Mysterious Wanderers of the Universe: What Are Asteroids

30th Oct 2022
Mysterious Wanderers of the Universe: What Are Asteroids

Astronomy has confirmed that the Earth is just a tiny grain of sand among hundreds of others that live in the vast ocean of space. But who are our closest neighbours in the Universe, and how well do we know them? These are the Moon, a natural satellite of the Earth, other planets of the solar system similar to the Earth, along with their satellites; as well as a lot of smaller celestial bodies, which scientists call asteroids.

What are the asteroids, where do they come from, what are they made of, and what does this neighbourhood hold for us? You will find the answers below.

What is an asteroid?

Asteroids are rocky and/or metallic celestial bodies no larger than planets and their moons. Most often, they have an irregular shape, sometimes close to spherical, and are covered with numerous impact craters. There are binary (double) and triple asteroids, in which two or three bodies of approximately the same size revolve around each other. Over 150 asteroids have their own small satellite (sometimes two), and one is even surrounded by rings like Saturn.

What size are they?

They can be as small as a boulder or as large as 600 miles across, like Ceres, discovered in 1801. Until 2006, it was considered the largest asteroid in the solar system, but later it was transferred to the category of dwarf planets. Since then, Vesta, which is half the size of Ceres, has been granted the title of the largest asteroid.

Where do they come from?

According to scientists, asteroids are leftover materials from the formation of our solar system. 4.6 billion years ago, the solar system was a large collection of gas and dust. As the protoplanetary disk, which later became the Sun, formed, large planets and planetesimals, smaller celestial bodies formed from most of the gas and dust. Frequent collisions in the early, chaotic solar system shattered planetesimals into smaller pieces that we today call asteroids.

Presumably, the moons of Mars (Phobos and Deimos), as well as most of the outer moons of Jupiter, Saturn, Uranus, and Neptune, were once asteroids until they were captured by the planets’ gravity and became their satellites.

Asteroids classification by location

Accumulations of asteroids are concentrated in different regions of the solar system, according to which they are classified.

Main asteroid belt

This is the largest cluster, which is located between the orbits of Mars and Jupiter and has more than 200 bodies with a diameter of over 60 miles, about 2 million of over 300 feet, and millions of smaller asteroids.


These are named after the Iliad characters. What are Trojan asteroids? These are stones that are located in Lagrange points, where the gravitational influence of the planet and the Sun is equivalent. Jupiter has over 10,000 Trojans. Neptune has 30 or more, Mars has nine, while Earth and Uranus have one each.

Hilda Group

This group lies behind the main belt between the orbits of Mars and Jupiter. It contains over 1,000 Trojan-like asteroids.

Near Earth Asteroids (NEAs)

They are called so because they revolve around the Sun at about the same distance as the Earth. Even though they are extremely few (only 27 thousand had been found as of 2022), these asteroids have their own classification:

  1. Аtiras – a class of NEAs whose orbits coincide completely with the orbit of the Earth, located at a distance of less than 1 AU. They are named after the asteroid Atira.
  2. Atens – these are Earth-traversing NEAs with a semi-major axis smaller than that of the Earth, with a distance of less than 1 AU. Named after the asteroid 2062 Aten.
  3. Аpollos – Earth-traversing NEAs with a semi-major axis greater than that of the Earth, with a distance of less than 1 AU. Named after the asteroid 1862 Apollo.
  4. Аmors – NEAs with orbits outside the Earth but inside the orbit of Mars. They are named after the asteroid 1221 Amor.

Some of the near-Earth asteroids are considered potentially dangerous for our planet. However, there is no real threat as of yet. The nearest such asteroid orbits 4.65 million miles from Earth and is less than 500 feet in diameter.

Sometimes the Kuiper Belt is mistakenly called an asteroid belt. The Kuiper Belt is an annular region extending beyond Neptune’s orbit at a distance of 4.5 billion to 7.5 billion km from the Sun. It is indeed a cluster of small celestial bodies, but they are all made of ice. And what are asteroids made from?

What are they made out of?

Most asteroids are grouped into three main classes based on their composition. There are other specific classes, but they include very few asteroids.

C-type asteroids (carbonaceous or chondrites)

They are named so because of the high content of carbon compounds and make up over 75% of all known outer belt asteroids. In composition, they are close to carbonaceous chondrites, the oldest meteorites found on Earth, with an age of 4.5 billion years. The most prominent representatives of carbonaceous asteroids are Hygiea, Matilda, and Isolda.

S-type asteroids (stony or silicic)

They make up about 15% of known asteroids and dominate the inner asteroid belt. These asteroids are composed predominantly of silicates, nickel, magnesium, and iron and show an almost complete absence of any carbonaceous compounds. This indicates that the stones at these asteroids have undergone significant changes due to partial melting and differentiation. Representatives of this class include Juno, Eros, and Iris.

M-type asteroids (metallic)

They make up about 10% of asteroids and are located in the middle part of the main belt. What are metallic asteroids made out of? Mostly nickel and iron. The largest representative of this class is the Psyche asteroid.

Among the specific classes, V-class asteroids (Vestoids) can be distinguished. This is a very rare type, accounting for about 5% of the main belt and some isolated cases in other parts of the solar system. Asteroids in the main belt are called Vestoids because they are fragments of Vesta ejected by the impact that created the Rheasilvia crater. Like S-type, they contain stony and iron chondrites but with a high content of pyroxene. Some V-asteroids were once subjected to high temperatures, which caused their iron to sink to the centre, pushing out the basaltic volcanic crust.

What colour are they?

The colour of asteroids directly depends on their composition. So, carbonaceous rocks are predominantly dark grey, and one needs a large telescope to see them in the sky. Silicon asteroids can be grey, green, blue, and red, while iron asteroids are predominantly red. Both are brighter than carbon asteroids, and some of them, for example, Vesta, can be seen even with good binoculars. Scientists claim that the older the asteroid, the darker it is. Accordingly, the grey colour becomes dark over time, and the red one acquires brown shades.

What colour are asteroids

What are they also known as?

Scientists sometimes refer to asteroids as minor planets or planetoids. In literature and daily life, they are often called shooting stars. The thing is, small fragments, called meteors, fall into the Earth’s atmosphere and burn out on their descent, leaving a bright trace behind. Some people also confuse asteroids with comets, but these objects are completely different in composition and location, relevant to the Sun.

What are the similarities and differences between comets, asteroids, and meteors can be clearly seen in the picture below.

types of space rocks

What asteroids are a threat to Earth?

This is a question that worries many people. In fact, small fragments of space rocks fall to Earth regularly, but we do not notice this because, in our atmosphere, they all burn up with almost no trace. However, history knows cases of rather large asteroids colliding with the Earth, which causes considerable damage to our planet. The last example is the Tunguska meteorite. Read more on this topic in our separate article — Will an asteroid hit the Earth?

Why do we need to learn about asteroids?

Scientists continue to study the Universe. And perhaps soon, we will acquire new information about what are asteroids and learn how to defend against a possible collision with them. Possibly, we might even figure out mining on Asteroids. Similar missions are already up and running.


In 2016, NASA sent Origins Spectral Interpretation Resource Identification Security Regolith Explorer (Osiris Rex) to retrieve soil samples from asteroid Bennu. This asteroid was not chosen by chance. Bennu belongs to the Apollos group, which is close enough to Earth; plus, it is a C-type asteroid, which allows obtaining carbonaceous matter that has been left on it since the solar system’s formation. The name of the Osiris mission is quite symbolic. It simultaneously refers to the ancient Egyptian god Osiris and Bennu, the symbol of his rebirth. The soil sampling took place on October 20, 2020, and the return to Earth is scheduled for September 2023.


In November 2021, NASA sent a DART mission to the Dimorphos asteroid to change a celestial body’s trajectory and work out a planetary defence mechanism. On September 27, 2022, a 570-kilogram probe crashed into an asteroid with a diameter of about 160 m at a speed of over 20,000 km/h. The impact is expected to alter Dimorphos’s orbit “by a fraction of a percent”. In the coming weeks and months, scientists will determine whether this mission was successful. And we will definitely inform you about the results, so follow our news updates.

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