The Growing Threat of Space Debris

Abstract

The danger to orbiting satellites from space debris has been known for decades, yet accidents and intentional antisatellite weapon tests still pose serious threats. Better rules are needed to govern increasingly busy near-Earth space.

The United States is heavily reliant upon satellites. These satellites save lives, strengthen the economy, and provide invaluable support to the military and intelligence services. To wit, there are approximately 300,000 emergency GPS receivers used in the United States to aid search-and-rescue teams, and the space industry supports about 729,000 U.S. jobs. 1 During the 2003 invasion of Iraq, precision-guided munitions, most guided by GPS-based satellite navigation, made up two-thirds of all the bombs used. 2

Yet satellites are also highly vulnerable. The space environment is a harsh vacuum that is constantly swept by solar storms, naturally occurring micrometeoroids, and a hail of fast-moving space debris. Most satellites travel at speeds ranging from 3.1 kilometers per second to 7.8 kilometers per second–many times faster than a bullet. 3 When satellites and space debris collide at such speeds, the results can be catastrophic. In peacetime, the consequences include damage or destruction worth many millions of dollars and a gap in satellite services. If a collision occurs during a crisis, it could be difficult to tell whether it was an accident or a purposeful act, which might exacerbate tensions or spark an armed conflict. Space debris also could damage communications and intelligence-gathering satellites when they are needed most.

Similarly, antisatellite (ASAT) weapon tests pose enormous risks to both satellites and the space environment more generally. Thus far, such tests have been relatively rare. But China's 2007 ASAT test and the U.S. destruction of a defunct spy satellite a year later have raised fears of further tests. 4 Both tests created debris–but because the United States destroyed its satellite at low altitude, the debris burned up in the atmosphere in less than a year. The debris consequences of the Chinese test were far more severe. It generated some 150,000 pieces of debris larger than 1 centimeter and roughly 2,500 pieces larger than 10 centimeters. (Pieces of debris between 1 and 10 centimeters can damage satellites but cannot be tracked reliably with existing space surveillance systems; pieces of debris larger than 10 centimeters can be tracked–and can destroy satellites outright.) The debris from the Chinese test is concentrated at altitudes near 850 kilometers and has doubled the likelihood that a satellite at this altitude will be struck by debris during a typical five- to ten-year service lifetime. 5

Recognizing the danger. Scientists have become progressively more aware of the risk posed by space debris since the start of the space age. A 1961 NASA technical paper concluded that naturally occurring space debris–such as micrometeorites and other small natural objects trapped in Earth's orbit–posed “a definite hazard,” but that the danger could be only “poorly assessed.” 6 By 1978, NASA scientists knew enough to understand that a chain reaction of debris collisions might render space operations prohibitively risky in highly populated orbits. 7 Around this time, NASA scientist Donald Kessler noticed that the upper stages of Delta space-launch vehicles that had been left in orbit were exploding, creating debris that could endanger upcoming space missions. NASA engineers soon developed a fix for the exploding rocket bodies–ensuring that no unused rocket fuel remained in the orbiting rocket stages after their missions were completed–and convinced colleagues in Japan, who were under contract to build Delta rockets, to make similar changes.

Eventually, awareness of the debris problem led the National Security Council (NSC) to request a U.S. policy study on ways to mitigate the problem. The study was published in 1989, after which the NSC offered other spacefaring nations briefings on the subject. Bilateral meetings were conducted with the European Space Agency, the National Space Development Agency of Japan, and the Russian Federal Space Agency. Later, the four agencies created the multilateral Inter-Agency Space Debris Coordination Committee. 8 Over time, space agencies from Britain, France, India, Germany, Italy, and Ukraine became involved in the committee, promoting broader and deeper knowledge about space debris. The group began making presentations to the U.N. Committee on the Peaceful Uses of Outer Space (COPUOS) in 1997. 9 Technical debris-mitigation guidelines were submitted to COPUOS in 2002 and were officially endorsed by the U.N. General Assembly in December 2007. 10 But this endorsement was not legally binding. Thus, implementation of debris-mitigation guidelines remains in the hands of the governments of spacefaring powers.

A continued threat. Unfortunately, existing efforts to prevent the creation of additional space debris are flawed in two important ways. First, destructive ASAT tests, each of which can create as much debris as is produced in decades of peaceful space operations, are addressed only tangentially by existing laws. Second, there is a lack of “rules of the road” for space. As more satellites populate orbits around Earth, the risk of collisions grows. For instance, on February 10, 2009, a satellite operated by the private company Iridium and a defunct Russian satellite collided, producing roughly 100,000 new pieces of debris larger than 1 centimeter in low Earth orbit, a region 200-2,000 kilometers above Earth's surface that is already densely populated with both debris and active satellites. 11 It caused a roughly 70 percent increase in the amount of dangerous debris at 570 kilometers, the altitude of the Hubble telescope. Fortunately, most of the debris at lower altitudes will burn up in Earth's atmosphere over the next decade. 12

It is estimated that a collision between an active satellite and a piece of dangerous debris will occur on average once every two to three years over the next decade.

Since the collision, the U.S. Air Force has taken important steps to address the problem. It has expanded the number of space objects it monitors to determine the risk of collision, providing the advance warning needed to plan avoidance maneuvers, if necessary. In the past it focused only on so-called high-value assets such as the International Space Station and certain military satellites. But now it reportedly performs analyses on some 800 satellites and plans on adding an additional 500 by early this year. 13 The air force also is taking steps to enhance its capacity to track space objects as small as 5 centimeters in diameter. 14 While these unilateral steps constitute a much needed improvement, it remains to be seen whether they will be sufficient to prevent future collisions or whether further unilateral or multilateral steps will be necessary.

It is estimated that a collision between an active satellite and a piece of dangerous debris (larger than 1 centimeter) will occur on average once every two to three years over the next decade. 15 NASA contends that existing satellite debris shields can protect against impacts with such dangerous debris. Even if true, there are more than 300,000 pieces of debris larger than 1 centimeter in low Earth orbit, and fewer than 20,000 of them are tracked regularly by the United States. 16

Aside from posing a risk to satellites, debris threatens better-protected manned spacecraft as well. On March 12, 2009, the crew of the International Space Station was forced to evacuate to a docked Soyuz spacecraft in response to a debris fragment's predicted close approach. Another piece of debris threatened the station four days later. Then, on March 22, the space station and the docked space shuttle were forced to change orbit to avoid an approaching Chinese rocket-booster fragment. 17 Although both manned and unmanned spacecraft can be maneuvered to avoid potential collisions if enough warning is provided, such maneuvers use limited fuel, which can shorten the operational lifetime of the spacecraft, and disrupt data and other satellite services. (In some cases it can take many hours to plan and execute such a maneuver; for the International Space Station, for example, it takes approximately 30 hours.) 18

Furthermore, because there is very little atmospheric drag at the high altitudes associated with low Earth orbit, debris can remain there for decades. 19 Independent studies predict that roughly one-quarter of the debris larger than 10 centimeters created in the Iridium collision will remain in orbit for more than 30 years. Roughly 15 percent of 1-10 centimeter debris is expected to remain in orbit even longer. 20

The threat to satellites in low Earth orbit is heightened because most are not in equatorial orbits, but rather in polar or near-polar orbits. 21 Because all satellites in such orbits cross above Earth's poles, the risk of collision near these two spots is dramatically higher than the risk of collision at any other point during an orbit, creating a polar bottleneck. The spatial density of satellites over the poles is approximately 10 times greater than that over the equator. As a result, the debris problem is exacerbated by two crowding problems: the concentration of debris at certain altitudes and the frequent, high-speed approaches occurring over the poles.

Mitigation efforts. NASA simulations indicate that even if no new satellites were launched, collisions would continue to occur in low Earth orbit and increase debris. 22 Since additional spacecraft (and the rockets needed to deploy them) will continue to be launched, the actual situation is likely to become much worse. Any additional destructive ASAT tests also will add to the problem. A single ASAT test that destroys a 1-ton satellite (the approximate weight of the satellite destroyed by China) in low Earth orbit would produce the same amount of debris that accumulates over several decades of normal space operations. 23 And satellites targeted by ASAT systems could be 10 times larger. In other words, a single ASAT attack could undo decades of careful debris mitigation.

Luckily, recognition of the space debris issue is now widespread, and many countries have expressed a willingness to take steps to address it. China's 2000 White Paper on Space Activities, for example, stated, “[Beijing will] continuously make efforts to explore, together with other countries, ways and means to mitigate and reduce space debris, and promote international cooperation on this issue.” 24 India's delegation promised in 2007 its “full support and cooperation” with COPUOS's efforts to prevent space debris. 25 The U.S., French, and Pakistani delegations all made similar statements in support of efforts to mitigate space debris at COPUOS meetings in 2007. 26

In addition, a number of diplomatic initiatives have been proposed to address the long-term debris consequences of destructive ASAT weapons. China and Russia have submitted a draft treaty at the U.N. Conference on Disarmament that would seek to prevent the placement, or use, of weapons in outer space. 27 The U.S. government and many independent experts, however, feel that the draft treaty has serious flaws–e.g., the absence of prohibitions on the research, development, and deployment of ground-based ASATs (such as the one tested by China) and the absence of any verification measures. 28 Another initiative is a European Union-sponsored voluntary code of conduct that addresses issues such as space-traffic management, debris mitigation, and destructive ASAT testing. By agreeing to certain rules, states would clarify responsible and irresponsible actions, facilitating responses to the latter, which would include debris-creating events. 29 But China, Russia, and others have expressed a desire for a legally binding agreement, a condition that the European Union code does not satisfy. A third initiative is a narrowly focused proposal that would ban destructive testing against space objects. Such a treaty would directly address a pernicious source of space debris without approaching many of the contentious issues entailed in the more comprehensive approach taken by China and Russia. The idea was broached at a 2008 workshop organized by the Henry L. Stimson Center (where one of the authors of this article is employed), but no country has formally endorsed it. 30

No international consensus exists on which of these approaches would enhance space security the most. China and Russia remain publicly committed to a comprehensive, legally binding treaty. The European Union is conducting rounds of consultations with other spacefaring nations on its code of conduct. And discussions on the narrow treaty proposal have so far been largely academic. It is unfortunate that at present, an international agreement addressing the largest potential source of debris, destructive ASAT tests, does not appear to be forthcoming. Without it, international security and the use of space are under threat.