Ted Postol, a researcher at the Massachusetts Institute of Technology, said the system includes a plan to intercept intercontinental ballistic missiles (ICBMs) just minutes after launch, while their rocket boosters are still burning. This should be simpler than targeting missiles in mid-flight because tracking a flaming rocket is easier than homing in on a relatively cool warhead.

But Mr Postol told New Scientist magazine that destroying just the booster could leave the warhead shooting across the sky. Precisely where the warhead would land would depend on when the booster was destroyed during its four- to six-minute burn, and would be difficult to control. He said that potentially the warhead could land anywhere between the launch site and the target city.

"Even if you knew all the details, you couldn't be sure of what would happen in any given engagement," said Mr Postol. He said that a nuclear missile fired at the US from North Korea could explode over Alaska or Canada, while one fired from Iraq might strike Britain or mainland Europe.

Mr Postol, a former science adviser to the departments of Energy and Defense, has long been a critic of the anti-missile defense system. Research into the system is still at a relatively early phase, though the Bush administration is adamant it will be operational. A variety of options are being considered, including using a powerful airborne laser mounted inside a modified Boeing 747 to intercept shorter-range missiles.

Mr Postol said that to destroy the warhead itself during the boost phase would need a larger and more maneuverable interceptor than anything the US is currently developing.

© 2001 Independent Digital (UK) Ltd

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