In the beginning, there were ballistic missiles, but no way to shoot them down. First generation anti-ballistic missile systems (ABM) were developed by the U.S. and the Soviet Union in the 1960s to shoot down incoming warheads using nuclear warheads to literally vaporize incoming attacks. This approach had harmful side effects  for the people and electronic devices in the area, but was considered better than nothing. Second-generation ABM systems started with the concept of getting a non-nuclear explosive warhead close enough to distribute a cloud of shrapnel to knock down an incoming warhead. Today's ABM systems are designed for hit-to-kill literally smashing into an incoming warhead to vaporize it. 

To counter ABMs, missile designers went to the MIRV (multiple independently targeted re-entry vehicles) concept. MIRV allowed a missile to carry three or more warheads on a "bus," and dropping them off when they got into the target area. Multiple incoming warheads meant that defenders had to shoot at each incoming radar target. Attackers also started adding "penetration aids" to help warheads get through without being intercepted. This included chaff to clutter up targeting radars and inflatable decoys that would appear as incoming warheads. Finally, and by as much accident as design, rocket boosters on shorter-range missiles such as the decades-old Soviet SCUD and various Iraqi copycat versions, would break up after lofting their payloads at their target, cluttering up a defender's radar with many bits and pieces and forcing the defender to shoot at larger broken-up pieces of the booster as it fell to earth, rather than the warhead. It caused a lot of problems for Patriot missiles trying to shoot down incoming Iraqi SCUD warheads during the 1993 Gulf War and missile designers, especially those working with copycat SCUD missile designs, took note. 

For a hit-to-kill missile system, this is a nightmare. To ensure a warhead kill, defenders would prefer to launch at least two missiles at each probable inbound warhead, but ABM missiles are expensive and if it's for a limited national missile defense system, there won't be a lot of missiles available to launch or time to launch them. 

The latest trick the U.S. Missile Defense Agency (MDA) would like to field is the Multiple Kill Vehicle (MKV), an ABM with lots of smaller-sized kill vehicles on board. Lockheed Martin is currently developing the MKV to fit into the same space that Raytheon's hit-to-kill vehicle takes up on the U.S. Ground-Based Interceptor being deployed in Alaska this year. The MKV would work a lot like a MIRV It would have a "bus" that would hold dozens of coffee-can-sized kill vehicles and a long-range infrared sensor to spot an approaching cluster of objects such as a bunch of MIRVs and decoys. The bus would assign each kill vehicle a target and provide in-flight targeting updates. With dozens of smaller vehicles, every probable target in an inbound missile attack could be hit, including warheads, decoys, and junk parts that simply look threatening. 

MDA would first use MKVs to complement existing single-shot kill vehicles and ultimately replace them. The MKV design is scalable, so it could be incorporated on smaller systems such THAAD using fewer kill vehicles. Even providing a smaller system such as THAAD or the Navy's Standard Missile with a couple of smaller kill vehicles would be beneficial. Flight tests are expected around 2010. Doug Mohney.