How often have shuttles been hit by debris and what was the most significant damage?
Debris hits occur on almost every flight. Usually these are very small microscopic particles that pose little or no danger to the spacecraft. The most significant damage to date has been impacts to the windows. After some flights, several windows have been replaced. There have also been impacts to the orbiter's radiator surfaces, although no coolant has been lost. On STS-79, for instance, there was a 1/8 inch diameter crater in one of the radiator panels. Even small particles pose a risk to the shuttle systems. To minimize this risk, the orbiter is placed in attitudes that protect critical systems whenever possible.