Defensive weapons that can intercept and destroy enemy missiles before they can harm the United States or its allies have been a key part of military strategy for decades, but the rules of the game are changing.
More countries have or are developing long-range missile technology, including systems that can carry multiple warheads, known as Multiple Independently Targetable Re-entry Vehicles (MIRVs) and/or decoys.
"Both China and Russia possess the MIRV capability for their ballistic missiles. In 2014, reports confirmed that Iran too had developed Multiple Re-entry Vehicles (MRVs) for their ballistic missiles. Cold War literature suggests that MIRVs are first strike weapons and could be strategically destabilizing," independent consultant Debalina Ghoshal wrote in a June 2016 report for the Federation of American Scientists.
"The United States realizes these threats and is working towards a robust missile defense system," she said.
Last year, the U.S. Missile Defense Agency awarded contracts to Raytheon, Lockheed-Martin and Boeing to begin designing what is known as a "Multi-Object Kill Vehicle" or MOKV, which could destroy several objects in space with a single launch.
"Ten years ago, we had a single kill vehicle on a single interceptor. Kill vehicles today are the size of a toaster..