Abstract
Even if small nuclear warheads are employed in U.S. defenses, there will be no immaculate interceptions.
A few years ago, during a discussion on missile defense between one of the authors and a retired air force general, the general ventured into the realm of physics. He had thought a lot about the prevailing wisdom that hit-to-kill interceptors were the best way to destroy incoming reentry vehicles carrying weapons of mass destruction, and he came to a conclusion: Prevailing wisdom was badly mistaken. A PhD physicist, he said, should be wise enough to see that any country really serious about destroying a missile in flight had to use the only tool likely to do the job: a nuclear weapon.
It made sense. A modest-sized nuclear weapon detonated at 100-200 kilometers (or more) above the planet would do no damage on Earth–not even flash-blind anyone who happened to be looking at it. In addition, a small nuclear device, perhaps with the yield of one of the old artillery-fired atomic projectiles in the range of 1 kiloton, would cause no measurable electromagnetic pulse (EMP) on Earth, so it would not harm electronics and would not damage satellites with radiation or radioactive debris. What's more, the United States has already built many low-yield nuclear weapons with explosive powers ranging from less than 1 kiloton to around 5 kilotons. Nuclear-tipped interceptors might be the best method to halt incoming ballistic missiles carrying weapons of mass destruction.
A lightweight exoatmospheric projectile—the interceptor element used in hit-to-kill defenses.
In theory, with no expected fallout, any policy-maker concerned about rogue states obtaining ballistic missiles would not and should not hesitate to authorize the use of nuclear-tipped interceptors far above the atmosphere to prevent an American city from being destroyed.
An April 11, 2002, Washington Post news story raised the retired general's hypothetical scenario to contemporary political reality. The Post reported that Defense Secretary Donald Rumsfeld had encouraged William Schneider, Jr., chairman of the Defense Science Board, to investigate the possibility of employing nuclear-tipped missile interceptors.
Within a few days, the administration backed away from Rumsfeld's forward-leaning position on nuclear-armed missile defense–but only slightly. In congressional testimony, Lt. Gen. Ronald Kadish, director of the Missile Defense Agency, said, “We have no part of our program that involves nuclear-tipped interceptors.” But, he admitted, “Some people are thinking about it.” (The United States has not fielded nuclear-armed missile defenses since the short-lived 1975 Safeguard system.)
A bipartisan uproar culminated in an October 2002 vote by Congress to ban spending on nuclear-tipped missile defense in the fiscal 2003 defense appropriations bill. The bill prohibits using funds “for research, development, test, evaluation, procurement, or deployment of nuclear-armed interceptors of a missile defense system.”
But even if Congress did shoot the idea down–at least temporarily–they left open the question of technical feasibility. Are nuclear-tipped interceptors, as the general suggested, the only real way to safely take down incoming missiles?
Before determining the effectiveness of nuclear-tipped missiles and their consequences on Earth and in space, we need to know what type of warhead would be used. The larger the yield of a nuclear warhead, the longer the range at which it can destroy a reentry vehicle, but increasing yield has disadvantages, too. High-yield weapons, in the megaton range, would inflict damage on the ground from the EMP; even comparatively low-yield devices in the 50-kiloton range would inject so much radiation into the Van Allen belts that most satellites, including those on which U.S. armed forces rely for communications, navigation, and intelligence collection, would be destroyed–not immediately but over a few months.
An April 2001 Defense Threat Reduction Agency report provides estimates of the anticipated damage from high-altitude nuclear detonations by other countries over their own territory or near their territory. If North Korea, for example, detonated a 50-kiloton device at 120 kilometers above its own territory, the estimated lifetime of a National Oceanic and Atmospheric Administration weather satellite in an 850-kilometer altitude orbit and 99-degree inclination (that is, somewhat retrograde) would be reduced from 48 months to 0.8 months; an Orbcomm communications satellite at a 775-kilometer altitude and a 45-degree inclination would see its lifetime degraded from 84 months to 0.5 months. By 55 days after such a detonation, the number of commercial satellites surviving in low-Earth orbit (LEO) would decline from about 450 (as of 2002) to zero. An Indian detonation of a similar device 250 kilometers above the Bay of Bengal would have similar results.
We assume similar results would be found for detonations at other combinations of latitude and longitude, including the detonation of nuclear-tipped U.S. mid-course defenses. The conclusion: High-yield weapons are not a workable solution.
We also assume that the damage done to the satellite environment from radiation varies approximately linearly with the yield of the weapons detonated. Fifty 1-kiloton explosions would have roughly the same effect on satellites as a single 50-kiloton explosion. (Because EMP is not cumulative, several small bursts will not add up to one large one.) For that reason, we believe that modern nuclear-tipped interceptors would use low-yield warheads so that they could engage more targets while limiting destruction of the space environment.
The question then becomes: What would a low-yield nuclear weapon exploded in space 100 kilometers, 200 kilometers, or 500 kilometers above the surface of the earth, do to its target? It would clearly do nothing to the earth and very little to the space environment.
The weapons effects of a nuclear device detonated in a vacuum are very different from those of a bomb exploded near the surface of the earth. Blast and shock require a medium such as air or water to produce and propagate them. But space is a near vacuum.
The large thermal fireball associated with a nuclear explosion comes from bomb X-rays heating a large volume of air to luminescence. But there is no air in space, so no fireball develops.
A nuclear weapon in space has exactly two ways to destroy a target: soft X-rays and neutrons. The X-ray flash of a nuclear blast in space is very short, and the X-rays comparatively low in energy. But the instantaneous power in the radiation is enormous. When low-energy X-rays hit the outer skin of a warhead they stop, and their energy heats up a very thin layer of material. That sheath explodes away from the reentry vehicle, producing an intense shockwave that travels through the warhead. The shockwave is so intense that it is likely to destroy the structure of the intercepted nuclear weapon. In addition, plasmas may form on the powered electronics in the reentry vehicle, causing them to fail from “systemgenerated electromagnetic pulse.”
The effect of X-ray photons depends on how “soft” the warhead is. Soft warheads are vulnerable at a radiant energy input of around 3 calories per square centimeter. (For comparison, desert grass bursts into flame when it experiences a thermal input of 6 calories per square centimeter.)
Weapons designers can harden a missile, but only at the cost of making the nuclear warhead heavier and significantly reducing its range. It is likely that hardening warheads to 100 calories per square centimeter is the outer range of what an entry-level missile and nuclear power can achieve.
The effect of X-ray photons also depends on how close the interceptor detonates to the incoming warhead. Estimating X-ray radiant energy in space is not difficult. By definition, 1 kiloton is the release of 1 trillion calories of energy. In a vacuum, roughly 85 percent of the nuclear yield appears in the form of X-rays. (In fact, only about half of the energy release is in the soft X-ray region, and hard X-rays penetrate too deeply into the skin or body of a warhead to produce an explosive “blow-off” and internal shock. Nevertheless, for the sake of this article and to be fair to nuclear proponents, we have used the full X-ray yield to determine the range of effectiveness.)
A 1-kiloton interceptor has a surprisingly long range against a soft warhead–somewhat more than 1.5 kilometers, or about a mile. The same weapon would be effective at only 250 meters against a “fully hardened” reentry vehicle.
Placing an interceptor reliably within 1.5 kilometers of a target using today's seeker and radar technologies is not overly difficult. It's almost as feasible to get the interceptor within 250 meters. Indeed, interceptors of the current mid-course guidance system have been able to hit their targets in artificial circumstances.
The neutron effects of a low-yield blast also can be effective against an incoming nuclear warhead. The neutron flash is intense, and neutrons, of course, can cause the uranium or plutonium components of the nuclear weapon to begin fissioning. Because the nuclear material is not in a critical configuration, imploded by the high explosives, no nuclear explosion will occur despite the possibly large number of neutrons interacting with the fissile material. However, every fission caused by a neutron from an interceptor's warhead will cause additional fissions; together, all of these nuclear reactions will release enough energy to heat the precisely shaped parts of the attacking weapon.
December 3, 2001: A launch vehicle at the Kwajalein Missile Range.
If the temperature rises high enough, the plutonium in the pit may change phase (and hence its volume) rapidly. Alternatively, either uranium or plutonium could be heated to a temperature at which it softens, and then the centrifugal forces inside the spinning reentry vehicle are likely to distort the material so that it cannot explode.
At this point in the investigation, using nuclear-tipped interceptor missiles may appear so workable that proponents are likely to be drawing up budget estimates.
Not so fast. While soft X-ray absorption and neutron effects theoretically can render nuclear missiles harmless, in practice the proposition begins to disintegrate.
A nuclear explosion in space will not cause a blast wave, so it will not rip apart a canister of germs or poison gas, allowing them to disperse harmlessly in space. Chemicals have to be destroyed by radiolysis, the breakdown of their molecules by the X-rays and neutrons hitting the warhead. Biological weapons would have to be destroyed by similar processes. These processes are not very efficient, and many nuclear designers have voiced doubts that all, or even most, of the chemical or biological agents would be destroyed. If the agents were packaged in submunitions, which would afford considerable shielding, accessing the canisters would be even less likely.
Based on the results of an August 2000 Lawrence Livermore National Laboratory study by Hans Kruger, a 1-kiloton fission warhead would have to get within 3 meters of its target to be assured of destroying biological agents. This demanding interceptor accuracy would only be relaxed slightly for a 1-kiloton fusion, or enhanced radiation, warhead, which would have to be placed within 16 meters of the target.
For a 1-kiloton interceptor, the lethal range for neutrons from a conventional fission device is likely to be on the order of 200 meters–much closer than the threshold for damage from X-ray-induced shock produced by the same type of nuclear explosion. Neutrons add nothing to the effectiveness of a nuclear-tipped interceptor–X-rays alone do the job, at a greater distance.
Identifying a target unambiguously in order to avoid having to shoot at everything in space is difficult. Many believe that the first nuclear interceptor fired would sweep the sky clean of decoys, even if it did not destroy the real target. That would pave the way for a follow-on interceptor to aim at the real target without worrying about distracting decoys. And there would be decoys.
Making a decoy balloon that resembles a warhead is fairly easy; it is even easier to make a warhead that looks like a decoy by enclosing it in a balloon. One of the easiest decoys to build is an aluminized plastic balloon inflated with a propellant such as lead azide or sodium azide–in short, an automobile airbag on a smaller scale. The aluminization makes the balloon radar-reflective; external coatings give it a proper infrared signature. The hardware to inflate an airbag typically weighs 1-2 kilograms, while the balloon plastic is much lighter, no matter what kind of film is chosen. Even garbage bags work, and the polyethylene film used in such bags is cheap, available, lightweight, and has good radiation resistance. Thus, a decoy ought to be transparent to X-rays and not provide material that can blow off, but it still must have a metallic coating on either the inside or outside to reflect radar waves.
So how tough are garbage bags? More specifically, how tough are 4-millimeter polyethylene bags sold cheaply on the Internet as “tough-tough” and really nice when you're crunching cardboard boxes into plastic bags? A realistic model of such a bag with 1 micron of aluminum evaporated onto both the inside and the outside will withstand up to 8 calories per square centimeter with no more than 10 percent of the polyethylene being melted. This makes a plastic balloon “harder” than a soft warhead.
A 1-kiloton interceptor that would destroy a soft warhead at 1.5 kilometers has to get within 1 kilometer of the balloon–or perhaps even closer, since 50 percent melt-through of the same bag requires 15 calories per square centimeter. Sweeping them from space is unlikely.
Proponents also believe that a nuclear detonation would discriminate between a balloon and a real reentry vehicle by accelerating the balloon to a speed that is detectable by radar–about 1 or 2 meters faster per second. So long as the balloon does not rupture, it should roughly maintain its shape well enough to fool ground-based radar for seconds or minutes (the blow-off of aluminum and plastic on the inside of the balloon will temporarily pressurize the interior, but the vapors are likely to condense out quickly).
A Minuteman II ICBM launched from Vandenberg Air Force Base as part of a missile defense test.
The net impulse transferred to such a balloon is quite small because the blow-off is comparable from both the side facing the nuclear explosion and the side facing away. It will be difficult, although not impossible, to distinguish a decoy surviving a nuclear detonation from a warhead that also survives.
Thus, launching low-yield nuclear weapons against a cluster of several decoys spread out over a constellation diameter of 3 kilometers and containing one real warhead is unlikely to improve discrimination, and unlikely to destroy many decoys.
Small nuclear weapons do not provide the proverbial broom with which to sweep the skies clean of attacking ballistic missiles and their accompanying decoys. Nuclear interceptors turn out not to be a panacea to make a ballistic missile defense practical.
High-yield warheads, which would destroy a warhead and an entire cluster of decoys, are risky because they would also destroy satellites on which the United States depends and would probably cause unacceptable EMP damage to the civilian infrastructure.
Low-yield warheads would reduce the requirements for interceptor guidance and make the endgame almost trivial if the proper target could be identified. They also are unlikely to inject too much radiation into the Van Allen belts, sparing the deterioration of LEO satellites. If discrimination fails, however, missile defense command must shoot at every probable target and even some improbable ones. And dozens of detonations of 1-kiloton interceptors would impair commercial satellites.
A simple model of radiation effects on satellites would lead to the suspicion that the damage depends primarily on the total nuclear yield in space due to all explosions, rather than on the size of either the average warhead or the biggest one used. If that is so, multiple low-yield interceptors are as bad for the space environment as a single larger-yield device.
Mini-nuclear warheads offer some advantages to defense, but they don't provide much of an edge. The gap between a nuclear defense being effective and being suicidal is too small to give useful encouragement to the fans of small nuclear weapons for missile defenses.
These are, of course, technical arguments. They do not prove that a nuclear missile defense system would not work, nor even that it would not work better than the Safeguard system from the 1970s, but they do show that nuclear-tipped interceptors don't do much better than hit-to-kill and other conventional systems.
These arguments do not even begin to engage the enormous political liabilities attending the development of a nuclear missile defense. If a practical and politically palatable missile defense is a national requirement, nuclear-tipped missile interceptors probably aren't the way to go.