The U.S. 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.

-- The amount of time remaining to defenders to correct for misses is greater than at later phases in the trajectory.

-- The amount of downrange space that can be protected by defensive efforts is maximized.

However, the advantages afforded by boost-phase or ascent-phase interception come at a price. Defenders must detect a launch and respond very quickly, because boosters will burn out and payloads will divide into many discrete objects in only a few minutes.

In fact, the timelines for successful interception are so compressed that a 2003 study by the American Physical Society found it would not be feasible to achieve boost-phase interception of missiles launched by North Korea or Iran under normal circumstances.

The society calculated there would be only 170 seconds from the moment of launch to booster burnout in a missile using solid propellant, and 240 seconds in a missile using less energetic liquid propellant. It concluded that in most scenarios there would be insufficient time for defenders to detect a launch and then intercept the missiles from likely defensive sites.

These findings are valid given the assumptions the study made, but there are several ways of improving the chances for defenders. First, missiles and their warheads can be engaged in the ascent phase, providing more time to respond and a wider range of basing options while still avoiding the challenge of numerous, hard-to-track targets.

Second, high-energy lasers can be used to eliminate any delays associated with intercepting missiles from distant defensive sites, since light travels at a speed of 300,000 kilometers per second.

Third, interception can be achieved from stealthy aircraft that hostile nations cannot detect even though they are loitering very close to launch sites.

Ballistic-missile ranges vary considerably, from less than 1,000 kilometers to more than 10,000 km. Regardless of their ranges, though, all of the ballistic missiles posing a major threat to U.S. security have trajectories exhibiting the same distinct phases:

Boost phase is the earliest segment of the missile trajectory, when rocket boosters are providing the momentum that will carry warheads to their targets.

Ascent phase is the segment of the trajectory after boosters have cut off.

The Promise Of Kinetic Energy Interceptors
Ascent phase is the segment of an intercontinental ballistic missile's trajectory after boosters have cut off, but before the payload has separated into warheads, decoys and countermeasures.

Midcourse phase is the longest portion of the trajectory, during which warheads coast through space before re-entering the atmosphere. Descent phase is the initial stage of re-entry, when contact with the upper atmosphere begins to separate warheads from lighter objects.

Terminal phase is the final, brief stage of the trajectory within the atmosphere immediately prior to detonation on target.

The Kinetic Energy Interceptor is a mobile system of agile, surface-launched missiles designed to intercept most types of ballistic missiles in the boost, ascent or early midcourse phases of their trajectory.

The program derives its name from the fact that the interceptor missiles use the force of impact -- pure kinetic energy -- to destroy their targets. It was begun in 2002 as a hedge against the possible failure of other approaches to boost-phase interception, and to cope with anticipated changes in the threat that would make midcourse discrimination of warheads more challenging.

The Kinetic Energy Interceptor is conceptually similar to a Patriot air defense battery, consisting of mobile launchers, interceptor missiles and a command unit. It initially would be carried by ground vehicles that can be delivered anywhere in the world within a few hours on C-17 cargo planes, but it is designed to also be deployed at sea.

The system's mobility was made possible by the fact that KEI is the first missile-defense program the United States has developed outside the constraints of the 1972 Anti Ballistic Missile Treaty. As a result, it is more flexible than earlier defensive systems, and can be quickly dispatched to the borders of hostile countries in periods of heightened tension. Proximity to the launch sites of ballistic missiles is essential if they are to be intercepted during the early phases of their trajectory.

Despite its superficial similarity to the Patriot, the KEI is unique in several ways that collectively make it much more capable than other missile-defense systems utilizing ground-based interceptors:

-- Its multistage interceptor missiles are faster than any other tactical missile used by the U.S. military, quickly accelerating to 8 kilometers per second so that the kill vehicles they carry have the speed necessary to destroy target missiles in the early phases of their trajectory.

-- Its command unit is able to receive and process sensor inputs from many different sensors elsewhere in the defensive network, including ground-based radars and satellites using infrared detection, eliminating the need to carry organic sensors with the KEI battery.

(Part 9: Further advantages of kinetic energy weapons)

(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.)