Changes

==Components of SDI==

==Components of SDI==

There are a limited number of ways to prevent a ballistic missile from detonating over one's territory.  A brief primer on nuclear attack will help explain the problems to be solved.

There are a limited number of ways to prevent a ballistic missile from detonating over one's territory.  A brief primer on nuclear attack will help explain the problems to be solved.

===Launch===

===The Problem===

Missiles with nuclear payloads are launched in two primary ways: as ground based intercontinental ballistic missiles, and as shorter range missiles, such as submarine-launched ballistic missiles and submarine-launched cruise missiles.   

Missiles with nuclear payloads are launched in two primary ways: as ground based intercontinental ballistic missiles, and as shorter range missiles, such as submarine-launched ballistic missiles and submarine-launched cruise missiles.   

*ICBMs are launched from the ground, travel into space, and return with the assistance of gravity.  Most contain "multiple independently targeted reentry vehicles" or MIRVs.  This means that a single ICBM can cause many nuclear warheads to detonate over multiple targets.

*ICBMs are launched from the ground, travel into space, and return with the assistance of gravity.  Most contain "multiple independently targeted reentry vehicles" or MIRVs.  This means that a single ICBM can cause many nuclear warheads to detonate over multiple targets.