Planes don't leave Earth because they rely on air for lift and propulsion, and space is a vacuum. This fundamental limitation prevents them from reaching the altitudes required for space travel.
The Role of Air
- Lift: Airplanes generate lift by using their wings to manipulate airflow, creating an upward force. This mechanism is entirely dependent on the presence of air. In the vacuum of space, there's no air to create lift. The X-15, a notable exception, was designed for high-altitude flight and briefly reached the edge of space, but it fundamentally depended on rocket propulsion, not air-based lift.
- Propulsion: Most airplanes use engines that require air for combustion. Without air, these engines would cease to function. Rocket engines, on the other hand, carry their own oxidizer and can operate in a vacuum.
Gravity and Earth's Curvature
While the curvature of the Earth is significant at high altitudes, it doesn't cause planes to spontaneously leave the atmosphere. Planes maintain altitude by constantly adjusting their pitch and thrust. The pilots don't need to explicitly account for the Earth's curvature for normal flight operations, but the curvature is inherently factored into navigational systems and flight planning. As stated in one source: "The plane will remain level at the assigned altitude and the instruments will reflect that." The perceived need for constant adjustments is a misconception arising from a misunderstanding of how altitude is maintained.
Spacecraft vs. Airplanes
Spacecraft, unlike airplanes, are designed specifically for space travel. They use rocket engines that generate enough thrust to overcome Earth's gravity and reach orbital velocity, allowing them to operate in the vacuum of space. Airplanes are simply not designed for these conditions.
In Conclusion
The simple answer is that the design and operational principles of conventional airplanes inherently limit them to Earth's atmosphere. They require air for both lift and propulsion, which is absent in space. Reaching space requires different technologies and designs, such as those incorporated in rockets and spacecraft.