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Technical Aspects of Ballistic Missile Defense

Richard L. Garwin

Presented at Arms Control and National Security Session, APS, Atlanta, March 1999

ABSTRACT Hundreds or thousands of ICBMs armed with nuclear warheads could destroy the United States; even a few could kill millions of people. Blocking or deterring the acquisition or use of such capabilities is highly desirable. This talk, however, deals with the technical problems and prospects for effective defense against ICBMs-- in boost phase, mid-course, and during and after reentry. Topics include detection, tracking, guidance, and the ability to destroy the target or render it harmless. For interceptor missiles: multi-stage propulsion, sensors, and guidance. For directed-energy weapons: generation, propagation, and kill. The revolution in microelectronics and computing, and the experience of decades render some of these problems trivial (in principle), but effective countermeasures vitiate many approaches. Three durable countermeasures against non-nuclear intercept include release of submunitions on ascent, large enclosing balloons, and anti-simulation.

Introduction
The guided ballistic missile was first used in warfare September 8, 1944-- the German V-2, bombarding English cities from the continent. The predecessor to the V-2 was the V-1, an early cruise missile with a pulse-jet engine (the "buzz bomb") that British defense forces became quite effective in intercepting. Toward the end of the V-1 campaign, British fighter aircraft would fly next to the buzz bomb and with the wing tip of the fighter would tip the wing of the buzz bomb in order to divert it into less populous areas. It was fortunate that the V-1 did not have a simple contact sensor to cause it to explode when touched in flight. There was no defense against the V-2, except to attempt to destroy it before launch; an effort out of all proportion to the damage caused by the V-2 was expended to this end.

On a flat, non-rotating Earth, without an atmosphere, a ballistic missile attains maximum range for a given launch speed with an elevation angle of 45 deg, and the range increases as the square of the initial velocity, as is evident from high school physics, R = V^2/g; for R in km, V in km/s, g is 0.0098 km/s^2

For missiles of intercontinental range, the fact that the Earth is almost spherical is important, since an initial velocity of 8 km/s that would correspond to a range of 6530 km on a flat Earth now corresponds to infinite range, since it is the velocity required to enter low-Earth orbit (LEO). Using conservation of momentum in the frame of the accelerating rocket (with velocity V(t) and mass M(t)) leads to the rocket equation

dV/dM = -V_e/M --> dV/V_e = -dM/M --> \Delta V/V_e = -\Delta ln M --> M_o/M_f = e^{\Delta V/V_e} (Eq. 1)

where M_o and M_f are the initial and final masses, and \Delta V is the "velocity gain" of the rocket. If Eq. 1 is used in the approximation of continuous staging, the Ve that appears in the formula is reduced from the actual Ve to compensate for the fact that staging is discrete and for the structure discarded at each stage. The limiting speed of a single-stage rocket is simply the rocket nozzle exhaust speed V_e times ln (M_o/M_f). For a rocket with no payload, the ratio M_f/M_o is the "structure factor". In the rocket literature, propellants are characterized by their "specific impulse"-- Isp, measured in seconds, which is simply V_e/g.

It should be observed that even though the mass that can be propelled by a rocket to a given high velocity is exponentially smaller as this velocity increases, according to Table 1 the efficiency of conversion of the thermal energy of the rocket fuel to kinetic energy of the payload remains high-- to velocities well beyond those of interest in ballistic missiles.

The first row is the final velocity V_f; the second \alpha. is the ratio of final velocity to rocket exhaust velocity V_e. The third row is the mass ratio m from the rocket equation, while the fourth row is the energy efficiency in converting to kinetic energy of payload the internal energy of a mass of propellant equal to the initial mass of the rocket less the final payload. Eq. 2 shows the relevant formula,

\epsilon == (K.E.)/(P.E.) = (1/2 M_fV_f^2)/(1/2 V_e^2 (M_o-M _f)) (Eq. 2)

Table 1: for final velocity Vf achieved by rocket propulsion with exhaust velocity V_e = 3 km/s, the payload fraction is \mu and the fraction of fuel total energy present in the payload kinetic energy is &epsilon..

with numerical values tabulated in the fourth row of the Table.

Table 1 assumes practical "staging", without which there is a firm limit on the speed that can be achieved, even with zero payload.

A "Scud" is a ubiquitous single-stage liquid-fueled missile manufactured in the Soviet Union or indigenously in many nations of the world. Hundreds of Scuds were exchanged between Iraq and Iran, and scores of "extended-range Scuds" armed with high explosive were launched by Iraq against Israel and Saudi Arabia. Iraq had lengthened the fuel tank of the Scud and thus increased its range in producing the al-Husayn missile.

THEATER MISSILE DEFENSE IN ACTUAL WARFARE. (1) On January 18, 1991, a U.S. Patriot air defense system launched its interceptors against an al-Husayn attacking an air base in Saudi Arabia, but it soon became evident that the al-Husayn warhead was not an easy target. The modifications to the Scud had affected the stability of reentry (which normally takes place in these missiles with the warhead still attached to the spent missile), so that the warhead broke off and descended in a tight helix, greatly reducing the effectiveness of the intercept. The missiles usually broke up into three pieces-- the warhead, the fuel tanks, and the

1. A useful reference is IEEE SPECTRUM, September 1997, "Ballistic Missile Defense: It's Back", with 5 articles on BMD. Also J. Pike, "Ballistic Missile Defense: Is the U.S. 'Rushing to Failure?', Arms Control Today, April 1998, Vol. 28, No. 3, p.9-13. (Available at http://www.armscontrol.org/ACT/april98/pikap98.html)

rocket motor, but the Patriot radar and computer were usually able to distinguish the warhead from the other potential targets.

The Patriot system uses a ground-based phased-array radar that continually scans in a flexible fashion a sector of the sky for approaching objects. It characterizes them and, either automatically or via manual intervention, launches one or two interceptors against the aircraft or missile. The interceptor itself is tracked by the radar and steering commands are automatically generated and transmitted in order to intercept the warhead within the atmosphere. The Patriot interceptor is endo-atmospheric, since it relies on aerodynamic forces to maneuver. The interceptor itself carries a receiver of the signal transmitted by the radar, so that in the vicinity of the offensive warhead the interceptor picks up the reflected radar return and relays it to the radar; "track via missile" approach greatly improves the performance of the interceptor against the targets for which it was designed. At the optimum time, high explosive in the interceptor warhead is detonated by the fuzing system, spraying the incoming warhead with steel pellets.

It is entirely reasonable to have a defense against warheads armed with high explosive, just as it is worthwhile to defend against aircraft delivering such weapons.(2)

U.S. defense against theater ballistic missiles is being improved with the substitution of the Patriot Advanced Capability-3 (PAC-3). This is a smaller interceptor that employs "hit-to-kill" technology and is to destroy the incoming warhead by the kinetic energy of its collision rather than by explosively driven fragments. A similar Navy system with endo-atmospheric capability is under development, with the first units being operational in late year 2000 and 2002 respectively.

Enlargement of the area defended from a single interceptor site can be achieved with intercepts outside the atmosphere, and an Army theater high-altitude air-defense (THAAD) system and corresponding Navy theater-wide system are to be available in 2008 and 2010 respectively. Of course, radars

2 I judge that no more than a small fraction of the offensive warheads were destroyed by Patriot intercept during the Gulf War, based on a review of the work published by T.A. Postol and his collaborators at MIT, as well as analyses published by Raytheon, the Patriot builder. See G.N. Lewis and T.A. Postol, "Video Evidence on the Effectiveness of Patriot during the 1991 Gulf War," Science and Global Security, Vol. 4, no. 1, 1993, pp. 1-63.

must be able to detect and to discriminate the threatening warheads at greater range for these systems to be effective.(3) And they must also perform technically against targets that may have features that reduce the effectiveness of the defense-- i.e., countermeasures.

Defense Against ICBMS
A typical intercontinental ballistic missile (ICBM) will have a range of 8000-10,000 km and a speed of around 7 km/s. Minimum propulsion for a given range is achieved with a reentry angle closer to 22 deg than to 45 deg because the range is not small compared with the radius of the Earth. Of course, the atmosphere of depth 10 tons per square meter exerts a substantial drag on the ballistic missile as it rises, so that a practical design does not achieve intercontinental speed within the dense atmosphere; this is accomplished by limiting the thrust and hence the acceleration of the missile. This also limits the "dynamic pressure" and the skin heating on ascent. But too low an acceleration implies excessive "gravity loss", since it is only the excess of vertical component of thrust over the force of gravity that actually results in acceleration of the missile. Even so, a typical large liquid-fueled missile may have a thrust of 1.3 times its gross weight, so that initially only 0.3/1.3 of the thrust accelerates the missile just after liftoff.

The kinetic energy of the RV, which is a good fraction of the total energy of the propellant of the entire ICBM, must be dissipated on reentry. ICBM warheads must be protected by a reentry vehicle (RV) against the heat and deceleration of the atmosphere. A typical peak deceleration is on the order of 60 g. However, only a small fraction of this energy need be absorbed by the material of the RV-- the rest being carried off by the wake of the reentry. Although initial ICBM RVs used a heat-sink approach, this was soon superseded by a much lighter protection system that uses ablative material that gradually sacrifices its heated surface layer and erodes in a controlled fashion on reentry. The RV shape approximates a sharp cone with a small nose radius.

An ICBM of concern to the United States is usually protected in a silo or concealed inside a mountain. It might need to be removed via horizontal tunnel and erected before launch, or it might be stored vertically, and launched in that position after opening a protective door. The missile can

3 See footnote re MIT charts.
be ignited in the silo, or it can be ejected more gently and ignited as it emerges. The first-stage booster, when fuel is exhausted, is then separated by explosive bolts, as are eventually the second stage and the third stage, so that the warhead travels on its own to the reentry point. In order not to prejudice accuracy because of the uncertain orientation of the warhead on reentry, the warhead can be fitted with monopropellant rocket jets to force it to pitch over to assume the appropriate orientation for reentering along its longitudinal axis. It is then often "spun-up" by additional jets, in order to maintain that orientation for the remaining 20 minutes or so of flight. Alternatively, the alignment with velocity might be delayed until the beginning of reentry, and the RV spun up at that time.

The United States and other countries with multiple warheads or "penetration aids" often use a "bus" (more formally a PBV, for Post-Boost deployment Vehicle) which carries the guidance unit for the missile and which has the job of accelerating each of the warheads (or decoys or penetration aids) sequentially to the proper velocity so that it will fall to its particular target. Bussing may take five to ten minutes in order to distribute the warheads and appropriate decoys to accompany them.

A ballistic missile defense system could interfere with the ICBM either before launch, or in boost phase (during the operation of the first, second, or third-stage engines) during the bus activity, or it could counter a warhead either above the atmosphere or on reentry, until it achieved its detonation altitude. The ICBM presents different vulnerabilities and opportunities for intercept in its various stages:

Pre-boost-phase Intercept. The United States tries to learn where all potentially threatening missiles are based. It could destroy them preemptively in the case of hostilities. However, some ICBMs are mobile, and if they are out of garrison and not otherwise observed, they are not vulnerable to such attack. Even if the location of the mobile launcher were known at the time of launch of ICBM, if it were in motion it would be safe from destruction by the nuclear warhead on a ballistic missile. But if the mobile launcher could be tracked continuously, then updates could be sent to a maneuvering warhead; the amount of divert fuel required against a moving ground target is negligible since the /Delta V is rigorously(4) less than twice the maximum velocity of the ground vehicle.

4 F.J. Dyson & W. Press, "Transverse Boost Requirements for ICBM Targeting of a Maneuvering Aircraft," Appendix A from JASON report JSR-79-03 (1979).

Boost-phase Intercept. The United States maintains in high-Earth orbit a set of Defense Support Program satellites (DSP) which for decades have reported in real time every ballistic missile launch of significant size. It was revealed officially that the U.S. observed in this fashion every Scud launched during the Gulf War. With a 6000-element linear infrared sensor that rotates once every ten seconds, DSP can determine the launch point with an accuracy on the order of a kilometer. Since a typical ICBM burns for about 250s, multiple observations are possible and pretty good trajectory information can be obtained in this way.

In the early seconds of boost, an ICBM is vulnerable to a command-detonated mine adjacent to the site or to a rocket-propelled grenade. Even short-burning Scuds could be destroyed by small homing interceptors launched by radio from as much as 50 km distance from the launch site. Normal ICBMs would be vulnerable in boost phase to ground-based interceptors (GBI) (or sea-based interceptors) from anywhere within a region of about 1000 km of the launch site. Such an interceptor would be launched by command on the basis of DSP data, without there ever having been a radar detection of the ICBM. Fitted with a sensor capable of detecting the missile flame, it could direct its limited field of view in the direction commanded according to the data from DSP, and accelerate toward a predicted intercept point. The prediction would need continued refinement, by observation from the interceptor of the current position of the ICBM booster.

But the interceptor would have to be launched from a site sufficiently close and have sufficiently high performance in order to reach the missile while it was still burning. Furthermore, the interceptor could not simply home on the flame but in the late stages of intercept would need to look "ahead" of the flame, in order to strike the solid missile and not sail harmlessly through the tenuous flame. This could be done either by blind reckoning because of the known shape of the flame, or by actual detection of the solid missile with a proper design of the interceptor seeker.

Because of the ocean area east and north of North Korea, North Korean ICBMs aimed at the United States are an ideal target for ground- or sea-based boost-phase intercept. Specifically, it should be possible to use an interceptor of the same gross launch weight as the GBI of the NMD program (about 14 tons, with 12.5 tons of solid fuel) to boost the kill vehicle (of perhaps 60 kg mass and containing some 15 kg of liquid fuel) to a speed similar to that of the ICBM-- 7 km/s, but with larger engines relative to the mass, so it will reach its final speed more rapidly. A simple calculation shows that the sea-based interceptor could be deployed as much as 2100 km downrange from the launch site and still be able to catch the ICBM while it is still burning. We assume a burn time of 250 s to ICBM speed of 7 km/s (an acceleration of three times that of gravity-- "3 g") while the interceptor acquires 7 km/s in 100 seconds--an average acceleration of 7 g. Because the interceptor must rise vertically in the lower atmosphere, it probably moves only about 250 km toward its target while it is burning, and then in the remaining (250-100) seconds moves some 1050 km. So in the burn time of the ICBM, the interceptor can reach out a total of 1300 km from its launch site. The ICBM at an average speed of 7/2 = 3.5 km/s in 250 s moves no more than 875 km from its launch site. The interceptor could be deployed 1100 km east or west of the ICBM trajectory, about 800-1000 km downrange. So there is plenty of room for U.S. navy ships to carry these interceptors. The ships need have no missile-tracking radars.

Such a sea-based boost-phase intercept system is not compliant with the 1972 ABM Treaty; but Russia and the three other parties to the Treaty might well agree to a specific exception, especially if this were combined with progress on lower missile levels in Russia and the United States. My own judgment is that the ABM treaty plays a valuable role in U.S. national security and in the reduction of Russian nuclear weapons, and that it should not be abandoned lightly. Alternatively, Russia and the U.S. might each deploy 15 test interceptors at a new joint ABM test range south of Vladivostok.

The Air Force is developing an airborne laser (ABL) for boost-phase intercept. It is a chemical oxygen-iodine laser mounted in a Boeing 747 aircraft that has the task of focussing through the turbulent atmosphere (further disturbed by the passage of the laser beam itself) in order to weaken or melt the structure on a missile during boost phase. The ABL laser operates at 1.317 \mu m, perhaps(5) at a power level of 3 MW. The ABL will operate at an altitude of 13 km-- most of the time above the clouds and, assuming that the laser works as planned and that laser beam propagation is as assumed, Forden assesses the range for "decisive engagement" from 320 km for the al-Husayn to 185 km for the North Korean Nodong 1300-km-range missile. This assumes that a substantial arc of the missile skin must be softened so that the missile collapses. For a less catastrophic criterion, Forden estimates the range limit from the missile launch point to be 320 km, and 1000 km or more for an ICBM (assuming the ABL downrange from the ICBM launch and so can attack at closer range). He notes that

5 G.E. Forden, IEEE SPECTRUM, Sept. 1997, "The Airborne Laser", pp. 40-49.

ABL could be stationed west, south, and east of North Korea for use against Nodong missiles and ICBMs.

BUSSING. This phase has no particular vulnerabilities and will not be further discussed. If one catches the bus toward the end of its maneuvers, one can counter in this way only a fraction of the RVs. Catching it at the beginning seems less likely than doing the boost-phase intercept.

Mid-course and Exo-Atmospheric Intercept. RVs falling through space are on a highly predictable trajectory, so that repeated radar observation, for instance, can refine that trajectory and contribute to the intercept capability. Intercept of a single RV is simpler with a nuclear-armed interceptor, with an effective kill range measured in kilometers. Several long-range weapons effects can be important in this regard(6) -- x-ray-induced blowoff of the external surface drives a shock into the material of the RV and can damage the warhead by "spall" or by deformation of the structure; the fissionable material can be melted by neutrons from the interceptor thermonuclear warhead.

Large and powerful radars are required to see a reentry vehicle at ranges of several thousand km, and such have long been deployed in the ballistic missile early warning system (BMEWS) in Alaska, Canada, Greenland, and Britain. The criterion is simply that the radar direct enough energy on the RV within the required search time for the reflected energy to the radar to exceed thermal noise. This is embodied in the radar equation:

E_s = E_o \sigma/R^2 d\Omega, (Eq. 3)

where the solid angle d\Omega = \lambda^2/A into which the radar energy is emitted is related to the antenna area A and the wavelength \lambda, on the assumption that the antenna is diffraction limited. The energy Es scattered by the target is proportional to the effective back-scatter cross section &sigma.. The energy received from the radar antenna

E_r = E_s A/4\pi R^2 = E_o A^2 \sigma/(4\pi R^4 \lambda^2)=NkT (Eq. 4)

where R is the range to the target, kT the thermal energy, and N the margin required for losses and to make thermal excursions above the signal threshold sufficiently rare.

Radar and detection theory are highly evolved and can take into account the fact that reentry vehicles do not suddenly appear in space.

The RVs have an easy ride through the vacuum of space, and an ICBM launch for which the RVs do not wish to be seen or identified can provide a vast amount of clutter or chaff

6 Bethe, H.A. and Garwin, R.L. "Anti-Ballistic-Missile Systems," Scientific American 218, 3, pp. 21-31, March 1968. that can mimic the radar return from an RV. Early-warning radars usually operate in the VHF/UHF range (typically 400 MHz), so that a half-wave dipole is 37 cm long. Vast numbers of such dipoles can be formed of metal, glass, or carbon fibers with a metal coat, and can thus provide a substantial radar return that masks real RVs. Similar radar clutter can be provided by inflated balloons of metal-coated plastic, and, for good measure, one can put a balloon around the RV itself-- a simple form of "antisimulation" to enable the RV to simulate a decoy that is easy to make.

The other approach to wide-field detection of RVs in space is via their thermal (infrared) radiation. An RV at the temperature of the Earth (with a black-body surface) radiates about 400 W/sq m, and in a 1-\mu m band centered at 10-\mu m about 40 W. In a typical example,(7) a 10-cm diameter telescope at a range of 1000 km from an RV with a black-body radiating area of 1 m^2 would collect about 1000 photons in a millisecond dwell time, for a scanning line of 5000 detectors. The focal plane could be cooled to liquid hydrogen temperature to reduce the self-generated "noise" from thermal radiation in the detector itself. In the modern era, one can use a "staring" array so that longer integration times are possible, although the motion of the light source across the visual field (because of relative motion of the RV and the observer) limits the integration time that can be used in this simple fashion.

Such telescopes are planned to be mounted eventually on the space-based infrared system in low-Earth orbit (SBIRS-low) which would observe in the thermal ir looking at warheads against the black background of space. (Of course many stars will be observed, but they are readily discriminated from warheads because they do not move.)

Detection of an RV from two or more satellites will fix it in space at the intersection of the two lines of sight, so that an interceptor can be directed accurately toward the RV which is moving in a Keplerian orbit. As the interceptor approaches, even a relatively crude ir telescope in the interceptor will be able to detect the RV. In the late stages of intercept, as the RV and interceptor coast into collision at a relative velocity typically of 11 km/s, small adjustments must be made in order to collide with the center of the RV, and not have a brushing collision or a total

7 S. Weiner,"Systems and Technology" in "Ballistic Missile Defense", A.B. Carter and D.N. Schwartz, Eds. (The Brookings Institution, 1984). This volume is an accessible compilation of technical and strategic aspects of BMD.

miss. The attached charts(8) show some of the elements involved in the modeling of the performance of an exoatmospheric kill vehicle (EKV) as a function of the assumed initial offset of the RV (the "impact parameter").

Endo-Atmospheric Intercept. Technically, intercept within the atmosphere is easier for the defense because the ICBM warheads are highly visible to radar and to optical sensors, because of the very hot "wake" produced by the Mach-23 RV as it enters the atmosphere. Balloons and light chaff(9) are no longer effective against sensors, because they will be retarded or destroyed on reentry. Within the atmosphere it is more difficult to make survivable and effective decoys that match the deceleration of the RV containing a nuclear warhead. And the interceptor can undertake much more aggressive maneuvers by aerodynamic force than it could conveniently with rocket propulsion in space.

On the other hand, the RV is decelerating rapidly rather than existing in a well-defined orbit; it may also be maneuvering violently, whether intentionally or not. Sensors on the interceptor are much more difficult, since its high speed through the atmosphere requires heat resistant windows and adds greatly to the background in detecting infrared from the RV. Radars must be more closely spaced to see RVs down to altitudes of reentry, and interceptors cannot drive out hundreds or thousands of km through the atmosphere. So while endo-atmospheric intercept is important for defense against missiles of theater range, it is of little interest in the context of a national missile defense of the U.S.

The Proposed U.S. National Missile Defense. The NMD system under development by the Defense Department, according to Lieutenant General Lester L. Lyles, USAF, Director of the Ballistic Missile Defense Organization (02/24/99) "would have as its primary mission the defense of all 50 states against a small number of intercontinental-range ballistic missiles launched by a rogue nation." But General Lyles goes on "Such a system would also provide some residual capability against a small accidental or unauthorized launch of strategic ballistic missiles from China or Russia. It would not be capable of defending against a large-scale, deliberate attack."

8 Yingbo He, "THAAD Interceptor and ABM Demarcation...", Presented at 6th ISODARCO-Beijing Symposium on Arms Control, Shanghai, October 1998 (used by permission of Yingbo He). 9 Illustrations in talk are from the MIT Defense and Arms Control Studies Program, Technology Working Group, used by permission of G.N. Lewis and T.A. Postol.

As described by General Lyles, the NMD system is intended to use a ground-based interceptor launched from a site within the United States (North Dakota or Alaska) to strike reentry vehicles above the atmosphere. With North Korea as an example, in order to be specific, these ICBMs would have been launched toward the North in order to fall on the United States. The rocket launch flame will be detected by the Defense Support Program (DSP) satellites in geosynchronous orbit within considerably less than a minute after launch, and an approximate location of the launch site and a direction of the missile is established in that way.

The upgraded ground-based early-warning radars operating typically in the frequency band of 420-450 MHz would some minutes later detect the threat missile and on the basis of these data confirming the DSP information, interceptors would be launched. While the interceptor is in flight, a ground-based X-band (10,000 MHz) radar with better resolution will track the reentry vehicles and to some extent discriminate them from other objects put into space by the missile (perhaps intentional decoys, certainly other parts of the missile) to guide the interceptor close enough to the target missile for the interceptor's sensor to acquire the warhead and "to discriminate the warheads from potential decoys." Several interceptors would need to be launched at each warhead in order to achieve the NMD requirement to have high confidence in no ICBM warheads impacting on U.S. soil.

In fact, there is no specific design for the NMD, as it is still evolving. Against the simplest threat of a few "rogue nation" ICBMs without countermeasures, it is expected that 20 GBI would be deployed, either at the Grand Forks (North Dakota) site the U.S. specified under the 1972 ABM Treaty, or at a site to be selected in Alaska. Against ICBMs launched from North Korea, the Alaskan site would give better protection to dwellers of Alaska and Hawaii, about 0.7% of the population of the U.S..

The three-stage GBI would deliver the exo-atmospheric kill vehicle (EKV) on a near-collision course with the target. The EKV is to have a multi- (possibly 4-) band ir telescope as well as a focal plane for visible light and is to be capable of transverse ("divert") accelerations of 3 g or more. The task of colliding with the RV is daunting-- especially at closing speeds of 11 km/s, but the real difficulty would arise from lack of cooperation by the RV. In July 1998, the nine-member Rumsfeld Commission to Assess the Ballistic Missile Threat to the United States (on which I had the privilege to serve) issued its unanimous report, judging that North Korea could have a true (but unreliable and inaccurate) ICBM within a couple of years-- specifically within five years of a decision to move forward with a program, assuming that it is thoroughly funded with a high priority. The Rumsfeld Commission also advised that there were other and earlier threats from missiles of shorter range launched from ships, and observed also that BW or CW agents could be packaged in the form of bomblets released early in flight, that would fly separately to the target region.

I elevate this last conclusion to the status of a likelihood. It is far more effective militarily for an ICBM payload of biological warfare agents to be arriving in the form of individual reentry vehicles (bomblets) spread over an area 10 or 20 kilometers in extent, rather than to be delivered as 100-500 kg of BW agent at a single point in the target area. Under the latter condition, a very narrow plume will be produced by wind-born BW, threatening people within the narrow plume. But if the same payload were dispersed in the form of bomblets, a large number of such narrow plumes, each equally lethal within its interior, would threaten people in the target area.

Given this undisputed increase in military effectiveness, any nation with the capability to make an ICBM and reentry vehicles would almost surely arrange to package the BW in the form of bomblets, released just as soon as the ICBM reached its final velocity on ascent. Placing the bomblets at predetermined positions in a rack within a spinning final stage, the release of the bomblets would then allow them to spread during their 20-minute or more flight to reentry, with the initial rotation rate determining precisely the spread, and the pattern being that in which the bomblets were stored in the missile.(10) This threat of BW bomblets released on ascent is to be expected whether or not a defense is deployed, but the proposed NMD would have strictly zero capability against these bomblets. First, there would be so many of them (with a loading of perhaps 1 kg of agent per bomblet) that it would exhaust any planned number of interceptors.

If North Korea obtains fissile material either from its own reactors or from abroad, so as to make a nuclear weapon that could be carried to intercontinental range by an ICBM, it would initially have what is probably an unreliable warhead on an unreliable missile. The warhead would be likely to miss its city target entirely. But would a defense make any

10 Those bomblets ejected to the side will land at ICBM range almost as if the displacement grew linearly with time, but those dispensed in the plane of the minimum-energy trajectory will land at the same point as the RV, but at an earlier or later time. To obtain a circular pattern from bomblets dispensed in an expanding circle requires the axis of rotation first to be tilted with respect to the velocity.

difference? Yes, if the launching country cooperated, but not if it wished to prevent the intercept of its nuclear warhead.

Because the NMD interceptors are all "hit-to-kill" so that they must collide with the warhead in order to destroy it, the attacker need not conceal the existence of the warhead but only its exact location. This is readily done by the use of an enclosing balloon made of aluminum-foil coated mylar that can be put together by anyone who buys this article of commerce and a hand-held tool for heat sealing the plastic to make a large balloon. Even a balloon ten meters in diameter, inflated after the RV separates from the missile, would render it unlikely that an interceptor would actually strike the warhead rather than plunging harmlessly through the balloon.

The balloon would be inflated in space by a tiny charge of gas-generating compound like that found in every automotive air bag, but instead of deploying in a 100th of a second or less, the balloon could deploy in a second. Since the launch country might fear that the interceptor striking the balloon might cause sufficient disruption to expose the RV, several balloons in sequence could be shrunk down on the RV (and would occupy very little space with the air removed by an ordinary vacuum cleaner). So each would be ready for deployment to hide the RV once again in case the balloon was intercepted.

Alternatively, the launching country could deploy ten or more such balloons over a region 10 km or more in extent, so that these would need to be attacked one at a time. Even the dynamics of a balloon bouncing around over an enclosed object could be simulated in the decoy balloons with an enclosed object that weighed extremely little in that case-- a heavier, small balloon just big enough to enclose the RV in the one balloon in which it exists. This is an example of the utility of "antisimulation", in which the warhead itself is modified to make it easier to simulate by a cheap and convenient decoy. Thus, it may be desirable to choose alternative approaches that make decoys easier to produce than to copy the "bus" concept. One approach is to have the warhead rotate only slowly in space, and then to align itself with its velocity as it begins to reenter the atmosphere.

The interceptor would normally track the RV by means of its infrared (heat) emission and it could readily distinguish an empty balloon from a balloon containing the RV, simply because the empty balloon would be colder, while the RV would not have had time to cool off during its 30 minutes or less of flight. But highly reflective aluminum not only reflects light (and infrared) but (by reciprocity and Kirchoff's law) it correspondingly radiates a lot less-- about 30 times less than does an unprepared surface. Furthermore, multi-layer insulation is an article of commerce that can reduce the emitted heat by another factor 50 or more. Finally, if the decoy maker wished to have even greater confidence that sensors would not be able to discriminate the decoy balloon from the balloon containing the RV, a small commercial lithium-metal battery weighing about 0.3 kg could be used to mimic the 40 watts of heat that would be emitted by the reentry vehicle shrouded in multi-layer insulation within its own balloon. The attached charts show a different approach-- in which a shaped shroud is made less visible at long range to the ir-homing telescope on the interceptor itself.

These achievements are easy relative to the scale and cost of the effort required to develop an ICBM, and if a country expects the United States to have this NMD at the time of its first ICBM, then I am confident that these countermeasures can and will be provided.

Conclusion

The field of ballistic missile defense is full of fascinating problems of physics and engineering, but to have an effective defense requires attention to what the other side can do to defeat the system-- countermeasures. Since it is a big effort to analyze every plausible option and to choose the best, in a small program it is often better to choose an approach that gets the job done and avoids the cost and delay of the universal analysis. The nature of the defense the U.S. might build and its effectiveness depend critically on the type of countermeasures that might be encountered, in conjunction with antisimulation that facilitates the countermeasures job. A realistic assessment is necessary before a decision is made to build a defensive system and before large expenditures lock in a system that might be ineffective, to the detriment of approaches that are less susceptible to countermeasures.

____________________________________________________________ Final

Richard L. Garwin
Thonas J. Watson Research Center (IBM)
P.O.Box 218, Yorktown Heights, NY 10598-0218
(914)945-2555, RLG2@watson.ibm.com

Missile Defense in Modern War

Gregory H. Canavan

It is now agreed that the US could be subject to missile threats within a decade; the Rumsfeld Commission argues that attacks from "rogue" states could occur in a few years. As the timing of these developments is uncertain, and the credibility of the organizations responsible for monitoring them has been eroded by recent events, a prudent hedge appears to be to provide some capability as soon as possible for as wide a range of threats as possible. However, it is also prudent to consider how those threats could evolve in time to assure that these development paths are adequate for a range of possible attacks. While nuclear and other weapons of mass destruction are likely, terrorist attacks from within, chemical and biological weapons, short-range missiles from ships close to shore, and others are no less pressing.

Missile threats will grow as long as there is no effective counter to the free ride they now enjoy. Planned defenses rely on the search and interceptor technologies found to be inadequate for similar threats in earlier decades. Satellites can provide better detection and warning, but cannot compensate for inadequate basing. Space- or ground-based boost-phase interceptors could overcome these problems and apply pressure to all phases of attack, but are not in development. Versions of each based on current technology would not significantly impact strategic systems. Their joint development could promote cooperation on a range of missile defense issues.

Theater missile defense developed in parallel with strategic, aircraft, and cruise missile defenses. While theater systems do not have to conform to ABM Treaty limits, their demarcation is not clearly defined, so theater interceptors were largely stripped of their ability to intercept missiles. Even so, the Patriot's residual capability had some effectiveness against missiles in the Gulf War and maintained Alliance morale and integrity. Patriots were directed by C-band phased array radars of a few meter aperture, tens of kilometers from the intercept. Even with semi-active "track via missile", that only gave resolutions of meters, which meant Patriots had to carry large warheads and fuse them precisely to closest passage. That made them sensitive to timing errors and unintentional evasive maneuvers executed by SCUDs breaking up during reentry. Those problems have been addressed by PAC 2's improved fuse and PAC 3's on-board K-band radar, which directs intercepts and provides precise angle and range measurements for higher order guidance for maneuvering targets.

Theater High Altitude Air Defense (THAAD) and Navy Theater-Wide (NTW) interceptors are designed to extend defended footprints by intercepting missiles outside the atmosphere, which makes lower-leakage multi-layer defenses possible. Both are commanded by on-board infrared cameras, which reduces guidance package mass and provides the higher bandwith needed for hit-to-kill intercepts of high velocity missiles-at the price of range measurements, which are not possible with passive sensors. On-board guidance also frees THAAD and NTW's large, high precision X-band radars from the need to command intercept end games, so they can search for long range targets and generate precision metrics for discrimination. Measurements of radar length, area, and motion-combined with infrared measurements of physical size and temperature, should largely eliminate the effectiveness of decoys in theaters. A 600 km SCUD has a maximum altitude of ~150 km, where the radar could detect light decoys by their differential deceleration by the atmosphere. Thus, only decoys that could replicate all radar dimensions of weapons would be effective. Such decoys are complicated, heavy, and reduce payload significantly, so aggressors could not be assured of their effectiveness.

THAAD is still in development. It has had more than the usual number of problems, which have been well publicized. It was unsuccessful in six attempted intercepts; however, the failures have been caused by faulty thrusters, connections, and other quality control problems. None suggest a fundamental flaw in its interceptor, sensor, and guidance, which appear adequate for the conventional, non-maneuvering targets for which they were designed. The combination of infrared guidance and hit to kill has had significant testing. The Homing Overlay Experiment (HOE 1984) used it to intercept a reentry vehicle at intercontinental range and speed. The FLAGE (1987), ERIS (1991), and ERINT (1993 and February and June 1994) intercepted tactical missiles. The US-Israeli Arrow interceptor, which complements an infrared focal plane with a ground-based radar range measurement, has achieved several hits, at least one direct.

These defenses should be adequate for unitary conventional weapons. Nuclear weapons require much lower leakages. Theoretically, they could be obtained by compounding the kill probabilities of the above concepts, but in practice they would probably require more direct counters. Alternatively, attackers could use multiple submunitions=bomblets containing chemical or biological weapons. It is possible to release such munitions within seconds to minutes after the completion of boost phase, which is far out of range of Patriot, THAAD, and NTW. Early dispensing of munitions could saturate, exhaust, or render cost- ineffective current theater defenses. At theater speeds, reentry protection requires little more than a coat of plastic on each cannister, so there are few technical barriers to dispersal after boost, which actually benefits explosive, chemical, and biological weapons, whose effectiveness increases with the total area covered and hence with the number of munitions.

Theater boost phase.The general solution to bomblets is to push intercepts back up the weapon's trajectory: to pre-launch or boost phase. However, success to date in locating theater missiles prior to launch is not encouraging. There are concepts for intercepting theater missiles in boost: air-or space-based lasers, ground- or space-based kinetic interceptors, and less-developed concepts. Each has its own technical and cost issues. Suppression of launches from a 1,000 km country could require 3-4 ABL (Air Based Laser) orbits. It takes 3-4 aircraft to keep one on orbit continuously, so for costs of ~$300M per ABL, inveestment cost would be ~$3-5B per country.

Space-based lasers (SBL) are well suited to counter fast burning theater missiles. As they avoid most of the atmosphere, propagation is less of a concern. However, target hardening remains an issue, and the vulnerability of the SBL itself becomes one. Its location, capability, and hardness are subject to inspection and test, so it might be possible for a rogue to attack the SBL with a lower level of technology. Because satellites move continuously on orbit, at any time most are somewhere else; thus, just the fraction overhead must be able to handle the number of missiles launched. Those "absent" cannot participate, which is generally viewed as a penalty, although if the SBLs elsewhere are over other areas facing missile threats, they would contribute to global defenses. About 20 five megawatt hydrogen fluoride lasers with 4 meter mirrors ("5-4 SBLs") would be required to cover the 30 degree latitude "SCUD belt" against the launch of a missile burning for 60 sec and hardened to 10 KJ/cm2. Theater launches are point like, so the number of SBL scales directly with the product of number of missiles and their hardness and inversely with their boost time and SBL brightness. Deploying technology in space is more difficult than on airplanes or ground platforms, and SBL development has been sporadic, so space lasers are about a decade less mature technically than airborne lasers. SBL costs scale roughly on brightness, and those in the 5-4 class might cost ~$0.5B each, so this constellation might cost ~$10B, which is much smaller than the costs estimated for strategic constellations, primarily due to the much lower launch rates.

Space-based interceptors (SBI) are small rockets, launched from space platforms, with sensors for self-directed hit to kill. A missile requiring 90 sec to accelerate could be reached by a promptly launched SBI with an acceleration of 10g's and a top speed of 10 km/s from a distance of r ~ 400 km. Each SBI platform could cover an area ~ pr2 ~ 500,000 km2, so about 2pRe2/pr2 ~ 500 such SBIs could cover the SCUD belt. Ranges are monotonic in acceleration: 5g SBIs could reach 90 sec missiles from about 200 km; 20g SBIs from about 800 km. The optimal combination of acceleration and velocity can be determined by trading off the greater number of slow SBI platforms that would be needed against the greater mass of faster SBIs. A 5 kg kill vehicle costing $5M with launch costs of $10K/kg produces a ~470 platform constellation of 10g, 7 km/s SBIs costing about $4B. If more than one missile is launched in a short interval, constellation size and cost increase proportionally. Iraq launched several SCUDS simultaneously in the Gulf War. That would be more difficult with longer range missiles. For longer range missiles SBI range and coverage increase roughly in proportion to missile range. Thus, absenteeism drops rapidly, making SBIs attractive and intra-theater attacks against allies unattractive.

Fast ground-launched GBIs with high accelerations and velocities can reach missiles in the boost phase in some theaters. It has been noted that "short- burning Scuds could be destroyed by small homing interceptors.from as much as 50 km distance from the launch site." With higher acceleration interceptors and longer burn times, larger ranges are possible. In Asia, engagements involve launches over large bodies of water, which could allow interceptors to be placed close to the missile launch area. If a "fast" 7g, 7 km/s GBI was placed between the launch area and the target and fired promptly on missile launch, the GBI could reach the missile by burnout from a range of ~760 km. The cost of a deployment per country, which would be dominated by launch platform, command and control, and operation during periods of crisis, could be ~$1B.

Unmanned aerospace vehicles (UAV) or pilotless aircraft carrying kinetic energy interceptors could in some geometries reach missiles in boost. If they remained behind front lines, their timelines would be similar to those for GBIs. However, it would be difficult to carry a fast interceptor on a UAV, so from the rear, they could only reach forward-deployed missiles. If dispersed over the missiles before launch, they could intercept a wide range of missiles in boost even with modest interceptors. However, that requires the violation of the opponent's airspace before operations, which is provocative and assumes a degree of survivability that is unlikely with either current UAVs or affordable future ones. The cost of the UAV and interceptor would be ~$1M. Overall costs would be dominated by command, control, and operations.

Assuming successful development, the concepts above can be roughly compared on the basis of their availability, coverage, robustness, and cost. Due to extensive prior development, SBI and GBIs could probably be available in 3-5 years, which is about when the inter-theater missiles for which they are best suited should appear in numbers. Their lethality is robust, and they should be difficult to decoy. UAV overflight is a cheap, near-term solution with significant political costs and risks due to platform vulnerability. SBIs' greatest sensitivity is to near-simultaneous launches and short burn missiles, which increase constellation number and cost nonlinearly. The cost of GBI and SBI defenses per country against inter-theater missiles are ~ $1B and $5B, respectively; thus, GBIs would be preferred to SBIs for trajectories they could access. For ~1,000 km missile ranges, SBIs would be cost competitive with ABL, if given credit for global coverage. For ranges over 2,000 km, SBIs would be preferred even if they were not given credit.

Battle management. Recent improvements in the internetting of sensors, battle management, and command and control have significantly multiplied the effectiveness of the interceptors described above. As little as a decade ago, it was not clear that the flood of sensor data from satellites, Patriot, Aegis, and THAAD radars could be combined computably. That is now done routinely in real time on operational platforms. The Gulf War was a watershed in information fusion as much as in combat tactics. It is now possible to internet and fuse as many radars and satellites as necessary to synthesize defended footprints as large as the theaters our troops operate in and the continents our allies stage from. An essential element of that progress is the development of the corps of highly skilled men and women who are the heart of these defenses.

National Missile Defense (NMD). The end of the Cold War and the transformation of the USSR into Russia shifted the threat from all-out strategic launches to accidental or unauthorized launches plus contingencies. Third world developments such as the North Korean missiles, China-Taiwan straits, and India-Pakistan tests have since complicated that picture. It is now generally agreed that within a decade the US could be subject to global missile threats with nuclear and other weapons of mass destruction with competent radar and optical penetration aids. The Rumsfeld Commission argues the timelines for those threats could be as little as a few years.

Ground based systems. That the Earth is round is a fundamental limitation on ground based systems, because in intercontinental attacks, the weapons are hidden from ground-based radars until they come within a few thousand kilometers. A radar in the center of the US can only see slightly beyond each coast, so interceptors committed by the ABM Treaty permitted configuration could only protect a fraction of its interior and none of its coast. Distributing the interceptors alone would not alleviate this sensor limitation. Ballistic missile early warning system (BMEWS) radars at the perimeter of the US can see objects a few thousand kilometers from the coasts, which is about the distance an interceptor would have to fly out from the center of the US to intercept them at the coast. However, the incoming weapon is at intercontinental speed, while the interceptor's average speed is about half that, so the weapon would arrive first. Central basing of GBIs would not protect the contiguous, let alone the non-contiguous states.

Radar performance degrades in environments disturbed by nuclear explosions. Hit- to-kill GBIs eliminate the nuclear weapon in the interceptor, but not that in the incoming RV, which could detonate on contact or command. That would produce widespread ionospheric disturbances that could interrupt radar or infrared sensors for times longer than the attack. The US has no relevant data on nuclear phenomenology at relevant intercept altitudes. While x-band radars are less susceptible to nuclear blackout, the Achilles heel of Sentinel and Safeguard was random refraction from multiple bursts, for which there is no experimental evidence. For attacks greater than a few weapons, this introduces a fundamental uncertainty into NMD.

NMD GBIs use solid rockets, passive infrared focal plane guidance, and hit to kill similar to those in THAAD and NTW. Their larger rockets produce velocities approaching the RVs to defend footprints of several thousand kilometers. They intercept exoatmospherically, where RVs are still cold, hence they must use more sensitive focal planes. Even with these extensions, GBIs only take advantage of only the last few thousand kilometers of the descending phase of the the RVs' trajectory. GBIs depend on detection and discrimination by ground based radars and satellites for target and trajectory information, which degrades in an unknown manner.

Non-nuclear weapons of mass destruction are possible. The damage from chemicals would hardly justify intercontinental launch, but biological attacks are so damaging that their use as intercontinental payloads has been described as "a likelihood." Per unit mass, biological agents are as destructive as nuclear weapons. They can survive transit through space and reentry. And they can do so subdivided into a large number of individual cannisters. The first makes them as serious as nuclear weapons. The second makes them a real threat. And the third makes them more difficult to defend against, as they represent a payload fractionation mechanism that could exhaust planned defenses. The cannisters could be too small and cheap to be effective targets, even if GBIs could detect and discriminate them. It is not clear how to address this threat. Smart rocks on GBI are a possibility, but in addition to the economic issues, leakage restrictions for effective defense of population would be difficult to meet. Even nuclear intercepts would be marginal, as lethality against biological agents requires ranges of ~100 m for neutron kill and 1,000 m for thermal and shock, while cannisters are dispersed more widely for efficient attacks on cities.

GBIs committed on the basis of DSP or SBIRS detection and track could intercept weapons about mid-way. That would provide global coverage that was largely independent of interceptor basing, although only with single-phenomenology infrared discrimination in the region where optical decoys are most effective. If satellites serve only as adjuncts to early warning or battle management radars, the required BMEWS confirmation of satellite detection would occur about midcourse, which would give GBIs ~15 minutes to fly out. At an average speed of ~3.5 km/s, that gives a range of ~3,000 km, which would still require the GBIs to be based on both coasts. Thus, satellite commitment and direction of interceptors are essential elements of robust global coverage. Due to the uncertainties in threats, discrimination, and nuclear environments, the prediction of the performance of current NMD concepts is uncertain for attacks larger than a half dozen nuclear weapons and questionable for cannistered biological threats, for which they were not designed. Thus, current NMD concepts would produce defenses with some new technologies but essentially the same weaknesses as those deployed and found inadequate against similar threats three decades ago. As current NMD concepts are represented as the best that can be done with US ground-based decending phase systems, it is appropriate to relax those restrictions to see what other concepts could contribute.

Boost phase defenses largely eliminate decoys and disturbed environments and operate before multiple weapons are deployed, so they could address the major uncertainties. "Pre-boost," i.e., destroying missiles before launch has not been effective in theaters, where launchers are difficult to find, but should be possible for intercontinental launchers, which are larger, fixed and take significant infrastructure and time. For those reasons, preemption is perhaps technically the most feasible solution for rogue threats today, but it involves action before the initiation of hostilities and is strategically destabilizing, so it is not elaborated in the discussion below, which concentrates on boost phase concepts.

Fast GBI (Ground Based Interceptor). A typical liquid-fueled, 3g intercontinental missile has a boost phase of ~270 seconds and burns out at about 490 km altitude 780 km downrange towards its target. An interceptor launched without delay from along its track with an acceleration of 7g and maximum velocity of 7 km/s could reach it by burnout from a distance of 2,300 km, which is larger than the distance to assumed threats such as North Korea and the Middle East. The interceptor flyout distance is ~1554 km, which is a survivable standoff range. It could be command guided, look for a bright plume rather than a dim body, and use laser hard body handoffs to reduce weight, cost, and signature and increase survivability. Such high-acceleration interceptors have been developed: Sprint produced 100g with 40 year old technology. Fast GBIs could be launched on DSP for detection and track, although SBIRS higher frame rate would be preferable. Either could discriminate any decoy short of a full first stage missile. Any delay between missile and GBI launch reduces range. A ~60 s delay for multiple observations and characterization reduces the range of a 7 km/s GBI ~20%, which corresponds to a 35% reduction in the area covered. The reduction in GBI flyout distance is ~30%, which reduces the standoff area available for survivability by 50%. The defense would have a kill probability less than unity; thus, it would not be a single-layer, stand-alone defense. However, it should put enough pressure on the boost phase to complicate attack planning and reduce the threat faced by downstream layers enough for the overall defense to have adequate performance and reliability.

Space based interceptors (SBI) were developed to perform boost phase intercepts over launch areas inaccessible with ground-based systems from survivable, non- provocative platforms. They provide a maximum of autonomy and a minimum of sensitivity to uncertainties in natural and disturbed environments as well as a capacity for intercepts in all phases of the defense. For current threats, survivability and autonomy are no longer as essential, but the ability to execute boost phase engagements and insensitivity to environments are. The requirements and timelines for SBI are much the same as for the fast boost phase GBI, as they require roughly the same acceleration and velocity to reach a given missile before burnout. SBIs' additional degree of freedom is that its platform constellation density can be adjusted for the most efficient combination of SBI velocity and acceleration, whereas the fast GBI's velocity and acceleration are constrained by the standoff distance required for survivabililty.

The command and control, onboard sensors, and response times for NMD SBI are similar to those discussed earlier for TMD SBI. An optimized SBI has an acceleration of about 5g, a speed of 7-8 km/s, and a range of about 1,500 km due to the missiles' long acceleration time. Thus, its absentee ratio drops to about 30 for single launches. The resulting constellation cost is ~$1B, which is less than that of TMD SBI. However, this absentee ratio is still an order of magnitude larger than that for GBIs; thus, which of the two is preferred depends on whether the GBI can reach the missiles and whether the SBIs are given credit for the global coverage they provide.

Lasers react faster than interceptors, but have greater sensitivity to hardening. Airborne lasers have marginal ranges for deep inland launches, but their mobility should make it possible for them to orbit close to the borders of launch areas. If so, for intercontinental launches, from below the missile's path they could irradiate missiles burning out ~500 km above, because in propagating upward the beam would encounter only a thin lens of turbulence, which maximizes range.

By firing downward, space based lasers avoid most of the atmosphere, so they have ranges of thousands of kilometers. Their dominant scaling is that the product of the number of SBLs and their brightness is proportional to the product of the missile launch rate and hardness. A half dozen 5-4 SBL could negate the launch of a single 10 MJ/cm2, 270 s intercontinental missile. That constellation would also place all launch areas at risk. The constellation could cost on the order of $3-5B. The number of lasers would increase with the number of missiles launched simultaneously and their hardness; however, the challenges of achieving simultaneous launch are greater for inter-continental missiles-as are hardening penalties. SBL for NMD are less developed than airborne lasers or fast GBIs, but should benefit from the commonality of technology and common use of constellations for theater and NMD applications. SBL survivability is a concern in that they could be attacked by SCUD-like missiles fired upward in their path. A combination of SBL and GBI could provide overall survivability, quick response for fast missiles, and inexpensive intercepts of hardened intra- and inter-theater missiles.

Cooperation. Interceptor mobility is a common feature of the promising concepts for intercontinental missiles. It is prohibited by the ABM Treaty; however, none of the concepts uses it in a way that would threaten strategic systems. Thus, it would appear inappropriate to apply this criteria to concepts designed for threats that are not viewed as strategic by signatories. As Richard Garwin has argued, "Such a sea-based boost-phase intercept system is not compliant with the 1972 ABM Treaty; but Russia and the three other parties to the Treaty might well agree to a specific exception, especially if this were combined with progress on lower missile levels in Russia and the United States." It would seem appropriate to waive the issue of mobility and compare the concepts on the basis of cost and effectiveness.

A current problem in NMD is how to provide defenses needed for rogue threats without degrading current defensive agreements with Russia, which views the ABM Treaty as binding on both. The US and Russia are apparently far apart on issues of strategic stability in the post-Cold War world and how the understandings embodied in the Treaty should guide NMD programs. However, discussions at the technical level indicate that were it possible for the two nations to jointly develop and control defenses against those rogue threats, that might free them to resolve bilateral strategic issues on a longer timescale with fewer distractions. Such joint defenses have been raised to Presidential levels in both countries.

The boost phase defenses discussed above are potential candidates for cooperation. The fast GBI should have high effectiveness and adequate survivability against rogue threats but none against a Russian attack, so it would present no threat to strategic systems. SBI should have even higher effectiveness, but its constellation of platforms could be inclined so that the SBIs presented no threat to US or Russian strategic systems. And because of the relaxation of the need for autonomy against such threats, the SBIs could be constructed at a lower level of technology, and hence possibly built and controlled jointly. Such defenses might provide solutions to the threats the US and Russia currently express concern about-and only those threats-which could clarify whether the US wants a defense against rogue threats or an entry into a wider defenses, as suggested, and whether Russia wants to guide US NMD towards permitted defenses or influence domestic politics.

Time never seems right for cooperation. A decade ago the common judgment was that Soviet Union wasn't mature enough; now Russia's drift to the right is cited as making technical interactions unsafe. In the few years at the beginning of this decade in which Russia offered cooperation, the US was occupied with its domestic economy. There is always a reason for delaying cooperative efforts. However, it is possible that US pressure for NMD could prevent the Russian Duma from ratifying START II and allow the General Staff to move back to heavy land- based MIRVed missiles-posssibly the most destabilizing option imaginable-and Russian intransigence on NMD could provide the leverage needed for the US to leave the ABM Treaty. These dangers are real. Cooperation on defenses that do not impact legitimate strategic concerns could significantly reduce them.

Summary and conclusions. If rogue missiles gain the ascendancy, it is likely that they will be used against the US and its Allies. If defenses gain the ascendancy, the US and its allies could maintain sanctuaries from which to provide a stabilizing influence. The best way to slow the development of missiles is to take away their effectiveness. Defenses with fundamental limitations will not serve that function; those discussed here could. It is too early to say which would be the best choice technically; that must await the completion of their development programs. Any of them could be an effective deterrent to theater and intercontinental missiles, if deployed soon. However, absent broader and faster development, that appears unlikely.

Gregory H. Canavan

Los Alamos National Laboratory

gcanavan@lanl.gov

National Missile Defenses:

What’s new? What’s not? And, most important, does it make sense?

Lisbeth Gronlund

For the past several years, Congressional Republicans have sought to mandate deployment of a national missile defense (NMD) system, designed to defend the United States against a small number of long-range ballistic missiles. According to its advocates, the system could defend against a small accidental or unauthorized launch by Russia; a deliberate, accidental, or unauthorized launch by China; and a deliberate attack by a hostile state that acquired long-range missiles in the future.

Until recently, the Clinton administration maintained that there was no current or potential missile threat to the United States that would justify the deployment of such a defense. At the same time the administration has pursued its "3+3" plan to spend three years developing a national missile defense–by 2000–that could then be deployed in another three years–by 2003, if a decision was made to deploy. Thus, for the first time since it deployed the Safeguard missile defense system in 1975, the United States has moved beyond conducting R&D, and is developing a specific NMD system to be deployed.

What’s New?

The administration made several changes in its NMD policy early this year. On January 20, Secretary of Defense Cohen announced that the deployment date would likely be delayed from 2003 until 2005, but that the deployment decision was still slated for summer 2000. This change in deployment date reflects the widespread realization that the timetable for the 3+3 program was completely unrealistic. Last year, the program was criticized as a "rush to failure" by a independent panel of missile defense experts headed by General Larry Welch, former Air Force Chief of Staff.

Cohen also declared that the administration expected there would soon be a threat to warrant deployment, and that the primary remaining criterion for deployment of a national missile defense would be technological readiness. This change in policy can largely be attributed to two things. First, a commission chaired by former Secretary of Defense Donald Rumsfeld concluded in its July 1998 report that North Korea and Iran could acquire the capability to strike the United States with ballistic missiles within five years of making a decision to do so (Iraq is also included in this category now that UN inspections have stopped), and that the United States might have little or no warning before missiles were deployed. Second, on August 31 North Korea conducted a flight test of a three-stage missile. Although the third stage malfunctioned, the flight test suggested that North Korea could plausibly develop an intercontinental-range missile in the next several years.

To keep the deployment option open, the Pentagon added $6.6 billion for deployment to its total budget for FY 1999-2005. With the nearly $4 billion already budgeted for development, this brings the total FY 1999-2005 budget for NMD to $10.5 billion.

The administration also sent a stronger message to Russia that it will seek to renegotiate the Anti-Ballistic Missile (ABM) Treaty to permit deployment of a US NMD system.(See box on ABM Treaty Restrictions.) President Clinton apparently informed President Yeltsin of US intentions to modify the treaty in a letter sent in January. However, in his January 20 press briefing, Cohen strongly suggested that the United States would withdraw from the treaty if Russia did not agree to the modifications. The White House has since been trying to soften that position, and continues to maintain that the treaty is a "cornerstone of strategic stability." It remains unclear what exactly US policy towards renegotiating or withdrawing from the treaty is.

What’s the Same?

Despite the change of deployment date from 2003 to 2005, the nominal decision date of June 2000 remains unchanged. The Pentagon will conduct a "deployment readiness review" to assess the technological readiness of the NMD system. The President will then use this information to make a deployment decision. The Pentagon plans four flight tests prior to its readiness review; the first of which is now scheduled for August 1999. However, all four of these flight tests will be performed using surrogate boosters and kill vehicles and not prototypes of the components that would actually be deployed. Tests of the new booster and the kill vehicle–described as two of the "most critical tests" by Director of the Ballistic Missile Defense Organization Lt. General Lyles–are not scheduled to take place until FY2001 and FY2003, respectively.

If the administration decides not to begin deployment of a US NMD system next summer, then development will continue so that a decision to deploy could be made in the future. This issue will then be revisited periodically.

The administration made its January announcements to give credibility to its NMD program, in the hope that it could buy itself some political breathing space. However, Congressional pressure for deployment has continued unabated. Senator Cochran introduced legislation mandating deployment of a national missile defense as soon as is "technologically possible," which passed in March (by a vote of 97-3) once the White House withdrew its threat to veto the bill. The House then passed (by 317-105) legislation stating simply "It is the policy of the United States to deploy a national missile defense." These two bills now have to be reconciled in conference between the Senate and House.

Does It Make Sense to Deploy an NMD System?

As with all policy issues, making a decision about whether to deploy an NMD system requires weighing the likely costs and benefits. On the benefits side, one needs to assess the likely operational effectiveness of such a system. On the costs side, it is important to understand and take into account the likely reactions of other states to a US deployment. We consider these issues below.

Technical Feasibility vs. Operational Effectiveness

The Pentagon has identified technical feasibility as the key remaining criterion for deployment of a national missile defense. The US NMD will use high-altitude hit-to-kill interceptors (which are designed to destroy their targets by ramming into them above the atmosphere), whose test record so far has been dismal. Since 1982, the United States has conducted 16 intercept tests of exo-atmospheric hit-to-kill interceptors. Only 2 of these 16 intercept tests scored hits, for a 13 percent success rate. And the test record is not getting better with time: the most recent successful high-altitude test occurred in January 1991 and the last 10 such intercept tests have been failures.

However, even if this test record were stronger, it would not address the more important question: Can a national missile defense be expected to work in the real world? In other words, will it be operationally effective?

Even if all four of the NMD intercept tests planned between August 1999 and June 2000 take place and are successful, the United States will not have learned or demonstrated anything about whether the system would be operationally effective. A fundamental problem is that in the real world defenses will not face cooperative targets. Effective countermeasures can be cheap and use simple technology–much simpler than that required to build a long-range missile in the first place. Despite decades of research, dealing with countermeasures remains the key unsolved–and likely unsolvable–problem facing missile defenses. Indeed, this is precisely why the current program objectives call only for defending against "simple" warheads–those without effective countermeasures. However, if the United States deploys a national missile defense, it must expect that any developing country willing to expend the resources to build or buy intercontinental-range missiles to deliver an attack would also make sure it had countermeasures to penetrate the defense.

For biological or chemical weapons, the warhead can readily be divided into dozens of small bomblets, or submunitions, that would be released from the missile soon after boost phase. Not only would these numerous targets overwhelm a national missile defense, but this is a more effective way to disperse such weapons than using a single warhead.

For nuclear weapons, which cannot be subdivided, the attacker can employ other strategies to defeat the defense. In particular, any defense using high-altitude interceptors has an Achilles’ heel. Above the atmosphere, where the interceptors would attempt to hit their targets, there is no air resistance and objects of different weights and shapes travel at the same speed and follow the same path. This allows a missile to carry a large number of lightweight decoys to confuse and overwhelm the defense. For example, one relatively simple approach would be to hide the incoming warhead in a mylar balloon, and then release numerous identical balloons along with it. The defense would be unable to find the real warhead, and would run out of interceptors by trying to shoot at all the balloons.

Thus, in the real world, defending against a handful of warheads delivered by long-range ballistic missiles may be no more realistic than Reagan’s dream of building an impenetrable shield against the Soviet nuclear arsenal.

National Missile Defenses in Context: The Blackmail Scenario

Some argue that the United States needs a national missile defense to avoid being "blackmailed" by states that might try to deter US intervention in a military conflict by threatening to attack US cities with missiles carrying weapons of mass destruction. For example, the argument goes, if North Korea invaded the South it could deter the United States from aiding its ally by threatening Los Angeles with a nuclear-tipped missile but, the argument continues, the United States would not be deterred if it had a national missile defense to protect Los Angeles. This is a dangerous proposition. No missile defense system would be reliable enough that US leaders should be willing to risk Los Angeles based on its effectiveness. In practice, the United States would still need to rely on diplomacy or other means to counter such a threat.

The Emerging Missile Threat in Context

If the United States moves forward with deploying a national missile defense, how might other countries respond? First, consider the emerging missile states that the United States fears might threaten it with long-range missiles armed with nuclear or biological weapons (North Korea, Iraq, and Iran). If these countries do seek to threaten the United States with weapons of mass destruction, they have other delivery options.

As the Rumsfeld Commission pointed out, short-range missiles–which are much easier to build than long-range missiles–could be launched from off-shore boats and thus might present a nearer-term threat to the United States than long-range missiles. But a national missile defense would be useless against such short-range missiles, which would land before the interceptors could reach them.

And, according to Gen. Henry Shelton, Chairman of the Joint Chiefs of Staff, the United States will need to "assess where the ballistic missile threat is in the grand scheme of things." Even if North Korea does deploy long-range missiles that can reach the United States, this would not fundamentally change its ability to threaten the United States with weapons of mass destruction. North Korea already has at its disposal other ways to get such weapons to the United States, namely, by terrorist-type attacks such as truck-bombs, suitcase-bombs or weapons brought into a harbor by ship. As Shelton put it, ". . . there are adversaries with chemical and biological weapons that can attack the United States today. They could do it with a briefcase–by infiltrating our territory across our shores or through our airports."

Russia and China:

Perhaps most serious, the negative reaction from Russia and China to the US deployment of a NMD system could result in large security costs to the United States. Unfortunately, Russian and Chinese political leaders are unlikely to be confident that US missile defenses can easily be countered with penetration aids of the type discussed above, and thus will also react in other ways to preserve their deterrent capabilities.

Russia is deeply opposed to US national missile defenses and such a deployment will almost certainly scuttle the ongoing nuclear arms reduction process, which would otherwise cut Russia’s nuclear arsenal by many thousands of warheads. Russia’s collapsing economy will not force it to disarm unilaterally–as many in Washington seem to believe–but only to keep its multiple warhead missiles and operate its still substantial nuclear forces less safely. Perversely, moving forward with a US national missile defense would likely block those steps that would be most effective in reducing the missile threat to the United States–very deep reductions in Russia’s nuclear arsenal. Moreover, US deployment could actually increase the chance of accidental or unauthorized attacks by inducing Russia to rely more on launch-on-warning of its nuclear forces to preserve its deterrent.

China, too, is deeply opposed to a US national missile defense system. It has only some two dozen long-range missiles now, but has said it would seek to upgrade its nuclear arsenal in the face of US defenses. And if the United States and Russia remain stuck at relatively high levels of nuclear weapons, it will be impossible to get China and the other smaller nuclear weapon states involved in the nuclear arms control process.

Ultimately, US deployment of a national missile defense would likely put political and military barriers in the way of deep reductions in nuclear weapons. This would be a very high price to pay for a system that is unlikely to work in the real world.

Lisbeth Gronlund

Senio Staff Scientist, Union of Concerned Scientists

Research Fellow, Security Studies Program, MIT

(617)547-5552;fax:(617) 864-9405

lgronlund@ucsusa.org

ABM Treaty Restrictions

Article 1 of the Anti-Ballistic Missile (ABM) Treaty flatly prohibits deploying a defense that can cover the entire territory of the country. The treaty does permit each country to deploy a missile defense system with up to 100 interceptors at a single site, but only for a defense of an "individual region." Russia is permitted a defense around Moscow, whereas the United States chose as its site Grand Forks, North Dakota to allow protection of its ICBM fields.

Moreover, it is not enough that the interceptors be limited to one site; the ground-based battle-management radar (which tracks the incoming missiles and guides the interceptor to near its target) must be located at the same site. In fact, this restriction is consistent with the constraint that only an individual region may be defended. Because of the curvature of the earth and the large size of the United States and Russia, a radar at any one site in either country cannot see missiles landing on the entire country, and thus a single-site system cannot defend either country.

If a sensor system other than the ground-based radar is used to provide coverage beyond the field-of-view of the radar then such a system would be substituting for the radar, and thus falls under the provisions of Agreed Statement D, which states that such systems would be subject to negotiation. Unless such systems are specifically permitted, they remain illegal. Thus, deployment of the space-based sensor system (the low-earth orbit component of the Space-Based Infrared System or "SBIRS-low") that the United States is developing to track missiles and provide information to defense interceptors is illegal, as Russia has argued.