The space environment around the earth is cluttered with millions of naturally occurring micrometeoroids and man-made orbital debris. The past 50 years of space exploration has unfortunately generated a lot of junk that threatens the reliability of spacecraft. Orbital debris includes things as large as spent rocket stages and old satellites, to objects as small as flakes of paint and small metal fragments. Even these tiny particles can seriously damage a spacecraft, because they are traveling at very high speeds. In low-Earth orbit (LEO) man-made debris travels around the earth at speeds of roughly 7.5 kilometers per second. That's almost 17,000 miles per hour! If two objects in orbit were to collide head-on, the impact velocity would be double: 15 km/s or 34,000 mph. In general, micrometeoroids are in orbit around the sun and can reach velocities in excess of 70 km/s, but they are usually much smaller and less dense than orbital debris.
The orbital debris environment is growing. More and more satellites are launched every year, and this adds to the orbital debris population. Obsolete satellites can explode or fragment, generating thousands of new and smaller debris particles. NASA is working with other space faring nations to reduce the production of orbital debris fragments. This activity is called debris mitigation.
Large orbital debris (>10 cm diameter) are tracked routinely by the U.S. Space Surveillance Network. Objects as small as 3 mm can be detected by ground-based radars, providing a statistical estimate of their numbers. Assessments of the population of orbital debris smaller than 1 mm can be made by examining impact features on the surfaces of returned spacecraft, although this has been limited to spacecraft operating in altitudes below 600 km. The large orbital debris objects can be avoided by maneuvering a spacecraft out of the way. Smaller particles cannot be tracked, however, and encounters with these debris comprise the greatest risk associated with spaceflight in the LEO environment. Current NASA Orbital Debris Program activities are documented in the Orbital Debris Quarterly Newsletter.
NASA has developed engineering models of the meteoroid and orbital debris environment. These models may be used to estimate the expected particle flux for given mission parameters. The estimated particle flux, impact velocities, and spacecraft structure can be used to determine where additional protection is needed. Spacecraft shielding must be designed to withstand hypervelocity impacts by these particles. Shield testing is required to verify shielding performance.