NASA has tested a prototype of an airplane that may one day fly across the surface of Mars. During the August 9 test, National Aeronautics and Space Administration engineers used a helium balloon to haul the glider to 103,000 feet. At that altitude, the atmosphere is as thin as it is on Mars. The plane was then dropped. After an initial 13,000-foot plunge, the plane swooped out of its steep dive into stable flight. Flying for the most part on autopilot, the plane took two hours and 22 minutes to spiral down to a landing in the grass at Oregon’s Tillamook Airport. It reached a top speed of Mach .82, or slightly less than the speed of sound.