LOS ANGELES—A spacecraft that must ultimately crash to succeed was launched late on Tuesday from California on a NASA mission to demonstrate the world’s first planetary defense system, designed to deflect an asteroid from a potential doomsday collision with Earth. The DART spacecraft soared into the night sky at 10:21 p.m. PT on Tuesday from Vandenberg U.S. Space Force Base, about 150 miles northwest of Los Angeles, carried aboard a SpaceX-owned Falcon 9 rocket. The launch was shown live on NASA TV. The DART payload, about the size of a vending machine, was released from the booster a few minutes after launch to begin a 10-month journey into space, some 6.8 million miles from Earth. Moments later the rocket’s reusable lower stage flew back to Earth and safely touched down on a landing vessel floating in the Pacific in what has become a routine part of the cost-cutting launch sequence …