Breaking the sound barrier by air, by land, and in free fall

On October 14, 1947, high above California's Antelope Valley, Charles "Chuck" Yeager became the fastest man alive. That day Yeager—an Appalachian farm boy-turned-fighter ace—flew an experimental rocket plane called the Bell X-1 through the sound barrier and into the history books. Fifty years and one day later (and only about 500 miles due north), another fighter pilot—RAF Wing Commander Andy Green—equaled Yeager's feat but on four wheels. Thrust SSC was the name of his ride, and it made Green the fastest man on Earth. It's a title he still holds.

But 65 years to the day after Yeager's supersonic flight, an Austrian skydiver named Felix Baumgartner got his own entry into the record books. Baumgartner rode a helium balloon from Roswell, New Mexico, (yes, that Roswell) 128,100 feet (39,045m) into the atmosphere and then stepped out of its gondola, breaking the sound barrier with nothing more than a pressure suit and the laws of gravity. Luckily for Green, Baumgartner became the fastest man en route to Earth.

All three of these historic supersonic firsts happened on (or about) October 14, but the pursuit of speed isn't some endeavor confined to a single day. The speed of sound—otherwise known as Mach 1 after an Austrian physicist—varies depending upon the medium through which that sound is passing. On a warm day at sea level, it's about 768mph, or 343.2m/sec if you prefer to think in SI units. Throughout the 17th century, scientists in England and France worked to calculate the speed of sound, getting ever closer before William Derham got there—or thereabouts—in 1709. Derham used a telescope, a pendulum, and his church tower in Upminster (now a far suburb of London) to arrive at the answer by observing the interval between seeing a rifle flash and hearing its crack. It would be more than 200 years before a human could attempt to travel that fast, however.