(Image: NASA)
IT’S like threading a needle while travelling faster than the speed of sound. Pilots at the US air force test pilot school flew this supersonic T-38C jet to the right place at the right time so that NASA could capture this photograph of the jet’s shock waves. When the waves are forced together, they merge into one, creating a sonic boom.
This is the first time shock waves have been photographed from the ground in such detail. The technique takes advantage of the sun both as a very bright light source and as a background. Edward Haering at NASA’s Armstrong Flight Research Center in California and his team used a special optical filter to reveal the sun’s textured surface, stippled with sunspots. This meant the team could make hundreds of detailed observations of each shock wave by observing how it distorted the pattern of sunspots.
(Image: NASA)
Skilful flying was needed to get the photo: the jet had to be within 90 metres of a specific point in the sky between the camera and the sun in a 2 minute window. Had it missed, the sun’s position in the sky would have changed because of Earth’s rotation and the opportunity would have gone.
The technique is called Background-Oriented Schlieren using Celestial Objects. Schlieren photography (from the German for “streak”) was developed in the 1860s to visualise air flow around a moving object. The new method could also be used to take photos silhouetted against the pocked surface of the moon.
This article appeared in print under the headline “Shock wave silhouette”
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