Orbital debris, or “space junk,” is any man-made object in orbit around the Earth that no longer serves a useful purpose. Space junk can be bad news for an orbiting satellite. On February 11, 2009, a U.S. communications satellite owned by a private company called Iridium collided with a non-functioning Russian satellite. The collision destroyed both satellites and created a field of debris that endangers other orbiting satellites.
To minimize the risk of collision between spacecraft and space junk, the U.S. Space Surveillance Network tracks all debris larger than 10 centimeters. These images represent all man-made objects, both functioning and useful objects and debris, currently being tracked. The images were made from models used to track debris in Earth orbit. Of the approximately 19,000 manmade objects larger than 10 centimeters in Earth orbit as of July 2009, most orbit close to the Earth, top image. The lower image shows all items in orbit, both close to and far from the Earth.
A distinctive ring marks the geostationary orbit, a unique place where satellites orbit at the same rate that the Earth turns, allowing them to essentially remain over a single spot on Earth at all times. This orbit is invaluable for weather and communications satellites. When satellites in geostationary orbit are taken out of operation, they are moved to another orbit to keep the geostationary orbit clear. The dots between the geostationary orbit and the low-Earth orbit are in an orbit used by GPS satellites or a highly elliptical orbit, called Molniya, used to monitor the far north or south. To read more about common satellite orbits, see Catalog of Earth Satellite Orbits on the Earth Observatory.
Though the black dots that represent objects in space swarm around the Earth, obscuring the surface in the lower image, the space junk situation is not as dire as it may appear. The dots are not to scale, and space is a very big place. Collisions between large objects are fairly rare. The orbit of each piece is well known. If any debris comes into the path of an operating NASA satellite, flight controllers will maneuver the satellite out of harm’s way. As of May 2009, satellites in NASA’s Earth Observing System had been maneuvered three times to avoid orbital debris. NASA flight engineers are carefully tracking the debris from the Iridium collision, since much of it is near the altitude at which EOS satellites orbit.
To read more about what it takes to maintain a satellite’s orbit, common Earth orbits, and the science behind calculating an orbit, please see the Earth Observatory series About Orbits.