A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution.

For example, the path of an object launched from Earth that reaches 100 km (62 mi) above sea level, and then falls back to Earth, is considered a sub-orbital spaceflight. Some sub-orbital flights have been undertaken to test spacecraft and launch vehicles later intended for orbital spaceflight. Other vehicles are specifically designed only for sub-orbital flight; examples include manned vehicles such as the X-15 and SpaceShipOne, and unmanned ones such as ICBMs and sounding rockets.

Sub-orbital missions for the Space Systems Lab and Kentucky Space are usually designed to test hardware that will be flown on an orbital mission later on, such as communications, or deployment systems and etc.