Timing is the fatal flaw in long-range missile defences that target ballistic missiles during their vulnerable launch phase, says a report from the American Physical Society.
The US government is developing a "layered" defense that aims to destroy enemy missiles at different stages of their trajectory. So far, the most advanced layer is mid- course defense, which targets warheads high above the atmosphere some 10 to 15 minutes after launch.
But it is much easier to detect and damage rockets while they are firing their boosters, than to target smaller, coasting warheads. Destroying missiles in their boost phase would also drop any hazardous debris, such as nuclear or biological material, close to the attacker's territory.
However, the long-awaited APS study concludes that those advantages are most likely to be unobtainable, because the three to four minute boost phase of most rockets leaves too little time for the interceptor missiles to reach their targets.
The Airborne Laser being developed by the US Missile Defense Agency could fire faster, but is hampered by only being able to deliver damaging power over 300 to 600 kilometres.
Sense, decide, calculate
Liquid-fuelled missiles, such as those now being developed by North Korea and Iran, have boost phases of 240 seconds. More advanced solid-fuelled rockets have even shorter burn times, just 170 seconds.
Sensors need time to detect the launch, decide it represents a threat, and calculate a path for the interceptor. That leaves 140 to 170 seconds for the interceptor to destroy a liquid-fuel missile, but only 90 to 120 seconds to hit a solid-fuel missile.
To achieve these deadlines, ground-based interceptors would have to be both extremely fast and sited within 400 to 1000 kilometres of the target missile's course. To hit a target missile accurately with enough energy to destroy it, interceptors would in some cases need to be larger and faster than the target missiles themselves.
"Such interceptors have never been built and would push the state of the art," the report says. Even if they were built, they could not intercept the shorter burning solid- fuelled rockets. "Even fast interceptors would be unable to catch solid-propellant ICBMs in time," said Frederick Lamb of the University of Illinois, co-chairman of the study group. US intelligence has estimated that North Korea and Iran could develop solid rockets in 10 to 15 years.
The APS analysis further warned that any interceptors which did succeed would not hit the missiles early enough to stop warheads close to the launch pad. Instead, they would probably land short of their targets in the US, Canada, or other countries.
Space-based interceptors could in theory reach targets faster. But the APS panel concluded that full coverage would need a fleet of 1600 interceptors, each weighing 1.2 tonnes. That would require launching spacecraft at five to 10 times greater than the present capacity of the US.
The panel did present one glimmer of hope, concluding that boost-phase defense might work against short- or medium-range missiles launched from enemy ships off the US coast. Existing US Navy Aegis missiles should be up to the job, if they were within 40 kilometres of the missile- carrying ships.
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