Size and composition

Asteroids vary greatly in size. The largest asteroid, Ceres, is 950 km in diameter and contains about one-third the total mass of all the asteroids. When it was discovered in 1801, Ceres was thought to be another planet because of its size. Ceres was classified by the IAU in 2006 as a 'dwarf planet' because it is too small to have cleared out the smaller chunks of matter orbiting around it in the asteroid belt.

Figure 8.8 The Hubble Space Telescope captured this image of Ceres in 2004. Notice how Ceres is more spherical in shape than the other asteroids. In 2006, Ceres was classified as a dwarf planet because it is part of a larger population of bodies with the same general orbital path. (Photo: NASA)
Figure 8.9 Comparative sizes of the Earth, Moon and Ceres.

The second largest asteroid is Vesta. Vesta has not been classed as a dwarf planet because its shape is not spherical: it has a large concavity and protrusion near its south pole. Vesta is thought to have an iron-nickel core, a rocky olivine mantle and a surface crust containing several impact craters. Fragments of Vesta are thought to be scattered thoughout the solar system.

Scientists have found out about the composition of asteroids by analysing the light reflected from their surface and from analysing meteorite fragments. There are two main types of asteroids, based on their composition. One group dominates the outer part of the belt. These asteroids are rich in carbon and their composition has not changed much since the solar system formed. The second group is located in the inner part of the belt, and is rich in minerals such as iron and nickel. These asteroids formed from melted materials.

Scientists believe Ceres and Vesta have followed different evolutionary paths. Vesta's origins seemed to have been hot and violent because it has basaltic flows on its surface. Ceres seems to have a primitive surface and evidence of water in its minerals. Vesta's physical characteristics reflect those of the inner planets, whereas Ceres is representative of the icy moons of the outer planets. By studying these contrasts and comparing these two asteroids, scientists hope to develop a better understanding of the transition from the rocky inner regions of the solar system to the icy outer regions.

Table 8.3 The 10 largest asteroids



Diameter (km)



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