Through the atmosphere

Most of the objects that fall on Earth are very small (not bigger than a meter) and are therefore destroyed before they reach the ground, giving birth to the beautiful phenomena of  meteors. In this case -as well as in the case when the body is too big to be destroyed- the atmosphere plays a very important role, acting as a real shield.

In this image, the distribution of the 136 meteoric explosions in the atmosphere recorded by the USA system of satellitary defense between 1975 and 1992. The totality of the events in that period is estimated to be 10 times greater.

(courtesy of the US Air Force)

During the crossing of the atmosphere, the falling NEO experiments friction with air, which gives birth to two fundamental physical phenomena that can lead to the complete destruction of the object, depending on the mass and on the type of body:  ablation and fragmentation. It is interesting to say that among bodies smaller than a 10 m diameter, only iron bodies pass through the atmosphere. However, even if the NEO completely destroys itself before coming to the ground, the impact event can still be very dangerous (as for example, in the Tunguska event)!

   

Ablation

The first possibility is that the heat caused by friction makes the body melt. Drops of melted material leave the main body of the falling object, eating it up. This process can bring to the complete destruction of the smallest bodies.
 
 

Fragmentation
 

For bigger bodies, a consequence of friction acting on its surface, is the augmentation of the external pressure on the front side of the object, while on the rear face of the body, the pressure tends to reduce. If the stress is bigger than the interior resistance of the material the impacting body is made of, this phenomenon can lead to fragmentation, with the destruction of the main body in smaller pieces.  Consequently, this process of fragmentation makes the area of the body on which frictions acts, grow up. For this reason, the process tends to continue, until the formed pieces slow down enough for the pressure on the surface of the body not to be able to make the process restart.  
 

Obviously, objects of different materials and masses act in different ways, for what concerns fragmentation: for example, it is possible to give some data about the height at which fragmentation begins, depending on the material of the body: 

sand 50-40 Km
rock 30-10 Km
iron 8 Km

Collateral effects

Whether the body is completely destroyed or not, different collateral effects follow the crossing of the atmosphere and can be explained as simple physical phenomena, such as the light trail left behind, or the bang that accompanies the phenomena.
Friction between the atmosphere and the falling body tends to slow down the body, making it loose part of its kinetic energy, partly transferring it to the air in the form of heat, partly irradiating it as light. For this reason, meteors are associated with very brilliant trails of light. This rapid lost of kinetic energy can also give birth to a mechanical shock wave in the atmosphere, that can be sometimes detected as a loud bang.