Objects out there are subject to perturbation by nearby stars. We believe that comets are remnants of the most primitive objects that formed the planetesimals. The spaces between planets is mostly devoid of matter. Heavier components sunk and lighter object floated in this soup. NASA keeps the Antarctic meteorites in a special storage facility in Houston.
Japan also has an Antarctic meteorite facility.
A few meteorites have been photographed with accurate measuring devices. All of these trajectories projected backwards had orbits in the asteroid belt. For every 'n' times the asteroid revolves about the sun, Jupiter revolves 'm' times - this acts like a pump and the asteroid's orbit gets perturbed. These asteroids either get swallowed by Jupiter, leave the solar system, or crash into the sun. Some may intercept the earth since they mostly orbit in the same plane as the earth.