SPECIAL EDITION: Lucy sees double: Small asteroid is orbited by a binary moon!

November 8, 2023   Issue #641

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Astronomy News

It’s a big Universe. Here’s a thing about it.

One thing about asteroids you can always count on: They’re full of surprises.

Like Dinkinesh, a small rock that was just observed by the Lucy mission: It not only has a moon, but the moon is a contact binary!

This is amazing, and super cool, so let’s back up a wee bit and figure out what got us here.

Lucy is a mission that is on its way to Jupiter’s neighborhood, to examine a series of asteroids that share the giant planet’s orbit. These are called Trojan asteroids, and are likely specimens from the very earliest days of the solar system. They’re like a fossil record, which is why the mission is named Lucy, after the fossil Australopithecus afarensis, nicknamed Lucy, that was found in Ethiopia in 1974.

Astronomers are not ones to waste a moment of mission time though, so they realized that there might be asteroids in the main belt between Mars and Jupiter that could be close enough to the mission trajectory to get a closer look. They found one, called 1999 VD57, which got the name Dinkenesh, which is the Ethiopian name for the Lucy fossil. It means “Marvelous One”.

And oh my, how aptly named.

Dinkinesh is a lumpy rock about 790 meters across orbiting the Sun in the inner region of the main belt. Lucy was able to pass it at a distance of just over 400 km, which is pretty close, enough to see details on the surface. But when the images were sent to Earth, scientists got a nice surprise: Dinkinesh has a moon!

This isn’t too shocking; some 15% of asteroids have moons. They likely form when two asteroids collide, and debris is blown off one of them. If the impact isn’t too violent, that debris can stay in orbit around the asteroid and coalesce to form a small moon. The DART mission last year was sent to just such an asteroid.

That image was taken when Lucy was pretty close to the asteroids, but it turns out there were more surprises in store. Six minutes after closest approach Lucy was still snapping images and took the one shown at the top of this article, where the moonlet is pretty obviously double. The angle had changed, and revealed the third rock; in the first image the second moonlet was hidden behind the first.

In the discovery image, you can see a dark band around the middle. My guess is that’s a shadow from the inner rock (the one closer to Dinkinesh) on the outer one, and they do indeed touch. We can’t see that because that contact point is in shadow.

Two rocks touching like this are called a contact binary, and those are common too. The comet 67P/Churyumov-Gerasimenko, visited by the Rosetta mission, was a two-lobed rubber-ducky shaped object. Arrokoth, seen by New Horizons in the Kuiper Belt past Neptune, is also a contact binary.

It’s not clear exactly how they form. It’s almost certainly due, again, to a collision. Some impactor hits a solid object, for example, and the debris can naturally form into two objects that then settle together. Or one chunk stays intact while another is blown off, and again they grow closer and eventually come into contact. The gravity of both is so low they don’t merge or even deform much, so you get two lumpy objects that simply… touch. Or maybe they just formed separately and somehow came together. They’re probably billions of years old. A lot of weird stuff can happen in that time.

While contact binaries have been seen before, one has never been seen orbiting another asteroid. So that’s extremely cool.

And a bit baffling. If it’s not clear how these form on their own, it’s even weirder to see one orbiting a bigger asteroid! This will send planetary scientists scrambling to their simulations. Dinkinesh is small enough that its gravity is pretty weak, but my gut says that will still interfere with the formation of a contact binary, but I could be wrong. Or maybe the moon existed as a contact binary and somehow fell into orbit around Dinkinesh. That doesn’t seem likely to me either; the moon(s) would have to shed a lot of energy to go from passing by to orbiting, and I’m not sure how that would happen without disrupting them. Again, planetary scientists will have a great time working on this. I look forward to reading the papers.

One other thing. It’s hard to tell from this single image, but it looks like the long axis of the moon points right at Dinkinesh. Again, that may be coincidence given the angle of the show here, but that’s what you expect from tides: The gravity of Dinkinesh would want to align the moon’s long axis that way. Maaaaaybe. I don’t know the detailed math, and the tidal force may be far too weak to do that. But it’s interesting.

And holy wow, what a scientific haul to stumble on for Lucy’s first targeted asteroid pass! This was only really supposed to be a warm-up for the real show when it gets to Jupiter’s Trojans, a chance to check out the instruments and tracking software. Everything worked, which is great, but this is a huge scoop of gravy for the mission.

A double scoop, even.

Et alia

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