While commercial airplanes soar high, the idea of them continuing into space is common. However, the transition to space travel involves fundamental shifts in physics, engineering, and environmental conditions. The capabilities allowing efficient atmospheric flight are precisely what prevent venturing beyond.
How Airplanes Achieve Flight
Commercial airplanes rely on two primary principles for flight: lift and thrust, both dependent on air. Lift is generated by wings, shaped as airfoils, creating a pressure difference. Faster air over the curved upper surface results in lower pressure, while slower air beneath creates higher pressure, pushing the wing upward.
Thrust, the forward force, is generated by turbofan engines. These engines draw in atmospheric air, compress it, mix it with fuel, and ignite it. The hot, high-pressure gases are then expelled rearward, creating thrust. Commercial planes typically cruise between 30,000 and 42,000 feet (approximately 9 to 12 kilometers), where air density provides the necessary lift and oxygen for combustion.
The Boundary of Earth’s Atmosphere
The Kármán line, approximately 100 kilometers (62 miles) above Earth’s mean sea level, defines the boundary of space for flight. This altitude marks where the atmosphere becomes too thin for conventional aircraft to generate sufficient aerodynamic lift. Although Earth’s atmosphere extends beyond this line, its density and pressure decrease rapidly.
Above the Kármán line, air molecules are too sparse for wings to generate effective lift. Air-breathing jet engines, used by commercial airplanes, also cannot operate in such thin air. They require a continuous intake of atmospheric oxygen for fuel combustion. Without enough air, these engines would cease to produce thrust, making atmospheric flight impossible.
Propulsion Systems for Space Travel
Space travel requires a fundamentally different approach to propulsion. Unlike jet engines that rely on external air, rocket engines carry both their fuel and an oxidizer, such as liquid oxygen, within the vehicle. This self-contained system allows rockets to operate efficiently in the vacuum of space, where atmospheric oxygen is absent.
Rocket engines generate thrust by expelling hot, high-velocity exhaust gases rearward through a nozzle. This action, based on Newton’s third law, pushes the rocket forward, enabling it to accelerate and maneuver without surrounding air. This principle allows spacecraft to achieve the speeds needed to overcome Earth’s gravity and enter orbit.
Designing for the Space Environment
Beyond propulsion, spacecraft design must account for extreme conditions vastly different from those encountered by airplanes. Space is a vacuum, posing challenges for structural integrity and thermal management. Spacecraft require robust structures to withstand intense forces and vibrations during launch.
Once in space, components must endure the vacuum, which can cause materials to degrade. Pressurized cabins are essential to protect humans. Space also presents extreme temperature fluctuations; surfaces in direct sunlight can reach around 250°F (121°C), while shadowed areas can plummet to -250°F (-157°C). This necessitates specialized thermal control systems and insulation.
Spacecraft also encounter harmful radiation from the sun and cosmic rays, requiring shielding for electronics and human occupants. Unlike Earth’s atmosphere, space lacks a medium for convection, meaning heat must be dissipated primarily through radiation. Finally, re-entry into Earth’s atmosphere generates immense heat due to friction, requiring spacecraft to be equipped with heat shields that commercial airplanes lack.