Skeptics and nay-sayers to the contrary, the U.S. ground-based mid-course interceptors are slowly but steadily moving towards fulfilling their mission of being able to destroy ICBMs in flight. But they will not be able to destroy more than half a dozen or a dozen ICBMs that could be launched now and at the most may be able eventually to deal with a threat from a nation with an ICBM force the size of China's, but not Russia's, with its more than 2,400 warheads.

That is because destroying an intercontinental ballistic missile in mid-flight is an infinitely more difficult task than destroying an orbiting satellite in orbit.

Consequently the Achilles heel, or vulnerability of GBI interceptors, is not that they cannot do the job, but that it takes so much effort to make a single one of them that can do the job.

After seven years in office, and after making ground-based missile defense a priority from its earliest days in power, the Bush administration can still today count fewer than 20 GBIs deployed in all at Vandenberg Air Force Base, Calif., and around Fort Greeley, Alaska, to guard the United States from the threat of nuclear-armed ICBMs being launched by so-called rogue states such as North Korea or Iran.

Disastrous development and testing decisions by Defense Secretary Donald Rumsfeld and his deputies slowed down the program disastrously. It has since moved back on track.

However, the underlying reason for the slowness is that it is an immensely difficult business to build an ultra-fast BBI interceptor capable of flying at 18,000 to 20,000 or more miles per hour to destroy an ICBM flying at speeds of at least 15,000 miles per hour and possibly even faster. That involves making an interception at combined speeds that total 16 to 19 times the velocity of a speeding bullet.

Yet the GBI must be guided by fragile and ultra-sophisticated electronics that will not be damaged by the tremendous acceleration forces to which they are subjected. The abstract physics of conceiving an anti-missile defense interceptor is therefore quite easy. But the blood and sweat engineering required to make the system work reliably every time is immensely slow and hard.

GBIs, therefore, are incredibly complex and expensive weapons to build. It is far cheaper to build lots more ICBMs for any nation that has the technology and the resources to do it.

Russia -- though not, so far, China, India, North Korea or Iran -- also has the option of installing lots of countermeasures or multiple independently targeted re-entry vehicles on its ICBMs, because by sheer weight of numbers they will be able to overwhelm the limited number of GBIs that even the United States can build to defend against them.

This cost and production problem does not mean that GBIs cannot or should not be built. But it does put realistic constraints on the circumstances under which they could be used.

The American pubic and it policymakers also need to be able to make the distinction between the dream of an absolute defense against any number of nuclear ICBMs being fired at the United States and the already existing reality of a limited defense that promises to be effective against one or several missiles that might be fired against American cities with nuclear warheads.

A comprehensive or "astrodome" defense against ICBMs remains a pipe dream more than a quarter-century after President Ronald Reagan made his famous "Star Wars" speech. But a limited defense against much smaller numbers of incoming ICBMs is a practical and achievable goal, and tens of thousands of dedicated U.S. Air Force and defense industry men and women are working flat out to make it so.

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