Asteroids are bodies of rock, airless and usually irregular in shape, which are too small to be considered planets. Material left over from the formation of the solar system, they orbit the sun in the same direction as the planets. Their size ranges from a few millimetres to hundreds of miles across, and the majority are found in the Asteroid Belt which circles the sun between Mars and Jupiter.
Above: An artist's impression of the Asteroid Belt between Mars and Jupiter, showing an asteroid breaking-up after being struck (Credit: NASA/JPL-Caltech).
200 million - the estimated number of asteroids with a diameter greater than 1 km (0.6 miles).
3 - the number of distinct types of asteroid, classified by their material composition; carbonaceous, silicaceous, and metallic.
75% - the approximate amount of known asteroids that are carbonaceous (C-type), meaning they are comprised of carbon-rich material.
3-10% - the amount of light reflected by carbonaceous asteroids, making their appearance very dark.
17% - the approximate amount of the Asteroid Belt represented by silicaceous (S-type) asteroids, which are comprised mostly of iron and magnesium silicates (just like Earth's mantle).
Above: 433 Eros, the second-largest near-Earth asteroid (after 1036 Ganymed) is an S-type asteroid (Credit: NASA/JPL/JHUA).
10-22% - the amount of light reflected by silicaceous asteroids.
8% - the approximate amount of the Asteroid Belt made up of metallic (M-type) asteroids, which are believed to be a mix of iron and nickel, not unlike Earth's core. Once molten, they cooled to form the solid bodies we find today.
50 m - the width (164 feet) of the M-type asteroid that struck modern day Arizona some 50,000 years ago, creating the 1.2 km wide (0.75 mile) Barringer Crater.
50,000 km/h - the estimated speed at which the Barringer astroid struck Earth (30,000 mph).
945 km - the width of Ceres, the largest body in the Asteroid Belt (587 miles), and the 33rd largest known body in the solar system.
30% - the approximate amount of the total mass of the Asteroid Belt represented by Ceres alone.
Above: An animation of the dwarf planet Ceres, the largest body in the Asteroid Belt. The mysterious bright spots which can be seen in the Occator crater are now thought to be ice (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).
130 - the approximate number of mysterious bright areas seen across the surface of Ceres, most within craters. Some are believed to be ice, whilst others are thought to be the remnants of mineral salts.
01 January 1801 - the date Ceres was discovered, by Italian astronomer Giuseppe Piazzi (1746-1826). Ceres is named after the Roman goddess of agriculture.
Ceres is the only asteroid in the Asteroid Belt with sufficient mass to have pulled itself into a spherical, planet-like shape. Because of this regular, round appearance Ceres is classified as a dwarf planet.
Above: An animated fly-past of Ceres, constructed from images captured by NASA's Dawn spacecraft in 2015. A star field has been added in the background (Credit: NASA/JPL-Caltech/LPI).
525 km - the width of Vesta, the second largest body in the Asteroid Belt (326 miles).
9% - the approximate amount of the total mass of the Asteroid Belt which is represented by Vesta.
Above: The colourful surface of Vesta, revealed in this composite of images captured in 2011 by the Dawn mission. The impact crater Sextilia can be seen bottom right (Credit: NASA/JPL-Caltech/UCLAMPS/DLR/IDA).
1,200 - the approximate number of smaller asteroids that have been created by the ejecta thrown off when other asteroids struck Vesta around 1-2 billion years ago, estimated by the number of impact craters found on its surface.
70 km - the width of the largest impact crater on Vesta (43 miles), one of the Snowman craters.
29 March 1807 - the date Vesta was discovered, by German astronomer Heinrich Wilhelm Matthäus Olbers (1758-1840). It is named after the Roman goddess of home and hearth.
Above: The scale of asteroids Ceres (middle) and Vesta (right) compared to our Moon (Credit: NASA/JPL-Caltech/UCLA).
512 km - the width of Pallas, the third largest body in the Asteroid Belt (318 miles).
7% - the approximate mass of the Asteroid Belt represented by Pallas.
28 March 1802 - the date Pallas was discovered, also by Olbers. It is named after the Greek goddess Pallas Athena (more commonly known simply as Athena).
© 2016 - Dave Fowler, History in Numbers. All third party trademarks are hereby acknowledged.