What Are The Types of Asteroids?

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While asteroids are small, they possess large amounts of scientific value where they’re helpful in determining our origins. But before we carry out research on them, it’s a good idea to learn how to classify them. Let’s explore the types of asteroids in this article.

Classification by Chemical Composition

First and foremost, we’ll talk about how asteroids are classified by their chemical composition. You may ask how the scientists are able to identify what an asteroid is made of without in-situ chemical analysis, but the chemical composition of the asteroid affects the distribution of light that is reflected from the asteroid. Scientists classified the asteroids based on the different spectral patterns they exhibit, leading to the creation of many categories. Most known asteroids belong to one of these three categories:

  • C-type asteroids (carbonaceous / chondrite)
  • S-type asteroids (stony)
  • M-type asteroids (metallic)

About 75% of all asteroids in the Solar System are C-type asteroids. Made of carbon-based molecules, chondrites, and silicates, their surfaces are very dark, only reflecting less than 10% of the light they receive. On the other hand, S-type asteroids are made of silicate materials and some amounts of metal. They account for about 17% of the asteroid population. Moreover, the M-type asteroids are composed of mostly metallic materials such as nickel-iron. They are also rarer, accounting for no more than a few percent of the asteroids in the Solar System.

The distribution of the semi-major axis and the geometric albedo of asteroids based on their spectral types
Blue: C-type asteroids (140 data points)
Green: S-type asteroids (338 data points)
Red: M-type asteroids (38 data points)
Brown: D-type asteroids (35 data points)
Purple: E-type asteroids (12 data points)

Classification by Location

Note: classify them by the location, the chemical composition, and the asteroid family

Asteroids can also be classified based on where they orbit the Sun. There are some that are closer to the Sun, while the others are farther away. Here are the main categories:

  1. Main-belt asteroids
  2. Near-Earth asteroids
  3. Planet-crossing asteroids
  4. Trojan asteroids
  5. Centaurs
  6. Trans-Neptunian objects
1. Main-belt Asteroids

As you might know, there is an asteroid belt that resides between the orbits of Mars and Jupiter. That’s where most known asteroids orbit the Sun, and they belong to the category of main-belt asteroids. According to the Small-Body Database (SBDB) of the Jet Propulsion Laboratory, the asteroids in the main belt can be divided into three groups:

  1. Inner main-belt asteroids
  2. Main-belt asteroids
  3. Outer main-belt asteroids

The classification of an object into one of these three categories depends on the object’s semi-major axis (average orbital distance) around the Sun. Inner main-belt asteroids are closer to the Sun — they tend to be about 1.5 to 2 AU (astronomical unit) from the Sun on average. The main-belt asteroids are a little farther, about 2 AU to 3.2 AU from the Sun, whereas the outer main-belt asteroids occupy the range between 3.2 AU to about 5 AU from the Sun.

Distribution of the main-belt asteroids
Distribution of the inner main-belt asteroids
Distribution of the outer main-belt asteroids
2. Near-Earth Asteroids

Other than those in the main asteroid belt, there are asteroids that orbit closer to the Sun. In particular, their perihelion (closest approach to the Sun) is less than 1.3 AU, allowing it to come within 0.3 AU of the Earth. According to the Small-Body Database from the JPL, there are four main classifications of the near-Earth asteroids by the location of their orbits:

  1. Atira asteroids
  2. Aten asteroids
  3. Apollo asteroids
  4. Amor asteroids

Unlike the sub-classifications in the above section, it might not be obvious what those names mean at first glance — these categories are named after a notable example, not after the location of that group. The bullet points below briefly explain the classifications:

  • Atira asteroids are ones whose aphelion (farthest point from the Sun) are less than 1 AU. That means they always occupy the space within Earth’s orbit. They are very difficult to find due to their proximity to the Sun, and only 31 examples are known as of October 2023.
  • The Aten asteroids and the Apollo asteroids are more concerning — they have perihelions of less than 1 AU and aphelions of more than 1 AU, so they can come quite close to Earth, almost crossing Earth’s orbit.
  • The Aten asteroids orbit closer to the Sun than the Earth on average, while the opposite is true for Apollo asteroids. 2620 Aten asteroids and 18709 Apollo asteroids are known as of October 2023.
  • The Amor asteroids are ones whose perihelion are more than 1 AU. They always occupy the space outside Earth’s orbit. 11957 examples are known as of October 2023.
A graph of the semi-major axes and the perihelia of near-Earth asteroids. Note that the semi-major axis might go up as high as about 5 AU, but the perihelion never exceeds 1.3 AU
3. Planet-crossing Asteroids

As the name suggests, planet-crossing asteroids are those that “cross” a planet’s orbit. By “crossing” a planet’s orbit, the perihelion of the asteroid is smaller than the aphelion of the planet, and/or the aphelion of the asteroid is larger than the perihelion of the planet.

As you might have noticed, we’ve already mentioned Earth-crossing asteroids before. The Aten and Apollo asteroids are such examples. But did you also notice a gap between the near-Earth and the inner main-belt asteroids? That’s where the Mars-crossing asteroids are, where their orbits “cross” that of the red planet.

Distribution of the orbits of Mars-crossing asteroids; note that the large red dot on the upper left is the perihelion of Mars in the x-axis and the aphelion of Mars in the y-axis. This indicates that the asteroids in this graph indeed “crosses” the orbit of Mars.
4. Trojans

And out of the asteroids that come close to the planets’ orbits, one category is special — the trojan asteroids. They are the asteroids that reside in the L4 and L5 Lagrange points, 60 degrees ahead of and behind the planet, respectively. These asteroids are co-orbital with the planet — this means, their orbital periods are the same as that of the planet.

Out of the trojan asteroids, most of them are co-orbital with Jupiter — those are the Jupiter trojans. According to JPL’s Small Body Database, there are 13027 known Jupiter trojans as of October 2023. Trojans are found surrounding the orbits of other planets as well — there are even two Earth trojans!

5. Centaurs

The types of asteroids we’ve discussed so far reside within Jupiter’s orbit — but what about those in the outer Solar System? If the asteroid orbits the Sun within the orbit of Neptune, it is known as a centaur. The orbits of most centaurs are very unstable, as they are planet-crossing asteroids — perturbations and close approaches with the planets tend to kick them out of their orbits in a relatively short time.

Distribution of the orbits of centaurs

As of October 2023, there are 733 centaurs listed in the Small-Body Database. That number might make the centaurs look like a very small population of the asteroids, although with the large sizes of the known centaurs, that number is probably severely limited by our observation capabilities, as they are already very far away from the Earth.

The relatively large sizes of the centaurs and the trans-Neptunian objects are demonstrated in the obvious negative correlation above graph; the larger the semi-major axis, the lower the absolute magnitude. A lower absolute magnitude often means a larger body.
This phenomenon is just as evident, if not more obvious, when the perihelion distance is taken into account.
6. Trans-Neptunian Objects

What about the asteroids outside the orbit of Neptune? These are the trans-Neptunian objects. Most of them orbit within the Kuiper belt, which is the donut-shaped region around 30 to 50 AU from the Sun, although there are a few exceptions that orbit farther out from the Sun.

Currently, there are 4487 trans-Neptunian objects in the JPL’s Small-Body Database. That is a small population compared to the known asteroids in the asteroid belt, but according to various studies (reference?) the Kuiper belt houses many more objects than the asteroid belt. (talk about the scale in detail)

The distribution of trans-Neptunian objects (note: about 200 objects have semi-major axes far beyond 100 AU, but those are excluded from the graph)

Classification by Asteroid Families

While the two classification schemes (chemical composition and location) are helpful, they often only act as general guidelines. For more specific classifications often used while researching asteroids, we need other schemes of classifications. One of them uses asteroid families.

The term “asteroid family” often refers to collisional families, where multiple asteroids are formed together by the breakup of a parent body, often through a collision. They don’t only have very similar orbital parameters and dynamical properties, but also have similar chemical compositions and spectral types as well. Unlike other schemes of classifications, more complicated algorithms (such as hierarchical clustering) are needed to determine the location of the collisional families. Examples of collisional families include:

  1. Vesta family
  2. Flora family
  3. Koronis family
  4. Eos family

Occasionally, an asteroid family can also be a dynamical group — where multiple asteroids follow similar orbits around the Sun. Unlike in collisional families, they may or may not have the same chemical composition. Instead of sharing a common origin, they often share a common dynamical pathway that brought them from their original orbit to their current orbit. Examples of dynamical groups include:

  1. Hilda asteroids
  2. Alinda asteroids
  3. Cybele asteroids

Conclusion

In this article, we’ve mentioned three ways that asteroids can be classified — by their chemical compositions, locations, and origins. We have also named a few common types of asteroids, such as:

  1. C-type asteroids
  2. S-type asteroids
  3. M-type asteroids
  4. Near-Earth asteroids
  5. Main-belt asteroids
  6. Trojan asteroids

If you would like to suggest improvements to this article, or if you have any questions, please put them in the comments below.

References

  1. (n.d.). Asteroids: Facts. Retrieved October 29, 2023, from https://science.nasa.gov/solar-system/asteroids/facts/
  2. (n.d.). Chapter: 4 Asteroids and Meteorites – The National Academies Press. Retrieved October 29, 2023, from https://nap.nationalacademies.org/read/6281/chapter/6
  3. (2022, September 6). LEARN – What are the different types of Asteroids? Retrieved October 29, 2023, from https://asteroidday.org/resources/event-resources/learn-what-are-the-different-types-of-asteroids/
  4. Bartlett, R. J. (2020, August 11). Different Types of Asteroids (C, S, and M) – The Definitive Guide. Retrieved October 29, 2023, from https://astronomysource.com/different-types-of-asteroids/
  5. (n.d.). Small-Body Database Query. Retrieved October 29, 2023, from https://ssd.jpl.nasa.gov/tools/sbdb_query.html
  6. (n.d.). Kuiper Belt: Facts. Retrieved October 29, 2023, from https://science.nasa.gov/solar-system/kuiper-belt/facts/
  7. Nesvorny, D., Broz, M., (2015, February 5). Identification and Dynamical Properties of Asteroid Families. Retrieved October 29, 2023, from https://arxiv.org/abs/1502.01628

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