text: The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt .
simplified: The belt used to contain many more asteroids than it does today , but Jupiter 's gravity disturbed the orbiting debris and much of it was sent into orbits which meant it was lost to the asteroid belt . The total mass of the material in the asteroid belt today is thought to be about 4 % of the mass of the Earth 's Moon .