Editor's note: On Friday (June 12) the X-59 broke the sound barrier for a second time, achieving a top speed of Mach 1.4 (about 924 mile per hour), according to NASA. The achievement came just a week after the X-59 first exceeded the speed of sound. This article has been updated accordingly.
NASA's X-59, an experimental supersonic aircraft developed in partnership with Lockheed Martin, has broken the sound barrier twice in a row, during test flights conducted roughly one week apart. The tests represent an important step toward "quiet supersonic" flight, in which a craft surpasses Mach 1 — the speed of sound — with a small thump rather than a deafening crash.
