Opinion & Analysis
Anti-Satellite Capabilities of Planned US Missile Defence Systems
By David Wright and Laura Grego
The Bush administration appears interested in pursuing an aggressive approach to the military use of space. Some indications of this approach appeared in the January 2001 report of the Commission to Assess United States National Security and Space Management and Organization, which was chaired by Donald Rumsfeld shortly before he became Secretary of Defense.1
The Commission's report stresses defensive space operations, noting that the United States is more reliant on space assets than any other country and stating that the country must have the capability to "defend its space assets against hostile acts" as well as to "negate the hostile use of space against US interests," and more generally to help "deter and defend against evolving threats directed at the US homeland, its forward deployed forces, allies and interests abroad and in space." However, the report is surprisingly vague about specific threat scenarios and responses, or the kinds of technologies and capabilities needed to defend against these threats.
The report also makes clear that it does foresee US offensive uses of space. It argues that space is another medium - like land, sea, and air - that will be used for war and that the United States must dominate this medium. It states that the United States must develop the capability for "power projection in, from and through space." Additionally, many defensive technologies are capable of being used offensively.
The Bush administration is currently increasing the funding for and widening the scope of research and development of space-relevant technologies. However, development of most of these new technologies into deployable offensive or defensive systems will take many years. As a result, what the United States can do in the near future is limited by available technology.
Both the United States and Soviet Union developed and tested dedicated anti-satellite (ASAT) weapons in the past, although further development of weapons that physically destroy satellites has been restricted by Congressional bans and voluntary Russian moratoria on testing them. Both countries retain some ASAT capability, including the ability to disrupt satellite functions without disabling them, such as temporarily blinding them with ground-based lasers or jamming their transmissions (even those in geosynchronous orbits).
In addition to this capability, some of the systems currently being developed to intercept ballistic missiles would have considerable inherent capability to be used as ASAT weapons, and could therefore significantly increase US ASAT capability.
Indeed, while the technologies being developed for long-range missile defences may not prove very effective at defending against ballistic missiles, some could be much more effective against satellites, since in many ways attacking satellites is an easier task. Satellites travel on predictable orbits that can be determined accurately by tracking from ground facilities, allowing the position of the satellite to be known at future times. The United States would have time to plan an attack, could choose the timing, and would have time to take as many shots as necessary to destroy it. In contrast, in a ballistic missile attack, the attacker would have the advantage of surprise and the defence less than 30 minutes to respond. In addition, an interceptor attacking a satellite would not have to deal with the severe countermeasure problem that would face a missile defence system. Current-generation satellites are not equipped to defend themselves. While future satellites might include defences of some type, it will be difficult to overcome the advantages that an attacker has.
Below we consider the ASAT capabilities of three missile defence systems the Bush administration is planning for early deployment. These are the Ground-based Midcourse Defense (GMD), the ship-based Aegis-LEAP system, and the Air-Borne Laser (ABL).2 We also consider briefly the ASAT capabilities of space-based missile defence systems.
Ground-Based Midcourse Defense (GMD)
The Bush administration plans to field five ground-based missile defence interceptors at Fort Greely in central Alaska by late 2004.3 These interceptors will consist of a three-stage rocket booster that carries a kill vehicle into space. The kill vehicle, which is intended to intercept above the atmosphere, carries its own fuel for manoeuvring, as well as optical and infrared sensors, which are intended to allow it to track and home in on an object, and destroy it by direct impact. As we discuss below, it appears that these interceptors could be effective as ASATs against a large fraction of satellites in low-earth orbit.
The planned burnout speed of the ground-based interceptors is reported to be 7 to 8 km/s. If launched straight up, this interceptor could lift the kill vehicle to a height of roughly 6,000 kilometers. It could therefore reach satellites in low-earth orbit, which are typically at altitudes less than 1,200 km, but not satellites in geosynchronous (36,000 km) or semi-synchronous orbits (20,000 km).4 If launched against satellites in low-earth orbit, the interceptor could use some of its speed to reach out laterally thousands of kilometers, allowing it to hit satellites on orbits that do not pass directly over the launch site. Thus, even interceptors at a fixed ground site in Alaska could reach a large fraction of satellites in low-earth orbit - especially those in orbits that pass over the United States.5
It is currently unclear what ground sensors will be part of the early deployment system at Fort Greely.6 However, the United States has extensive space tracking assets, and this continual monitoring would provide a good approximation of a satellite's location at any time. This information could be used to launch interceptors toward them and position the kill vehicle so that its sensors could detect the satellite.
The sensors on the kill vehicle are intended to detect the light and heat from a warhead in the midcourse phase of its flight, and therefore should also be able to detect satellites. The detection range would vary according to a number of factors, such as the temperature of the warhead or satellite, which would depend on whether or not the object was in sunlight and what its surface properties were. In recent tests of the GMD system, the detection range has apparently been several hundred kilometers.7 Based on these numbers, it is reasonable to assume that the detection range against a satellite could also be hundreds of kilometers. Moreover, the closing speed between the kill vehicle and target would be similar in both cases, since a satellite in low-earth orbit would travel at 7.3 to 7.8 km/s, depending on altitude, while an intercontinental range missile would have a maximum speed of about 7.2 km/s.
In the longer term, the ASAT capability of the GMD system would increase if additional interceptors and sensors were added to the system. The X-band radars that are to be part of the GMD system should be able to track satellites in low-earth orbit at least as well as they could missile warheads, and the system could use that information to launch the interceptors. It is not publicly known whether decisions have been made about future increases in the number and locations of interceptors.
The Bush administration is also developing a ship-based missile defence that is intended to intercept above the atmosphere. Until recently, this system was called Navy Theater-Wide, but is now referred to as the Sea-Based Midcourse Defense (SMD) segment, and is being restructured from the old programme. The system currently under development would only be able to attack those satellites in the lowest orbits, as discussed below.
The system that may be fielded in the next few years is called Aegis-LEAP (previously, it was called Navy Theater-Wide Block I), and is intended to intercept missiles with ranges up to 1,000 to 2,000 kilometers. The interceptor consists of a modified version of the anti-aircraft missile used on Aegis cruisers, called the Standard Missile, topped with a Lightweight Exo-Atmospheric Projectile (LEAP) kill vehicle. Like the GMD kill vehicle, the LEAP vehicle is intended to home on a missile warhead and destroy it by direct impact. The US Missile Defense Agency (MDA) has argued that if the testing programme goes well, it will field a small number of Aegis-LEAP interceptors in the 2004-6 timeframe.8
The Aegis-LEAP system reportedly has a burnout speed of 3 km/s. Fired vertically for use as an ASAT, the kill vehicle would be able to reach altitudes of 400 to 500 kilometers and attack satellites at those altitudes. However, these low altitudes include a relatively small number of satellites: a few imaging satellites and some military communication and electronic intelligence satellites in highly elliptical orbits, most of which currently are owned by the United States. As noted above, since the interceptors could move to different locations, it would have essentially global coverage against satellites at these altitudes.
The eventual goal of the SMD programme is to develop a system intended to intercept long-range missiles, using an interceptor similar to those of the ground-based system. Such a system would therefore have similar ASAT capabilities to those discussed above for the GMD system, with the added advantage that the launch site of the interceptors could be moved, providing even better coverage against satellites.
Air-Borne Laser (ABL)
The Air-Borne Laser is intended to consist of a modified Boeing 747 carrying a powerful (megawatt class) chemical laser and a beam director that can aim the laser and allow it to track a moving missile during its boost phase. It is intended to disable the missile by dwelling on the missile body long enough that the heat causes sufficient damage to the body to terminate its acceleration, causing the warhead to fall short of its target. Publicly available analyses estimate that the full-power laser being developed may be able to destroy missiles at a range of a few hundred kilometers with a dwell time of 10 to 20 seconds.9 Considerable uncertainties remain, however. An intercept test using a half power laser against a short-range missile is planned for late 2004.
If ABL can be developed for the anti-missile role, it would also be able to damage satellites in low-earth orbit. While the system was originally intended as a defence against short-range missiles, it has more recently been discussed as being used against long-range missiles as well. Using it against long-range missiles has several advantages, a key one being that the longer boost time allows more time for the laser to attack it.
The ABL is intended to fly at an altitude of about 13 kilometers. While a 300-kilometer range Scud missile burns out at 25 to 30 kilometers altitude, a long-range missile would burn out at 200 kilometers or higher. As a result, if ABL is to be able to attack long-range missiles, the beam director must be able to point the beam upwards, which would allow it to target satellites as well.
If the beam director is able to hold the laser beam on an accelerating missile body at a range of several hundred kilometers, it could also hold the beam on a satellite at an altitude of several hundred kilometers. If the beam had sufficient power to destroy missiles, it appears likely that it could physically damage satellites in low orbits, especially since the beam could dwell for longer times than it could on missiles. It would certainly be able to temporarily blind reconnaissance satellites, and could likely damage their sensors and permanently blind them. This mission would require less power than destroying a missile.10
Surface-based lasers could also be used to blind or attack satellites, and would not have the same weight or operational constraints as those on an air-borne platform. However, the military may find that having an air-mobile laser may be useful in some situations. Determining to what extent stationing the laser high in the atmosphere is an advantage, for example, to avoid cloud cover and atmospheric distortion of the beam, requires further analysis.
Space-Based Missile Defences
The final case we consider is the ASAT capability of space-based systems being developed for ballistic missile defence. Unlike the systems discussed above, these systems are not intended for near-term deployment. Moreover, MDA has reportedly decided not to emphasize space systems, although some funding for continued development is likely to continue. Two systems are currently being funded: the Space-Based Laser and kinetic-energy "hit-to-kill" systems, which are the follow-ons to the "Brilliant Pebble" system of the early 1990s.
The Space-Based Laser would presumably have significant capability against satellites if it could be made to work against ballistic missiles. However, the technology is far from ready and funding for the programme is low enough that it is likely to stay in a pre-deployment phase for many years. As a result, we do not consider this system in any detail.
A constellation of space-based kinetic interceptors could not be deployed for many years, although small numbers of prototypes could possibly be deployed earlier. These systems are important to consider since they could have the capability to attack, with little warning, satellites in geosynchronous or semi-synchronous orbits, which could not be reached by the other missile defence systems we have discussed.
Several types of space-based kinetic missile defences have been considered in the past 15 years. The GPALS (Global Protection Against Limited Strikes) missile defence system being developed in the previous Bush administration was intended to include a constellation of 1,000 Brilliant Pebble kill vehicles. These small satellites were intended to intercept missile warheads during the midcourse phase of their flight. The system currently under development is intended to attack a missile during its boost phase.
As part of this system, satellite kill vehicles would be placed in orbit where they would remain until a missile launch was detected. A kill vehicle near the missile launch site would then use its on-board propulsion and sensors to accelerate out of its orbit and home on the missile, attempting to destroy it by direct impact. The orbital speed of the kill vehicle would be roughly 8 km/s, and the propulsion system is intended to accelerate it an additional 6 km/s to allow it to reach a boosting missile in the relatively short time available. As a result, the kill vehicle would have a total speed of up to 14 km/s. Calculations show that such a speed would allow it to travel from low earth orbit to geosynchronous orbit in just over an hour, and still have a speed of nearly 10 km/s at that altitude.
Whether a kill vehicle designed solely for missile defence could be used to attack satellites in this way depends on the details of its design, such as the type of sensors it contains, the amount of fuel for manoeuvring it carries, and the length of time it is designed to operate (a matter of minutes to reach a boosting missile versus an hour to reach geosynchronous orbit). It is clear, however, that these are engineering decisions rather than technical obstacles, and that these capabilities could be built into the kill vehicle to give it the capability to be an effective high-altitude ASAT.
The location of a satellite being attacked could be determined sufficiently accurately from ground observations to launch the interceptor and allow the on-board sensors to detect the satellite when it was close enough. Once the interceptor's sensors had detected the satellite, it could home in on it just as it would on a missile target.
The detection range would depend on a number of issues, including the type of sensors on the interceptor. Since geosynchronous satellites are in the sunlight during all or nearly all of their orbit, they would reflect sunlight and would be expected to have a relatively high temperature, both of which could be used for homing.11 The interceptor's sensors, designed to detect the missile plume during boost phase, may not be suitable for detecting a satellite, but lightweight sensors exist that could be used, such as those on the LEAP vehicle.
An important final point is that the number of space-based kill vehicles in a constellation designed to defend against missiles would be large - likely hundreds. This is because the time available to attack a boosting missile is short and thus a kill vehicle is limited in how far it can be from the missile, so the kill vehicles would need to be closely spaced above potential target areas. However, since the vehicles would be in low-earth orbit, they would move with respect to the earth's surface; multiple kill vehicles would therefore need to be in orbit so that as one moved from above a potential launch site another would move to replace it. As a result, there would be more than enough kill vehicles in a constellation to destroy the fewer than 100 military satellites that Russia currently has in orbit.12
The United States currently has some capability to disrupt or destroy satellites as a result of the aircraft-launched ASAT it tested several times in the 1980s, as well as ground-based lasers and electronic jammers. Therefore, the ASAT capability added by missile defence systems would not be unique. However, current US ASAT capability is fairly limited and, based on current funding levels, dedicated ASAT systems appear not to be high priorities. Some of the planned missile defence systems, on the other hand, would add significant ASAT capability to the US arsenal and have strong political and financial support. This fact should be kept in mind when analysing US capabilities and developing policies relevant to restricting ASATs.
Notes and References
1. The Commission's report is available on the website of the Federation of American Scientists (FAS), http://www.fas.org/spp/military/commission/report.htm.
2. The Theater High-Altitude Area Defense (THAAD) system is also designed to operate above the atmosphere, but the speed of its interceptor is slow enough (roughly 2.3 km/s) that the maximum altitude it could reach would be less than 350 kilometers, where there are very few satellites. Other theater missile defences, such as Patriot PAC-3, are intended to operate at low altitudes and would not be able to threaten satellites.
3. For details, see Lisbeth Gronlund and David Wright, "The Alaska Test Bed Fallacy: Missile Defense Deployment Goes Stealth," Arms Control Today (September 2001), available at http://www.armscontrol.org/act/2001_09/gronlundwrightsept01.asp.
4. Important satellites in or near semi-synchronous orbits include the US Global Positioning System and its Russian equivalent, GLONASS.
5. Further analysis is needed to determine if there are satellites in low-earth, low-inclination orbits that might be out of reach of the Fort Greely interceptors.
6. The existing Cobra Dane radar on Shemya Island, which will be part of the Fort Greely system, has a rather restricted field-of-view that limits its ability to track warheads or satellites. Other options under development, such as sea-based X-band radars, would not be available by 2004 when the interceptors are to be placed at Fort Greely.
7. Press briefing on IFT-6 by US Maj. Gen. Willie Nance, 9 August 2001, available on the website of the Department of Defense at http://www.defenselink.mil. The briefing stated that detection of the target set by the kill vehicle occurred 94 seconds before intercept, and that the mock warhead was discriminated at 31 seconds before intercept. For a closing speed of 7.4 km/s, these times correspond to distances of 700 km and 230 km, respectively. The longer distance probably refers to detection of the large balloon decoy, which is several times brighter than the warhead. The shorter figure presumably means that the sensors have collected enough data to match the warhead to data stored prior to the test. The detection range of the warhead would therefore fall between these two figures. In these tests, the warhead has been in the sunlight.
8. The Aegis-LEAP system has successfully intercepted targets in three tests, but analysis shows that the tests conditions are highly artificial. See David Wright, An Analysis of the 25 January 2002 Test of the Aegis-LEAP Interceptor for Navy Theater-Wide, Union of Concerned Scientists Working Paper (March 3, 2002), available at http://www.ucsusa.org/global_security/missile_defense/index.cfm.
9. Geoffry Forden, The Airborne Laser: Shooting Down What's Going Up, Center for International Security and Arms Control, September 1997.
10. Satellite sensors can be temporarily blinded by very low-power lasers. A laser ASAT test in 1997 showed that a 30 watt, ground-based chemical laser was able to temporarily blind an Air Force satellite orbiting at 425 kilometers altitude (John Donnelly, "Laser of 30 Watts Blinded Satellite 300 Miles High," Defense Week, 8 December 1997, p. 1).
11. For a discussion of the temperature of objects in space, see Appendix A of Andrew Sessler et al., Countermeasures, Union of Concerned Scientists and MIT Security Studies Program, April 2000.
12. John Pike, "The Military Uses of Outer Space," in SIPRI Yearbook 2002 (Oxford University Press, 2002).
David Wright is Co-Director and Senior Scientist in the Global Security Program at the Union of Concerned Scientists (UCS) and a Research Scientist in the MIT Security Studies Program. Laura Grego is a Postdoctoral Science Fellow at UCS.
© 2002 The Acronym Institute.