Why Do Asteroids Clump together in Families?

We only know about 1.2 million asteroids in this vast Solar System. But if you want to find asteroids similar to each other, you may get more luck than it looks. So why do some asteroids share properties and clump together in asteroid families? Let’s find out in this article.

Before we explore the properties of asteroid families, we first want to explain that it is an umbrella term for two completely different things. They are the dynamical groups and the collisional groups, respectively. As you would know from their names, dynamical groups only share dynamical properties, while collisional groups are created from a large impact event. Let’s explore how to find them and what they look like in the rest of this article.

The explanation of dynamical groups is simple. They share similar orbital elements, like semi-major axes, eccentricities, or inclinations. They all undergo the same dynamical process and share the same fate, so it’s beneficial to categorize them as groups even though the members could be completely unrelated.

An excellent example is the Alinda family, named after the near-Earth asteroid 887 Alinda. Each member takes approximately 4 years to orbit the Sun, and if you consider Jupiter’s orbital period of slightly less than 12 years, you would notice that these asteroids orbit the Sun about three times as Jupiter orbits the Sun once. This is a 3:1 resonance with Jupiter, which is unstable. Alinda asteroids in this resonance have their eccentricities pumped up as their perihelia approach that of the inner planets. Eventually, a close approach breaks this object off the resonance. In fact, this resonance is one of the Kirkwood gaps of the asteroid belt, whose semi-major axis is nearly devoid of asteroids. The Alinda family happens to be the transient population transiting from the gap to other populations.

The other type of asteroid families are the collisional families. These are clusters of asteroids that formed from a collision. When two asteroids crash together, countless fragments get splashed out from the scene. They come in all sizes and shapes, sometimes as small as dust particles, or as much as tens of kilometers across. Some of those rocks recombine together due to their gravity, forming rubble-pile asteroids. And some fragments are solid bodies from the collision that could become the largest fragments we can see today.

One spectacular example is the Vesta family, which contains about 16,000 known asteroids (Licandro et al, 2017). Most of the asteroids within the family are V-type asteroids, which contain just 6% of asteroids. How could this happen? This may have come from a collision involving Vesta, which is one of the largest asteroids in the asteroid belt. During the collision, it released so many fragments that thousands are large enough for us to see today. In fact, we may have already spotted the crash site, which is probably the gigantic crater on the south pole of Vesta.

Given that asteroids can essentially clump together, can you find a family easily? Unfortunately, it might be more complicated than you think. Dynamical families consist of asteroids with similar orbits; theoretically, every asteroid belongs to a dynamical group. However, most of them, like the near-Earth asteroids and the centaurs, serve as classifications of objects that are already well-established long ago. New groups are only worth mentioning when the objects have a notable dynamical fate, or significant clumping occurs. For example, in the outer asteroid belt, one significant cluster of asteroids is called the Hilda family. The asteroids there are in the 3:2 resonance with Jupiter, and given its population of more than 5000, it’s very stable there.

Collisional families are even harder to find. After the collision event, every fragment is subject to different perturbations, putting them on different orbits. This also requires the asteroids to be of similar spectral types, as they all formed from the same parent objects. Thus, the recognition of collisional groups requires a statistical analysis of both spectral types and orbital parameters, and looking for any significant clustering. As a principle, the older the collisional family, the harder it is to find, and the weaker the clustering. As time goes on, the members eventually reach drastically different orbits, making them seem as if they are completely unrelated.

One example is the relationship between 2 Pallas and 3200 Phaethon. Pallas is in a highly-inclined orbit in the middle of the asteroid belt, while Phaethon is in the near-Earth population, grazing very close to the Sun and reaching the asteroid belt in a highly elliptical orbit. But studies show that Pallas is the parent body of Phaethon. That’s because it is very close to a dynamically unstable zone, where a resonance with Jupiter pumps up the eccentricity of anything there. Thus, the objects reach the near-Earth population in a very short period of time and orbit the Sun in Phaethon-like orbits. This dynamical instability happens can happen to other objects as well, so it’s possible that many asteroid families have gone unnoticed because of this phenomenon.

Finally, if you would like some examples, here are some prominent or well-known asteroid families (Nesvorny et al, 2015):

• Nysa family
• Vesta family
• Flora family
• Eos family
• Koronis family
• Eunomia family

In this article, we’ve learned what asteroid families are, how they form, and how astronomers identify them. Remember that they are very useful tools to group asteroids together, and to simplify efforts to explain the formation of asteroids and the Solar System! If you would like to learn more about them, please visit the webpages in the references below.

1. de Leon et al. (n.d.). “Origin of the near-Earth asteroid Phaethon and the Geminids meteor shower.” Retrieved December 27, 2022, from https://www.aanda.org/articles/aa/full_html/2010/05/aa13609-09/aa13609-09.html
2. Licandro et al. (2017, January 17). “V-type candidates and Vesta family asteroids in the Moving Objects VISTA (MOVIS) Catalogue.” Retrieved December 27, 2022, from https://arxiv.org/abs/1701.04621
3. Nesvorny et al. (2015, February 5).”Identification and Dynamical Properties of Asteroid Families.” Retrieved December 27, 2022, from https://arxiv.org/abs/1502.01628
4. (2019, December 19). “4 Vesta”. Retrieved December 27, 2022, from https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/4-vesta/in-depth/