How can asteroid fragments enter Earth-crossing orbits?

Asteroid fragments from the main belt can enter Earth-crossing orbits primarily through two mechanisms: gravitational resonance and the Yarkovsky effect.

Gravitational Resonance

The most significant process involves Kirkwood gaps, which are regions in the asteroid belt where an object's orbital period is a simple fraction of Jupiter's period. When an asteroid fragment enters one of these resonances—such as the 3:1 resonance—it experiences regular, repeated gravitational "nudges" from Jupiter. These perturbations make the fragment's orbit chaotic, causing its perihelion to shift inward over millions of years until it crosses Earth's orbit.123

The Yarkovsky Effect

For smaller asteroids, the Yarkovsky effect plays a crucial role. This is a small force resulting from the anisotropic emission of thermal radiation from an asteroid's surface. This force causes small fragments to drift gradually through the belt until they reach a resonance zone, where Jupiter’s gravity then takes over to push them into Earth-crossing paths.14

While direct collisional ejection can also place fragments into new orbits, it is generally considered less efficient than resonance mechanisms for supplying the observed quantity of meteorites.3