The president did not actually describe any specific technology underlying his hope of defending the country against nuclear attack. But administration officials in background briefings after the speech suggested that a primary emphasis be placed on directed energy weapons, one possibility being space-borne lasers. Therefore, it may be illustrative to consider the prospects of this particular technology for providing an effective defense for the country.

A space-borne laser system is by no means the only approach to ballistic missile defense but, among the exotic technologies being considered, it is the most mature and best understood. The Defense Department has invested some $1 billion in high-energy laser technology in the last decade, during which time substantial technical progress has been made. Even more technical progress may be confidently predicted in the coming decade, especially with the projected increase in funding. Still, the most optimistic forecast I can make is that this technology could produce an operational system capable of degrading a nuclear attack, but not capable of protecting the nation from devastation in the event of a massive nuclear attack. To understand this conclusion, it is instructive to consider the operational concept of such a system.

A space-based laser would be designed to attack an ICBM by burning a hole through the rocket during the period that the missile was still under powered flight. The ICBM would thus be destroyed, not only before it reached its target but before it even had a chance to release its multiple warheads. To hit the ICBM target with enough laser energy would require having the laser on a low-altitude satellite "battle station" that must be located over the launch area when it fires its laser beam. Because of the orbital motion of the satellite, not one but a whole constellation of satellites--about 20--would be necessary to shoot down any particular ICBM at any given time that it might be launched.

A few seconds would be required to detect, track, lock on, and dwell on the target long enough to burn a hole through it. Therefore, any given laser is tied up for several seconds in this operation, which has to occur during the few minutes the ICBM is in powered flight. The 20 satellites required for continuous coverage of the launch area could attack in sequence perhaps a few tens of ICBMs that were launched simultaneously, but they could not handle a mass attack of even a few hundreds of ICBMs from one geographical area. Therefore, the base number of 20 satellites would have to be multiplied by about 10 to deal with a mass attack. In other words, several hundred satellites continually orbiting the Earth would be needed to maintain enough laser beams to deal with a mass attack against the United States.

The necessary laser weapons in these several hundred battle stations would be immensely complex. The lasers would require an operational pointing and tracking accuracy of a few inches at a range of a few hundred miles; that is, better than one part in a million accuracy, requiring a feasible but difficult and expensive development program. Once the beam is properly pointed, it must have sufficient energy to burn a hole in the missile skin. This would require a more than tenfold increase in power over what has already been demonstrated for high-energy lasers. Finally, the reflecting mirror of this whole system would need to be several times larger than any that has been built so far, even on the ground. I believe that these problems would eventually yield to a determined and expensive development program, but this new generation technology would have to be demonstrated before we could begin to build the hundreds of operational laser weapon systems and put them in space.

A laser system with these capabilities would likely be too large to be launched from the space shuttle. For each of the several hundred battle stations, four or five shuttle launches may be required to place its components in orbit for assembly in space. (During this assembly phase, the system would be extremely vulnerable to attack or disruption.) My most optimistic view is that such a program would cost well in excess of $100 billion in today's dollars and could not reach a beginning operation status until some time in the next century.

If we spend two decades developing, testing and then deploying a system to defeat the Soviet ICBM and SLBM forces, they certainly have ample time to consider, develop and deploy a variety of countermeasures. Some of these are straightforward. Against lasers, for example, infrared decoys might be used to simulate the heat signatures of missile launches. Another countermeasure would be to rotate the ICBM in flight or coat the ICBM skin with the same kind of heat-absorbent material already used on reentry vehicles so that still higher levels of energy would be required to burn through the skin, requiring increases in laser power or in the mirror size of the laser weapon. Direct countermeasures against the space stations also might be possible, including space mines and anti-satellite satellites. The space- based laser perhaps would be most vulnerable to an attack by ground-based lasers.

Even if the technology development is successful beyond my expectations, the ultimate operational problems are a major concern. Whatever exotic technology we finally settle on, we must believe that, like every other weapon system, it will be subject to some countermeasures. And because of the measure-countermeasure contest, our defensive system will have some variable level of effectiveness at any given time. In World War II, the best air defense systems achieved about 10 percent effectiveness. The program manager of the space-borne laser program has estimated that it might achieve 50 percent effectiveness. If by remarkable improvements in defense technology we were able to deploy an antiballistic missile system with 95 percent effectiveness and during this period the Soviets made no changes in their present force of ICBMs, they would still be able to place a residual force of 300 ICBM warheads on our cities, each of which was 30 times larger than the atomic bomb that devastated Hiroshima. Therefore, we would still want some deterrence in addition to our defense; that is, we would still want to maintain offensive nuclear forces to threaten retaliation. So, unless a defensive system were perfect--which is as unachievable as the perpetual motion machine--it would not replace offensive, retaliatory forces, only supplement them, and the task of maintaining that deterrent would be made immeasurably more difficult by the existence of a Soviet missile defense built to match ours.

This need for deterrence, not hoping for perfect defense, is the the inevitable consequence of the enormous destructive force of the excessively large numbers of nuclear weapons possessed both by the Soviet Union and the United States. Maintaining our security through the threat of nuclear retaliation puts us in an agonizingly uncomfortable position. If we could find a safe way out, we should seize it. But we should not delude ourselves. Pursuing the unattainable risks diversion from real priorities--better conventional defense (including using our technology as leverage), secure and stable retaliatory deterrence, and the search for arms control.

It has always been tempting to solve the problems posed by nuclear weapons by wishing them away. But we cannot uninvent the nuclear bomb--we cannot repeal E MC2.