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Short Wave

Where did our moon come from?

22 Apr 2026

Transcription

Chapter 1: What is the main topic discussed in this episode?

0.031 - 23.466 Brittany Luce

This year, for the first time in NPR's history, public media is operating without federal funding. That means NPR needs your support now more than ever. I'm Brittany Luce from It's Been a Minute. Please do your part to keep independent, reliable news coverage strong and support the podcasts that get you through the day by making a gift for public media giving days. Head over to donate.npr.org.

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24.928 - 28.654 Brittany Luce

You're listening to Shortwave from NPR.

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30.018 - 44.392 Emily Kwong

Hey, ShortRivers, Emily Kwong here, and I don't know about you, but nothing grounds me quite like looking at the moon, this beautiful, powerful companion to Earth. And for the first time in over 50 years, humans have traveled around the moon.

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44.412 - 56.375 Unknown

3, 2, 1, booster ignition. And lift off. And we have a beautiful moon rise. We're headed right at it. We have reached the closest point of our destination to the moon.

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56.715 - 60.662 Emily Kwong

And back on the Artemis II mission.

60.722 - 70.318 Unknown

Houston Integrity, splashdown, sending post landing command now. A new chapter of the exploration of our celestial neighbor is complete. Integrity's astronauts, back on Earth.

70.298 - 90.376 Emily Kwong

And all of this moon joy has got me thinking about everything the moon has been through. Because she's been through a lot. Yes. Regina Barber, my shortwave co-host and astrophysics queen. Yes. Also our resident moon connoisseur. True. Mistress of moons. I like that one. Okay, speaking of names, other moons have names, like Europa.

90.616 - 97.766 Regina Barber

Yeah, so Europa is one of Jupiter's large moons. It's my favorite moon. But today's episode is all about Earth's moon.

Chapter 2: What is the origin story of Earth's moon?

98.166 - 110.042 Emily Kwong

Some people call it Luna, but its name is the Moon with a capital M. With a capital M. What is then the leading hypothesis on how she was born? Through a collision.

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110.022 - 122.548 Kelsey Preisel

by a giant impact with a Mars-sized body named Theia and Earth that then shot out magma from the Earth and that balled up and formed the moon. Wow.

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123.63 - 128.52 Regina Barber

Yeah, that's Kelsey Preisel, a volcanic rock planetary chemist at Purdue University.

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128.5 - 136.94 Emily Kwong

I have so many questions. Like, where did Thea come from? Why did she come for us? And how did the moon come to stay in our orbit and be so round and so beautiful?

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137.422 - 144.619 Regina Barber

Em, I love your enthusiasm. I'm here for you. And actually, I had my own questions too. So we'll tackle them all today.

144.599 - 175.08 Emily Kwong

So today on the show, our moon. We try to answer one of the most enduring questions. How was the moon made and how do we know? And what does all of this have to do with water's origin story? You are listening to Shortwave, the science podcast from NPR. All right, Gina, I know you love Europa, one of the moons of Jupiter. Yes. But where does our homegrown moon rank for you?

175.12 - 186.504 Regina Barber

It's my second favorite. I actually have a phase of the moon tattoo. It's the first tattoo I've ever got. It's a cool tattoo. Thank you. And Em, you might think this love comes from childhood because it's so deep.

186.785 - 186.925

Mm-hmm.

186.905 - 193.917 Regina Barber

But kind of like Kelsey, the planetary scientist we heard from earlier, I learned a lot of what I know about the mood now when I was in college.

Chapter 3: What is the leading hypothesis for the moon's formation?

350.153 - 370.78 Regina Barber

Yeah, there's one other hypothesis that takes into account the very similar composition between Earth and the moon. This one suggests Earth spinning so quickly when it was forming that stuff just like flew off. So imagine like I'm spinning on ice or something and I'm spinning so fast that my arm just flies off. And your arm would become your moon. Exactly.

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371.22 - 375.726 Regina Barber

And TAB is actually more willing to entertain this idea than the non-collision options.

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376.267 - 385.852 Tab Preisel

Hmm. I live in the realm that everything is still possible. And it's really, really hard to completely objectively rule something out to 100% certainty. So I'm still happy with that.

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386.193 - 407.042 Regina Barber

But he also said that there's been a lot of computer simulations that just don't support that our Earth would have been stable. spinning that fast enough to do that and still have like the Earth-Moon system we have today. And it just doesn't fit. So this brings us to why scientists think the most likely Ceres events was that Theia, this early proto-planet, smashes into Earth and makes the moon.

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407.022 - 424.111 Emily Kwong

This is the part of the story that I don't know why it just amazes and terrifies me. It's like this idea of another body coming closer and closer and then smashing into Earth. Like, can you imagine what that looked like from the volcanoes here? They were probably like, ah, what is that?

424.271 - 436.546 Regina Barber

Well. Well, I mean, we were still a proto-planet ourself, you know, like it was still that roller derby. All these things were happening. We were getting pummeled by like asteroids. I don't know. So baby Earth was like, no, just more of the same.

436.626 - 437.407 Emily Kwong

Sounds about right.

437.567 - 458.387 Regina Barber

Same old, same old. I can take it. Yeah. Yeah. So scientists think that Theia was possibly Mars size due again to computer simulations. But of course, there's like debate over that. The exact size, the speed, if there were more objects than Theia, what angle Theia or maybe these other objects came in at. More than Theia? Like Theia's? Many Theia's?

458.407 - 458.508 Emily Kwong

Yeah.

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