The acronym HALO stands for a High Altitude, Low Opening parachute jump from an airplane. A NYTimes article reports that Felix Baumgartner, one of the world's most experienced extreme skydivers, plans to take things to new heights later this year by jumping from a helium-filled balloon in the stratosphere, at least 100,000 feet above the earth. Wearing a suit not unlike that of an astronaut, he will quickly reach descent speeds in excess of 690 miles per hour, becoming the first skydiver ever to break the speed of sound (which varies with altitude and the density of the air). If all goes according to plan, his freefall will last five and a half minutes before his parachute opens, bringing him safely to the surface.
That's the theory. No one knows for certain what the effects will be upon the human body as it passes through the shock wave in transiting the sound barrier. But as the article makes clear, the project is proceeding with every attention to safety and proper science. The article's companion videos, The Science Behing a High Altitude Jump and The Highest Parachute Jump Ever, are well worth watching.
I'm reminded of two similar scenes -- the head first freefall jump in the 2009 movie Star Trek, and also a riveting scene from a science fiction story (title and author long since forgotten), in which a military astronaut escapes his endangered spacecraft by the simple expedient of leaping from it in an intentional HALO jump. His special suit protected him from heat buildup and air pressure differentials, and he landed safely from 25 miles up. The sheer audacity of the jump took my breath away. And still does.