Skip to main content

Where is space?

February 22, 2016
Rendering of all the satellites orbiting earth

Space. It's the final frontier and also happens to be where NOAA operates the Nation's environmental and meteorological satellites.

But where is “space” exactly? This may seem like a simple question, but any answer beyond “up” may be more complicated than you think. Although most people are generally in agreement that space begins when Earth’s atmosphere ends— where exactly that is depends on who you ask.

International law states that outer space shall be free for exploration and use by all, but there is no definitive law stating where national air space actually ends and outer space begins. This leaves the door open for a variety of interpretations.

A common definition of space is known as the Kármán Line, an imaginary boundary 100 kilometers (62 miles) above mean sea level. In theory, once this 100 km line is crossed, the atmosphere becomes too thin to provide enough lift for conventional aircraft to maintain flight. At this altitude, a conventional plane would need to reach orbital velocity or risk falling back to Earth.

The world governing body for aeronautic and astronautic records, the Fédération Aéronautique Internationale (FAI), and many other organizations use the Kármán Line as a way of determining when space flight has been achieved.

Image at NESDIS
NOAA hurricane hunter WP-3D Orion and Gulfstream IV aircraft in flight. Commercial planes fly at an altitude of 30,000 to 40,000 feet (5 to 6 miles) Credit: NOAA
Rendering of Satellites
Earth's atmosphere is our natural shield against the harsh conditions of space— including everything from meteors and falling satellites to deadly ultraviolet radiation from the sun. It also contains the air we breathe, the weather we experience and helps to regulate planetary temperatures. Credit: NOAA

The U.S. military and NASA define space differently. According to them, space starts 12 miles below the Kármán Line, at 50 miles above Earth's surface. Pilots, mission specialists and civilians who cross this boundary are officially deemed astronauts.

If we define getting to space by the strictest of terms—that is, by escaping Earth's atmosphere completely—you may have to travel over 600 miles or more to the outermost layer of the atmosphere to reach it. Here, the atmosphere becomes incredibly thin and starts to give way to the stronger, more violent solar winds of the sun.

This way of defining space complicates things a bit, though. At that altitude the International Space Station (orbiting between 205 to 270 miles up), the space shuttle (which orbited 200 miles up) and some of NOAA's polar-orbiting satellites (orbiting 540 miles up) would not be considered spacecraft!

In recent years, scientists have tried to determine a definitive ‘edge of space' through various studies of the atmosphere. In 2009, researchers at the University of Calgary designed and launched the Supra-Thermal Ion Imager , an instrument developed to measure the transition between the relatively gentle winds of Earth's atmosphere and the more violent flows of charged particles in space. According to their data, the edge of space begins at 118km (73 miles) above sea level.

Where is space? Until now, a simple nod to the sky may have sufficed, but it seems that with each passing year the final frontier is becoming a little more accessible. In today's world of potential commercial passenger space flights, missions to Mars and unimaginable technological advancements, outer space may be getting closer than we ever thought.

"Freedom of Space"

The concept of "Freedom of Space" began in the 1950's with the dawn of the space age, the Cold War and President Dwight D. Eisenhower. With the threat of nuclear war looming, President Eisenhower and his advisers sought to ensure international acceptance of "freedom of space" for the unspoken use of reconnaissance satellites.

His interpretation of space treated it much like the high seas. Outside of national territorial airspace, nations were free to conduct peaceful space operations without concern for international borders. Well aware of the President's unstated motivation to operate high altitude spy satellites safely, the Soviet Union adamantly opposed the idea, seeking to establish much higher airspace boundaries.


Photograph of William H. Pickering, James van Allen, and Wernher von Braun hold aloft a full-scale model of Explorer I satellite.
William H. Pickering, James van Allen, and Wernher von Braun hold aloft a full-scale model of Explorer I, the United States’ first satellite, at a press conference announcing its successful entry into orbit. Credit: NASA

Despite such protests, Eisenhower announced that the United States would launch a scientific Earth satellite as part of the 1957 International Geophysical Year (IGY).

The U.S. hoped that a science satellite, already part of the IGY plan and far less controversial than a spy satellite, would make the world's first foray into space and set the precedent for freedom of space.

Ironically, it was the Soviet Union who inadvertently set the precedent with the launch of Sputnik-1, the world's first artificial satellite. As Sputnik, and the satellites that followed during the IGY, crossed international borders without protest, the concept of Freedom of Space" was tentatively established.