<P> The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt . Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth . Primarily because of gravitational perturbations, most of the material was ejected from the belt within about a million years of formation, leaving behind less than 0.1% of the original mass . Since their formation, the size distribution of the asteroid belt has remained relatively stable: there has been no significant increase or decrease in the typical dimensions of the main - belt asteroids . </P> <P> The 4: 1 orbital resonance with Jupiter, at a radius 2.06 AU, can be considered the inner boundary of the asteroid belt . Perturbations by Jupiter send bodies straying there into unstable orbits . Most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System . The Hungaria asteroids lie closer to the Sun than the 4: 1 resonance, but are protected from disruption by their high inclination . </P> <P> When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a "snow line" below the freezing point of water . Planetesimals formed beyond this radius were able to accumulate ice . In 2006 it was announced that a population of comets had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans . According to some models, there was insufficient outgassing of water during the Earth's formative period to form the oceans, requiring an external source such as a cometary bombardment . </P> <P> Contrary to popular imagery, the asteroid belt is mostly empty . The asteroids are spread over such a large volume that it would be improbable to reach an asteroid without aiming carefully . Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff . Over 200 asteroids are known to be larger than 100 km, and a survey in the infrared wavelengths has shown that the asteroid belt has 0.7--1.7 million asteroids with a diameter of 1 km or more . The apparent magnitudes of most of the known asteroids are 11--19, with the median at about 16 . </P>

Where is the kuiper belt in relation to the asteroid belt