While it might seem straightforward to ask how far it is from Earth to space, the answer is more complex than a simple number. Earth’s atmosphere gradually thins, making the transition to the vacuum of space subtle. This gradual change has led to various definitions of where space truly begins.
The Kármán Line
The most widely recognized boundary separating Earth’s atmosphere from outer space is the Kármán Line, approximately 100 kilometers (62 miles) above mean sea level. This altitude is named after Theodore von Kármán, who calculated that beyond this height, the atmosphere becomes too thin for conventional aircraft to generate sufficient aerodynamic lift. At and above the Kármán Line, a vehicle needs to travel at speeds comparable to orbital velocity to stay aloft, transitioning from aircraft to spacecraft. The Fédération Aéronautique Internationale (FAI) officially adopted the 100-kilometer Kármán Line as the boundary of space for its record purposes.
Earth’s Atmospheric Layers
The Kármán Line serves as a defined boundary because Earth’s atmosphere lacks a distinct physical edge. The air becomes progressively less dense with increasing altitude. Scientists categorize Earth’s atmosphere into several main layers, each transitioning into the next without a sharp cutoff.
The lowest layer is the troposphere, extending up to about 12 kilometers (7.5 miles), where most weather phenomena occur. Above it lies the stratosphere, reaching around 50 kilometers (31 miles), known for containing the ozone layer. The mesosphere, from about 50 to 80 kilometers (31 to 50 miles), is where most meteors burn up.
The thermosphere extends from roughly 80 kilometers (50 miles) to 700 kilometers (440 miles), and it is within this layer that the Kármán Line resides. Finally, the exosphere, the outermost layer, can stretch up to 10,000 kilometers (6,200 miles) and gradually merges with interplanetary space.
The Nuance of Defining Space
While the Kármán Line is widely accepted, the definition of “space” remains complex and context-dependent. Some organizations, like NASA and the U.S. military, use a lower boundary of 80 kilometers (50 miles) for defining space. This highlights that “space” is often more of a functional or legal distinction rather than a precise physical location.
Even above the Kármán Line, some atmospheric drag persists, impacting satellites in low Earth orbit. Satellites, including the International Space Station, orbit well above the Kármán Line, typically at altitudes of 400 kilometers (250 miles) and higher, to minimize atmospheric drag and maintain stable orbits. The true transition from Earth’s atmosphere to the vacuum of space is a gradual continuum.