The U.S. Missile Defense Agency does not expect any particular layer or weapon system to function perfectly, but if each one can achieve some reasonable degree of success, then the number of warheads that actually reach targets will be very small. The MDA's budget for all its activities currently totals about 1 percent of annual defense spending.

The Sea-Based X-Band Radar is a mobile missile-defense radar with sophisticated capabilities for tracking ballistic targets and distinguishing warheads from non-threatening items such as decoys. The radar was developed by the Missile Defense Agency to support testing of defensive systems and to participate in the protection of U.S. territory, forces and allies.

Ballistic missiles and the warheads they carry traverse several distinct phases between the moment when they are launched and the moment when they impact on their targets. During the first three to five minutes of flight, booster rockets are firing, generating the velocity required so warheads can reach distant targets.

This portion of the trajectory is called boost phase, and it typically ends at altitudes of no more than 300 miles. Boost phase is followed by a brief period called ascent phase, when rocket motors have ceased firing but warheads and penetration aids are not yet fully separated from the final stage of the missile.

Ascent phase is followed by midcourse, the period when the payload is coasting through space that comprises most of the time between launch and impact. This is followed by a brief descent phase and then the terminal phase, a period of less than a minute when warheads re-enter the atmosphere and reach their target areas.

Each of these phases has special characteristics that demand unique defensive responses.

For example, midcourse provides the longest time for defenders to sort out the elements of a threat, but warheads are harder to discriminate from decoys and debris while they are in space, and require very sophisticated defensive sensors to successfully engage. In contrast, the terminal phase allows defenders to use atmospheric filtering to sort out warheads from decoys and debris because they traverse the atmosphere at different rates, but defenders have less than a minute to intercept warheads before they impact on target.

The Missile Defense Agency describes boost phase and ascent phase as the "ideal" time to intercept ballistic missiles, given the characteristics the missiles exhibit during the earlier stages of flight:

-- The flaming booster stages present a bright signature that can be easily tracked from thousands of miles away.

-- The warheads and decoys have not fully separated from the missile, so there are fewer targets to engage.

-- (Part 7: How to maximize the amount of down-range space that can be protected by defensive efforts)

(Loren B. Thompson is chief executive officer of the Lexington Institute, an Arlington, Va.-based think tank that supports democracy and the free market.)

(United Press International's "Outside View" commentaries are written by outside contributors who specialize in a variety of important issues. The views expressed do not necessarily reflect those of United Press International. In the interests of creating an open forum, original submissions are invited.)