The environment of space above the earth is cluttered with millions of naturally occurring micrometeoroids and man-made orbital debris. The past 40 years of space exploration has unfortunately generated a lot of junk that threatens the reliabiity of spacecraft. Orbital debris includes things as large 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.

The animation above is a computer model of the objects being tracked by USSPACECOM radar, including large pieces of orbital debris. The smallest object which is tracked by USSPACECOM is roughly 20 cm in diameter. These 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. NASA has developed an engineering model of the meteoroid and orbital debris environment. This model may be used to determine the expected particle flux for given mission parameters. Spacecraft must be designed to withstand hypervelocity impacts by these small, untrackable particles. Testing is required to investigate hypervelocity impact phenomena, and to develop effective shielding. Current NASA Orbital debris program activities are documented in the Orbital Debris Newsletter.