The topic is global missile defense. But just how do you defend the whole globe? The best way — perhaps the only effective way — is from overhead. Yet, the Pentagon plans to spend as much as $20 billion over the next 10 years to develop a very high-speed interceptor that theoretically can stop an intercontinental ballistic missile (ICBM) in the boost phase — not from overhead — but from a land- or sea-based platform.

But surface-based boost-phase intercept is really hard to do. You must be very close to the target — perhaps too close for safety. You need the cooperation of allies to launch from or over their territory. You need a very fast interceptor. You need incredibly rapid reaction times — which may be unattainable. You need near perfect communications and flawless command and control. You need to be in just the right place at just the right time. And you need to know where your adversary will launch from — which may not be possible if he has mobile missiles.

Add to those challenges the multi billion dollar cost of boost-phase defense and the question is: why go down this road when so much else needs to be done?

For example, while the first mid-course interceptors are in their silos, 10 more are to be installed during 2005 , and more after that. The sea-based Standard Missile-3 must be bought and deployed, and the Theater High-Altitude Area Defense system will be in testing next year. Major radar upgrades are underway, including the one at Fylingdales in the United Kingdom, and new space-based sensors are being developed.

A multitude of activities are required to get the initial land- and sea-based defenses operational. Completing these programs in blocks 2004 through 2012 with spiral upgrades is a top priority for America’s national defense.

For U.S. allies, it is important to get defenses against regional threats into the field. Japan, Taiwan and the Dutch are buying Patriot PAC-3s. Turkey, India and the Gulf Sheikdoms also want missile defenses. With U.S. help, Israel is upgrading the Arrow, and the Medium Extended Air Defense System is going to NATO. Aegis ships are being deployed with allied navies. These regional defenses, together with the U.S. national missile defense, help prevent missile proliferation, intimidation and blackmail while defending against existing threats.

Of course, we also must plan for future threats. The administration’s answer is the Kinetic Energy Interceptor (KEI). But this high-cost effort will take much of the money needed for future missile defense development in other areas. The Pentagon asked for half a billion dollars for KEI this year and was to request $1 billion next year. And to what end? Experience shows it is not easy to develop a new booster for a new mission.

The ground-based midcourse defense was delayed by booster development problems, and that was supposed to be a piece of cake. How hard will it be to develop a high-acceleration booster that can go much faster than anything that now exists? And where can KEI be deployed close enough — within a few hundred kilometers — to hit a long-range missile in the boost phase? It can’t stop a Russian accidental or unauthorized launch, or deter Chinese threats. It might be able to reach a North Korean missile from South Korea or the Sea of Japan, but that is by no means a sure thing, especially if the missile is launched toward Europe or over the Pole. And besides, South Korea probably would not allow KEI to be deployed there.

Just where would such a weapon be based in the Middle East? In Iraq or Afghanistan with ongoing insurgencies? In Turkmenistan, Azerbaijan or other republics where Moscow still has influence? Even Turkey, a longtime NATO ally, refused to let coalition troops transit that country. One plan is to deploy KEI when hostilities appear imminent. But how many governments would allow interceptors to be based on their soil to attack launches in a neighboring country? Some would, but the number will be small, and permission granted may well be revoked when tensions rise.

The timelines for a surface-based boost-phase weapon are extremely stressful. The crew must be on duty day and night, in good weather and bad, 24 hours a day, seven days a week . They will have less than 3 minutes to detect a launch, decide to intercept, fire their weapon and catch a rapidly accelerating missile. A decision must be made in seconds by a duty officer or automatically by computer. In either case, you run the risk of destroying a satellite launch.

There also is a plan to put KEI on ships. But these 36-foot tall rockets will not fit in the launch tubes of existing Aegis ships, and launching such large rockets from the deck would be both difficult and dangerous. Is the Navy really willing to design its new cruiser, at a cost of billions of dollars, around this mission? Would the Navy subordinate its many other missions to missile defense picket duty in a remote corner of the globe? It is at least doubtful.

Another issue is that of burn-time. Liquid-fuel ICBMs burn for about 3 minutes, but solid-fuel ICBMs may burn for only 2 minutes. Russia is deploying mobile solid-fuel ICBMs, and China is testing them. It is only a matter of time before this technology spreads to other countries. So the time available to intercept in the boost phase actually may be less than 2 minutes.

Because the boost-phase window is so short, the best intercept would be at the speed of light, which means by laser. That is why the Airborne Laser is so important. Just recently the laser achieved first light on its first try. That was a major milestone. If the Airborne Laser works as intended, and it has made remarkable progress in recent months, it promises a paradigm shift in missile defense, which is why it is worth the investment. Congress understands that, and this year fully funded the Airborne Laser, while deeply cutting the KEI program.

The other overhead option is to base interceptors in space. While this is controversial, space-based weapons must be considered, because they do not need foreign bases and could be placed within range of every trouble spot on Earth. Interceptors on orbit are the only boost-phase defense against long-range missiles that would be both global and persistent. During the Strategic Defense Initiative, much of the technology needed for space-based interceptors was developed. That work should now be brought up to date.

KEI also is touted as a good defense against the launch of a Scud-type missile from a ship off shore. But this is a hypothetical threat. Missile defense should focus on real threats. Besides, missile defense can’t do everything. It is better to let the Navy deal with tramp steamers and other suspicious ships, and the Proliferation Security Initiative, supported by 62 countries, is an excellent step in that direction.

In addition, KEI is being sold as a future defense against missiles of all ranges, in all phases of flight. But weapons designed to do everything usually don’t do anything very well. You may remember the problems with the multi role F-111 bomber.

Instead of concentrating its assets on KEI, the Pentagon should accelerate the simple plan to enlarge the second stage of the existing Standard Missile-3 to a 21-inch diameter engine, thereby increasing its speed by up to 50 percent. And Japan says it is willing to share the cost of that upgrade. It would give ship-based defenses a good capability against longer-range missiles, using existing launch tubes and without having to design a new ship around them. A destroyer in the Mediterranean that can stop intermediate-range missiles would be especially important for the defense of Europe.

Also, creating a Ground Based Interceptor site in Europe, which already is in the U.S. defense budget, should be a priority both to protect the NATO allies and improve the defense of the United States. President Bush has made the defense of America’s allies a key part of the missile defense effort. A Ground Based Interceptor site on the continent, along with ship-based defenses off shore, would go a long way toward meeting that commitment.

Another program that should be fully funded is the relatively inexpensive Multiple Kill Vehicle, which can help solve the problem of discriminating among decoys in the midcourse. If successful, it could preclude the need for the more expensive KEI program to deal with decoys. Yet another important effort is the plan to give the Theater High-Altitude Area Defense a larger footprint and a real terminal ballistic missile defense capability. And finally, the Missile Defense Agency should develop an architecture for a space-based missile defense, and conduct a demonstration of the technology, so it will be ready to deploy space-based defenses, when and if the threat justifies them.

The U.S. budget deficit will limit the money available for missile defense in the next few years. Those scarce funds should not be spent on a KEI program that offers, at best, a marginal improvement, while drawing funds from the spiral development of the initial defenses. The Bush administration should take a hard look at where the missile defense program is going, and make sure it concentrates on deploying and upgrading the weapon systems that are needed now, not those that may or may not be needed a decade or more in the future.

James Hackett now is a defense consultant and writer based in San Diego and previously worked as a national security official in the Nixon and Reagan administrations. This commentary is adapted from his Nov. 17 remarks in London at the sixth Missile Defence Conference at the Royal United Services Institute for Defence and Security Studies.