, a writer and producer on the show, took me on a tour of the set while they were shooting the second season. I happened to see some set design pieces lying around, one of which was a Borg distribution node. I of course recognized it right away. He had to swear me to secrecy, since the episode using it (“Regeneration”, duh) wouldn’t air for a few months yet. That was fun.

Anyway, if you’re an Enterprise fan you should listen. I’ll note they aren’t always, um, too kind to the show (especially when it deserves it; the writing could be a little uneven), but it’s funny and fun. FWIW the “Second Contact” episode for this is up now too.

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!