The Slow Dances of Asteroid Moons

An astronomy pop quiz for you: How large does a planet have to be before it can have a natural satellite of its own?

It’s a trick question, you see. There’s no lower limit. In fact, moons aren’t just the province of planetary-mass bodies like Jupiter, Pluto, and our own Planet Earth—asteroids have them, too. Even very small asteroids. As of 2025, more than 450 minor planets are known or suspected to have natural satellites; for today’s post, we will explore this overlooked world of moons in miniature, and probe the limits of what’s possible in our Solar System.

All objects, no matter their size or distance, attract each other through gravity. Technically, I’m exerting a gravitational pull on you right now, and vice versa; the force is just too small to be measurable. In any case, the overwhelming gravity of Earth would make it a moot point. But suppose I placed two small objects together in deep space, far away¹ from any interfering forces. Suppose one object is, say, a BiC mechanical pencil (23 grams), and the other is my car, a 2008 Scion xD (1,210 kg), floating two meters away. Would it be possible to make the pencil orbit the car?

Turns out, it would. Per the universal law of gravitational attraction, but sparing you the math, the two bodies would pull on each other with a force of 4.64 x 10^-10 Newtons. That’s not much holding them together! So much as a breath would send the BiC flying away from the Scion, never to return. Left to their own devices, though, they would gradually fall towards their common center of mass (which would be well inside the car, owing to its much greater size). And with an extremely slight nudge, I could launch the pencil into an orbit around my Scion, whereby the force of gravity would keep it traveling in an endless circle. In other words—an exceptionally small moon. It would travel along its orbit at a stately 0.0002 meters per second, and complete one revolution every 17 hours.

That was an extreme and somewhat silly example. Yet, some real-life asteroid pairs aren’t all that much larger in the scheme of things. Take 152830 Dinkinesh and its moon Selam, pictured above. NASA’s Lucy spacecraft flew by them in November of 2023. Selam is a minuscule 220 meters across; Dinkinesh, the primary, is 790 meters across, which is small enough to fit comfortably in New York’s Central Park.

Both objects are rubble pile asteroids, amalgamations of rocks bound only loosely by a feather-light gravitational force. It’s believed that an impact several million years ago blasted Dinkinesh to pieces; some of those settled back down into place, forming the rubble pile, while others, moving fast enough to orbit but not fast enough to escape the parent asteroid’s gravity, coalesced to form Selam. This would have been a slow, slow process, due to the subtlety of the attractions involved. Even now, Selam takes a whopping 57 hours to orbit Dinkinesh, despite being only three kilometers away.

Another closely explored asteroid-moon pair is 65803 Didymos and its moon Dimorphos. They’re both rubble piles, like Dinkinesh and Selam. Unlike Dinkinesh and Selam, they orbit close to Earth. Didymos is 850 meters across, while Dimorphos is a measly 177 meters—small enough that NASA used it as target practice, smashing it with the DART probe to see if they could meaningfully deflect a hazardous near-Earth object. It turned out that they could; after the impact, the orbital period of Dimorphos around its parent asteroid was half an hour shorter. A follow-on mission by the European Space Agency, Hera, is headed out there to inspect the damage.

Besides asteroid/small asteroid pairs, there are other, more exotic arrangements out there, which have yet to be explored up close. A handful of triple and even quadruple asteroid systems have been discovered. 130 Elektra, a 262-kilometer-wide asteroid orbiting between Mars and Jupiter holds the record with three moons. All of them are much smaller than their parent body, at only a couple kilometers across.

Other objects are classed as double asteroids, meaning they are similar in mass and orbit around a common center of gravity. In these cases it’s hard to say which one is the “moon.” One example is the relatively large asteroid 90 Antiope, first discovered in the outer Asteroid Belt in 1866. More than a century later, in the year 2000, astronomers with more advanced equipment found that it was actually two bodies, closely orbiting each other. The larger object, which kept the name Antiope, is 88 kilometers across. Its companion, provisionally designated² S/2000 (90) 1, is only slightly smaller at 84 kilometers.

All these discoveries I’ve mentioned are surprisingly recent ones. Until 1993, the idea that asteroids could have moons of their own was just speculation. In August of that year, the spacecraft Galileo flew by 243 Ida on its way to Jupiter, snapping pictures of what was only the second asteroid to be seen up close. But Ida came with a surprise: it had another asteroid orbiting it. The moon, Dactyl, was only found several months after the flyby, when JPL scientists chanced across it in overlooked footage. Nobody had expected it to be there. Some in the scientific community would have argued that asteroid moons were impossible—yet, there it was, a puny 1.6-kilometer rock circling lazily around its much larger companion. Over the decades since, we’ve discovered hundreds like it.

Nature has a way of being more complicated than we initially think. Asteroids are no exception to that rule. They may be airless, desolate space boulders, but they have surprises in store, too. As we’ve seen in today’s post, they can replicate in miniature the more familiar patterns of their much larger neighbors. Just as the Moon orbits Earth, and the eight planets circle around the Sun, so, too, do the Solar System’s tiniest worlds have companions of their own. There’s a certain poetry to that, no?

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I appreciate the read on this one, folks! More adventures in astronomy to come, so check the box below to make sure you’re subscribed—and I’ll see you next time.

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1. How far away? In practice, you probably couldn’t get away with this anywhere in the Solar System—the Sun’s gravity would interfere. So let’s assume these objects are in an infinite, totally empty void. ↩︎
2. Though “90 Antiope” is also used to refer to the binary system as a whole. ↩︎