HALO stands for High Altitude Low Opening. In essence jumping out of a plane from above 28,000 feet plus (the norm is around 12 – 15,000 feet). Due to the high altitude the temperature is somewhere around 25 to 35 degrees below zero. Without oxygen, you would likely be unconscious pretty quickly.
There are a few extreme examples recently of HALO skydiving – most famously Felix Baumgartner, followed by Alan Eustace.
After flying to an altitude of 39,045 meters (128,100 feet) in a helium-filled balloon, Felix Baumgartner completed a record breaking jump from the edge of space. Felix reached a maximum of speed of 1,357.6 km/h or 843.6 mph before being slowed by the atmosphere during his 4:20 minute long free fall. The 43-year-old Austrian skydiving expert also broke two other world records (highest free fall and highest manned balloon flight), leaving the one for the longest free fall to project mentor Col. Joe Kittinger.
Jumping from a balloon at 135,890 ft (41,419 m) above Roswell, New Mexico, Eustace also set new world records for vertical speed and free fall distance.
Eustace and the StratEx (Stratospheric Explorer) program team also took a slightly different approach. Rather than being protected from the environment by a custom-built fibreglass pressurised capsule like Baumgartner, Eustace rose to the stratosphere in a pressure suit dangling from a helium-filled scientific balloon.
The ascent took two hours and seven minutes, with Eustace spending around half an hour enjoying the view before cutting himself free of the balloon at 9:09 MDT on October 24 and arriving back on solid ground 14 minutes and 19 seconds later.
Eustace reached a maximum speed of 1,323 km/h or 822 mph. He covered 123,414 ft (37,607 m) in just four minutes and 27 seconds.