Nuclear warheads, rocketing across oceans in less than 30 minutes, would be tough to take down. Existing U.S. missile defenses aim to confront the warheads in space, in the middle of their ballistic trajectories, disabling them with a colliding interceptor launched from land or sea. Experts liken the challenge to hitting a bullet with a bullet. Even in controlled Department of Defense tests, success has been as chancy as a coin toss.

Now, President Donald Trump’s administration wants to lift those chances by putting sensors and interceptors in space, and by going after warheads during their boost phase, when they are rising more slowly and still attached to large and noticeably hot booster rockets. Last week, the Pentagon released its Missile Defense Review—its first strategy update since 2010. The plan would expand existing ground-based missile defenses, but also add sensor-laden satellites, laser-equipped drones, and missile-carrying fighter jets.

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