*2.2.2 Asteroid Belt*

Several thousand bodies (asteroids) reside between 2 and 3.5 AU between the orbits of Mars and Jupiter [15]. This region is known as the Asteroid Belt. The largest of these is Ceres that contains about one third of the Asteroid Belt mass, and has a radius of about 500 km that is smaller than the Solar satellites summarized in **Table 4** [15]. Solar probes have investigated the properties of Asteroid Belt bodies. For example, NASA's space probe Dawn orbited Vesta that is one of the largest asteroids, and also investigated Ceres [11]. Some asteroid bodies have orbits that intersect Earth's orbit and present a potential collision hazard [7].

Some Solar System planets contain small moons that have a rocky composition that is similar to the Asteroid Belt bodies. It is possible that some of these moons were once asteroids that were gravitationally captured by the planets during an early phase of Solar System evolution when orbits were less stable than the current configuration.

**Table 5** lists the largest asteroids and their physical characteristics. A portion of the data was derived from the Dawn spacecraft. The mass values are rough estimates provided by NASA [25]. Ceres is classified as a dwarf planet [7]. Vesta, Pallas, and Juno and are the second, third, and fourth heaviest asteroids, respectively.


**Table 5.**

*Selected Asteroid Belt data derived from Ref. [25].*

Ceres is believed to have a rocky core [7]. An icy mantle and crust surround the core. The mantle could be as thick as 120 km and contain a volume of water greater than the Earth's oceans. Ceres has a surface temperature of about 233 K [7].
