NASA’s futuristic X-59 jet is about to face its biggest challenge yet: breaking the sound barrier for the first time. After a successful series of test flights that pushed the aircraft to near-supersonic speeds, engineers are preparing to fly it faster than Mach 1 and eventually up to Mach 1.6 at 60,000 feet. The sleek experimental aircraft is designed to replace the thunderous sonic boom with a much quieter “thump,” a breakthrough that could help bring supersonic passenger travel back over populated areas.