An artist's rendering of orbiting space debris around Earth, courtesy of the European Space Agency.
China made global headlines in January 2007 when it used an anti-satellite (ASAT) missile to destroy one of its old weather satellites orbiting 537 miles above Earth, but national security wasn’t the only critical issue at stake. China’s ASAT missile test created the largest amount of space debris in history, making the threat much more severe, according to several speakers at a session on the growing problem of space debris at the APS April Meeting in St. Louis.
Space debris has long been cause for concern. In the mid-1990s, the United Nations deemed it a significant enough risk to implement mitigation measures aimed at reining in the proliferation of space debris. (New updated measures were re-introduced in June 2007.) Mitigation held the density of space debris to constant levels throughout the 1990s, but in recent years, the number of fragments has begun to climb again. There are now more than 150 million pieces floating in space, most measuring less than 2 inches across.
Space debris mostly consists of a mix of discarded objects: spent rocket stages, defunct satellites, fragments from explosions of various space equipment, paint flakes, dust, even the occasional glove, camera, or jettisoned garbage bags. Most of these are at low enough altitudes that they drift through space for awhile between re-entering the atmosphere, often burning up in the process.
However, the greatest concern is debris that results from explosions, such as when rockets or spacecraft with unspent fuel collide with other objects, thereby producing a great number of fragments. According to Geoffrey Forden, an MIT physicist, the Chinese action produced more than 2300 pieces larger than a golf ball, and over 35,000 pieces larger than 1 centimeter. We are in danger of a runaway escalation of space debris, he said.
The density of debris is fast approaching supercriticality, according to David Wright of the Union of Concerned Scientists, which maintains a satellite database tracking all those bits of debris. This situation could result from the destruction of an orbiting object into many smaller fragments of debris, initiating a chain reaction as that debris collides with other satellites in nearby orbits, breaking them into fragments in turn and compounding the problem further.
At high altitudes, the debris can stay in orbit for decades, accumulating to the point where there is a much higher risk of collisions with satellites. In fact, it may already be too late: Wright cited a 2006 study by NASA’s Orbit Debris Program that found certain parts of space particularly the 900 to 1000 km band, or Lower Earth Orbit (LEO) have already reached supercritical debris densities. NASA estimates that an active satellite in LEO will collide with a piece of debris larger than 1 centimeter every five to six years.
With their high speed in orbit, even fairly small pieces of debris can damage or destroy satellites in a collision, said Wright. Orbital speeds in LEO are typically greater than 7 kilometers per second, 30 times faster than a jet aircraft, and the relative speed of a piece of debris approaching a satellite in an intersecting orbit may be 10 km per second or higher, said Wright.