Just days after NASA’s experimental X-59 plane flew at supersonic speeds for the first time, the space agency has announced another milestone for the research aircraft.

The plane is designed to travel faster than the speed of sound without creating loud sonic booms—instead it generates what NASA describes as a “sonic thump.”

On Friday the X-59 flew at Mach 1.4, or around 924 miles per hour, at an altitude of 55,000 feet. This is the speed and altitude the plane must reach in order to fly over populated areas. Future flights will give NASA crucial data about how quiet people perceive the plane to be.

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When aircraft travel faster than the speed of sound, the pressure waves they produce are funneled toward the rear of the aircraft, where they form a cone; if the aircraft flies low enough for the cone to glance off Earth, the result is a sonic boom. These loud bangs (and the high cost of the technology) were the reason why the world’s only supersonic passenger plane, the Concorde, was retired in 2003.

The X-59 is designed for a quieter faster-than-the-speed-of-sound flight. It has a long nose that dissipates the pressure waves to the point that the aircraft may produce only a “supersonic thump,” which NASA has said will sound somewhere between a distant thunder crack and a car door slamming down the block.

An F-15 research jet has been chaperoning the X-59 on its flights so far; because the F-15 is a traditional supersonic aircraft, its sonic booms have obscured whatever noise the X-59 might make. NASA plans to mount a shock sensor to the F-15 to measure the X-59’s shock wave signature during future flights to see if it is performing as desired. After that, the space agency will perform more flights at different altitudes and speeds, as well as measure the aircraft’s acoustic signature.

“Each flight brings NASA one step closer to flying the X-59 over communities and gathering feedback that could help shape the future of commercial supersonic flight over land,” NASA said in a statement.

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