Final answer:
The main asteroid belt is a region of leftover material from the solar system's formation, situated between Mars and Jupiter and stretching from 2.2 to 3.3 AU from the Sun. It contains numerous widely spaced asteroids with stable orbits.
Step-by-step explanation:
The main asteroid belt is best described as (a) material left over from the formation of the solar system. The asteroid belt is a vast region located between the orbits of Mars and Jupiter, where most asteroids are found. The main belt extends from 2.2 to 3.3 astronomical units (AU) from the Sun, signifying the space where the orbits are generally the most stable.
Asteroids in this region revolve around the Sun in the same direction as the planets, predominantly lying near the plane in which Earth and other planets orbit. Although the asteroid belt contains a significant number of asteroids, they are spaced out, with millions of kilometers separating them, allowing spacecraft to pass through without collision risk.
The asteroid belt's presence reflects the early solar system's conditions, whereby these small bodies could maintain stable orbits due to the gravitational influence of Mars and Jupiter and the large gap between these planets. These asteroids are also noteworthy as they mainly consist of stony or metallic substances without an atmosphere.