What are asteroids?

Asteroids are small, rocky bodies that orbit the sun. Most of them reside in the asteroid belt, which is a region located between Mars and Jupiter, but asteroids can also be found all throughout the solar system and in the oort cloud. 

Asteroid Size?

Asteroids range in size from Vesta—the largest at about 329 miles (530 kilometers) in diameter – to bodies that are less than 33 feet (10 meters) across. The total mass of all the asteroids combined is less than that of Earth’s Moon.

In simpler terms, asteroids’ size can range from space pebbles to about a thousand kilometers in diameter.

Where did asteroids come from?

Asteroids are thought to be remnants from the formation of our solar system 4.6 billion years ago. They are believed to be the material (gas and dust) that did not compile into planets and instead became smaller objects. 

Why didn’t a planet form in the asteroid belt?

Jupiter’s gravity is so strong that it makes asteroid orbits within the Kirkwood gaps unstable. It’s these gaps that prevented a single planetary body from forming in the asteroid belt. 

Asteroid Classification

The three broad composition classes of asteroids are C-, S-, and M-types. The C-type (chondrite) asteroids are most common, consisting of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system. The S-types (“stony”) are made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). 

The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.

C Type- 

> are carbonaceous asteroids

> Outer asteroid belt

S Type-

> a stony composition

> Inner asteroid belt

M Type- 

> are asteroids of partially known composition

> Some, but not all, are made of nickel–iron, either pure or mixed with small amounts of stone. 


“In Depth.” NASA, NASA, 19 July 2021, 

“What Is an Asteroid?” NASA, NASA, 26 Aug. 2021, 

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