When a plane goes fast enough, it compresses the air it is flying through so much that it can change its density.
NASA and the US Air Force have been trying to visualize this effect for years so they can build better supersonic aircraft and enable them to go faster than the speed of sound.
Until recently, these kinds of tests were contained to wind tunnels on the ground.
There, researchers used the schlieren technique, invented by German physicist August Toepler in 1864, to understand more about how air was traveling around supersonic aircraft.
Schlieren imaging is a way to see the differences in air density, using a particular setup of lenses and cameras.
Now, NASA researchers have adapted this method to visualize supersonic aircraft in flight. Bringing the Schlieren method into the air has been challenging because the aircraft carrying the imaging equipment has to fly right above the plane it’s recording, and travel just as fast — which, during supersonic imaging, is faster than the speed of sound.