ABSTRACT
For human-rated suborbital flights in particular, space starts as expected at 100 km altitude — this is a nice round number and one that is within reach for the first families of suborbital spacecraft, such as Virgin Galactic’s second SpaceShipTwo and the XCOR Lynx. In fact, long duration orbital free flight is only possible quite a bit higher. Due to residual atmospheric drag, the lowest altitude at which an object in a circular orbit around the Earth can complete at least one full revolution without propulsion is approximately 150 km. For the Space Shuttle’s reentry into the atmosphere, the “Entry Interface” (or EI) was at 400,000 feet altitude, or about 120 km above the surface of the Earth, and the deceleration due to the spaceplane’s penetration into the high layers of the atmosphere at 26 times the speed of sound and at a 40-degree angle of attack, started to be felt by the crew shortly after this boundary was crossed.
