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The moon gets pummeled with a lot of rocks from space. Without an atmosphere like Earth’s every single bit of cosmic debris slams into the moon’s surface. Most are teeny tiny and make microcraters (also called zap pits, yes, seriously) on rocks, but bigger ones make larger impact craters on the surface. The bigger the space rock the more rare it is, so substantial crater formation isn’t terribly common.

The Lunar Reconnaissance Orbiter (or LRO) takes images of the lunar surface all the time, and has been since it entered orbit around the moon in 2009. It has found many new small craters because it sees the same spots on the moon over and over again; when a new crater appears it can be pretty obvious in a before-and-after comparison.

And sometimes they’re really obvious: sometime between April and May 2024, an asteroid probably about 20 meters across (so, about the same size as the Chelyabinsk impactor, the one that blew up over Russia in 2013) slammed into the lunar surface, carving out a crater 222 meters across and about 30-40 meters deep. That’s an area of about 10 acres, so yeah, decently big. A crater this large only forms on the moon about once per 130 years, so it was pretty cool LRO happened to be up there to see it.

Informally named McGetchin crater (after an Apollo-era geologist), it was first seen in ratio images, where one image is divided by another to look for changes. Those are taken six months apart, so once scientists saw the new crater they found lots of other images of the area, and were able to narrow the impact date down further. They also commanded LRO to make more observations to get as much info on the crater as they could.

It yielded a lot of very interesting science [link to journal paper]. For example, debris from the crater was found as far as 100 kilometers from the impact site! Rocks and dust were thrown a long way, and that means people looking into future exploration and inhabitability of the moon need to be very thoughtful about such events (as well as dust thrown around as rockets land). Small impacts won’t throw material as far, but happen more often, so both could affect how we build lunar structures.

Usually, material is ejected at low angles to the surface, around 2°, creating a low arc of ballistically flung debris. However, there’s a ridge eight km west of the impact site, and ejected material was found at its base, meaning some shrapnel from the impact was thrown at angles of at least 13°. That’s a huge difference, and could be due to the shape of the impacting asteroid and the angle at which it impacted.

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I’ll add that high-velocity impact physics is extremely complicated and very difficult to test experimentally (accelerating something to those speeds is really hard), so the details of how the crater formed aren’t clear. The crater itself will be carefully studied to help us understand how these structures form.

The energies involved are huge, a large fraction of a megaton (a million tons of TNT). It takes a lot of energy to carve out that much rock. Boulders over 10 meters across were thrown over 200 meters away! Yes, gravity is lower on the moon, but a rock that big is still pretty dang hefty.

You can find more details on the LRO blog and in the science paper linked above.

A final thought: the crater is located near the moons’ equator, about ¾ of the way from the center to the eastern edge. That doesn’t mean it’s easily visible from Earth: Even Hubble can’t resolve craters less than about 90 meters across, so this crater would only be a few pixels wide in such an image. Unless you have an 8-meter ‘scope lying around with advanced optics this crater is too small to be seen in detail from Earth. Now, it might appear as a dimensionless dot somewhat brighter than the surrounding area, but that’s about it. I’d be curious if any amateur telescopes could spot that, actually. It might be possible.

But that puts the moon into perspective: it’s covered in craters, literally; the ground is saturated with them. The vast majority, billions if not trillions of them, are too small to resolve from Earth! We can only see the biggest ones, and even then there are too many to individually name. 

I like Earth’s atmosphere; it lets us breathe and can be a pretty color during the day. But this really brings home the fact that our atmosphere also protects us exceedingly well from invading space rocks. Or, at least, the smaller ones, of which there are many.

The images here are from the LRO images blog, which is a terrific resource and well worth putting in your RSS feed reader.

Via a follower on Bluesky I found out that the folks at Complexly (Hank and John Green’s outfit that make Crash Course videos) have edited together all 46 episodes of Crash Course Astronomy that I wrote and hosted into one giant marathon video!

So if you have eight hours 21 minutes 25 seconds with nothing to do then why not learn about the universe?

And you will almost never hear anyone say this, but… yes, do read the comments! People love astronomy, and it’s really, really uplifting to see so many folks being so positive about science. My thanks to the Complexly team who helped create this series, and for getting tens of millions of people (!!) to think about the stars. 

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!