NASA's X-59, an experimental supersonic aircraft developed in partnership with Lockheed Martin, has broken the sound barrier for the first time. The test represents 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.

On June 5, test pilot Jim "Clue" Less flew the craft from Edwards Air Force Base in California for an 81-minute round trip, NASA announced in a statement. During that time, the X-59 reached a speed of Mach 1.1 ‪—‬ 713 mph (1,147 km/h) ‪—‬ at an altitude of 43,400 feet (13,200 meters).

However, despite passing this key test, the craft’s "quiet" supersonic capabilities are still not well defined; during this flight, the X-59 was trailed by an F-15 fighter jet, whose loud supersonic boom cloaked the noise from the X-59. Further testing planned for later this year will better demonstrate the experimental aircraft’s progress on the noise-reduction front.

Breaking barriers

The world's first supersonic flight took place in 1947, when U.S. Air Force test pilot Charles "Chuck" Yeager flew the Bell X-1 aircraft over California's Mojave Desert. Yeager piloted the craft to a speed of Mach 1.06, opening a new frontier of possibilities in aviation — but not necessarily for commercial air travel.

"We always kind of joke that the X-1 broke the sound barrier and now we're trying to fix it," Catherine Bahm, NASA's Low Boom Flight Demonstrator project manager, who oversees the X-59's development, told the BBC in 2023.

When a plane moves forward, it pushes aside the air molecules in front of it. This causes the air to vibrate, creating sound waves that spread from the aircraft like the wake trailing a boat. When a craft breaks the sound barrier, the air pressure waves behind it compress and build into a shock wave, which releases energy as a thunderous "sonic boom."

A sonic boom can exceed 110 decibels ‪—‬ as loud as a fully operational steel mill and above the threshold where most people feel ear pain. Sonic booms have even been known to damage property, such as by shattering windows and, in at least one case, fish tanks. For this reason, in 1973 the Federal Aviation Administration banned non-military aircraft from breaking the sound barrier above U.S. soil.

Although Air France and British Airways' Concorde, which ran between London or Paris and New York City from 1976 to 2003, was the world's first commercial supersonic flight, that aircraft was allowed to operate only because it broke the sound barrier over the Atlantic Ocean. It was ultimately decommissioned after one flight crashed during takeoff from France's Charles de Gaulle airport, killing everyone aboard.

The X-59's ultra-long, skinny nose is designed to minimize the shockwave its sonic boom creates. A plane that produces a quieter sonic "thud" could allow for faster-than-sound commercial domestic flights over land. That would mean passengers could, for example, get from Los Angeles to New York City in under three hours (compared with the current five-to-six-hour flight time). However, NASA has not yet released information about how many decibels X-59's first sonic boom produced.

Beyond its commercial possibilities, quiet supersonic flight has obvious military applications, as it makes secret missions significantly stealthier. NASA plans to run X-59's first "mission conditions" test flight, which will reach speeds of Mach 1.4 at an altitude of approximately 55,000 feet (17,000 m), in the coming weeks.