Aerospace engineer David Graham and his three colleagues had a deadline, and a little brown tortoise was putting it in jeopardy. In a few hours, as the sun rose over the Mojave Desert on an August morning last year, two Northrop Grumman F-5E fighter jets would come racing over the horizon. Flying 30,000 feet above Harper Dry Lake and traveling at 920 mph, the airplanes would be trailing long sonic booms–the distinctive aural signatures of supersonic flight that ordinarily make high-speed passages over land impossible.