What are Asteroids? - Facts & Explanation

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Millions of asteroids dot our solar system, bumping and crashing into each other as they orbit the sun. This lesson will define and describe the properties of asteroids and how they have shaped the Earth.


An asteroid is an object made of rock and/or ice that orbits the sun. Planets also fit this definition, but an asteroid lacks some of the features needed to be a planet. For example, asteroids lack a consistent spherical shape as seen with planets. Also, planets are large enough to clear other objects out of their immediate path. This does not occur with asteroids, which can be seen by studying the asteroid belt.

The surface of asteroids is littered with craters from impacts with other asteroids, as shown in the image below:

Asteroids commonly have impact craters

The largest asteroid currently known is Ceres. With a diameter of 590 miles, Ceres orbits over twice the distance from the sun as Earth does.



Asteroid Belt

Just past Mars lies a ring of over two million asteroids orbiting the sun. These asteroids formed along with the formation of the solar system 4.5 billion years ago. Unlike planets, the asteroids and small rock fragments here were prevented from combining into a planet by the gravity of Jupiter.

Location of the asteroid belt
asteroid belt

With so many asteroids orbiting the sun, it's inevitable that they bump and collide into each other. At times, these collisions are strong enough to knock an asteroid out of the asteroid belt and into open space. If conditions are just right (or wrong?), their path can put them on a collision course with Earth. These impacts occur and can have disastrous effects, including the extinction of the dinosaurs.

Trojan Asteroids

While most asteroids are found in the asteroid belt, other asteroids can lie in the orbits of moons and planets. Asteroids that lie in the same orbital paths of planets, but are at a different location, are able to survive because they do not collide with the planet.


Throughout Earth's history, and even most recently as 2013, impacts from asteroids have caused hysteria, destruction and even complete extinctions.


On February 15, 2013 an asteroid entered Earth's atmosphere over Russia, traveling at over 41,000 miles per hour. The heat from air molecules caused the meteor to explode while in flight, causing a large explosion in the atmosphere. The explosion caused broken glass and some buildings to collapse and roughly 1,500 people were injured. In the city of Chelyabinsk, small meteorite fragments have been found.


A much larger impact caused the Mesozoic Era to come to an abrupt end 66 million years ago. An asteroid about six miles in diameter slammed off the coast of Mexico causing destruction worldwide and eventually causing the extinction of the dinosaurs.

Artist rentition of Chicxulub impact


Commonly there is confusion about the difference between an asteroid, meteor and meteorite. They are the same thing, but have different names for their different locations.

  • An asteroid is a rock orbiting in space
  • A meteor is an asteroid that has entered Earth's atmosphere.
  • A meteorite is a meteor that has actually struck the surface of the Earth.

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