Asteroid Belt


The Asteroid Belt is the region of interplanetary space between Mars and Jupiter where most asteroids are found. It contains irregularly shaped chunks of debris called asteroids.

asteroid belt photo

The Space Asteroids are made of rock and metal, mostly nickel and iron. Contrary to popular imagery, the asteroid belt is mostly empty.

Facts about the Asteroid Belt

* Area: The main asteroid belt extends from 255 to 600 million km (2.15 to 3.3 astronomical units) from the Sun and may contain over a million objects bigger than 1 km across.

* Diameter: The largest objects are Ceres (1,003 km), Pallas (608 km) and Vesta (538 km).

* Total Mass: The total mass of all the asteroids is less than that of the Moon. There are 26 known asteroids larger than 200 km across.

* Location: The Asteroid Belt is a region between the inner planets and outer planets where thousands of asteroids are found orbiting around the Sun.


How big are the asteroids? Ceres is the largest asteroid in the asteroid belt and is the only dwarf planet in the belt. Pallas is second largest and the second asteroid to have been discovered. More than half the mass of the main belt is contained in the four largest objects: Ceres, Vesta, Pallas and Hygiea.


The current main belt consists primarily of three categories of asteroids: C-type (carbonaceous asteroids), S-type (silicate asteroids) and M-type (metallic asteroids).


Scientists have detected water-ice and carbon-based organic compounds on the surface of asteroid Themis. The discovery of water ice on 24 Themis was announced in April 2010. Two teams of researchers independently verified that the asteroid 24 Themis is coated in a layer of frost. This discovery has changed scientists perspectives on asteroids.

The discovery may also be a boon to NASA’s new space exploration program which is aiming to send astronauts to visit a near-Earth asteroid in the future.


Scientists believe the asteroids are the pieces of a planet that never formed. One possible theory is the ongoing gravitational tug-of-war between Jupiter and Mars has prevented the pieces from bonding together, hence, this planet was never created.



The first flybys of asteroids were performed in 1991 and 1993 by NASA’s Galileo spacecraft and in 1996 by the Near Earth Asteroid Rendezvous (NEAR) spacecraft.

In the 1980’s, the Soviet Union was planning to send a probe to Vesta. No information available on its status.

NASA’s Dawn mission is on a 3-billion-km (1.7-billion-mile) journey to the asteroid belt to orbit asteroid Vesta and dwarf planet Ceres. Scientists hope to study the conditions of the solar system’s earliest days.

More Facts on the Asteroid Belt – Did you know?

Other regions of small solar system bodies include the Centaurs, the Kuiper belt and scattered disk and the Oort cloud. Beyond the orbit of Neptune lies an even large and more populous region of minor bodies known as the Kuiper belt.

When was the Asteroid Belt Discovered?

The process of discovering the asteroid belt began in the late 1700s. The first asteroid was discovered by Italian astronomer Giuseppe Piazzi was named Ceres in 1801. Ceres is now regarded as a dwarf planet. Ceres was given dwarf planet status in 2006, along with Pluto, Eris, Haumea and Makemake.


Asteroids and Dwarf Planets and How to Observe Them (Astronomers’ Observing Guides) by Roger Dymock

Guide to the Universe: Asteroids, Comets, and Dwarf Planets (Greenwood Guides to the Universe) by Andrew S. Rivkin

Impact!: The Threat of Comets and Asteroids
by Gerrit L. Verschuur

The Asteroid Belt Links

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