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June 2, 2022
After China's Test: Time For a Limited Ban on Anti-Satellite Weapons
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Geoffrey Forden

China 's Jan. 11 test of a sophisticated hit-to-kill anti-satellite (ASAT) weapon should have shattered complacency about the dangers posed by these arms. Much press commentary has focused on the threat to U.S. military systems, but these are less vulnerable than is popularly perceived. The real danger lies less in the military realm than in the long-term risk to civilian communications, weather forecasting, and pure scientific research conducted by all space-faring nations.

The possibility of great harm to the major civilian economies and a lack of real military utility should bring all nations together to outlaw these weapons. To date, it has proved difficult to achieve international consensus on banning these systems and is likely to remain so. China , for one, is concerned about the U.S. missile defense system, and the Bush administration wants to keep open the option of fielding these weapons.[1] If these disagreements can be overcome, however, a technical agreement detailing limits on “closing speed” and maneuvering provides the appropriate basis for a verifiable and robust ban on the most dangerous of these arms.

The Chinese ASAT Test

Closing speed is the key to understanding the sophistication of China 's ASAT capabilities. This relative speed between the interceptor and the satellite determines the complexity of the ASAT weapon's onboard tracking and guidance systems and the control of its rocket engines. After all, one cannot simply “plug in the satellite's coordinates” because one risks making an error of at least several kilometers in locating where the satellite is at any given moment.[2]

China 's ASAT weapon hit its target, the obsolete Feng Yun-1C weather satellite, almost head on with a rapid closing speed of just more than 8 kilometers per second.[3] To accomplish this, it almost certainly used an onboard optical tracker. This is basically a video camera that would see the satellite as a bright star, albeit one that moved very fast relative to the other stars. If so, China has been developing this weapons system for quite some time with previous flight tests of the tracking system, perhaps mounted on experimental satellites.

China 's ASAT weapon, unlike those tested by the Soviets, for example, appears not to have used an exploding warhead. It relied instead on the interceptor's substantial kinetic energy; at the time of the collision, it packed as much energy as 10 times its weight in TNT. No wonder it created substantial debris, more than 1,000 pieces large enough to be tracked from Earth. Debris from this collision has been observed at altitudes as great as 3,600 kilometers, four times as high as the original target satellite. It is possible that some pieces actually escaped the pull of the Earth's gravitational field altogether.

Although we cannot determine the ASAT weapon's mass precisely, we do know from an analysis of the resulting debris pattern that it was less than 600 kilograms, possibly much less.[4] Therefore, at least three such interceptors could be placed on China's smallest space launch vehicle capable of lofting satellites into geostationary transfer orbits. At these orbits, they could attack more strategically important satellites, such as communications and early-warning satellites or, at somewhat lower orbits, GPS satellites.

Assessing the Threat

Even though this was a test of a very sophisticated weapon, it was still only a single successful test. China , with its history of deliberate weapons development, is unlikely to feel confident in this system until it has undergone a significant number of additional tests against similar targets. China would then almost certainly want to test many of the ASAT weapon's subsystems in geostationary transfer orbits so Beijing could have confidence in attacks against higher-orbit satellites. China might choose to do so using close flybys that would not create the debris or international uproar its last test did.

Contrary to some analysts' assertions,[5] China would then likely have an ASAT system capable of threatening all U.S. space assets, not just those in low-Earth orbit. China has already mastered the techniques of placing satellites in medium[6] and higher orbits: first placing the satellite and its booster's third stage into low-Earth orbit, then using the third stage to boost the satellite into a highly elliptical transfer orbit,[7] and finally using the satellite's onboard engine to place it in a higher-altitude circular orbit. An ASAT attack against a navigational satellite or higher communications satellites would almost certainly involve the first two steps.[8]

At higher altitudes, moreover, the final attack is easier because at these altitudes satellites need to move less quickly to stay in orbit because of the Earth's weakening gravitational field. Likewise, an ASAT weapon does not need to approach its target satellite with as great a closing speed (information graphic available in the print edition). Thus, an attack on a geostationary satellite would be considerably less stressing for an ASAT weapon's tracking, guidance, and control systems than the scenario already successfully tested by China 's ASAT system.

It might be possible to protect low-Earth-orbit satellites either by passive countermeasures (maneuvering out of the way of the interceptor) or active defenses (destroying the incoming interceptor before the collision). Active defenses are possible at such low altitudes because most of a suborbital interceptor's debris would fall to earth within minutes. Unfortunately, neither measure is effective at higher altitudes and could be counterproductive. If it missed the first time, an ASAT weapon placed in an elliptical transfer orbit could simply wait for its next pass. For a geostationary satellite, the interceptor would have another shot about 24 hours later. Furthermore, to escape, the target satellite would undoubtedly have had to accelerate at several times that of gravity, likely causing booms or large, high-gain antennas to shear off. If on the other hand, the defender was foolish enough to try to destroy the interceptor, it would simply create a shotgun blast of debris traveling in essentially the same trajectory as the interceptor; eventually this widening swarm would destroy the target. The advantage is definitely on the side of the attacker.

On the other hand, an attacker would have to destroy a considerable number of satellites in order to have an immediate effect on military operations. There are on average about 10 GPS satellites visible at any given time and point on the Earth's surface even though a high positional accuracy requires only six. An attacker would have to destroy at least six satellites to affect precision-guided munitions even momentarily because other GPS satellites would soon appear as their orbits took them into view. A country would need to disable nearly one-half of the United States' 24 NAVSTAR/GPS satellites currently in orbit to eliminate the ability to employ precision-guided munitions for more than a few hours each day.[9] Likewise, the United States has a number of alternatives for communications satellites in the short term. Other space assets, such as weather and mapping satellites, although important in the long term, are not as time critical.

Missile Defense and ASAT Systems

Any attempt to ban ASAT weapons development will have to figure out how to square such an agreement with the existence of U.S. ballistic missile defenses. Although the effectiveness of these defenses against missiles has been questioned, there is no doubt that they could hit a satellite in low-Earth orbit. Their tracking, guidance, and control systems have been developed and successfully tested against incoming warheads in engagements that have closing speeds in excess of 11 kilometers per second. Such closing speeds are much higher than those it would encounter against even the lowest satellite and certainly higher than those the Chinese overcame in their January test.

Missile defenses also pose an obstacle to making diplomatic progress on ASAT weapons systems. The United States believes these defenses are critical to protecting itself from attacks by rogue states, but China fears they could also be used to deter it in any conflict with the United States, such as over Taiwan.[10] In recent years, China, at first alone but later with Russia, has made several proposals to the United Nation's Conference on Disarmament on possible elements for a future treaty banning the weaponization of space. At times, the proposals have taken in all U.S. missile defenses, not merely U.S. consideration of deploying space-based interceptors.

Beijing 's and Moscow 's June 2001 proposal, for example, required signatories not to “test, deploy or use in outer space any weapons, weapon systems, or their components.”[11] As part of the proposed treaty, a list of definitions was offered that included defining outer space as starting at an altitude of 100 kilometers and a weapon as any device or facility that could “strike, destroy or disrupt directly the normal functions of a target.” These definitions are hardly controversial, but they would ban the United States from even testing its current defense shield, which is supposed to strike and destroy an incoming warhead at altitudes far higher than 100 kilometers.

In what could very well have been a response to these difficulties, China , in collaboration with Russia , proposed a new draft in June 2002.[12] This draft obligated signatory countries to “[n]ot place in orbit around the Earth any objects carrying any kinds of weapons.” Because the U.S. system is tested and deployed on suborbital boosters, as is China 's ASAT system, it would be allowed under this first part of the treaty. The draft treaty then goes on to ban “the threat or use of force against outer space objects.” Because the treaty does not define either “outer space” or “object,” there is a certain amount of ambiguity about whether it allows the U.S. missile defense system. It is difficult to imagine an interpretation of these obligations that would allow the Chinese ASAT system.

Proposed Solutions

Codes of Conduct: Creating an International Taboo

Many feel that these definitional problems are impossible to overcome if international agreement is to be reached.[13] In answer to this, the Henry L. Stimson Center , in collaboration with a number of other nongovernmental organizations, has proposed a Code of Conduct for Space-Faring Nations. This code is still evolving,[14] but its key feature is a pledge to avoid creating persistent space debris by following the guidelines of the Inter-Agency Space Debris Coordination Committee (IADC).[15] Such a pledge would go a long way in protecting the world's economic interests in outer space by creating an international taboo against creating dangerous space junk. It would be an effective first step in banning the weaponization of space if it can strengthen the political commitments to the IADC's guidelines, guidelines with which the major space-faring nations' technical experts have already agreed.

Another important aspect of the proposed code is the call for nations of the world to share space surveillance data. Through a series of radars, ground-based optical telescopes, and even a camera onboard a satellite, the United States observes and tracks almost all the objects in space with diameters greater than 10 centimeters. The parameters necessary to calculate the orbits for most of these are provided on a website open to the public.[16] Other countries also maintain such observations but do not share them. It would be an important confidence-building measure for all countries to share this information. It would even improve satellite tracking because satellites are occasionally “lost” for days or months at a time because of a lack of observations at a crucial moment.[17] The situational awareness of objects in space that tracking provides is important for two reasons: in avoiding collisions between satellites, particularly for geostationary satellites and for the International Space Station, and preventing false alarms by the North American Aerospace Defense Command early-warning radars misidentifying a satellite for an incoming warhead.[18]

The Stimson Center 's code has been designed to avoid bans on activities that would simulate attacks on satellites because of the definitional problems discussed above. As a result, countries unfortunately could still test complete ASAT systems under the proposed code by using close flybys.

A Treaty Banning ASAT Testing

Other analysts have attempted to make progress with proposals banning the testing, development, and deployment of ASAT systems above some threshold altitude.[19] Such methods certainly avoid the missile defense problems that have stymied previous treaty attempts, but they also leave open the development of these weapons at lower altitudes, even if combined with a code of conduct for lower altitudes. It would, unfortunately, be a relatively minor step to move an ASAT weapon that had been developed for lower altitudes and mount it on a more powerful rocket, especially for countries such as China or India that have already orbited geostationary satellites.

A better approach might be simply to ban one spacecraft from approaching another orbiting spacecraft[20] at excessive speeds. A technical annex to the treaty, one that could be adjusted by a standing committee of experts, might define these as closing speeds greater than 100 meters per second if they are within 100 kilometers of each other. These speeds and distances are great enough not to interfere with much of the normal operating procedures in space and yet would still obstruct the development of the tracking, guidance, and control of any ASAT weapon. At the same time, they do not prevent the testing and deployment of ground-based missile defenses because the target is not in orbit.

Space is far from empty, however. For instance, within a single 100-minute orbit, an equatorial satellite “violated” the proposed treaty limits several times by passing closer than 100 kilometers (at closing speeds of more than 100 meters per second) to 18 cataloged space objects, including two functioning satellites. Of the 16 pieces of debris, six were from the satellite destroyed in China 's ASAT weapons test, which, for this orbit, increased by 50 percent the risk of collision with a large piece of debris.

To prevent such false violations, the treaty should be limited to cases where spacecraft were maneuvering within this region, which is the essence of the tracking-guidance-control system. Thus, although it would still be possible to develop individual components of an ASAT system such as the optical tracker with in-orbit tests under this proposed treaty, it would not be possible to gain enough confidence in the complete system to deploy a weapon.

Space-based satellite surveillance, which has already been implemented on a single satellite, could be used to detect spacecraft maneuvering in close proximity to other satellites by observing the exhaust plumes from the interceptor's jets.[21] The satellite tracking system at present, however, could not verify this ban because it does not have the space-based surveillance assets necessary for such continuous coverage. The United States would need to implement a complete constellation of satellites dedicated to tracking other satellites, as proposed by the Congressional Budget Office in 2000.[22]

What Is Not Covered by the Proposed Treaty

The proposed treaty discussed here is aimed at stopping the testing and deployment of some of the most dangerous ASAT systems currently on the horizon: high-speed kinetic-kill ASAT weapons. It does not stop the development of other types of ASAT weapons, such as the space mines with which the Soviets experimented in the 1980s. These weapons slowly approach their targets and then detonate. It is very difficult to ban the development of such slow-speed approaches because they have a number of legitimate peaceful uses. For instance, the International Space Station is regularly resupplied by unmanned Soyuz spacecraft.

Micro- or even smaller satellites, which would be nearly impossible to track, are also being developed to service the International Space Station.[23] These too are not covered by the proposal when used as space mines. Microsatellite know-how, however, possibly will be turned into high-speed kinetic-kill ASAT weaponry sometime in the future and would be covered by the treaty; it would just be difficult to detect. This is an example of why space-tracking technology must continue to be improved for verification purposes as well as for keeping our space situational awareness up to date.

This discussion has focused on the kinetic-kill type of ASAT weapon that China tested in January, but significant damage to low-flying satellites can be caused by blinding lasers, which China also has allegedly used.[24] This type of weapons system should also be banned, but specialized methods of verification would need to be developed.

The time is right for a treaty banning the testing of the most dangerous ASAT systems. The world has expressed grave concern at the space debris China 's last test created that put at increased risk both manned spaceflight and commercial space assets.[25] If the United States acts now while it is still technologically dominant in space, it could prevent other countries from gaining the experience and confidence needed to field such weapons. China , for its part, has shown the world that ASAT weapons are not a Western monopoly, and if it believes in its rhetoric of the past decade, it could negotiate an end to an entire class of weapons.

Geoffrey Forden is a research associate with the Science, Technology, and Society Program at the Massachusetts Institute of Technology. He served as chief of the multidisciplinary analysis section of the UN Monitoring, Verification and Inspection Commission (UNMOVIC).


1. Michael Krepon, “Weapons in the Heavens: A Radical and Reckless Option,” Arms Control Today , November 2004, pp. 11-18.

2. The parameters determined by North American Aerospace Defense Command (NORAD) tracking of satellites give an uncertainty about where the satellite is at any given moment, of about 10 kilometers along the orbit and approximately 2 kilometers transverse to that.

3. This is a combination of the satellite's orbital speed of 7.4 kilometers per second and the interceptor's speed of 1.8 kilometers per second at the time of the interception.

4. For a technical analysis and explanation of how these estimates were made, see Geoffrey Forden, “An Analysis of the Chinese ASAT Test,” Jane's Online, April 2007.

5. See Michael O'Hanlon, “A Space Weapons Race Is Not the Answer for America ,” The Financial Times, January 22, 2007.

6. A medium-Earth orbit is defined as any orbit greater than 2,000 kilometers in altitude and less than geostationary Earth orbits at 35,786 kilometers in altitude.

7. The lowest point in a transfer orbit is at the original low-Earth orbit's altitude while its highest point is usually just greater than the desired end orbit.

8. The U.S. national missile defense booster and interceptor, if used as an ASAT weapon, could directly attack satellites as high as 18,000 kilometers. Although this altitude is well above 2,000 kilometers, it is lower than the altitude of NAVSTAR/GPS navigational satellites.

9. Geoffrey Forden, “Sensitivity of GPS Coverage to Loss of One or More Satellites,” Technical Appendix D, in Ensuring America's Space Security: Report of the FAS Panel on Weapons in Space (October 2004).

10. See Hui Zhang, “Action/Reaction: U.S. Weaponization and China ,” Arms Control Today , December 2005, pp. 6-11.

11. Hu Xiaodi, “Possible Elements of the Future International Legal Instrument on the Prevention of the Weaponization of Outer Space,” CD/1645, June 6, 2001.

12. Hu Xiaodi and Leonid Skotnikov, “Possible Elements for a Future International Legal Agreement on the Prevention of the Deployment of Weapons in Outer Space, the Threat or Use of Force Against Outer Space Objects,” CD/1679, June 28, 2002.

13. Michael Krepon, e-mail to author, February 10, 2007.

14. Previous versions of the Stimson Center 's code also asked states to forgo “simulating an attack on a satellite.” Stimson Center , “Model Code of Conduct for the Prevention of Incidents and Dangerous Military Practices in Outer Space,” May 19, 2004.

15. The Inter-Agency Debris Coordination Committee is composed of representatives of national space agencies including NASA, the European Space Agency, Russia's space agency, and China's space agency.

16. Space-Track, located at http://www.space-track.org. Space-Track does not list the parameters for classified U.S. satellites. Most, if not all, of these are tracked by amateurs in the backyards using very inexpensive equipment. See Visual Satellite Observer's Home Page, located at http://www.satobs.org/satintro.html.

17. Grant H. Stokes et al., “The Space-Based Visible Program,” Lincoln Laboratory Journal, Vol. 11, No. 2 (1998), pp. 205-229.

18. A false alarm of a nuclear attack, fortunately caught before it triggered a “response,” was caused by a satellite appearing to a radar in Moorestown , New Jersey , to rise from Cuba during the Cuban missile crisis. See Scott Sagan, Limits of Safety (Princeton, NJ: Princeton University Press, 1993), pp. 130-131.

19. For some previous proposals for high-altitude bans, see Donald Hofner and Bhupendra Jasani, “An Arms Control Proposal Limiting High-Altitude ASAT Weapons,” in Strategic Defenses and the Future of the Arms Race: A Pugwash Symposium, eds. John Holdren and Joseph Rotblat (London: MacMillian Press, 1987), pp. 226-239; Ashton B. Carter, “Satellite and Anti-Satellite: The Limits of the Possible,” International Security, Spring 1986, pp. 46-98.

20. The operative phase here is “orbiting spacecraft.” “Orbiting” would mean making more than one circuit of the Earth, and “spacecraft” is used to avoid the thousands of times per day of accidental close encounters with space debris. A standing panel of experts would have to be created to discuss such definitions in the light of experience.

21. The existing space-based tracking asset has already demonstrated a rudimentary capability to do this. See Stokes et. al., “Space-Based Visible Program,” p. 218.

22. Geoffrey Forden, “Option 3-08: Establish a Space-Based Capability to Search for and Track Adversaries' Spacecraft,” in Budget Options for National Defense ( Washington , DC : Congressional Budget Office, March 2000), pp. 37-39. The United States has undertaken the development of such a constellation in the form of the Space Based Space Surveillance Pathfinder single satellite project scheduled for launch in 2007. See Boeing Integrated Space Systems, located at http://www.boeing.com/defense-space/space/satellite/sbss.html.

23. See NASA Exploration Systems, located at http://exploration.nasa.gov/programs/station/STP-H2-MEPSI.html.

24. “Top Commander: Chinese Interference With U.S. Satellites Uncertain,” Inside Missile Defense, October 22, 2006.

25. See “European Union Expresses Concern Over Chinese ASAT Test,” Defense Daily, January 24, 2007.