Share This!
Text SizeAAA Share Email

Policy Implications of Space Weapons

This is Section 2 of a report published by the American Academy of Arts and Sciences (AAAS), The Physics of Space Security: A Reference Manual, which was authored by UCS scientists David Wright, Laura Grego, and Lisbeth Gronlund.

This report discusses how the laws of physics apply to operations in space and to interference with such operations. The essential technical facts that emerge from that discussion are summarized in Section 3. This section lays out some of the policy-relevant implications of these technical facts for the operation and utility of various systems, including space-based weapons, as well as antisatellite weapons (ASATs), both ground based and space based. Space-based weapons can be designed to destroy their targets in one of several ways: with direct impact,¹ an explosive warhead, or a laser. Space-based lasers intended to damage parts of satellites other than the sensors are not discussed in this report because the technology for these weapons will not be available in the foreseeable future.

The first five points below concern the types of missions for which space is or is not well suited. Points 6 through 9 address antisatellite weapons, the ASAT capabilities of systems designed for other purposes, and the vulnerability of satellites. Point 10 concerns the overall military utility of space-based weapons. Each point notes the section or sections of the report where the issue is discussed.

1. Space basing is uniquely well suited to a wide range of civilian and military applications. (Sections 5, 6, 7)

Space offers several features not available from the ground or air. Satellite-based sensors can see much larger areas of the Earth than sensors closer to the Earth can see. This allows large-scale simultaneous observation of the Earth's surface and atmosphere, and communication between and simultaneous broadcast to large parts of the Earth.

Because the atmosphere blocks transmission of many types of electromagnetic waves, some kinds of astronomical observations can only be made from space.

Moreover, space is much better suited to some types of operations than to others. Electromagnetic signals (light and radio waves) can be transmitted over large distances almost instantaneously and with very little energy cost. Space therefore favors activities that entail sending and receiving electromagnetic signals over activities that involve transporting large amounts of mass from the Earth into space or that involve significant maneuvering in space, which can require a large mass of propellant.

As a result, the applications for which space basing is uniquely well suited include:

  • large-scale environmental monitoring of, for example,
    atmospheric behavior, climate change, and deforestation
  • large-scale weather monitoring for weather forecasting
  • astronomy
  • global communication, broadcast, and data transfer
  • highly accurate navigation and position determination
  • reconnaissance on a global or large-scale basis
  • detection on a global basis of missile launches, to provide early warning of attacks and information about the missile testing programs of nations

Some of these tasks could, as discussed later, be accomplished by ground- and air-based alternatives, if urgently needed, albeit on a regional rather than global basis.

2. Space basing is poorly suited to the mission of attacking ground targets using kinetic energy or explosive weapons. This mission can be done as well or better from the ground, and acquiring a prompt attack capability from space would be much more costly. (Section 9)

Space-based ground attack weapons could offer global reach, obviating the need to forward-base weapons, providing prompt attack capability, and shortening any warning of an attack. However, space-based weapons fare poorly when compared with other long-range means of attacking ground targets. In particular, intercontinental-range ballistic missiles can provide the same prompt, global reach, but are less expensive and more reliable than space-based weapons.


Deploying a space-based system would be tens of times more expensive than deploying a comparable system using ballistic missiles. This is a consequence of the fact that any satellite system with prompt global coverage would require numerous satellites to ensure that at least one is in the right place at all times. The exact number of satellites will depend on the altitude of the orbit and the reach of each weapon, but tens of satellites would be required for prompt attack of one target. For example, the constellation considered in Section 9, which could attack any point on the Earth within about 30 minutes, would require nearly 100 satellites. If the promptness requirement was relaxed to a 45-minute response time, roughly 50 satellites would still be required.

For comparison, a missile capable of putting a given mass into low earth orbit can deliver the same mass to a range of 20,000 kilometers—halfway around the Earth. The flight time would be roughly 45 minutes. This one ballistic missile could therefore provide global coverage with the same response time as a constellation of 50 satellites requiring 50 comparable missiles to launch them into space.

For the five nuclear weapon states, the relative cost of a space-based system would be even higher, because they already possess intercontinental-range ballistic missiles that could provide a prompt ability to attack ground targets globally.


Unlike ground-based weapons, space-based weapons that have been launched must remain operational without either routine or emergency maintenance. As a consequence, space-based weapons would be less reliable and an attacker would have less confidence in using them for an attack than ground-based missiles. If a space-based weapon in the proper position for an attack failed, other satellites in the constellation could be used in its place but could not meet the same promptness criterion, because they would take time to move into position.

3. Space basing is unsuited to ballistic missile defense using kinetic interceptors. Not only would such a system be expensive, its intrinsic vulnerabilities would allow an attacker to readily negate its defensive capability. (Section 9)

The global coverage space-based weapons can provide is also a key motivation for deploying ballistic missile defense interceptors in space. The United States is conducting research on various types of missile defense systems designed to attack long-range ballistic missiles during their boost phase (the time when the rocket booster is still burning), which lasts only minutes. The short time available means that interceptors must be located close to the launch site of the missile; against large countries it may not be possible to deploy ground or air-based interceptors close enough. In contrast, a constellation of space-based interceptors in low earth orbits could provide global coverage. Thus, in principle, a space-based boost-phase missile defense system could offer capabilities that would not be available with a ground- or air-based system.

However, because of the short response time this mission requires, the system would be intrinsically vulnerable to debilitating attack and to being overwhelmed. Any country with the capability to launch a long-range ballistic missile could also develop an effective capability to destroy satellites in low earth orbit using ASATs launched on short-range missiles. Once one or more space-based interceptors were destroyed, producing a hole in the defense constellation, an attacker could launch a long-range missile through this hole. If the defense used one of its interceptors to protect itself, it would still remove the interceptor from the constellation and create a hole.²

Alternatively, an attacker could overwhelm the defense. A defense system designed to intercept one ballistic missile launched from any given region would require many hundreds or even a few thousand orbiting interceptors, depending on the design of the constellation and the interceptors. Increasing the defense capability so the system could attack two missiles launched simultaneously from the same region would require doubling the total number of interceptors in the constellation. Because the system costs would increase rapidly with the number of interceptors, any plausible defense system would be designed to intercept only one or two ballistic missiles launched simultaneously. Thus, any country launching more than one or two missiles roughly simultaneously from the same region would penetrate such a defense, even if it worked perfectly.

4. A nation could not use space-based weapons to deny other countries access to space, although it could increase the expense of such access. (Section 9)

At first glance, it might appear that the first country to deploy a system of space-based missile defense interceptors with global coverage could control access to space by intercepting space launch vehicles, which are similar to long-range missiles. However, as discussed in Point 3, the vulnerabilities of a space-based missile defense system would render it ineffective at prohibiting satellite deployments by other countries.

Any country that can launch satellites has the technical capability to attack space-based interceptors to create a hole in the constellation. While requiring a country to attack the defense system prior to launching a satellite would increase the cost of placing a satellite in space, it could not in this way deny the country the ability to launch satellites.

5. Ground-based and space-based ASATs have relative advantages and disadvantages depending on the method of attack. Jamming or dazzling attacks would only be practical using ground-based ASATs, whereas high-power microwave attacks would only be practical from space. Kinetic energy attacks could be conducted from the ground or space; in this case, space-based ASATs may offer advantages for very prompt attacks or for simultaneous attacks on numerous satellites, but would be less reliable than ground-based ASATs because they must operate without maintenance. (Sections 11, 12)

Ground- or space-based ASATs can be used to damage or interfere with satellites. However, some means of attack are only feasible or practical using one or the other. For those types of attacks that could be conducted from both ground and space, an attacker's preference would depend on a wide range of factors, including the scale and time requirements of the attack, whether the attack is to be covert, and to what extent cost is a factor (keeping in mind that only space-faring nations would be capable of deploying space-based ASATs).

A trailing space-based kinetic energy ASAT (which would be deployed in the same orbit as the target satellite to trail it) could carry out an attack almost instantaneously from the time a decision was made to attack, assuming the attacker was able to communicate with the ASAT. This would not necessarily be the case for either space-based ASATs that are designed to attack satellites in other orbits or for ground-based ASATs: both types of ASATs may need to wait hours for the target satellite to be within range of attack. Similarly, it would take several hours for a ground-based kinetic ASAT to reach a target in geosynchronous orbit. Moreover, by deploying multiple trailing ASATs, an attacker could destroy or interfere with multiple satellites essentially simultaneously.

On the other hand, space-based ASATs have some significant disadvantages compared with ground-based ASATs. Placing ASATs in space is much more costly than basing them on the ground. Once in space, these systems must work without maintenance, and reliability becomes an issue.

6. Satellites are intrinsically vulnerable to attack and interference. However, satellite systems can be designed to be less vulnerable than the individual satellites that compose the system. Moreover, air- and ground-based backup systems can provide some of the militarily relevant, time-urgent capabilities that would be lost if the satellite system was disrupted or destroyed. (Sections 10, 11, 12)

Because there is no place to hide in space, satellites are inherently vulnerable to interference and direct attack. However, steps can be taken to reduce the vulnerability of the system, including hardening satellite components, employing antijamming techniques, building redundant ground stations, developing the capability to quickly replace satellites, and distributing the task of a single satellite among clusters of smaller satellites. The commercial communications satellite industry routinely deals with the failure of satellites. It places spare satellites in orbit to allow rapid substitution when satellites fail, and can reroute communication traffic around a failed satellite.

Moreover, for many military missions, ground- and air-based components can serve as a backup on a regional rather than global level. The United States is not only the country most reliant on space-based systems, but also the one most capable of building alternative air- and ground-based backup systems.

7. Deploying defensive "bodyguard" satellites to protect other satellites against ASAT attacks cannot provide confidence in the survivability of those satellites. Doing so will therefore not preclude the need to take into account the vulnerability of satellite systems and to have backup systems for any essential military capability the satellites provide. (Section 11)

As discussed above, satellites are vulnerable to many types of attack and defending them is inherently difficult. Equally important, nations will not be able to rely on bodyguard satellites to protect their satellites from direct attack or interference by a determined adversary, because the limited amount of real-world testing that would be feasible would provide little confidence in the capability of the bodyguards.

8. No country can expect to have a monopoly on deployed ASATs. Space-faring nations have an inherent capability to deploy effective ASATs. Many other countries have the capability to develop means to destroy satellites or disrupt their performance, although the options of these countries will be limited relative to those of space-faring nations. (Sections 11, 12)

The technology required to build effective ground- and space-based ASATs is within the capability of any space-faring nation. These countries have the ability to place objects in orbit or lift them to geosynchronous altitude, to track objects in space, and to develop homing interceptors. They could develop systems to attack satellites in geosynchronous orbits as well as low earth orbits. They could also deploy ASATs relatively quickly in response to the deployment of ASATs by another country.

Other countries also have anti-satellite capabilities. Satellites in low earth orbit can be reached by ground-based ASATs using missiles that are much less capable than the launchers needed to deploy the satellites. Countries with short- or medium-range missiles can reach satellites in low earth orbit at an altitude of roughly half the range of these missiles. However, such countries would not necessarily have the ability to develop homing interceptors. For a destructive ASAT, these countries might therefore be limited to releasing clouds of pellets in the path of a satellite—a method whose effectiveness is uncertain and potentially limited.

It is also within the capability of many countries to use other methods of interference. For example, ground-based transmitters can be used to interfere with, or jam, satellite communications. While such jamming is unlikely to prevent a well-protected satellite from communicating, jammers can cause the satellite to use antijamming techniques that can significantly reduce the amount of information it can transmit. On the other hand, during a conflict such active methods of interference can be located and attacked.

Countries that possess both nuclear weapons and short- or medium-range missiles could explode a nuclear weapon in low earth orbit. The x-rays released would destroy unshielded satellites in low earth orbit that were in the line of sight of the explosion, and the explosion would generate persistent radiation that would last months to years and would damage unshielded satellites in low earth orbit. While high-altitude satellites would not be directly affected by the explosion, this radiation environment could make it more difficult for them to communicate with ground stations.

9. Several types of systems not designed as ASATs have an inherent ASAT capability. (Sections 9, 11)

Missile defense systems designed to intercept long-range ballistic missiles outside the atmosphere during the midcourse of their trajectory have significant ASAT capabilities against satellites in low earth orbits. These satellites orbit at altitudes similar to the altitude at which the defense is designed to intercept missiles. Unlike the case of an attacking missile, the trajectory and appearance of the satellite would be known in advance and the future trajectory would be predictable. Moreover, even highly controlled intercept tests of the defense against ballistic missile targets would provide confidence that the system would work against satellites, since the information provided to the defense about the missile target in these tests is comparable to what would be available in advance about satellite targets. Using interceptors against a satellite
rather than against a missile warhead is also easier in that the attacker could take multiple shots at the satellite if the initial attack was not successful.

Defender satellites would also have an inherent ability to serve as ASATs. Such satellites would need to carry enough fuel to maneuver to intercept attacking ASATs, and this fuel could also provide the maneuvering capability to serve as a kinetic energy ASAT.

10. Being the first to deploy space-based weapons would not confer a significant or lasting military advantage. (Sections 5, 9, 11)

In ground wars, there may be clear advantages to being the first to occupy and exploit a strategic location. Digging in and preparing defenses may make it difficult for an adversary to take control of the area. There are no such strategic locations in space, and a defender will not have the kinds of advantages in space as on the ground. No foreseeable space-based technologies would allow one country to prevent another from deploying space weapons or would allow it to reliably protect its satellites. 

1. Weapons that destroy by direct impact are called kinetic kill weapons. The kinetic energy of the fast-moving weapon and/or target provides the energy to destroy the target.

2. Instead of designing an ASAT weapon, an attacker could launch a ballistic missile without a warhead, thus forcing the defense to waste one of its interceptors and create a hole.

Powered by Convio
nonprofit software