Boost phase missile defense entails the destruction of an enemy missile during the earliest stages of its flight, while it remains within the Earth’s atmosphere. A viable boost phase defense has long been considered the “holy grail” of BMD, as boosting missiles are much slower and easier to track than missiles during the midcourse or terminal stage, which makes them more vulnerable to interception. Boost phase defense also overcomes the challenges of discriminating between lethal warheads and debris, as the missile is largely intact at this stage and has not had the opportunity to deploy decoys. Early intercept during the boost phase also has the added benefit of causing the missile’s ordinance to fall back on the aggressor after the boosters have been disabled or destroyed.

The main challenges for boost phase defense include the short window of opportunity between launch detection and the missile entering the midcourse phase. Kinetic interceptor systems must be placed either very close to a missile’s launch point or be fast enough to cover the necessary distance before the missile enters midcourse. This has proven problematic from both a geographic and engineering standpoint. The small window of opportunity also requires a dense system of early warning sensors to ensure the maximum time possible to conduct an interception.

Boost phase defense is not a new concept and featured prominently in the Strategic Defense Initiative (SDI), launched by President Reagan in 1983. Defense Secretary Caspar Weinberger envisioned a constellation of orbiting interceptors that would intercept Soviet ICBMs in the boost phase to prevent the Multiple Independent Reentry Vehicles (MIRVs) and countermeasures from deploying. The Missile Defense Act of 1991, however, forced an end to serious exploration of space-based systems as Congressional pressure limited research to terrestrial systems that fell within limitations imposed by the Anti-Ballistic Missile (ABM) Treaty.

Currently, directed energy technology—or lasers—is the most sought-after future boost phase missile defense option. However, complex financial, technical, deployment, and manufacturing challenges need to be overcome before this variant of boost phase missile defense can be employed. Space-based intercept systems also have exhibited promise as an effective boost phase missile defense option, however, highly expensive R & D has hamstrung such initiatives.

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