Do you want to build a snowman?

No? Okay, do you want to build a moon that vaguely resembles a snowman?

Also no? Well that’s okay,  because the solar system’s already made one to gawk at.  It’s a tiny moon called Selam, orbiting a slightly less tiny  asteroid called Dinkinesh. It was discovered back in November 2023, thanks to a visit from NASA’s Lucy spacecraft.

And astronomers figure it  was Dinkinesh that made its snowman of a moon…by spinning  so fast it broke into pieces! [♪ INTRO] The fun thing about the discovery  of this snowman moon is that Lucy wasn’t originally meant to visit  the Dinkinesh-Selam system. For one thing, it’s located in the  vast asteroid belt between Mars and Jupiter, and Lucy’s primary  mission is to visit the Trojans. No, not the residents of ancient  Troy who were a bit too keen to roll a giant wooden horse  inside their city walls.

These are the Trojan asteroids…  a group of asteroids that share Jupiter’s orbit, but due to  Jupiter’s and the Sun’s gravity, are clumped into one general spot in that orbit. And Lucy’s headed there because  we’ve never explored this particular group of rocky  space balls up close before. Scientists are hoping they’ll tell us more about how the solar system formed.

But first it’d have to make  it through the asteroid belt. And the original plan was to fly by one of those space potatoes as a sort of practice run. But in planning Lucy’s route, they  realized they could take a swing by Dinkinesh and test some of the  spacecraft’s autonomous systems.

You know, before it needed to use  them on “more important” targets. So they weren’t expecting to  discover Dinkinesh was hiding a tiny moon, or any of the other weird  features they came across! For example, they found a huge  ridge on Dinkinesh’s surface.

It’s not the only asteroid with this  kind of feature, but its ridge is also lying perpendicular to a long trough. And because it’s on top of that trough, it means the ridge formed  sometime after the trough did. But it could be immediately after, or a long time.

We’d need more missions to figure out how old both of these features are. As for what caused them,  I’ll be coming back to that. Meanwhile, Selam appears to be  tidally locked to Dinkinesh.

That means that the same side  of Selam is facing Dinkinesh as the two little worlds spin around each other. It’s also orbiting its asteroid  buddy in the wrong direction, at least compared to the direction  they’re both going around the Sun. And that suggests that Selam  didn’t form alongside Dinkinesh, but that Dinkinesh captured it.

After modeling all this moon motion, astronomers are pretty sure Selam is made of two separate spheres that got  a bit too close to each other and fused into that two-lobe snowman shape. If you’re wondering why they’re not 100% sure, it’s because when Lucy did  its flyby, the part where the two halves were supposed to be  in contact was hiding in shadow. But if it really is the case, that  would make Selam the first contact binary moon that we’ve ever found.

The other non-moon contact binaries  in our solar system include the Comet 67P/Churyumov–Gerasimenko  and the asteroid Arrokoth… both of which also had robot visitors. But how did Selam get to be this shape? Well, scientists think that  like a bunch of other asteroids, Selam is basically a pile of rubble held together.

Which raises the question:  Where did that rubble come from? Earth’s moon is almost certainly the  product of our planet getting hit by a Mars-sized body. Did Dinkinesh  get hit by a large space rock, too?

It turns out, no. Somehow, Dinkinesh  started spinning so fast that it shed a bunch of debris into space…  which then either rained back down forming that ridge, got lost  to space, or clumped up into two decently sized orbs that then  kissed hard enough to make Selam. But how do you make an asteroid spin that fast?

Fire a bunch of BBs at it at just  the right angle over and over? No. All you need is the Sun.

See, the Sun is constantly spitting out radiation. And when that radiation hits  something, be it an astronaut, or a solar sail, or a full-sized planet,  it exerts a tiny bit of pressure. For bodies that are massive enough, it doesn’t do a whole heck of a lot.

But for small bodies, like plenty of asteroids, it can get them spinning  faster than they had been. Astronomers call it the... YORP effect.

And models suggest that if  a small rubble pile asteroid gets spinning so fast that it  makes one rotation every two hours, it can get ripped asunder. As for Dinkinesh, we can  see evidence of this spin-up not just in the debris left behind.  It’s still spinning really fast, making about one rotation every four hours. So maybe far enough in the future, Selam will have another moon buddy.

But while everyone waits to  see if that could ever happen, Dinkinesh and its snowman of a moon are telling us a lot about the early solar system. Because the Earth, if not all  the planets, probably started out as a bunch of smaller rocky  bodies coming together. So when we look at worlds  like Dinkinesh and Selam, we learn about our own planet.

But maybe it’s okay to just want  to find all the weirdest-shaped space rocks our solar system managed to make, too. Now if you’re the crafty sort, you  could try making your own miniature Dinkinesh-Selam system out of  cool looking rocks you found. You could even glue them to some kind of safety pin set up and make your own bling.

But if you prefer your pins a little  more stylized, and a little less DIY, you might be interested in our latest  piece of limited edition merch. Head on over to Complexly.store/SciShow  to pre-order yours today! [♪ OUTRO]