The near-Earth space environment where the International Space Station, Space Shuttle, and numerous satellites orbit the earth is cluttered with man-made orbital debris in addition to naturally occurring micrometeoroids. Though most of the micrometeoroid and orbital debris (MMOD) particles are small, they are traveling at such high speeds that even small MMOD particles can cause damage when they collide with a spacecraft. Depending on the size of the impacting particle and impact location, the damage could functionally compromise the vehicle, or worse, result in catastrophic failure.
For this reason, there is an entire science devoted to investigating and assessing the MMOD risk to spacecraft, as well as developing and testing spacecraft shielding designs to reduce the MMOD risk to spacecraft.
View of tracked orbital debris objects in low Earth orbit (LEO) and in geosynchronous orbit (GEO). Approximately 19,000 objects larger than 10 cm are known to exist. The estimated population of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles smaller than 1 cm probably exceeds tens of millions.
So how much junk is floating around up there? How did it get there? There is a major research effort devoted to quantifying and understanding the particulate environment in orbit around our planet.
The physical processes involved in hypervelocity impacts are complex, yet fascinating. Take a look at some examples that illustrate just how lethal even very small particles can be when they collide at hypervelocities (greater than 3 km/s).
Spacecraft do get hit by orbital debris. One satellite sent to measure various aspects of the near-earth environment returned to earth with millions of impacts, ranging in size from microscopic surface abrasions, to large craters and holes. Here are some examples of both documented and suspected impact events that have occurred on orbit.