Once the boost phase finishes, the missile releases its payload, which continues in unpowered flight outside the atmosphere.
This midcourse phase lasts about twenty-five minutes for an ICBM, making the midcourse the longest phase of missile flight.
This offers several advantages for the defense. There is sufficient time for high-level decision making to be integrated in the command/control system. Multiple shoot-look-shoot intercept attempts may be possible. Midcourse defenses may be based far from the state of concern, possibly reducing system vulnerability.
On the other hand, midcourse defenses are particularly susceptible to countermeasures. In the near-vacuum of space where the midcourse occurs, light and heavy objects travel on the same paths. Thus, a dense warhead will appear the same as a much less dense balloon decoy. An adversary capable of building an ICBM would probably be able to send many light balloon decoys to accompany and simulate the ICBMís warhead. In addition, it would be able to place the warhead inside another balloon (anti-simulation). If the defense is therefore unable to distinguish the warhead from the decoys, its required number of intercepts could be multiplied to such a degree that the defense is overwhelmed.
If the offense were delivering biological weapons, it could split the payload into a hundred or so very small bomblets. This would certainly render a midcourse defense useless. Small biological bomblets were successfully developed by the United States for aircraft delivery in the 1950s. Use on ballistic missiles would require heat shielding for re-entry, but the additional weight required for such shielding is relatively small.
An excellent analysis of midcourse countermeasures can be found at UCS.
Midcourse defense was the focus of the Clinton administrationís NMD program. More recently, proposals have been made to use the Navyís NTW theater system as a midcourse NMD. The links below give detailed technical descriptions of these systems:
Clinton Administration NMD System
Navy Theater Wide as a Midcourse NMD