Where does the atmosphere end and space begin?

Deciphering where space commences seems straightforward, but it's not as easy as it appears. The conventional belief might be that space starts where the atmosphere ceases, which isn't entirely incorrect. However, pinpointing this transition is challenging, with most scientists placing this boundary comfortably within the atmospheric limits.

Most commonly, "space" is defined to begin 100 kilometers above the Earth's surface, a boundary known as the Kármán line. This line is named after Theodore von Kármán, a Hungarian physicist-engineer who identified that aerodynamics cease to function beyond this altitude. As Paul Newman, NASA's Goddard Space Flight Center chief scientist for earth sciences, puts it, "The Kármán line is the altitude where an airplane can no longer generate sufficient thrust to stay airborne."

Another important distinction at the Kármán line is the separation of gases. Newman explains, "Below 100 kilometers, gases are thoroughly mixed by turbulence, with nitrogen at about 78 percent and oxygen at about 21 percent. Beyond 100 kilometers, the gases start to separate diffusely due to gravity. This separation altitude is known as homopoeia, as everything following this is homogeneously mixed."

The Fédération Aéronautique Internationale (FAI), the international governing body for air sports, officially recognizes this altitude as the commencement of space. Dr. Terry D. Oswalt, chairman of the Department of Physical Sciences at Embry-Riddle Aeronautical University, notes, "The commercial space industry, too, seems to be adopting this as their target for tourist flights."

The recent launch of Jeff Bezos' Blue Origin test of the New Shepard capsule at 65.8 miles and Sir Richard Branson's Virgin Galactic's promise to take its guests above the 100-mile mark with the SpaceShipTwo capsule are prime examples.

However, the Kármán line is not universally accepted as the standard definition. Oswalt explains, "Despite the U.S. signing the FAI agreement establishing 100 kilometers as the definition of where space begins, it still confers 'astronaut' status to anyone who ascends above 80 kilometers or 50 miles—the altitude where the thermosphere begins and the temperature starts to increase."

A revised definition was proposed in 2009 by researchers at the University of Calgary, who suggested a more accurate boundary at 118 kilometers based on their Supra-Thermal Ion Imager data. Below this line, ions move calmly, while above it, they move erratically, indicating the boundary between the atmosphere and space.

Interestingly, the 118-kilometer mark is far from the outer limits of Earth's atmosphere, which consists of five layers—the troposphere, stratosphere, mesosphere, thermosphere, and exosphere—stretching approximately 650 kilometers above the Earth. Scientists are yet to define the precise point where the atmosphere ends, as it gradually thins out until it vanishes.

Then there is the case of the International Space Station (ISS), which, by most accounts, is certainly in space. It orbits at an average altitude of about 400 kilometers, well above the Kármán line and the University of Calgary boundary.

With astronauts experiencing microgravity there, one would think that 400 kilometers would certainly be above the boundary between atmosphere and space. Well, not quite. The station-which is in the thermosphere-is in constant free fall toward Earth, and is still affected by our planet's gravity and atmosphere. The ISS slows down as it passes through very thin layers of atmosphere and requires regular altitude and velocity corrections to stay in orbit.

Even astronauts floating inside the ISS still experience gravity. Their weightlessness is similar to the experience of passengers aboard a plane in parabolic flight, in which pilots maneuver a plane through the air in a giant arc. At the apex of the arc, passengers experience a few seconds of weightlessness. If you are falling at the same speed as everything around you (that is, without air resistance), you feel as if you are floating.

To reach a point where Earth's gravity no longer has some grip, you would need to move away to about 21 million kilometers. That's 87 times farther than the Moon.

"Each of the definitions of the boundary of space depends on what processes in the atmosphere you consider important for the problem at hand," Oswalt says. "For example, if you consider the point at which an airplane can no longer fly important, then that becomes your definition of where space begins. If the risk of radiation from solar flares and cosmic rays is the main concern, then that's where space begins. If reaching orbit is important, your definition might be 160 kilometers, where the friction with the atmosphere is low enough to maintain what is called 'low earth orbit.' "

This means that the boundary of space is not a barrier and looks more like a moving target, changing as we reach new milestones and achievements in spaceflight.

The only question that remains is how far will we push it?

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