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Elsa’s Tree | Aliens? | CATerpillars

Elsa’s Tree | Aliens? | CATerpillars

Issue #236

Steve Burgess
December 20, 2025

In partnership with

Tis the season…to get sick? I’ve been feeling ill all week, but science doesn’t stop! We have a pretty cool interstellar visitor so I asked Gemini to give us the low down on the higher up.

I specifically tasked with with mirroring casualSTEM’s style.

Plus: We got great human-made Clickables and a photo-version of “st3v on site”!

—Steve ☄️

p.s. I made some light edits to Gemini’s output in our main section. Playing with AI is fun, but it’s still easier to make manual edits sometimes!

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photo form National Geographic

Probably Not Aliens

3I/ATLAS is officially our third-ever confirmed interstellar visitor, and yesterday, December 19, 2025, it made its big "hello" by reaching its closest point to Earth. Think of it as a frozen time capsule from another star system that just happened to swing by our neighborhood.

What is 3I/ATLAS?

The name is basically its passport. 3I marks it as the third interstellar object ever detected (after 'Oumuamua and Borisov), and ATLAS is the telescope system in Chile that caught it sneaking in back in July. Unlike the weird, rocky "cigar" that was 'Oumuamua, this is a classic comet—a "dirty snowball" that’s currently melting and glowing as it gets closer to our Sun.

Why it’s a big deal

It’s moving at a blistering 130,000 mph. That’s way too fast for our Sun’s gravity to trap it, meaning it’s on a "hyperbolic" path— passing through once and will never be seen again.

Because it’s currently "off-gassing," we can actually see what it’s made of. NASA has already detected hydroxyl (OH), the chemical fingerprint of water. This is huge because it’s literal proof that the same water that fills our oceans exists in other, distant star systems. It also has a distinct green glow from diatomic carbon reacting with sunlight, basically acting as a neon sign that says "alien organic chemistry."

The current status

Right now, every major telescope (including James Webb) is staring at it to see if its chemistry matches our local comets. It’s currently hanging out in the pre-dawn sky near the constellations Virgo and Leo. It isn't quite bright enough to see with just your eyes, but it’s a prime target for anyone with a decent backyard telescope.

Read about it (article) | Hear about it (Short Wave podcast)

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