Space launch is the earliest part of a flight that reaches space. Space launch involves liftoff, when a rocket or other space launch vehicle leaves the ground, floating ship or midair aircraft at the start of a flight. Liftoff is of two main types: rocket launch (the current conventional method), and non-rocket spacelaunch (where other forms of propulsion are employed, including airbreathing jet engines or other kinds). Issues with reaching space Definition of outer space There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely: The Fédération Aéronautique Internationale has established the Kármán line at an altitude of as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about , as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself. Up until 2021, the United States designated people who travel above an altitude of as astronauts. Astronaut wings are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety". NASA's Space Shuttle used , or , as its re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces. In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over . Energy Therefore, by definition for spaceflight to occur, sufficient altitude is necessary. This implies a minimum gravitational potential energy needs to be overcome: for the Kármán line this is approximately 1 MJ/kg. W=mgh, m=1 kg, g=9.82 m/s2, h=105m. W=1*9.82*105≈106J/kg=1MJ/kg In practice, a higher energy than this is needed to be expended due to losses such as airdrag, propulsive efficiency, cycle efficiency of engines that are employed and gravity drag. In the past fifty years spaceflight has usually meant remaining in space