1. 1. Discovery
  2. 2. What is it?
  3. 3. Asteroid Types
  4. 4. Activity
  1. 5. Largest Asteroids
  2. 6. Exploration
  3. 7. Video Gallery
Asteroid Belt IllustrationAsteroid Belt Illustration /

As everyone knows, our solar system is divided into three sections. The inner solar system (Mercury to Mars), the outer solar system (Jupiter to Neptune), and the Trans-Neptunian (beyond Neptune). However, not everyone knows that there is a physical "wall" that separates the inner and outer solar system that lies between Mars and Jupiter. This "barrier" is known as the asteroid belt.


The asteroid belt was discovered by an Italian priest and astronomer Giuseppe Piazzi, entirely by accident in 1801. His first discovery was Ceres, the largest asteroid in the belt, orbiting between Mars and Jupiter which led to closer observations by himself and other astronomers. As a result, other asteroids were discovered and the belt was realized.

What is it?

Artist's concept of Asteroid beltArtist's concept of Asteroid belt /

The asteroid belt is a ring of asteroids that encircles the inner solar system around the sun. The asteroid belt was formed from debris left over from the formation of our solar system. Basically, they are the hunks of rock that did not get the chance to become part of or form a viable planet, the leftovers if you will. Gravitational perturbations from Jupiter prevented these debris' from being able to form into planets and collisions between them were too violent for fusion so instead, they just broke apart into smaller pieces. As a result of the pull between Jupiter and Mars, these objects settled in an orbit between them creating a sort of divider between the inner and outer solar system.

Asteroid Types

951 Gaspra, the first asteroid imaged by a spacecraft951 Gaspra, the first asteroid imaged by a spacecraft /

There are three types of asteroids that occupy the belt, C-type, S-type, and M-type. Although there are many other categories and subcategories of asteroid types, these are the three most commonly found within the belt.

C-type (carbonaceous) are carbon-rich asteroids that have slightly reddish hue but are very dark with a low albedo (reflective) surface. These asteroids take up most of the outer layer of the asteroid belt and are approximately 75% of the visible asteroids seen.

S-type (silicate) are silicate-rich asteroids and more commonly found in the inner part of the belt. S-type asteroids makeup about 17% of the belt and have a fairly high albedo. They lack in the carbonaceous material that would have existed in primordial times most likely due to melting and reformation.

M-type (metal) are rich in metals, primarily iron-nickel, and take up about 10% of the belt. These may have been planetary cores that were beginning to form but never completed their development.


Chelyabinsk meteor (2013) originated from the asteroid beltChelyabinsk meteor (2013) originated from the asteroid belt /

Although the asteroids in the belt are thousands to millions of miles apart, collisions do occur on a frequent basis. During these collisions, smaller pieces are created called meteorites and have a high chance of falling to Earth. It is believed that 90% of the meteorites found on Earth originated from these collisions in the belt.

Largest Asteroids

The total mass of the asteroid belt equals 2.8×1021 to 3.2×1021 kilograms. However, there are four distinct objects within the belt that take up half of that; Ceres, Vesta, Pallas, and Hygiea. Here is a brief overview of these objects.

CeresCeres /

Ceres is considered the largest asteroid but is classified by the IAU (International Astronomical Union) as a dwarf planet. It was able to achieve a spherical body under its own gravity and had the size requirements to be considered a planet, but did not clear its neighborhood of objects. It comprises a third of the mass of the belt at 8.958×1020 kg.

Color image of Vesta taken by DawnColor image of Vesta taken by Dawn /

Vesta — is the second largest asteroid in the belt and the first object in the belt to have been visited by a spacecraft. At 2.67×1020 kilograms it is also one of the brightest asteroids in the belt with an albedo of 0.4322.

Model of the asteroid PallasModel of the asteroid Pallas /

Pallas — is what scientist refer to as a protoplanet. Basically, it is a planet that did not complete its formation. Pallas takes up 7% of the total mass of the belt at 2.108×1020 kg and is the third largest of the asteroids in the belt.

Hygiea — is the fourth largest asteroid in the belt at a mass of 8.67×1019 kg which is approximately 2.9% of the total mass of the belt. Hygiea is very dark due to its low albedo carbon-rich surface so though it is the fourth largest asteroid, it was only detected after several smaller, brighter asteroids were found.


NASA's Galileo is the first spacecraft to get up close to the asteroids in the belt in 1991. It was also the one that first imaged an asteroid with a moon orbiting it in 1994. In 2001 NASA's NEAR spacecraft began studying Eros in the belt for a year. It was the first spacecraft to successful land on an asteroid though it was not designed to do so. This gave it the chance to explore closer into the surface, sending back large amounts of data for scientist to study and give us a better understanding of the composition of asteroids.

In 2006, Hayabusa (Japan) was the first spacecraft to successfully land and launch from an asteroid, returning to Earth in 2010. It brought back with it samples of the asteroid and they are currently being studied today. These samples are helping with the possibility of future projects and missions into the asteroid belt to open the door for resource mining in the future.

Artist's concept of the Dawn spacecraft with Vesta and CeresArtist's concept of the Dawn spacecraft with Vesta and Ceres /

NASA's Dawn launched in 2007 and was on a mission to explore Vesta and Ceres, the two largest asteroids. Dawn reached Vesta in 2011 and Ceres in 2015 and has sent back numerous data and images for scientist to research.

Resource corporations began to see the potential in mining these asteroids and companies such as Plantary Resource, Inc. were formed with initial plans to one day mine them for resources. NASA has even begun exploring this idea and has presented many future missions to gather more data to initiate plans to begin the mining proposals. There have even been discussions about the Mars colony missions proposed for the future as possible miners. Colonist on Mars would be close enough to the asteroid belt to begin surveying and mining the asteroids. In which case, our resources would come from off world means, preserving what little we have left here.

See also: Asteroids, Objects