Defensive systems plays a very important role in intercepting the missiles and destroying it before it can harm the country. So Defensive systems are as important as attacking systems. With the current increase in the development of long range missile(Multiple Independently Targetable Re-entry Vehicles or MIRVs), the threat to the US nation is increasing.
In 2015, Raytheon, Lockheed-Martin and Boeing were contracted by U.S Missile Defense Agency to design MOKV also known as Multi Object Kill Vehicle which can destroy several objects in space. Technically, each MOKV include sensors, steering, propulsion system and communication system that helps to maintain communication and hit the target accurately.
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The biggest technological challenge is to differentiate between decoys and bombs e.g balloons may have hydrogen bomb aboard. The US military will most probably do POC(Proof of Concept) in the late 2017 and non-intercept flight test in 2018 and If all the test are successful then Missile Defense Agency would conduct an intercept test in 2019.
Source : http://www.livescience.com/57053-military-missile-us-countermeasures-space-weapons.html