What’s What with the Asteroids

Article August 30, 2025

Some weeks back I wrote a post focusing on the many different terms we use to define things in the outer reaches of the solar system. At the time I said “Don’t even get me started on the asteroids—that’s a whole ‘nother post.”

Well, here’s that post, inspired by the fact that last week’s Spacing Out newsletter featured a whole raft of asteroid stories. There are a whole lot of asteroids in our solar system and we have several different terms that we use to keep track of what asteroid is where and made of what (yes, yes, they’re all have rock in them, but of course it’s not as simple as that). So let’s continue the vocab lesson and talk about all the (many) ways we can talk about the rocks, metal chunks, pebbles, boulders, and rubble piles that run around our solar system!

Asteroid

Let’s start with what an asteroid actually is. I personally like NASA’s definition: “Asteroids, sometimes called minor planets, are rocky, airless remnants left over from the early formation of our solar system about 4.6 billion years ago.” Rocky, you will note, not necessarily rocks (as much as I love to refer to asteroids as rocks, it’s not always the most accurate description for things that can be largely made of metal or be a loosely gravitationally bound pile of pebbles). Also the term minor planet, which I dove into in my outer solar system vocab post, pops back up here.

We often think of asteroids as being found in the gap between the orbits of Mars and Jupiter where, if you remember back to the diagrams of the solar system in your elementary school textbooks, the asteroid belt is found. And indeed, most asteroids do dwell in this general area. But it’s by no means the only place you’re going to find asteroids, and one of the ways we classify these objects is by just where it is they’re spending their time.

Groups

The main belt asteroid Donaldjohanson, as seen by the Lucy spacecraft on its way out to Jupiter’s Trojan asteroids, is a C-type member of the Erigone asteroid family. Credit: NASA/JPL

Asteroids get sorted into different groups based on where they orbit, and there are so many groups that I can’t list them here. Most groups are named after the first asteroid of that group to be identified, and frankly a stupid number of those asteroids happened to have names that started with the letter A.

Atiras are asteroids whose orbits are entirely inside of Earth’s orbit. There are two different classes with orbits that can potentially cross Earth’s. Depending on how far they are from the Sun at their closest point, these are either Atens or Apollos. Amors are ones that come in close to Earth but never actually get as close to the Sun as Earth’s orbit. Arjunas have an orbital radius similar to the Earth’s, which is also the purview of the Atens and Apollos so some asteroids will wind up classified as more than one group. And then there are the ‘Ayló’chaxnims which are asteroids whose entire orbit is inside the orbit of Venus (of which we currently know of exactly one, the asteroid ‘Ayló’chaxnim itself. Theoretically there are more though). I’m telling you, it’s a lot of As.

Then you’ve got your Hungarias, Hildas, Griquas, Cybeles, and so, so many others. Mostly these are all defined by the average radius of their orbits, similar to all those A groups already mentioned. Some group definitions also have requirements about the eccentricity and/or inclination of the orbits (how circular or tilted they are).

This diagram shows where the Jupiter Trojan asteroids, which are the ultimate target of the Lucy mission, can be found. Credit: Nature

I have a soft spot for the asteroids known as Trojans, the most prominent of which are Jupiter’s Trojans. Trojan asteroids are ones that actually share a planet’s orbit, held in place by the gravity of the planet. Jupiter has many, many Trojans. The whole point of the Lucy mission, currently making its way out to Jupiter orbit, is to check these guys out. However it can generally be assumed that the other gas giants will probably have at least a few (currently unknown) Trojans, and Earth and Mars are each known to have a couple.

So these groups are one way to classify an asteroid purely by looking at where it is and how it orbits. But often many individual members of those groups will share an origin story, so within the groups you will find what are called asteroid families.

Family Ties

Asteroids have been around for a long time. Like…a long time. As the NASA definition states, they’ve been here since the solar system began. But that doesn’t mean they haven’t changed in the intervening several billion years.

Rocks in space hit each other from time to time. They used to do it a lot more before the solar system began to settle down (we had some Wild West years towards the beginning there) but they still do occasionally. This results in big rocks becoming many smaller rocks, which can spread out as they are individually affected by gravity, solar radiation, and of course whatever event knocked them loose in the first place.

Vesta is the second largest known asteroid and is the parent body for the Vesta asteroid family. Credit: NASA/JPL/MPS/DLR/IDA/Björn Jónsson

But those rocks that used to be one rock and are now many rocks will still have a lot in common, because they have the same origin. That’s what asteroid families are—asteroids that are suspected, at one time, to have been a part of the same parent body.

In some cases those big parent bodies are still around today. Vesta, the second largest asteroid in the solar system, is the source of over 10,000 smaller asteroids flying around. The Vesta family, as it’s called, is one of the largest known.

In other cases the family will be named after one of the larger fragments of a much bigger parent that is no longer around as a single object capable of identification. The thousands of members of the Baptistina family, for instance, probably originated from a rock that was over 100 miles (160 km) across when it started out. The namesake of the family, the asteroid Baptistina, is somewhere around 10 miles (16 km) across, the largest known remnant of the fractured parent.

It should be noted that only a small fraction of asteroids have ever been able to be sorted into families. Most remain unknown orphans.

Chemistry

You’d think having two ways to classify asteroids would be enough, but can I offer you a third way? Remember when I said before that asteroids can be made up of different proportions of stuff, so calling them all “rocks” is not necessarily the most accurate? Well another way to sort asteroids is by what they’re made of. While there are numerous minor groups and subtypes, there are three main categories of asteroids based on their makeup: C-type, S-type (which I’ve also seen called K-type), and M-type, for carbonaceous, siliceous, and metallic.

Bennu is the asteroid that was sampled by the OSIRIS-REx spacecraft in 2020. It is an example of a rubble pile asteroid, a loose conglomerate of smaller rocks clinging together thanks to gravity. Credit: NASA/Goddard/University of Arizona

The vast majority of asteroids can be found in the main asteroid belt, which is also where the conditions were right for making C-types asteroids, so it is perhaps not surprising that something like 75% of known asteroids are C-types. These are made mostly of clay and rock, but have a whole bunch of carbon in them (hence the term “carbonaceous”). This tends to make them dark in color. Bennu, the rubble pile asteroid visited by the OSIRIS-REx spacecraft, is a C-type.

Since the carbonaceous C-types are rich in carbon, you can probably guess that the siliceous S-types are rich in types of silicon and you’d be right. These are often also called “stony” asteroids, because these ones are primarily made up of rock (of which silicon is a key component). You can also find metals mixed in, mostly nickel and iron. These rocks make up something around 17% of known asteroids and are brighter and denser than their C-type cousins.

In Star Trek M-type planets are the most Earthlike, but that’s definitely not the case with asteroids. These are rocks for which the word “rock” only kind of applies. They’re chunks of ancient iron and nickel forged in the fires of the early solar system. Just how they got this way is unknown—some of them may have tried to become planets and failed, leaving just the dense metallic core behind. Others may have just formed so close to the Sun that anything more volatile than metal was stripped away. However they got that way, they’re definitely the metalheads of asteroids!

The In-Betweeners

Having now done a post about icy outer solar system objects and this one about asteroids, it only feels fair to wrap up by talking about the things that are neither entirely one nor the other. These objects are found amongst the outer planets, which is too close to be properly classified as Kuiper Belt Objects.

The first known Centaur, Chiron, as seen by the Hubble Space Telescope. Credit: HST/Karen Meech

When the first of these things was found in 1977, it was named Chiron after the centaur from Greek mythology, and it was classified as an asteroid. And indeed, it seems to show a lot in common with C-type asteroids. But a few years after it was discovered, Chiron was seen emitting a gas halo and growing a tail, which is the behavior of a comet.

It turns out there’s an entire population of these things which in some ways act like asteroids and in other ways act like comets. Appropriately enough, given that the first one was named for the half-man, half-horse character of Chiron, these objects are officially known as Centaurs—not all of one thing but a bit of two things.

I love the Centaurs because A) they’re weird and I love weird space things and B) I feel like they sum up a solid lesson for those who follow space science: you can come up with as many different ways to categorize things as you want, but odds are something will come along that won’t quite fit any of them. But if you ask me, that’s part of what keeps things interesting!