Editing Spaceflight (section)

=== Sub-orbital ===
{{Main| Sub-orbital spaceflight}}
[[Image:X-15 flying.jpg|thumb|The [[North American X-15]] in flight. X-15 flew above {{cvt|100|km}} twice and both of the flights were piloted by [[Joseph A. Walker|Joe Walker (astronaut)]].]]

On a [[sub-orbital spaceflight]] the spacecraft reaches space and then returns to the atmosphere after following a (primarily) ballistic trajectory. This is usually because of insufficient [[specific orbital energy]], in which case a suborbital flight will last only a few minutes, but it is also possible for an object with enough energy for an orbit to have a trajectory that intersects the Earth's atmosphere, sometimes after many hours. [[Pioneer 1]] was NASA's first [[space probe]] intended to reach the Moon. A partial failure caused it to instead follow a suborbital trajectory to an altitude of {{convert|113854|km|mi|sp=us}} before reentering the Earth's atmosphere 43 hours after launch.

The most generally recognized boundary of space is the [[Kármán line]] {{cvt|100|km}} above sea level. (NASA alternatively defines an astronaut as someone who has flown more than {{cvt|50|mi|km|order=flip}} above sea level.) It is not generally recognized by the public that the increase in potential energy required to pass the Kármán line is only about 3% of the orbital energy (potential plus kinetic energy) required by the lowest possible Earth orbit (a circular orbit just above the Kármán line.) In other words, it is far easier to reach space than to stay there. On May 17, 2004, [[Civilian Space eXploration Team]] launched the GoFast rocket on a suborbital flight, the first amateur spaceflight. On June 21, 2004, [[SpaceShipOne]] was used for the first [[Private spaceflight|privately funded]] [[human spaceflight]].