Science Technology & Military Experimentation

A White Paper on the Defense Against Ballistic Missiles

Author: Hans Mark

The United States needs a strong, effective missile defense system to meet the threats and uncertainties that lie ahead.

In the past three years, very significant progress has been made on the development of defenses against ballistic missiles. A number of systems have been tested successfully, and it has been established that "hit-to-kill" technology is feasible. By this I mean that we have shown it is possible to intercept an incoming warhead with a kill vehicle carried by an antiballistic missile (ABM). This has been achieved with several different systems employing, in some cases, different technologies. We have also demonstrated for the first time that a high-power laser is capable of shooting down tactical ballistic missiles. My purpose in writing this paper is to step back and take an overall look at the Ballistic Missile Defense Organization (BMDO), starting with the threats and then turning to military considerations. Finally, I will comment on the programmatic situation. The BMDO is seriously underfunded. My hope is that the recommendations in this paper will be useful in persuading the political leadership to provide the support necessary to a working system that can be deployed in the coming years.

I have divided the military threats into three categories: near-term threats (the next 10 to 15 years); far-term threats (the next 15 to 30 years); and other threats (less likely threats that still should be considered). The threats are listed in no particular order of priority.

Near-Term Threats (10 to 15 years)
North Korea could pose a threat to South Korea, Japan, Taiwan, and other territories in the neighborhood with short- and intermediate-range ballistic missiles that have been tested. The North Koreans have also probably developed nuclear, chemical, and biological warheads that can be carried by these missiles. To the best of my knowledge, these have not been tested.

China could pose a threat to Taiwan, Japan, South Korea and other territories in the neighborhood with short- and intermediate-range ballistic missiles that have been tested. China has also tested nuclear weapons that can be carried by these missiles and probably has chemical and biological weapons. The Chinese could also pose a threat to Russia, India, Pakistan, and the Middle East with these weapons. Finally, the Chinese have tested a long-range missile that could eventually threaten the United States. However, in my judgment, threats from China are politically less likely than from some of the others listed in this section.

The Middle East. Israel has nuclear weapons and probably also chemical and biological weapons. Israel also has the means to deliver the weapons over the ranges compatible with the distances of likely enemies (up to 1,000 miles). Iraq has some ballistic missiles acquired from the Soviets and modified to increase their range. Iraq used ballistic missiles in combat in 1991 against both Israel and Saudi Arabia. Iraq's program to develop nuclear weapons was probably within 18 months of producing enough weapons-grade uranium-235 to manufacture a few nuclear weapons when the facilities were destroyed following the Gulf War. Iraq also used chemical weapons in combat against Iran and probably has the capability of manufacturing biological weapons. In short, Iraq remains the most dangerous threat to other countries in the Middle East as well as the countries of Eastern and Central Europe. Iran has short- and intermediate-range ballistic missiles acquired from China and probably nuclear-weapons-grade fuels and nuclear weapons components acquired from the states of the former Soviet Union. In my judgment, Iran is the most likely nation to test nuclear weapons in the next 10 to 15 years. Iranian weapons and delivery systems could pose a threat to Israel and Eastern Europe in the near term. Over the years, Syria and Libya have both harbored ambitions of acquiring nuclear, chemical, and biological weapons and their delivery systems. Both were client states of the Soviet Union, but neither has the indigenous capability of manufacturing or maintaining advanced, complex weapons systems. Both countries have exhibited aggressive intentions in the past, but probably neither is capable of implementing them now. Algeria, although not strictly in the Middle East, is a large and capable country that could become a threat if a radical Moslem fundamentalist government is installed. Egypt could also become a threat if the successor to President Hosni Mubarak changes the policy of peace with Israel. In my judgment, this is not likely, and Egypt will continue to be a stabilizing influence in the Middle East.

India and Pakistan. Both of these nations have tested nuclear explosives, and both are capable of weaponizing them-in fact, they may already have done so. Both also probably have the ability to manufacture chemical and biological weapons. Both nations have ballistic missiles that can deliver these weapons over a range of 2,000 to 3,000 miles. Pakistan's missiles were obtained from China or North Korea; India has the indigenous capability of developing and manufacturing long-range ballistic missiles. India and Pakistan have developed these weapons because they fear each other. At the present time, neither nation seems to be a threat to other nations in the neighborhood. An open question is what would happen if India and Pakistan used nuclear weapons against each other. Would the United States have to intervene to stop further nuclear exchanges? Would the potential deployment of ABM weapons by the United States neutralize the threat of nuclear weapons being used by other nations against each other in the near term? Even though neither of these nations constitutes an immediate threat to the United States or its allies, we must think about that possibility.

Far-Term Threats (15 to 30 years)
Russia. Russia remains the most serious threat to the United States because it is still the only nation in the world that can destroy the United States with a surprise nuclear strike. I have listed Russia as a long-term threat because, for the foreseeable future (10 to 15 years), Russia's political priorities will probably be focused on internal development.

China. China presently has the capability of delivering single-warhead nuclear weapons over intercontinental distances. In all probability, China will not be a near-term threat to the United States for various political reasons. However, in the long term, China must be considered an increasing nuclear threat to the United States.

Other Threats
A number of other nations around the world have the technical capability of developing nuclear explosives and the ballistic missiles to deliver them but have not done so for various reasons. Among these are Japan, Germany, Italy, Sweden, Spain, Brazil, and possibly a few others. In the next 15 to 30 years, alliances and politics could shift, however, and a complete reversal is at least possible. Thus, the possibility of weapons proliferation to these nations should be considered. All of the nations listed above, as well as many others, also have the capability of developing chemical and biological weapons. Thus, in the long term, we must consider the proliferation of these weapons around the world. Hence the development of appropriate defenses becomes even more important.

Other Means of Delivery
Many people believe that there are easier and less expensive ways of delivering nuclear, chemical, and biological weapons than ballistic missiles. Aircraft, trucks, ships, and even trains are all possible means of delivery. So-called "suitcase" bombs can "easily" be hand carried. Similar means of delivery are even more effective for chemical and biological weapons. My first answer has always been that, if it is indeed easier to deliver these weapons by other means, why do all nations-even smaller nations such as Iraq, Iran, North Korea, and Pakistan-that are developing or trying to develop a nuclear weapons capability also acquire ballistic missiles in one way or another. To my mind, the answer has always been very clear: A ballistic missile is the only means of delivery against which there is no workable defense. Once a missile is properly launched, the laws of physics guarantee that it will get close enough to its target to inflict serious damage. In the case of a very expensive and probably scarce weapon, such as a nuclear bomb, the certainty that the weapon will get to its target must be a major factor in the mind of any military commander.

All other delivery systems are less certain. Airplanes would be next on the list of priorities for delivery systems. But most nations have some kind of air defense system, which might create uncertainty in the mind of someone with a small number of weapons to expend. In my judgment, the U.S. air defense system is not as good as it should be. In fact, for a long time I have advocated improving our defenses against intrusions into our air space by unauthorized aircraft to discourage a rogue state or a terrorist group from expending a very valuable weapon. A comprehensive air defense system could be designed in a way that would also improve our air traffic control system.

Delivery of nuclear weapons by ship or by truck is possible, of course, but could be substantially hindered by careful inspection of incoming cargoes, which would create uncertainty in the minds of the people attempting to "import" weapons into the country. Finally, suitcase bombs are very hard to produce-only a very sophisticated design would be small enough to carry the bomb. A suitcase bomb would also be quite radioactive, so it would be relatively easy to detect.

Chemical and biological weapons would obviously be easier to smuggle into the country. However, in light of past experience, their effects would probably be much less devastating than the detonation of a nuclear explosive.

Response to the Threats
I have spent considerable time and space discussing threats because an ABM program must be structured to deal with them. The general principles that govern our thinking about the program should combine what we think we know about the threats and the technical means available to deal with them. Here are the three most important principles:

  • The protection of troops in the field and warships at sea should have first priority. During the Gulf War in 1991, U.S. forces in the field were attacked by ballistic missiles carrying weapons. In fact, the majority of U.S. casualties in that conflict were caused by an Iraqi SCUD warhead that struck an American barracks building in Saudi Arabia.
  • The most effective time to destroy a ballistic missile is in the boost phase, or ascent, of the trajectory. At that point, the missile is easy to detect because of its large infrared and visible light signal caused by the rocket plume. In other words, in the initial phase of the trajectory, a missile is much "softer" (i.e., easier to destroy) than a warhead entering the atmosphere. Furthermore, if ballistic missiles can be reached in the boost phase, ABM systems designed to shoot down incoming warheads may not be necessary at individual targets around the world. In other words, shooting down ballistic missiles as they are being launched would maximize the defended area.
  • For a number of reasons, the platforms on which ballistic missile defense systems are mounted should be easy to move. First, we cannot know ahead of time where troops will have to be deployed. Moveable defensive systems could be brought in along with the troops or ships to be defended. Second, mobility may be critical to placing ABMs where they could intercept threatening missiles in the boost phase. Finally, mobile ABM systems would provide a significant diplomatic advantage for the United States because they might be used to defend our friends and allies around the world against ballistic missile attacks.
Programmatic Considerations
In this section, I assess the technical status and the potential military value of various programs against the threats I outlined above. I will also discuss three programs, the Tactical High-Energy Laser (THEL), the Airborne Laser (ABL), and the Space-Based Infrared Satellite (SBIRS-High and SBIRS-Low), that are closely related to the development of a defense against ballistic missiles but are not included in the budget of the BMDO.

Tactical Systems
Tactical systems are primarily designed to defend troops in the field and ships at sea against short-range ballistic missiles. The highest priority systems should be Patriot Advanced Capability-3 (PAC-3) and Minimum Extended Air Defense System (MEADS), the Navy Area Defense, and the ABL.

PAC-3 and the related MEADS systems are the ground-based antiballistic missiles systems nearest to deployment. Two tests of the PAC-3 missile with the K-band active seeker radar and a unique, rapid-reaction divert system have been successful. In addition, in 1994 and 1995, there were three successful "hit-to-kill" intercepts by Extended-Range Interceptor missiles (ERINT), the immediate predecessor of the PAC-3 system. MEADS is a ground-based defensive system designed to work against short-range ballistic missiles and cruise missiles. A joint program with several NATO nations, MEADS will use a PAC-3 missile and a European radar system on the ground. The PAC-3 system has been successful technically and should be fully funded and deployed at the earliest possible time. The related MEADS system should also be fully funded.

Navy Area Defense is a system based on the Navy's Aegis fleet of defense cruisers and destroyers. The cruisers are equipped with 122 missile launchers, the destroyers with 90. All of the ships are equipped with S-band radar to perform detection and fire control functions called SPY-1. The missile used for the intercepts, the Standard Missile-2 (SM-2), has undergone a number of upgrades and "block" changes over the years. The SM-2 is equipped with active radar and an infrared seeker called the SM-2 Block IVA. The Aegis system has been tested numerous times as a defense against aircraft and short-range Terrier missiles. The warhead has a small explosive charge to perform a shrapnel kill. A successful hit-to-kill intercept using the SM-2 Block IVA missile was conducted at White Sands Missile Range (WSMR) in January 1997. Several other successful tests at WSMR were conducted in 2000. Because of the positive legacy of the Aegis program, there is good reason to believe that this program will be successful and that it will be an important addition to the nation's ballistic missile defense capability. It should be fully funded and deployed with the fleet as rapidly as possible.

The ABL originated in 1972 with the initiation of the Airborne Laser Laboratory (ALL), a proof-of-concept experiment that involved putting a large carbon dioxide laser on a KC-135 aircraft and conducting a number of flight experiments. The ALL demonstrated, among other things, that a large gas-dynamic laser could be operated successfully on an airplane, that the laser beam could be successfully transmitted through the boundary layer that surrounds the aircraft, that it could be pointed in all directions without undue distortion, and that an optical system could be built to provide fire control and fire direction. The ALL program was successfully completed when the KC-135 equipped with the laser disabled five Sidewinder missiles using a closed-loop fire control system.

In the 1980s, a more capable laser became available, the Chemical Oxygen Iodine Laser (COIL), which operates at a wavelength of 1.3 microns rather than the 10.4 microns for carbon dioxide lasers. Thus, atmospheric transmission would not be a limiting factor. The Air Force, which initiated the ABL program in 1994, planned to put a large COIL laser on a Boeing 747-400 cargo airplane that could be deployed in areas where our forces were engaged in combat and threatened by theatre ballistic missiles. The mission of the ABL aircraft would be to shoot down theatre ballistic missiles during the boost phase of their flight. The estimated range of COIL is sufficient to accomplish this objective in most circumstances. The aircraft would constitute part of the defense system of the warfighting forces against ballistic missiles.

A jitter in the optical system caused by vibration and the spreading of the beam caused by atmospheric turbulence has been addressed by a series of experiments and by extensive calculations. In addition, a number of countermeasures have been considered, such as special construction materials and coatings for the missile. Analysis shows that it would be hard to defeat the ABL by such passive means at militarily interesting ranges. The system performed successfully in closed-loop fire control experiments at the Oscura Peak Laser Test Facility in New Mexico over a path length of more than 50km.

Based on these results, there is every reason to believe that the ABL will work as predicted. Recently, the Air Force drastically reduced funding for the ABL program to cover shortfalls in other programs the Air Force believes should have a higher priority. Some of these reductions have been restored by Congress, and a missile shoot-down is scheduled for 2004. The ABL is the only long-range ABM weapon based on genuinely new technology that has an excellent chance of working as advertised. The ABL program should be fully funded and transferred to the BMDO so that it can be evaluated against other weapons designed to counter tactical-ballistic missiles.

The Tactical High-Energy Laser (THEL) is not part of the BMDO program. For some years, the U.S. Army's Missile Defense Command, in collaboration with the Israeli Air Force, has been developing a laser designed to shoot down Katyusha artillery rockets, Russian-designed guided ballistic missiles with a range of tens of kilometers. The Katyusha is cheap and is available in large quantities from a number of sources around the world. It has been used by Arab forces based in southern Lebanon against targets in northern Israel, hence the Israeli interest.

THEL is a hydrogen-deuterium fluoride (HF-DF) chemical laser that can produce a beam with a wave length of 3.4 microns and a continuous wave beam energy in the megawatt range. Against Katyusha rockets, the THEL has an effective range of a few kilometers. In June 2000, the first THEL (Fire Unit One) was tested at the WSMR Laser Test Facility. The laser promptly shot down a Katyusha in flight by heating the case of the missile sufficiently in one to two seconds to detonate the missile's explosive charge. A few weeks later, the THEL shot down two Katyushas fired within a second or two of each other. This test demonstrated that the acquisition, pointing, and tracking system of the THEL worked well enough to bring down Katyushas fired in a salvo. By September 2000, the THEL had engaged and destroyed 13 Katyusha missiles in flight. Even though the THEL Fire Unit One is not yet a deployable weapon, these tests demonstrate that a mobile THEL would be of great military value. The THEL would be the first practical directed-energy weapon.

Theatre-wide Defense Systems
Theatre-wide defense systems are designed to deal with threats that cover an entire theatre of operations rather than a single tactical situation. These missiles would have ranges of 500 to 3,500 miles, rather than the 300 to 800 miles characteristic of the tactical missiles we have considered so far. Longer range missiles have trajectories that reach altitudes of 500 miles or more, in contrast to short-range missiles that stay mostly within the atmosphere. Thus, ABM systems designed to shoot down missiles that pose theatre-wide threats must be capable of exoatmospheric, as well as endoatmospheric intercepts. The tactical systems discussed in the previous section are intended to perform only endoatmospheric intercepts. For this reason, tactical ABM systems are usually referred to as lower-tier systems, and theatre-wide systems are called upper-tier systems. Two upper-tier systems are discussed in this section, the Army's Theatre High-Altitude Air Defense (THAAD) system and the Navy Theatre-wide (NTW) System.

The THAAD system, the most advanced and the most sophisticated in the BMDO inventory, consists of a very capable X-band radar for fire control and an interceptor missile capable of exoatmospheric and some endoatmospheric intercepts. In addition, THAAD has a good ground-based control computer. THAAD has had a troubled history, however. Eight of the first nine test shots failed. Only in late 1998 and early 1999 were two successful hit-to-kill intercepts conducted at the WSMR. The early failures were caused primarily by poor quality control in the manufacture of the missiles, but a number of design flaws in various missile components also became apparent. Therefore, the test program using the old test missiles was terminated in the summer of 1999, an engineering development phase of the program was initiated during which the missile will be redesigned, and other components of the system will be improved. The THAAD system is a ground-based system that can be moved from place to place, but not easily. The PAC-3 defense system is genuinely mobile; in contrast, THAAD is movable. The THAAD system is the most advanced of the theatre-wide ABM defense systems. The first units will probably be fielded in 2007 or 2008. The program is now properly funded to achieve its objectives.

The NTW is an outgrowth of the Navy Area Defense System described above. The essential difference between the area-defense and the theatre-wide defense systems is a new and more powerful missile called the Standard Missile-3 (SM-3) Block I, which is capable of reaching a final velocity of about 3.5 km/sec and is designed to execute exoatmospheric intercepts. The missile is a three-stage solid-fueled vehicle with a unique solid-state third stage with two combustion chambers that permit a high degree of thrust control. The radar system is based on the SPY-I S-band radar with some upgrades. The high-range resolution radar system will make it possible, using phase-control techniques, to use the SPY-I radar to discriminate between warheads and decoys, in spite of the relatively long wavelengths at which the system operates.

Like the Navy Area Defense system, the NTW missiles and radar are carried by Aegis cruisers and destroyers. The great advantage of the NTW is its mobility. The Aegis ships can be moved easily and can provide defensive cover for any region in the world within a few hundred miles of the sea. Thus, the NTW could become an important diplomatic lever in terms of enabling the United States to defend friends and allies around the world.

Some debate has arisen about the concept of operations for sea-based systems. The Aegis ships were originally intended to defend valuable ships in naval task forces in the open ocean against attacks by relatively short-range air-to-air and surface-to-air missiles. After the end of the Cold War in 1991, the Soviet navy-at least the surface units-were no longer a threat. Thus, the idea that the Aegis cruisers could be modified to add an ABM defense capability became more attractive. Originally, the capability was considered an add-on, but in the past year, the Navy has been considering dedicating some ships in the Aegis fleet to the ABM defense mission rather than combining them. This would simplify software development and probably avoid delays in fielding the first missile-defense-capable ships.

A series of tests of the NTW system, originally planned for a hit-to-kill intercept in 2000 as part of the so-called Aegis Leap Intercept (ALI) test series being carried out in the Pacific to test both the SM-2 Block IVA missile of the Navy Area Defense System and the SM-3 Block I missile intended for NTW, had to be postponed because of technical problems with the solid divert and attitude control system (SDACS), a solid-fueled unit that is the fourth stage of the SM-3 Block I missile. The fourth stage, which contains both the SDACS and the seeker, constitutes the missile's kill vehicle. The SDACS unit has a unique design that is, unfortunately, very difficult to implement. In the past year, several ground-based tests of the SDACS system have been either partial or complete failures.

In an encouraging test conducted on January 25, 2001, the U.S.S. Lake Erie fired a complete SM-3 Block I missile against a target launched from the Barking Sands Missile Range on the island of Kauai. The missile came within a few hundred feet of the target. However, the SDACS was not activated because of the problems I have mentioned, so no hit-to-kill was attempted. Nevertheless, it was a successful test of the missile system and the SPY-1 guidance radar. On February 4, 2001, there was a successful ground test of the SDACS unit. Based on this test, a sea-based hit-to-kill attempt might be made later this year.

One limitation of the NTW missile defense system is that it is capable only of exoatmospheric intercepts. Thus, the differences in trajectories between the warhead and decoys in the upper atmosphere cannot be used to discriminate between them. To assist both THAAD and NTW in exoatmospheric intercepts, a space-based infrared satellite (SBIRS) system is being developed that can detect temperature differences between the warhead and the decoys above the atmosphere.

The SBIRS system consists of two satellite constellations, one in Molnya orbits, which are highly elliptical with the apogee (and thus, the long residence time) above the northern hemisphere, and the other in a roughly circular orbit at an altitude of about 500 miles. The satellites in the Molnya orbits are called SBIRS-High, and those in the circular orbits are called SBIRS-Low. The SBIRS-High system uses an existing constellation of satellites in Molnya orbits to carry the infrared radiation detectors. The primary purpose of SBIRS-High is to detect missile launches. SBIRS-Low satellites can also detect launches, but their more important mission is to discriminate between warheads and decoys by pointing the infrared detector at a point above the horizon through which the warhead and the accompanying decoys will pass. The data gathered in this way is then passed to the interceptor in real time.

The SBIRS program is currently managed by the Air Force. SBIRS-High is expected to be deployed in the near future and is more or less on track. The history of the SBIRS-Low system has been troubled, however, and there have been delays partly because of technical problems but mostly because of the relatively low priority assigned to the development of the system by the Air Force. Based on a number of test flights in the past few years of satellites that have successfully demonstrated the capability of very sensitive infrared detectors mounted on satellites (MISTI-3 and MSX, both satellites flown in 1996 and 1997), there is not much doubt that an operational system that can improve exoatmospheric discrimination can be developed. Because of the importance of exoatmospheric discrimination to the theatre-wide defense systems managed by the BMDO, the SBIRS-Low project should be transferred from the Air Force to the BMDO.

National Missile Defense
Besides the technical problems posed by defending large areas against an attack by intercontinental missiles, the most difficult problem considered in this paper, a national missile defense system must be governed by the ABM Treaty that was concluded with the Soviet Union in 1972. No discussion of a national missile defense system would be complete without considering how it might be affected by the treaty.

The ABM Treaty limits the deployment of defenses against ballistic missile attacks intended to inflict heavy casualties on the civilian population, part of the doctrine of Mutually Assured Destruction that governed the deployment of nuclear weapons by the United States and the Soviet Union during the Cold War. The theory was that neither side would attack the other if unacceptable casualties would be inflicted on the aggressor. Some have argued that the ABM Treaty was concluded at a time when it was not technically feasible to shoot down ballistic missiles, and, thus, neither side gave anything really important away. As Winston Churchill said in a speech to the Parliament in 1955: "We are entering an era in which terror will be the sturdy handmaiden of peace."

The writers of the treaty, however, realized that technology would not stand still and that someday means might be found to build a successful defensive system. Thus, the ABM Treaty not only limits deployments but also forbids conducting certain experiments necessary to the development of antimissile technologies. In other words, the ABM Treaty would have to be renegotiated for the United States to deploy a national missile defense system. In fact, President Clinton initiated a process to negotiate changes in the ABM Treaty. Given the history of these negotiations, success is likely to depend on the kind of system proposed.

The ABM Treaty as originally written permitted the deployment of one ABM system with no more than 100 missiles in the continental United States. The treaty also limited radar systems, space-based fire control systems, and how they could be deployed. The Clinton administration's plan to deploy a ground-based ABM defense system in Alaska with a worldwide network of radars and space-based infrared detectors to detect and track incoming missiles would have required that the ABM Treaty be modified. The Russians and some of our European friends and allies have already expressed serious concerns about the Alaska-based system. In addition, a number of U.S. senators have objected to renegotiating the ABM Treaty. Partly because of these considerations and partly because of technical failures (two successive intercept attempts by a kill vehicle failed in 2000), President Clinton cancelled the proposed deployment late last year.

In my opinion, negotiations with the Russians to modify the ABM Treaty to make the deployment of a national ABM defense system possible should be initiated as soon as possible. President Clinton clearly understood this when he raised the matter with Russian President Vladimir Putin at a meeting in June 2000. As expected, President Putin's reaction was negative. Even if the likelihood of reaching an agreement with the Russians is small now, it is vitally important that we do whatever is necessary to find out their current thinking on this subject. In addition, I believe it is very important that we look for an alternative to a land-based system that might be militarily more effective and that might also be more acceptable to the Russians because it would not threaten its land-based strategic deterrent rocket forces.

An attractive alternative to the land-based system based in Alaska might be a modification to the NTW proposed by a group at the Lawrence Livermore National Laboratory. The modification would provide the NTW system with a limited capability of shooting down intercontinental ballistic missiles in the boost phase, or ascent, of their trajectories by adding a fourth stage to the SM-3 Block I missile that would raise the burnout velocity of the missile from about 3.5 km/sec to about 5.5 km/sec. The total velocity increment of about 2.0 km/sec would leave 0.5 km/sec for maneuvering during the end game of the intercept. The fourth stage would be a small, pressurized, hydrogen peroxide rocket or one that would use a hypergolic mixture for propulsion. An important feature of the fourth stage is that it would fit into the same shroud on the missile that houses the current seeker and kill vehicle. The fourth stage would also weigh about the same as the currently existing kill vehicle. Thus, and this is the important point, the proposed upgrade could be made without any other significant changes to the missile. Figure 1 shows the trajectory diagram for an interceptor with a burnout velocity of 5.5 km/sec. An Aegis ship-based SM-3 Block I missile with the proposed liquid-fueled fourth stage would be capable of performing ascent and boost-phase intercepts of missiles launched as far away from the ship as 1,000 km (600 miles). With a space-based sensor providing the missile launch cue, the SPY-1 radar could provide guidance to the intercept point. No discrimination capability would be necessary if the intercepts were performed while the missile was in the boost or ascent phase.

Ship-based ABM systems designed to protect large territories and population concentrations are also prohibited by the ABM Treaty. However, the Russians might be persuaded to accept a sea-based system rather than the Alaska-based system because a sea-based system with an SM-3 Block I missile with a burn-out velocity of 5.5 km/sec could not possibly seriously threaten Russian or Chinese strategic land-based nuclear missile forces. Those missiles would be launched from silos beyond the range of the sea-based SM-3 Block I missiles. Aegis ships equipped with SM-3 Block I missiles plus the liquid-fueled fourth stage could deal with threats from North Korea and the Middle East because all of the conceivable launch sites would be close enough to the sea. Figure 2 shows the region an Aegis cruiser would have to occupy to shoot down a Taepo Dong missile launched at the United States from Pyongyang. The area increases as a function of the burnout velocity of the interceptor missile. The Aegis ships could not defend against a Chinese or Russian threat because the launch areas would be too far inland. To hit missiles launched from these sites, new ships with larger launch tubes would have to be developed. This argument would probably make more sense to the Russians, and eventually to the Chinese, than a guarantee on our part not to expand a land-based system.

The proposed fourth stage of the SM-3 Block I missile is a relatively small item that would probably require between $200 and $300 million for the development of hardware and tests necessary to make sure it works. The BMDO has allocated a small fund to study the possibility of developing the liquid-fueled fourth stage for the SM-3 Block I missile.

The arguments in favor of a sea-based ABM defense system are compelling. Unlike land-based defenses, ships are mobile and can, therefore, be moved easily. In addition, for the most likely threats (from North Korea and the Middle East), the ships could be placed so ascent and boost-phase intercepts would be possible. The alternative I have outlined should be seriously considered and carefully analyzed before a commitment to a purely land-based system is made. I believe very capable land-based high-frequency radars and sea-based missile launchers would be the best combination for an ABM defense system.

Summary and Recommendations
Earlier in this paper, I listed three principles that should govern the development of the architecture for the national ABM defense system: (1) the defense of troops in the field (an operational consideration); (2) intercepts in the ascent or boost phase of the missile trajectory (an organizational military and technical consideration); and (3) mobile launch platforms (an operational and diplomatic consideration).

At the beginning of this paper, I pointed out that all of the current BMDO programs have funding problems that could lead to serious compromises in U.S. capability. The PAC-3 system does not have enough missiles, and deployment delays have occurred with THAAD and the Navy's Area Defense system. Deployment of the NTW is not funded at all after fiscal year 2002. Finally, the ABL and the SBIRS, which are critical to the architecture of the ABM defense system, are included in the Air Force budget but are not properly funded. All of these funding problems could be solved by reallocating the funds currently allocated to the cancelled Alaska-based system. In order of priority, I recommend that the funds from the cancellation of the Alaska-based system be used for the following purposes:
  • We should fully fund the PAC-3 program by funding the number of missiles originally planned and also the missiles allocated to MEADS. We should also help the European partners of the MEADS system with the development of the radar and fire-control systems.
  • We should fully fund the ABL and THEL. These weapons, the first fielded directed-energy weapons, promise to be important tactical assets and would be a unique addition to our arsenal. These weapons are based on genuinely new physical principles.
  • We should make the decision now to fund the NTW fully. The allocation of these funds should be determined by the results of the ALI tests that will be conducted later this year. Fully funding the NTW would include accelerating the development of SBIRS-Low and the fourth stage of the NTW to defend the United States against attacks by a limited number of intercontinental ballistic missiles launched from the most likely threat areas.
  • We must immediately revive negotiations with the Russians to modify the ABM Treaty. If, after a reasonable period of time, these negotiations fail, the United States should seriously consider unilaterally abrogating the ABM Treaty.
About the Author: Hans Mark, NAE, is professor and John J. McKetta Centennial Chair in Engineering at The University of Texas at Austin. He was director of Defense Research and Engineering, U.S. Department of Defense, from 1998 to 2001.