<P> The asteroid belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3: 1 Kirkwood gap (2.5 AU), and the outer belt formed by those asteroids closer to Jupiter's orbit . (Some authors subdivide the inner and outer belts at the 2: 1 resonance gap (3.3 AU), whereas others suggest inner, middle, and outer belts; also see diagram). </P> <P> The high population of the asteroid belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main - belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years . A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family). Conversely, collisions that occur at low relative speeds may also join two asteroids . After more than 4 billion years of such processes, the members of the asteroid belt now bear little resemblance to the original population . </P> <P> Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to a few hundred micrometres . This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids . Due to the Poynting--Robertson effect, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun . </P> <P> The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light . This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic . Asteroid particles that produce the visible zodiacal light average about 40 μm in radius . The typical lifetimes of main - belt zodiacal cloud particles are about 700,000 years . Thus, to maintain the bands of dust, new particles must be steadily produced within the asteroid belt . It was once thought that collisions of asteroids form a major component of the zodiacal light . However, computer simulations by Nesvorný and colleagues attributed 85 percent of the zodiacal - light dust to fragmentations of Jupiter - family comets, rather than to comets and collisions between asteroids in the asteroid belt . At most 10 percent of the dust is attributed to the asteroid belt . </P>

Where are most of the asteroids located in the solar system