Asteroid ‘Families’ Reveal Hidden Histories and Impact Risks across the Solar System

By Phil Plait edited by Lee Billings

We’ve all seen this happen in a science-fiction movie: our plucky heroes jump into their ramshackle spaceship and escape the bad guys by flying through the treacherous asteroid belt, where huge rocks tumble and spin so close together that the crew has to constantly dodge, duck, dip and dive to avoid being smashed to atoms.

It’s exciting, but it’s wrong: asteroids so closely spaced together would grind one another to dust in short order, making it extremely unlikely that you’d ever find such a situation near a star. In our own solar system, there are pretty good odds that you could stand on the surface of an asteroid and not even be able to see another one! Big ones tend to be many millions of kilometers apart.

Yet they do interact if they are given enough time. Even in the sprawling main asteroid belt between the orbits of Mars and Jupiter, collisions are inevitable. In fact, we’ve managed to see some small asteroid smashups; bigger rocks are far more rare, so larger collisions are proportionally less common. But they still happen, too—spacecraft reconnaissance of large asteroids shows that they are riddled with ancient impact craters. And when two space rocks go “bump” in the main belt, their high orbital speeds mean the collision can occur at velocities far higher than that of a rifle bullet. Shrapnel is inevitable because big impacts blow lots of asteroidal real estate out into space.

What happens to that ejected debris? In many cases, these fragments stay on much the same orbital path as the parent asteroid, though they gradually separate from it as a result of slight velocity differences. After millennia, the ejecta might be clear across the sun from its source. You might think this is problematic for anyone trying to track down different types of asteroids to figure out how they all fit together—and it is! But this problem of orbital mechanics provides its own solution, too.

That’s because the chaos of collisions scarcely seeps into some parts of an asteroid’s orbit; two fragments from an asteroid may end up hundreds of millions of kilometers apart, but their distance from the sun and the shape and orientation of their orbits remain similar. One of the most important conserved characteristics is orbital inclination: changing the tilt of an object’s orbit via impact is quite energy-intensive, so even after a big collision, the daughter asteroids that have been blasted into space still retain a very similar inclination. Such enduring features are collectively called an asteroid’s orbital elements and allow us to tease order out of the chaos.

Japanese astronomer Kiyotsugu Hirayama was the first to realize this, noting in 1918 that many more asteroids seem to share orbital elements than would be expected by random chance. He called such groupings asteroid “families,” the term we still use today.

Families are named after the largest asteroid in the group; Hirayama initially identified three such families, belonging to the asteroids Koronis, Eos and Themis.

Today we know of more than a million asteroids in the main belt, with more found all the time—the newly commissioned Vera C. Rubin Observatory discovered more than 2,000 asteroids in its first 10 hours of observing the sky! As our catalogs swell with newfound asteroids (and as the availability of requisite computing power grows), orbital patterns are getting easier to see, and more families can be flagged. Astronomers currently recognize a few dozen large asteroid families, but quite a few smaller ones are known as well. In a paper published in the journal Icarus in August 2025, a research team announced that its orbital-element number-crunching had revealed an amazing 63 new families.

Finding asteroid families is a boon for planetary scientists seeking shortcuts to discovery: the properties of a small asteroid may be almost entirely unknown, for instance, but if that space rock belongs to a family with bigger, more well-studied members, we can better guess what it looks like. Confirming those guesses—making sure the objects really are related—usually requires taking spectra, the time-consuming process of breaking an object’s incoming light into individual colors to reveal its composition.

Care must be taken, though. Some very large asteroids are differentiated, which means that when they formed and were still molten, heavy metals and other dense materials sank toward the center, while lighter, rocky material floated nearer to the surface. A large enough impact could excavate an asteroid’s depths and shallows alike, creating a family with a mix of compositions. The Vesta family is an example of this. (Vesta is the second-largest object in the main asteroid belt after Ceres, and both Vesta and Ceres are actually considered to be protoplanets by planetary astronomers.)

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As a bonus, some meteorites on Earth have been identified as being from Vesta because they have very similar compositions, and they likely made their way down to Earth when the gravitational effects of Jupiter dislodged them from the main belt. They can be studied in detail in laboratories, giving us even more insight into that family.

Another team of astronomers published a paper in August 2025 in the Planetary Science Journal on James Webb Space Telescope spectra of Polana, a 55-kilometer-wide asteroid in the main belt. The spectra show that it’s the likely parent of the near-Earth asteroids Ryugu and Bennu. If those latter two names ring a bell, that’s because they’ve both been visited by spacecraft that gathered and returned samples to Earth for study.

Finding this particular branch of an asteroidal family tree is more than a mere academic exercise: Both Ryugu (about 1 km wide) and Bennu (0.5 km wide) are potentially hazardous asteroids, meaning they could collide with Earth sometime in the distant future. By knowing the parent bodies of such threatening asteroids, we can better understand how they find their way to the inner solar system from the main belt to pose threats in the first place, which in turn can help us defend our planet from future worrisome space rocks.

And of course, the scientific benefits to understanding asteroid families are worth the investigation, too. Asteroids are leftover rubble from the formation of the solar system itself, so studying them is quite literally studying our own family tree, with an occasional extra benefit of finding—and hopefully avoiding—potentially apocalyptic space rocks.

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