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Defense Against Nuclear Attack
Deterrence served as a form of nuclear defense during the Cold War—no one wanted to risk the threat of massive retaliation by initiating a nuclear attack. Nuclear weapons were owned by largely rational governments who understood the consequences of using them. Today we face quite a different set of threats, a mix of traditional nation-states that are deterred by the assurance of massive retaliation, rogue states that might not be so deterred, and international networks of terrorists that are willing to risk annihilation for the opportunity to inflict serious damage upon the United States. The inexorable advance of technology around the world has made it possible for almost any determined country to acquire at least a primitive nuclear capability and for terrorist groups to be given a weapon by a sympathetic sponsor.
We divide the problem of defense against nuclear attack into three parts: defense against clandestine attack, defense against attack by aircraft and cruise missiles, and defense against attack by ballistic missiles.
Clandestine Attack
Every year hundreds of tons of illegal drugs and other contraband are smuggled into the United States by land, sea, and air. Thousands of trucks cross our border every day from Canada and Mexico, few receiving more than a cursory inspection. The bulk of international trade occurs via seagoing containers, some eleven million of which enter the country every year. The Department of Homeland Security has installed detectors at ports, but we are still far from confident that nuclear materials could not be smuggled in by sea.
Hundreds of thousands of packages are shipped daily by air, again with only random inspection of what’s inside. Anyone can, after a few moments’ thought, come up with a viable scenario by which something as small as a nuclear weapon might be smuggled into the country and secretly placed in any of our major cities. The good news is that we can take several measures to lower the probability of success and indeed to discourage people from even trying.
Our best defense against any form of attack is good intelligence—knowing the intentions of an enemy in time to thwart its attack. One of the reasons that the individuals associated with the attacks of September 11, 2001, were able to remain hidden is that very few of them knew the full extent of the plot. Experience with clandestine activities suggests that the probability of a security leak grows when the number of people in the know exceeds thirty to fifty. Some communication, some financial transaction, some package will be intercepted to alert the authorities to the plot. Dividing the team into small parts, each operating independently of the others and ignorant of the total plan, can aid in keeping activities secret, but an operation of the size required to steal or accept a nuclear weapon and deliver it to a target would likely be large enough to arouse suspicion.
Unfortunately, experience tells us that we cannot rely on our intelligence agencies—which have been surprised time and again over the past decades—to give us advance warning of an impending nuclear attack. The unique destructive capability of nuclear weapons demands that we assume that we will not know about an attack and that we use every means at our disposal to detect a weapon smuggled into the country. Here we are fortunate, in that a number of existing and developing technologies can detect nuclear materials in buildings, on ships and aircraft, and even in moving vehicles. Long past are the days when white-coated men used clicking Geiger counters to detect the telltale radiation associated with nuclear materials. Sensitive detectors can be placed at tollbooths, under roadways, and at airports to sound the alarm should any nuclear material pass nearby. Other types of detectors search the huge cargo holds of container ships filled with steel containers stacked one upon the other with only inches of space in between. At the other end of the spectrum, there are miniaturized detectors hardly bigger than a cell phone that can be issued to local law enforcement personnel to wear on their utility belts.
All these are passive systems in that they detect the radiation given off by the uranium or plutonium in the bomb. Another approach is to use beams of neutrons or other subatomic particles to scan the interior of containers, vehicles, or even buildings to stimulate radioactive material into giving away its location. This is referred to as “active interrogation.” Active detection systems are more effective than passive systems, but their use must be limited to avoid exposing people to unsafe levels of radiation.
An interesting cross between passive and active detection uses naturally occurring cosmic rays as the “active” beam, minimizing exposure to the public. Even though the number of cosmic rays reaching the surface of the earth is small, when one of them strikes a heavy nucleus like uranium or plutonium, it gives off a unique signature that stands out from the natural background. This method is not as effective as active interrogation, which uses a much more powerful beam to stimulate the nuclear material, but it may be better than purely passive detection.
There are limits to what can be achieved by any type of detector. When I was director of the Defense Threat Reduction Agency, Deputy Undersecretary of Defense Mike Wynne accosted me in the hallway of the Pentagon. “I want you to develop a detector that we can put onto a satellite to detect smuggled nuclear material anywhere in the world,” he said. “I don’t think that’s possible,” I responded, “for fundamental reasons.” “Wrong answer, Younger!” he exploded. “Go back and think harder!”
I did go back and I did think harder. I enlisted the help of experts in the field and was able to show that even a very large detector in space—one that was tens of yards across that worked with perfect efficiency—could not pinpoint the location of nuclear material. The signal reaching orbit was simply too small to permit detection, and the amount of naturally occurring radiation in space was just too great to pick out such a tiny signal.
INTERCEPTING AND NEUTRALIZING a smuggled nuclear weapon involves more than just having the right detector—the operational side of the problem is at least as complex as the detection. Foremost among these challenges is distinguishing between the radiation emitted by a nuclear explosive and that from innocent radioactive sources. Small radioactive sources are routinely used in many medical facilities. Capsules containing small amounts of radioactive material are used to log oil and gas wells, and it is not unusual to find several of these in use within a single large field. Radioactivity occurs in the most innocuous of places, including sand, concrete, and even kitty litter. (All these arise from natural radioactivity found in the earth.) Granite buildings emit radiation owing to the presence of the radioactive gas radon in the stone. Cancer patients returning from radiation therapy have set off nuclear detectors by the residual radiation left in their bodies. Some advanced detectors can distinguish between the types of radiation emitted by these innocent sources and those emanating from a nuclear explosive, but at present they are expensive and additional work is needed before they are sensitive enough and cheap enough to permit widespread deployment.
A further complication of detecting a nuclear weapon is that the enemy might use shielding to reduce the radiation signature from the weapon. For a sophisticated nuclear device stolen or bought from an advanced nuclear nation, a few hundred pounds of lead would be sufficient to reduce the radiation profile to nearly undetectable levels; for more primitive weapons, of the type that an entry-level nuclear power might construct, the amount of shielding could easily reach several tons, making the weapon difficult to move and hide.
However, heavy shielding could itself alert authorities that something was amiss—if the manifest lists baby clothes and the container is grossly imbalanced, then alarms could ring. A big black spot on an x-ray—indicative of materials like lead—would also be a giveaway. There are ways around even this limitation, such as placing a nuclear device in the hold of a ship transporting thousands of tons of grain, oil, or other bulk materials, but again this begins to stretch the limits of credibility. It is much more difficult to find a nuclear device that is heavily shielded, but detectors are being developed by government laboratories to address just this possibility and tests have already been conducted by the coast guard.
Detection is only the first step in safely disposing of a smuggled nuclear weapon. We must also train first responders—police, firefighters, and other emergency services personnel—on what to do with a nuclear device after they find it. How do you anticipate and deal with booby traps in a vehicle that might detonate the bomb when the door is opened for inspection? What about the possibility of a timer on the bomb that could make the search time-urgent and preclude more thorough methods? Having a detector is only half of a practical system for intercepting nuclear materials—what happens after discovery is at least as challenging.
The Defense Threat Reduction Agency of the Department of Defense has tested a number of promising technologies for detecting nuclear materials in real-life situations. Practical demonstrations were done with various detector configurations, alarms, response forces, and disposal techniques. DTRA found that some detectors did not work as well as expected, whereas others worked much better and showed considerable promise for widespread use at civilian facilities. In one case, a roadside detector was able to identify nuclear material in vehicles passing at high speed, enabling law enforcement personnel to be notified to stop the suspect vehicle.
DTRA used some of these detectors to find and recover stolen radioactive material during the Iraq War of 2003. A highly radioactive source—one that emitted much more radiation than any nuclear weapon would—was stolen from an Iraqi military training facility. DTRA personnel mounted nuclear detectors in a helicopter that flew a low-altitude search pattern across the desert. They found the source in short order, but unfortunately only after one Iraqi died of radiation poisoning and several others were severely injured by keeping the source—which was warm to the touch—close to them.
The Domestic Nuclear Detection Office of the Department of Homeland Security built on the work of DTRA to construct a comprehensive plan for the defense of the country against smuggled nuclear material and weapons. The best scientists from universities and government laboratories have been engaged to develop new detection technologies. They have conducted a series of tests to verify that these technologies work under operational conditions. Experiments done at the Nevada Test Site scanned large tractor-trailer trucks as they passed a tollboothlike building—the figure opposite shows one of these systems in action. Thousands of smaller detectors have already been purchased by law enforcement, National Guard units, and other groups, and more capable detectors are in development.
Just as in any other type of smuggling, it is impossible to create a leakproof nuclear detection shield around the United States. Just as drugs and other contraband are smuggled into our country, so too could a clever and determined adversary succeed in bringing in a nuclear weapon. However, we are much better off than we were even a decade ago, and if government programs are even modestly successful, we will be safer still in coming years.
Delivery by Aircraft or Cruise Missile
Any country that has the capability to construct a primitive nuclear weapon would likely have aircraft that could deliver it, at least over short ranges. Aircraft are the preferred means for delivering nuclear weapons in both India and Pakistan, both of which are struggling to perfect ballistic missiles for nuclear use.
Short-range cruise missiles, which are available for purchase on the world arms market, could be adapted to carry a small nuclear warhead. Many cruise missiles are self-contained units launched from a truck chassis, so they could be fired from a ship outside U.S. territorial waters, solving the range problem while keeping the probability of detection to a minimum. They can be programmed to follow a preset course to their destination, flying very low to avoid detection, but their small payload capacity precludes the use of the heavy warhead designs typical of new entrants to the nuclear club. Add to this the possibility that the relatively slow missile could crash or be shot down en route to the target, and it becomes less attractive to a developing nuclear power.
For now, the most likely air delivery vehicle for a proliferant is the manned aircraft. However, they are challenged by their limited range—only a few advanced countries have military aircraft with the capability to reach the United States from their home base. Developing nations’ air forces are typically limited to small fighter-bombers that have ranges measured in the hundreds of miles, planes that could not cross an ocean without refueling. There are few islands in the right places that could provide the secrecy needed for such an operation, and in-flight refueling is a skill possessed by only a few countries.
One option for solving the range problem would be for a country to use a civilian airliner to carry a nuclear warhead, but again the problem is in the details. If tensions were high enough to threaten war, commercial flights would come under closer scrutiny and might be canceled altogether, as has happened many times in the past when we closed our skies to the airlines of a suspect country. Any country that would place a nuclear explosive on one of its own commercial aircraft would know that the source of the nuclear explosion would be immediately discovered and that massive retaliation would swiftly follow. (Smuggled nuclear weapons on innocent aircraft would fall under the clandestine delivery scenario described above.)
The United States keeps close track of the airspace surrounding North America as part of activities at NORAD, the North American Air Defense Command, a joint venture between Canada and the United States that uses massive radars to track every air vehicle approaching the continent. All legitimate flights file a formal flight plan before takeoff, so NORAD can quickly identify any unexpected activity in time to query the aircraft about its intentions. If NORAD is not satisfied with the answers, fighters are sent to intercept and, if necessary, shoot down the suspect plane.
During the early part of the Cold War the United States had ground-based interceptor missiles to defend against air attack. These have long since been dismantled. Military aircraft from the Air National Guard and the air force are the primary means of intercepting suspicious flights, but portable surface-to-air missiles can be used to protect high-value targets in times of crisis. These missiles include the long-range Patriot, famous for its service during the first Iraq War, and several short-range, shoulder-fired missiles for the defense of individual buildings. Although the Patriot’s record at intercepting SCUD missiles was less than perfect, it is quite effective against slower-moving aircraft.
Despite the sophistication of their radar and tracking computers, NORAD and its counterparts in other nations occasionally miss things, as was demonstrated in 1987 when a young German man flew a light plane from Helsinki to Moscow’s Red Square. The Soviet Union had layer upon layer of air defenses but they were still unable to detect, let alone shoot down, the small private plane that invaded its airspace. The reason was that radar has difficulty finding aircraft flying very low (literally “below the radar”) and those that hug hills and valleys to avoid detection.
Smugglers fly tons of illegal drugs into the United States every year using private aircraft big enough to carry a small nuclear explosive. Small planes suffer the same range problems as those discussed above so they would need to come into the United States from either Mexico or Canada, again requiring a sophisticated operation in which the weapon was transported to the takeoff point and then flown the final distance.
Defense Against Ballistic Missiles
Ballistic missiles are the vehicles of choice for the delivery of nuclear weapons across great distances. They can reach their target within an hour of launch, are very difficult to intercept, and have payload capacities large enough to carry a small nuclear weapon. All the major nuclear powers have most of their nuclear warheads mounted on ballistic missiles. As we have already discussed, mounting a nuclear weapon on a ballistic missile involves some very complex engineering, but we can expect more countries to solve this problem in the not too distant future.
There are two principal challenges associated with intercepting a ballistic missile in flight: finding it and hitting it. The United States has a system of satellites that can detect the launch of a missile and track it over at least the early part of its flight, long enough for ground-based radar systems to take over. One of the methods used is to look for a very bright light source moving up from the surface. There are many bright sources of light on the surface of the earth—for example, stadium lights, fireworks, reflections of sunlight—but very few of them move with the characteristics of a ballistic missile, so discrimination is relatively straightforward. Also, most countries announce peaceful launches of research rockets and larger missiles intended to place satellites in orbit, if for no other reason than to warn aircraft to stay out of the launch area. Any unexpected launch of a missile raises immediate suspicions.
Even an announced launch can sound the alarm, as happened in 1987 when Norway launched a small research rocket, and Russian forces, missing the notification, went on high alert, suspecting attack. It was only after Russian radar personnel determined that the rocket was not headed toward them that the error was corrected.
Once detected, there are three options for intercepting a missile: during the boost phase early in the flight, during mid-course when the uppermost stage of the missile is flying in space, and during the terminal phase when the warhead is streaking down toward its target. Each has its own set of advantages and complications.
The advantage of boost-phase intercept is that the missile is moving relatively slowly and is still physically large since it has not yet jettisoned its vulnerable first and second stages. The disadvantage is that the boost phase of missile flight lasts for only a few minutes, placing enormous burdens on the defender’s command and control systems. The missile would have to be detected and confirmed, and the order given to shoot it down within two or three minutes for there to be any chance of success, a tall order for even the most alert commander.
ONE PROMISING TECHNOLOGY to defend against short-range ballistic missiles is the Airborne Laser being developed by the U.S. Air Force. This massive device, mounted in a 747 cargo plane, uses a powerful laser beam to shoot down enemy missiles shortly after they are launched. If a sufficiently intense spot can be kept on target for several seconds, it can burn though the missile’s thin aluminum skin and detonate the propellant or otherwise so distort the metal that the missile will break up in flight. Although development has been slower than anticipated, the basic technology of aircraft-mounted lasers appears sound.
Lasers, however, have limitations. Some lasers are unable to penetrate clouds or rain, limiting them to fair-weather battles. Even clear air turbulence can disrupt the exquisite laser pulse and hence its effect on the target. Most important, airborne lasers are only effective when they are aloft near a missile launch—and they can remain on station for only twelve to eighteen hours at best. You might solve some of these problems by mounting the laser in space, where it could shoot from above the clouds, but that would be in violation of treaties forbidding the placement of weapons in space, and it would also require solving a whole other set of technological problems related to the longevity of highly complex systems in earth orbit.
The United States has placed greatest attention on defeating ballistic missiles during the mid-course phase of flight, that is, after the missile has reached space and when an accurate course has been determined by ground-based radar. From a practical standpoint, this is about the soonest that the threat can be confirmed and permission can be obtained to fire an interceptor missile. The challenge of mid-course interception is the high velocity of the missile—by that time the target is moving at speeds of several miles per second. And since it has jettisoned its first and second stages, it is only a small target in a very big sky, reminding one of the phrase “hitting a bullet with a bullet.” Even the smallest error in aim would result in a miss and a catastrophe as the warhead sped unimpeded toward its target. The engineering demands associated with mid-course interception are severe, but tests have already demonstrated our ability to detect a launch and fire a missile to successfully intercept a mock enemy warhead. Knowing that mid-flight interception is possible is a tremendous technical advance. It may be difficult, it may take time, and it may be expensive, but it is possible.
The National Missile Defense architecture involves myriad detectors, communication systems, and interceptors intended to protect all fifty states from long-range attacks. The Space-Based Infrared Systems, one in high orbit and one in low orbit, are intended to detect launches anywhere in the world. Advanced radar systems then take over to track the target and send data to interceptor missiles in Alaska (initially) and elsewhere.
Hitting a warhead the size of a trash can moving at several miles per second was still a daunting engineering challenge when the first interceptors were being designed, but that was not the only problem facing missile defense. To deploy the new system, the United States needed to withdraw from the Anti–Ballistic Missile Treaty of 1972, a treaty that set strict limits on where and how many defensive missiles the United States and the Soviet Union could have. Although the treaty was no longer relevant to a post–Cold War international environment, Washington’s announcement that we would withdraw prompted howls of protest from Russia and China. Russia threatened to field a new type of nuclear warhead that could evade interception, and the Chinese said that they would be forced to greatly increase the size of their relatively small nuclear arsenal. Both countries worried that such a shield around the United States could render their own missiles obsolete, giving the Americans the opportunity to strike without fear of retaliation.
In fact, the U.S. missile defense system contains only a few missiles that would be no match against the hundreds of Russian or even dozens of Chinese weapons. Also, the number of interceptor missiles fielded by the United States was comparable to the number already in place in the Russian defensive system around Moscow. The fundamental worry of Russia and China was that the initial American interceptor missiles were only a harbinger of more advanced systems to come, systems that could upset the delicate balance of mutually assured destruction that was the foundation of the theory of deterrence. International tempers eventually cooled, and when the United States formally withdrew from the treaty, the story rated no more than a short article on an inside page of the newspaper.
Other ballistic missile defense schemes appear promising. The U.S. Navy has tested a very capable ship-based system that would enable short-range interceptor missiles to be located off the coast of suspected trouble spots. Ships can loiter for weeks or even months, enabling them to provide an additional layer of defense to interceptor missiles in Alaska. They don’t suffer the problem of remaining aloft like the Airborne Laser, nor do they have problems with clouds or rain. We must remember, however, that all these technologies were designed to thwart an attack involving one or at most a few attacking missiles—they cannot and were never intended to protect against a massive launch of the type feared during the Cold War.
There have been several proposals to place small interceptors in space where they would be prepositioned to attack missiles in the mid-course phase of flight. The “Brilliant Pebbles” scheme proposed by the Lawrence Livermore National Laboratory in the 1980s envisioned putting thousands of small independent rockets in low earth orbit, each equipped with on-board sensors and computers. Extravagant claims were made that these interceptors could be mass-produced at low cost and that they would be smart enough to distinguish between an enemy ballistic missile and a peaceful manned space launch. Impressive progress was made, but the technology of the time was inadequate to the challenge.
The last opportunity to destroy an incoming weapon—terminal phase interception—occurs when the warhead is in its final free-fall from high altitude to its target. Terminal phase interception has the advantage that the interceptor need fly only a short range to the incoming missile, but it has the major challenge of hitting an oncoming warhead moving at more than one mile per second. This would be hard enough if the warhead was flying a predictable ballistic trajectory, but it becomes Herculean when the warhead has the ability to maneuver, as the Russians have claimed. Add to this the possibility that the warhead might detonate when it detects an approaching missile, and terminal defense becomes much less attractive. At best, the defensive missile would cause the enemy nuclear detonation to occur at several tens of thousands of feet rather than at the surface.
It is worth repeating that ballistic missile defense is not new—during the Cold War the United States and the Soviet Union had anti–ballistic missile defense systems that were remarkably advanced for the time. Both systems had nuclear-tipped missiles since neither had the technology capable of hitting a high-speed missile with a conventional explosive warhead. A combination of mid-course and terminal interceptors gave two chances to kill the incoming warhead. The United States dismantled its ABM system in 1976; the Soviet system is thought to be still partly operational.
The Changing Nature of Nuclear Defense
The assurance of massive retaliation was considered sufficient to deter any but the most foolhardy regime from attacking the United States during the Cold War. Today we face a range of adversaries from terrorist groups to rogue nation-states, some of which might be willing to risk their own destruction to inflict damage upon the world’s sole superpower. If the United States augments traditional deterrence with missile defenses, it sends a signal to would-be attackers that they could suffer a devastating response without their weapons even hitting America, something that might discourage them from developing nuclear weapons in the first place. Why bother to go to the trouble and expense of a nuclear capability when its chances of success are small?
Improved relations between Russia and the United States have reduced the probability of a massive attack involving hundreds of nuclear weapons. However, the spread of missile technology around the world, including major programs in North Korea and Iran, has introduced a new class of threat. The consequences of failure to detect and destroy an incoming nuclear weapon are extraordinary and suggest that even an imperfect system might be worth the investment. The United States must have active efforts in every area of nuclear defense: strong inducements to prevent proliferation, penalties if these inducements fail, treaties to control weapon types and numbers, detection programs to find any weapon that might be smuggled into the country, and active systems to shoot down air-or missile-delivered weapons.