The average collisional velocity in LEO is 10 km/sec.
Most hypervelocity impacts produce circular craters, irrespective of the angle at which the impactor strikes the target.
If an impactor penetrates a target, the egress hole will be irregular and considerably larger than the ingress hole.
As of early 2010 the current orbital space object density (>10cm in size) in LEO is approximately 10-8 per cubic kilometre.
The average debris impact produces approximately 100 pieces with sizes greater than 10 cm.
Catastrophic impact generally occurs when the mass of the impactor is greater than 1/1000 the mass of the target.
The average collision frequency with a piece of space debris may be calculated from the product of the target area, the debris density and the collisional velocity:
For the International Space Station in the early part of the year 2010, this works out at one collision with a 10 cm piece of debris every ten years.
In general bodies above 10 ton will definitely partially survive reentry whereas objects less than 1 ton will not.
However, some particular classes of objects of much lower mass do make it (partially) to the Earth's surface. These include objects made of high melting point materials (eg titanium spheres), and objects with a very high area to mass ratio (eg thermal blankets).
The reentry interface altitude, where significant ablation of a reentering orbital object occurs is generally around 75 km.
Australian Space Academy
For more detailed information on various aspects of this topic see the space debris section of this web site.