Mechanisms that make NEOs' orbits chaotic

There are several mechanisms that act on the minor bodies of the solar system, making their orbits chaotic and giving birth to NEOs. These mechanisms can be separated into physical (such as collisions) and dynamical (such as the gravitational action of a nearby planet, or the phenomena of resonances).


Collisions
 

A laboratory experiment on fragmentation:
(Courtesy of Ian Giblin, Ph.D Thesis, 1994
The most important of the first group is the physical phenomena of collision, which often happens between the very numerous asteroids that inhabit the main Belt. The physic behind these collisions is very complicated, and the destiny of the fragments produced can be very different: these new fragments can either escape toward a new life, as single asteroids, either give birth to dynamical families of asteroids. Normally, only on very rare occasions these collisions seem to be enough energetic to make asteroids become NEOs.

Fly-by of a planet
 
In normal conditions, the Sun is the main attractor of the solar system, and the orbit of any body of the solar system can therefore be calculated, in a first, very approximated determination, considering the only gravitational attraction of the Sun. The main exception to this approximated law takes place when the body (which can be an asteroid or a comet) comes near another planet. In this case, when the body enters the zone where the attraction of the planet is fundamental (the sphere of influence of the planet) its original orbit is drastically changed.  

Resonances
 
Resonance is an additional phenomena which is very efficient in making the orbit change into a chaotic one and its parameters vary. The characteristic timescale are much longer than our lives (millions or hundred of millions of years) but, due to the high number of objects involved, these phenomena are fundamental in the transferring objects from stable to unstable orbits.