Directing Meteoroids To Earth

There is compelling evidence that nearly all meteoroids that reach the ground and can be studied as meteorites are derived from the asteroid belt. Ideally, scientists would like to know which asteroids are the sources of particular types of meteorites and the mechanisms by which meteorites are transported from the asteroid belt to Earth. The question of which meteorites came from which asteroids may not be comprehensively answered until asteroids are explored by spacecraft. (One exception is the HED meteorites and their relationship to the asteroid Vesta.) Nevertheless, there is considerable information about how they have gotten to Earth.

Only two processes are known that can put meteoroidal fragments into Earth-crossing orbits on the short times-cales indicated by their cosmic-ray exposure ages. These processes are direct collisional ejection from the asteroid belt and gravitational acceleration by dynamic resonances with the planets. As mentioned above, collisions at velocities of 5 km (3 miles) per second are relatively common in the asteroid belt. In such a collision, some material is ejected at the velocity needed to put it into an Earth-crossing orbit, but the quantity is small, and most of it is pulverized by the associated shock pressures. High-velocity ejection is the likely explanation for those meteorites determined to have come from Mars or the Moon, but it completely fails to provide the observed quantity of meteorites from the asteroid belt.

Resonance mechanisms are believed to be of much greater importance in sending material toward Earth. These resonances efficiently expel material from the belt, producing the Kirkwood gaps. One of the most prominent of these gaps lies at a distance of about 2.5 AU from the Sun. An asteroidal fragment orbiting the Sun near 2.5 AU completes three revolutions in the time that Jupiter, the most massive planet in the solar system and a strong source of gravitational perturbations, executes one revolution. It is thus said to be in a 3:1 resonance with the planet. The regular nudges resulting from the resonance cause the orbit of the asteroidal fragment to become chaotic, and its perihelion (the point of its orbit nearest the Sun) becomes shifted inside Earth's orbit over a period of about one million years. Numerical simulations on computers support the idea that the 3:1 resonance is one of the principal mechanisms that inject asteroidal material into ultimately Earth-crossing orbits.

If gravitational resonance with Jupiter is an efficient mechanism for removing material from the asteroid belt, one might expect the region close to a strong resonance to be cleared of material over the lifetime of the solar system so that by now nothing would be left to send into Earth-crossing orbits. A number of processes, however, cause asteroids to migrate within the asteroid belt, thereby maintaining a constant supply of material to the resonances.

Meteoroids less than a few hundred micrometres across—i.e., interplanetary dust particles—come to Earth from the asteroid belt via a rather different mechanism than the larger ones. Interaction with solar radiation causes them to spiral into Earth-crossing orbits from the asteroid belt through a process called Poynting-Robertson drag. The time it takes a particle to traverse the distance from the asteroid belt to Earth depends inversely on its radius and where in the asteroid belt it started out. For 10-50 ^m (0.0004-0.002 inch) dust particles, traverse time is calculated to be about 100,000 years. (Particles that are much smaller than a micrometre are actually blown out of the solar system by radiation pressure from the Sun.) Estimates of cosmic-ray exposure ages for micrometeoroids collected on Earth are broadly consistent with their traverse times calculated from the Poynting-Robertson drag process. In principle, some dust particles could be much younger than the calculated traverse times, either because they were produced in collisions of larger Earth-crossing objects or because they are not asteroidal in origin but rather have been shed by comets during comparatively recent passages through the inner solar system.

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