The question of where Earth truly ends has no single, simple answer because the planet’s boundaries are defined by multiple distinct physical forces. Earth’s influence gradually fades into the vacuum of space, creating concentric and overlapping regions. The limit depends entirely on the property being measured, such as the density of the atmosphere, the strength of the magnetic field, or the dominance of the gravitational pull. Each boundary marks a transition where Earth’s immediate physical laws give way to the dynamics of the greater solar system.
The Definitive Marker: The Kármán Line
The most conventional and internationally recognized boundary between Earth’s atmosphere and outer space is the Kármán Line, situated at an altitude of 100 kilometers (62 miles) above mean sea level. This demarcation is not based on a physical barrier but rather a functional calculation, named after Hungarian-American engineer Theodore von Kármán, who theorized about the limits of flight.
The line represents the altitude where air density becomes so low that an aircraft would need to travel faster than orbital velocity to generate sufficient aerodynamic lift. Above 100 kilometers, a craft must rely on rocket propulsion and orbital mechanics rather than wings and air. While some American organizations, like NASA, have historically used a lower boundary of 80 kilometers (50 miles), the 100-kilometer altitude is the standard adopted by the Fédération Aéronautique Internationale (FAI) for aeronautical and spaceflight record-keeping. The Kármán Line functions as the legal and engineering limit of Earth’s airspace.
The Farthest Reach: Earth’s Exosphere
Despite the Kármán Line, Earth’s gaseous envelope does not abruptly terminate there. The outermost layer is the exosphere, which begins at the exobase, an altitude that can vary from about 500 to 1,000 kilometers depending on solar activity. Within the exosphere, atmospheric particles are so sparse that they rarely collide, instead following ballistic trajectories.
This tenuous region consists primarily of hydrogen and helium, which slowly leak away into interplanetary space. The most distant visible part is the geocorona, a vast cloud of neutral hydrogen that glows in ultraviolet light. Observations by the SOHO spacecraft show that the geocorona can extend up to 630,000 kilometers, placing the Moon (orbiting at 384,400 kilometers) well within the physical limits of Earth’s atmosphere. The exosphere represents the true physical extent of Earth’s gaseous matter.
The Invisible Barrier: The Magnetosphere
A more extensive and dynamic boundary is the magnetosphere, the region of space where Earth’s intrinsic magnetic field controls the motion of charged particles. Generated by the convective motion of molten iron in the outer core, this field acts as a vast, comet-shaped protective shield. Its function is to deflect the solar wind—a constant stream of high-energy plasma emanating from the Sun—preventing it from eroding the atmosphere.
The solar wind compresses the magnetosphere on the day side, creating a boundary called the magnetopause at about 10 Earth radii (roughly 65,000 kilometers). On the night side, the solar wind stretches the magnetic field lines into a colossal structure known as the magnetotail. This tail can extend for hundreds of Earth radii, reaching distances well past the Moon’s orbit. The magnetosphere defines the extent of Earth’s electromagnetic influence in space.
The Limit of Influence: Gravitational Reach
The farthest concept for defining Earth’s end relates to its gravitational dominance. While gravity technically extends infinitely, its effective reach is defined by the Hill Sphere. This sphere represents the volume of space where an object is gravitationally bound to Earth rather than to the much more massive Sun, and it is the practical limit for a stable orbit.
For Earth, the Hill Sphere extends approximately 1.5 million kilometers. Any object within this volume, such as the Moon or artificial satellites, is primarily influenced by Earth’s gravity. An object attempting to orbit outside this sphere would find its trajectory dominated by the Sun’s pull, eventually being captured into an independent solar orbit. This boundary, far beyond the atmosphere and the magnetosphere, delineates the ultimate extent of Earth’s control over nearby space.