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http://www.thebulletin.org/issues/2002/so02/so02forden.html September/October 2002, Volume 58, No. 5, pp. 48-53 Laser defenses: What if they work? By Geoffrey Forden The idea behind missile defenses has always been to save people from the disastrous consequences of nuclear attack. How ironic it is, then, that some of the defensive systems the United States is actively planning—including the U.S. Air Force’s Airborne- and Space-Based Laser systems—could well result in the deaths of many innocent people. In the case of a successful laser intercept, the difference would be that those who were killed would not be among those who had been targeted. The number killed would vary dramatically, depending on what country launched the missile and at what point in its powered flight the boost-phase defense struck. If a missile aimed at a major U.S. city were launched from a point in the Middle East, the destruction of the missile would cause an intact warhead (or warheads) to fall short of the target, landing and exploding somewhere in Europe or in Turkey instead. On the other hand, a warhead carried on a missile launched from North Korea, aimed at the same U.S. city, could very likely come down in an isolated area of the Pacific Ocean, resulting in far fewer immediate deaths. Layered defenses President Bush has declared that he wants “layered” missile defenses. Basically, this means using multiple methods for shooting down missiles in various stages of flight in order to improve the odds of actually hitting them. Over the years the Defense Department has studied a variety of systems that would try to engage incoming missiles at all stages of flight. These include boost-phase defenses, which would attempt to engage the missile when it is under power and actively accelerating; mid-course defenses, which would attempt to intercept a warhead as it coasts through the near-vacuum of space in free-fall; and terminal-phase defenses, which attempt an intercept as the warhead reenters the atmosphere. Occasionally, “post-boost-phase” and “pre-boost-phase” defenses are also mentioned. Post-boost-phase systems would try to destroy a warhead or its “bus” (a small platform that is capable of some maneuvering and provides the final alignment of the warhead with the target) after the main stages of the missile have burned out but are not yet separated. Such a system could in theory be effective if it could target the actual warhead rather than the missile. Striking the booster, the much larger part of the target, would be more likely, because doing so would be easier. It would also be ineffective. The term pre-boost-phase missile defense refers to preemptive attacks before missiles are launched. Such a defense presents a unique set of policy issues that will not be discussed here. Missile defense as we know it The current plan for the “Ground-based Midcourse Defense Segment,” formerly known as National Missile Defense or NMD, is to station ground-based interceptors in Alaska and perhaps in North Dakota. These interceptors are intended to collide with incoming warheads as they coast through space. One of the problems with this plan, as many critics have pointed out, is that it would be relatively easy for an attacker to institute “countermeasures,” especially the use of tens or perhaps hundreds of lightweight decoys, such as balloons shaped like warheads, which can be deployed from the same missile along with the true warhead.1 In the vacuum of space, they will travel along the same trajectory, making it extremely difficult for the system to distinguish between them and the actual warhead. A global network of sensors, both ground-based radars and satellite-based infrared telescopes, will be designed to help discriminate between the decoys and the warhead.2 But the sheer number of decoys that can be deployed represents a tremendous challenge. In a July 14, 2001 flight test, the battle-management computer network was overwhelmed by the number of tracks being reported to it.3 Managing the number of tracks, if not identifying the actual warhead, though, is an engineering challenge that can eventually be overcome. There are much more fundamental problems associated with discriminating between decoys and warheads, however. System engineers hope that two different sensing technologies— radar and infrared—will complement each other in the discrimination process. But here the attacker has a tremendous advantage. Once the missile defense system is built and its operational parameters are fixed, the attacker can change the appearance of both the warhead and the decoys. For instance, the warhead could be placed inside a balloon; decoys might have various coatings that would change their appearance to both the radar and infrared sensors; and both the decoys and the warhead could be heated or cooled to change their appearance to the infrared sensors that will be installed on both tracking satellites and the interceptor. The defense can never be flight-tested against the tremendous range of countermeasures, and the currently planned system will have to rely on computer simulations of a variety of countermeasures and then simulate the system’s response to them.4 Terminal defenses Terminal missile defense systems—in which the defense engages an incoming warhead after it has reentered the atmosphere—are already being developed for theater missile defense. These systems include the Patriot, PAC-3 (Advanced Patriot), and THAAD (Theater High Altitude Area Defense), although THAAD is intended to have some mid-course interception capability as well. The major advantage of these systems is that they will aim to engage an incoming warhead after the atmosphere has stripped away any balloons or other lightweight decoys that may have been employed in mid-course. Unfortunately, however, intercontinental-range warheads re-enter the atmosphere at speeds greater than 7 kilometers per second, leaving very little time between the time the decoys are stripped away and when the warhead strikes the ground.5 This limits the system both in terms of how large an area can be defended and its necessary location near where the interceptor missile is based. In fact, differences of just a few seconds in when the decoys are stripped away by the atmosphere can reduce the defended area from something the size of the greater Washington, D.C. area, to nothing. If the warhead is sufficiently aerodynamic, the upper atmosphere will not significantly slow it down, and the terminal defenses may have no capability at all. Developing newer, higher-accelerating interceptors may ease this restriction somewhat. On the other hand, both Russia and the United States have developed decoys that are capable of reentering deep into the atmosphere, raising the question of whether terminal defenses will remain free of the decoy problem. The boost phase Boost-phase missile defenses, which attack the missile during the first few minutes of flight, are being actively considered and—at least in the case of theater missile defense—developed. These defenses have a major advantage over mid-course and terminal defenses—they engage a missile while it is under power and still relatively slow moving. The target is very conspicuous with a bright plume, and the missile is comparatively fragile with large, thin-skinned fuel tanks. Another unique feature of the boost-phase period is that the trajectory of the warhead is not yet finalized—the debris from a successful engagement would land a considerable distance from the intended target. The perceived advantages of boost-phase missile defenses have led to a wide variety of proposals aimed at implementing the concept. These proposals include stationing interceptor missiles relatively close to suspected launch sites, placing interceptor missiles in Earth orbit (the “Brilliant Pebbles” concept), and mounting powerful chemical lasers in Earth orbit or on high-flying aircraft.6 Those defenses based on intercepting missiles with other missiles have the theoretical capability of targeting the warhead directly. Of course, turning theoretical capability into engineering reality would likely require large and sophisticated interceptors and enormous booster rockets. This in turn would place substantial limits on basing schemes. Laser-based systems, unfortunately, do not even have that theoretical capability. They take the easier approach of trying to destroy the larger and far more fragile missile body. Systems like these will have the potential to cause any warheads on intercepted missiles to fall short of their targets. The U.S. Air Force’s Airborne Laser is currently the most fully developed boost-phase missile defense system that could lead to warhead shortfalls, with a prototype entering final development. The Space-Based Laser is a variant that is being discussed as a next-generation system. Both of these systems would be likely to cause substantial damage to third parties if U.S. missile defenses caused warheads to simply fall short of their intended targets. That is because the lasers will not be powerful enough to attack the warhead, which will be thermally insulated for its re-entry into Earth’s atmosphere. Instead, a high-powered laser beam must be aimed at a section of the missile’s fuel tanks, eventually heating them enough to cause a rupture in the missile’s skin. There are three ways a laser attack might cause liquid-fueled missiles—the design most likely to be fielded by a developing nuclear power—to fail. First, a missile may lose thrust when the pressure in the fuel tank drops, with a subsequent decrease in range. Second, changes in the ratio of fuel to oxidizer delivered to the engine might cause it to burn up from an excessively high combustion-chamber temperature. And third, there could be a catastrophic structural failure caused by a sufficiently large hole. (This would be the most likely failure mode for more advanced solid-fueled missiles using composite skins like fiberglass.) In the latter case, the force of the missile’s own acceleration acting along the length of the missile could cause the rocket to bend in two. However, most large, intercontinental-range liquid-fueled missiles do not rely solely on their outer skin to supply structural support. Instead, there is a system of internal support rings and struts that carry a significant portion of the load, making catastrophic failure much less likely. We know that one thing will not happen: The missile will not explode. There have been enough accidents involving space-launch vehicles with holes in their sides, including the Challenger space shuttle tragedy, to know that rocket fuels do not explode in the upper atmosphere, even if they are subjected to enormous temperatures. This is true for both solid- and liquid-fueled missiles. All these circumstances would combine, if laser defenses were used, to leave the warhead intact—although in some cases a happy accident might cause debris from a structural failure to hit and destroy the warhead. Falling short What would happen to a warhead after a successful boost-phase missile defense engagement? Of course, if an interceptor should hit a warhead directly—as opposed to its booster—then the warhead would be destroyed outright or might burn up relatively harmlessly in the atmosphere. However, a nuclear warhead involved in a successful laser engagement, or in an intercept by a defensive missile that did not strike the warhead, stands a good chance of reentering intact, raising two critical questions: Would it detonate? And where would it land? Chances are that a U.S. or Russian nuclear warhead would reenter the atmosphere but crash to the ground without detonating. And although the crash would spread radioactivity over a small area, perhaps several hundred square meters, such an incident would in no way be comparable to the death and destruction that would accompany a nuclear explosion. Detonation would be avoided because both the United States and Russia reportedly use “environmental sensors” on their nuclear warheads. The warheads cannot detonate until after a certain set of measurable events have occurred: The warhead must experience a certain maximum acceleration or G-force corresponding to the expected powered flight of its missile, followed by an appropriate period of weightlessness as it coasts through space, followed by another period of high acceleration as it reenters the atmosphere. Successful boost-phase defenses would terminate the missile’s initial thrust before it reached peak values, and thus fail the first acceleration requirement; lower peak accelerations would also reduce the time the warhead spent in freefall, meaning it would be likely to fail the second environmental requirement. Finally, a considerable reduction in deceleration during reentry would occur because the missile’s speed would be significantly reduced. All these combine to make it highly unlikely a sophisticated warhead from either the United States or Russia would detonate. There are sound reasons to believe that would not be the case with a warhead from a rogue nation just developing nuclear bombs. A rogue would be unlikely to include this type of safety device on its weapons: First, an inexperienced nuclear power might be afraid that such sophisticated safety devices would fail to arm the warhead even after a successful flight. After all, many Defense Department scenarios postulate an attack by a rogue state after it has conducted few, if any, flight tests—and flight tests would be needed to test the reliability of environmental sensors. Second, a rogue nation would not consider these types of sensors to be as important to it as they are to the United States and Russia, both of which leave their nuclear weapons on fast-reaction alert for months or even years. Instead, a rogue might assemble a nuclear warhead right before launch, thereby all but eliminating the chance of an accidental detonation. Finally, a rogue nation might not be altogether concerned about the possibility that a warhead would detonate short of its intended target. Its leaders might hope that the United States would be blamed for causing any resulting casualties. If a warhead survives a successful missile defense engagement—and that seems highly likely for the airborne and the space-based lasers—then it still has to make it safely through reentry. It might, for instance, tumble as it reenters the atmosphere, having been thrown into a spin by the collapse of its booster. However, there is no fundamental law of nature that says warheads cannot tumble as they reenter. It has been reported, for instance, that one early version of the SS-18 warhead tumbled as it started to reenter without ill effect—except, of course, on accuracy.7 Furthermore, the current missile defense system has had great difficulty in discriminating decoys from real warheads in the case of tumbling warheads. It is at least possible that a country attacking the United States would design warheads to tumble as they reenter.8 Dealing death and destruction A number of assumptions are required to estimate the possible number of casualties associated with the shortfall of a nuclear weapon. First, let us assume that the warhead would have a yield of about 20 kilotons (roughly the equivalent of 20,000 tons of conventional high explosive). There are a number of physical reasons why an unsophisticated bomb—one whose yield has not been “boosted” by tritium—might have roughly this yield. These theoretical reasons have been augmented by the revelation that South Africa’s nuclear bomb had a yield of about 20 kilotons.9 Second, let us assume that the warhead would detonate at or near the surface of the Earth, generating large amounts of prompt fallout. If we add to these factors an estimated 15-mile-an-hour wind, we can roughly estimate the number of prompt deaths from both blast and radiation. Under the specified conditions, blast and fire effects would kill anyone within a 2,400-foot radius of detonation, and half the people within an additional area, from 2,400 to 6,000 feet out from ground zero. Using an optimistic assumption—that survivors could be evacuated within four hours of the blast—then exposure to fallout would kill everyone in a swath three miles downwind and a little less than a mile wide, at its widest. Half the people in a swath extending from three to 10 miles downwind would die. Survivors, of course, would run the risks of long-term illnesses, including cancer, as a result of their exposure. The actual number of people killed by the shortfall of a nuclear warhead would depend on its intended trajectory, when the engagement occurred, and the exact effect the engagement had on the missile—whether it completely terminated thrust or merely reduced it. However, a range of estimates of potential casualties can be arrived at by calculating the warhead’s trajectory and assuming that thrust is completely terminated at various times, using estimates of population density at the point of impact.10 Estimates for trajectories originating in Iraq and North Korea against Washington D.C. are shown here. Collateral casualty estimates are very different for the two launch points. Most boost-phase missile engagements with missiles launched from North Korea would stand a good chance of causing the warheads to fall in the ocean, with few if any immediate casualties. Even so, the timing of the engagement would have to be carefully orchestrated to avoid those parts of Russia and China that would be overflown by the missile. Of course, exploding a nuclear bomb over an unpopulated region of the Pacific would still have environmental implications for the world, but they would be far less deadly than if the bomb were to explode over a populated region. If missiles launched from the Middle East were successfully intercepted by a boost-phase missile defense, it would be very likely that their warheads would kill thousands of innocent civilians in Syria, Turkey, or Europe. It would be very unlikely that a warhead would fall within the borders of the country launching a missile because of the time it would take for the missile to climb out of the denser part of the atmosphere (for a laser engagement) or for an interceptor to reach it. No way to be sure The consequences of successful boost-phase missile defense engagements can only be discussed in terms of probable outcomes. There would always be the chance that in a laser defense the incoming missile would be so completely destroyed that a piece of it would strike its own warhead and destroy it, or that a warhead sent tumbling by the defensive engagement might burn up as it reentered the atmosphere. Or perhaps the attacking country would employ sophisticated sensors that would prevent the warhead from detonating after an abbreviated trajectory. But there are strong reasons to believe that the probability of any of these events occurring, thus preventing the detonation of an incoming nuclear warhead, would be small. If that is the case, then it is almost certain that—for an attack launched from the Middle East—thousands of Europeans who happened to live along the flight path of the incoming missile would be killed. The United States must consider that probability when it creates any defensive shield. After all, the countries that would be at risk are the same allies the United States would need in the battles after the missile was launched, and they may want to believe that the United States is as concerned about their citizens’ lives as it is about its own. The results of my calculations do not necessarily mean that boost-phase missile defenses will inevitably cause unintended casualties. There are other defenses that may engage the incoming missile during its boost phase by attacking the warhead directly rather than the booster’s fuel tanks. However, these consist of interceptor missiles stationed close to a suspected launch site, and they would require a large interceptor with sufficient motor size to be able to change course at the last second; almost certainly too large to be stationed in space. If the United States does decide to deploy such a system, it would be forced into basing the system in countries close to the trouble spot, requiring collaboration with a number of countries. And perhaps collaborative missile defense would offer the best missile defense—politically as well as technologically. <A HREF="http://www.ctrl.org/">www.ctrl.org</A> DECLARATION & DISCLAIMER ========== CTRL is a discussion & informational exchange list. 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