10/01/2007 - 22:00

Space debris from antisatellite weapons

The term "outer space" has always inspired visions of vast regions of nothingness. So even 50 years into the space age, people still tend to view space like the early pioneers viewed the Wild West--sparsely populated, and therefore in little need of laws or regulations, and so boundless that no one worried about ruining the environment.

Of course, over time, things changed in the Wild West. As society and commerce grew, people found that laws were essential to avoiding conflicts, resolving disputes, allowing equitable distribution of resources, and protecting the environment. In the five decades since the Soviet Union launched Sputnik I, the space around Earth inhabited by satellites has become sufficiently crowded and satellites have become important enough to modern society that a Wild-West view of space is outdated. Some 4,500 launches have taken place since Sputnik, and there are currently 850 active satellites in space, owned by some 50 countries, as well as nearly 700,000 pieces of debris large enough to damage or destroy those satellites. (For information about the satellites that are currently active, see the Union of Concerned Scientists' database.) Space commerce generates more than $100 billion a year in revenue. In every region of the globe, many aspects of society are becoming increasingly dependent on the services satellites make possible, and militaries are becoming increasingly reliant on them for a range of uses, including communication, reconnaissance, navigation, and weather monitoring.

Given its importance and changing nature, space requires international laws, regulations, and operational guidelines governing its use to enhance commercial and scientific uses, avoid and settle conflicts, and limit debris production. Parts of this structure exist, but its development hasn't kept pace with the growing complexity of space activities. Moreover, much of what exists isn't legally binding and doesn't have enforcement mechanisms.

One important piece of space law, the Outer Space Treaty, turns 40 in October. While it establishes the fundamental principles for governing space, these principles need to be articulated in laws and regulations to address contemporary challenges and new technologies. Fortunately, the international community is beginning to discuss important issues such as space traffic management. However, international negotiations on military issues in space haven't taken place since the early 1990s.

To preserve the long-term use of space, it's particularly important to address how to control the production of orbital debris. Due to their high speed in orbit, even small pieces of orbiting debris can damage or destroy a satellite. Since debris at high altitude can remain in orbit for decades or longer, it accumulates as more is produced, expanding the risk of collisions with satellites. If the amount of debris at some altitudes becomes large enough, it could become difficult to use those regions for satellites. Currently, there isn't an effective way to remove large amounts of debris from orbit; as a result, controlling the production of debris is essential for preserving the long-term use of space.

There are two main sources of orbital debris: (1) The accidental breakup of objects placed in orbit by routine activity; and (2) the creation of debris by the testing or use of destructive antisatellite (ASAT) weapons.

The international community is addressing the first issue by developing debris mitigation guidelines. The United States wrote and released its own guidelines in 1997, which call for measures such as designing satellites and rocket stages to limit the release of debris when placing satellites in orbit and depleting propellant from nonoperational satellites or stages to reduce the risk of explosions. By calling for spent stages and satellites to be removed from orbit, the guidelines also attempt to control the number of large objects in space that could break up due to collisions.

Following Washington's lead, other countries developed similar national guidelines, and the international Inter-Agency Space Debris Coordination Committee (IADC) adopted a consensus set of debris mitigation guidelines in 2002. Earlier this year, the U.N. Committee on the Peaceful Uses of Outer Space adopted guidelines based on the IADC measures (PDF).

This is progress, and there are indications that international guidelines are helping curb the growth of debris. However, the guidelines aren't legally binding and lack enforcement mechanisms. Requiring such actions (in particular venting unused propellants) could likely have prevented the explosion of the Russian Briz-M rocket stage in early 2007, which may have created as many as 1,000 pieces of debris large enough to be tracked from the ground (5 to 10 centimeters in size or bigger). Debris of this size can destroy a satellite in a collision.

But more importantly, the debris created by the testing and/or use of kinetic energy ASAT weapons, which destroy satellites by colliding with them at high speed, could overwhelm these reductions, since such breakups can create enormous amounts of orbital debris. There is no legal restriction on the testing or use of such weapons, and there are no international negotiations dealing with such weapons.

Somewhat surprisingly, the enormous debris consequences of testing or using destructive ASAT weapons isn't widely understood since estimates have only recently been published, which helps account for the lack of attention the issue has received. (See "The Consequences of Using Kinetic Energy Antisatellite Weapons" and "Space Debris.")

These estimates illustrate that the destruction of a single 10-ton satellite (comparable to a U.S. reconnaissance satellite) would double the total amount of large debris currently in low Earth orbit (i.e., at altitudes below 2,000 kilometers). In particular, it could produce 250,000 pieces of debris larger than 1 centimeter and 5,000 pieces larger than 10 centimeters. Because of their military value, reconnaissance satellites are seen as potential ASAT targets during a conflict.

The amount of debris from the destruction of one such satellite would negate the debris reduction that would be achieved in several decades of debris mitigation measures described above.

Moreover, this debris wouldn't be spread uniformly throughout low Earth orbit, but instead it would be concentrated near the altitude at which the original satellite was orbiting, significantly increasing the debris density in that region. Because of the low atmospheric drag at high altitudes, the debris resulting from the destruction of satellites orbiting at altitudes greater than about 800 kilometers could remain in orbit for decades or centuries. Because of asymmetries in Earth's gravitational field, for a satellite in a near polar orbit (such as a spy satellite), the resulting debris would spread out into a shell around Earth over time. This debris could therefore threaten all satellites whose orbit carried them through that altitude.

The concern about ASAT weapons was highlighted by a Chinese test of such a system in January 2007. The satellite destroyed in this test--a defunct weather satellite named the Feng Yun 1C (FY-1C)--was relatively small, with a mass of less than a ton. And yet, the Chinese test appears to have increased the amount of debris (size greater than 1 centimeter) in low Earth orbit by 15 to 20 percent, becoming the worst debris-producing event on record. The satellite was orbiting at about 850 kilometers, so the resulting debris is concentrated in a region of space that's heavily used by satellites and already crowded with debris. In addition, because the atmospheric density at this altitude is so low, the debris will decay from orbit slowly, and a large fraction will remain in space for decades.

How did this test change the risk to satellites? Before the Chinese test, the chance that any given satellite near the altitude of the FY-1C would be hit by debris larger than 1 centimeter--large enough to cause severe damage--was approaching 1 percent over the satellite's lifetime, generally 5 to 10 years. Since debris from the Chinese test is concentrated near this altitude band, the threat will nearly double for the next 5 to 10 years.

Fortunately, the threat to satellites remains relatively small, even in this heavily used altitude band. However, at these altitudes, the threat from debris is becoming comparable to other problems that limit a satellite's lifetime, and significant increases in debris could make it a major problem for satellite operators.

The long-term consequences of additional debris would be even worse. A recent NASA study showed that prior to the Chinese test parts of space have already reached "supercritical" debris densities. Supercritical means that the density of objects has become so large that collisions between objects are frequent enough that they produce additional debris faster than drag removes debris from the region. Those additional debris particles further increase the collision probability, leading to a cascade effect as the large objects in orbit are ground into smaller fragments.

In particular, the NASA study showed that in the heavily used altitude band from 900 to 1,000 kilometers, the number of large (greater than 10 centimeters) pieces of debris is expected to more than triple over the next 200 years--and that's assuming no additional launches of objects into this band. The study estimates that the amount of large debris in low Earth orbit is expected to increase by nearly 40 percent during that time, still under the assumption of no additional launches.

By significantly increasing the number of objects near 850 kilometers, the debris from the Chinese test will increase the rate at which these collisions take place, and therefore, speed up debris growth. The destruction of additional satellites--especially larger ones--near this region could make the debris situation much worse, both in the near and long term.

Failing to preserve the long-term utility of space is incredibly shortsighted. While progress is being made on some aspects of this problem, the international community should prioritize outlawing destructive ASAT weapons. Banning them will require negotiations and may not be possible without broader discussions of space security. Such negotiations and discussions have been blocked for more than a decade in the Conference on Disarmament, and the United States has been a key obstacle. Given their mutual interests in space, the United States and other countries must find a way to start such a dialogue.

As we embark on the next 50 years of the space age, creating an international consensus to further develop a legal framework for space and to establish mechanisms to deal with issues such as space debris is a key challenge. Doing so will likely require a change in the way we view space.