Debris in space is a problem that has received a great deal of attention over the past few years and has resulted in design and operational philosophy changes to the Space Station program.
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. Even small particles pose a risk to the shuttle systems.