Early in the Space Age, little thought was given to objects left in orbit as part of satellite launches. But as the number of those objects has grown, at first steadily and then very rapidly, through the 50-plus years since the launch of Sputnik, concerns about the polluted orbital sphere have grown accordingly. A series of notable events in recent years has focused attention on the problem.

Any human-made object in orbit that does not serve a useful purpose is considered debris. Common kinds of debris include satellites that have reached the end of their lives; the rocket stages used to place satellites in orbit; bolts and other objects released during satellite deployments (known as mission-related debris); and fragments from the intentional or accidental breakup of large objects. It also includes the rare failed spacecraft that has stalled in orbit, such as the Russian Phobos–Grunt probe, which was bound for a Martian moon but instead is expected to crash to Earth in the coming days.

In recent years the number of large objects tracked by U.S. military sensors and listed in the official government catalogue has reached an all-time high of around 15,000 pieces of debris, plus about 1,000 active satellites. This growth in space debris has become a concern because of the threat posed to satellites and to piloted spacecraft. The very high speed of objects in orbit means that debris as small as a centimeter can seriously damage or destroy a satellite. And debris can linger in orbit for decades or longer at high altitudes so it builds up as more is produced.

The risk of collision between large objects in low Earth orbit—below 2,000 kilometers in altitude—has doubled in the last five years. The chance that an operating satellite in the heavily used altitudes between 800 and 900 kilometers will be hit by a large piece of debris in its lifetime is now likely a few percent. While still a small risk to any individual satellite, it is becoming comparable to other threats to satellite life, such as electrical failures, that satellite owners work hard to minimize.

Moreover, recent studies by NASA and others show that low Earth orbit is already supercritical: the density of debris has become so great that collisions in orbit generate additional pieces of debris faster than atmospheric drag removes it from orbit. Additional debris particles further increase the frequency of collisions, leading to a slow-motion cascade, known as the Kessler Syndrome, that would cause the amount of debris to increase even if humankind ceased rocket launches entirely.

This fact has an important implication: remediation measures, such as removing large, massive pieces of debris from orbit, are needed. Researchers are actively developing various strategies for remediation, but no quick or inexpensive fixes are in sight.

The immediate challenge of minimizing the creation of debris is also crucial. New orbital debris emerges from two broad types of activity. The first is routine space pursuits and the accidental breakup of objects in orbit. The second is the intentional destruction of satellites by anti-satellite (ASAT) weapons.

Concern about debris growth led the U.S. and other countries to create the Inter-Agency Space Debris Coordination Committee (IADC); around the mid-1990s the international community started taking serious steps to reduce the amount of debris created in normal space operations. One major change was venting from satellites the residual fuel that could explode. Such efforts have helped, but the other category of debris—that from intentional destructions—has the potential to nullify any gains. This is evident in the figure below, which shows the growth of debris in the official U.S. government catalogue.

The red trend line in the figure above shows the historical trend in debris growth from the beginning of the Space Age through the mid-1990s. Over the following decade, the growth rate decreased and instead followed the blue trend line. This change reflects the debris mitigation efforts that began during the 1990s, and the graph shows that both fragmentation debris and mission-related debris remained roughly constant. Had this trend continued, those 15 years of debris mitigation efforts would by now have resulted in some 3,000 fewer large objects in space compared to business-as-usual—that is, an extrapolation of the red trend line.

Unfortunately, several significant events between 2007 and 2009 instead increased the current debris total to about 2,000 objects above the red trend line. The jump in 2009 was due to an accidental collision between a U.S. Iridium satellite and a Russian Cosmos satellite. But the very large jump in early 2007, which represents more than 3,000 large pieces of debris, resulted from a Chinese ASAT test that purposely destroyed the defunct one-ton Fengyun 3C weather satellite.

ASAT weapons can completely fragment a satellite, creating huge amounts of debris. And the natural targets of such an attack could be much larger than the satellite destroyed in the Chinese test. U.S. spy satellites, for example, have masses well over 10 tons. Based on NASA’s debris model and the Chinese ASAT test, the destruction of a single 10-ton satellite could produce as many as 750,000 pieces of debris larger than a centimeter, instantly doubling or tripling the amount of debris of this size in low Earth orbit.

Unless the international community addresses the threat of intentional satellite destruction, all the other efforts to preserve the space environment may be futile. This is a race that the slow and steady approach of debris mitigation may not win.