Where Does Space Begin?

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Space begins about 100 km (62 miles) above Earth's surface. Astronauts routinely live and work in space, but must live in special environments and wear space suits to work where there's no air to breathe and temperatures are extreme. NASA

Where does space begin? It's a good question that doesn't have a definite answer. Here's why. Space has long been defined as "the area beyond Earth's atmosphere". However, that is a slippery definition. There is no specific boundary that defines where space begins. There's no line in the atmosphere that has a sign that says, "Space Thataway!".  

The line between space and "not space" is really determined by our atmosphere.

It gradually gets thinner as you move away from Earth. You can find traces of the gases we breathe more than a hundred miles above our planet, but eventually, they thin out so much that it's no different from the near-vacuum of space. Some satellites have measured tenuous bits of Earth's atmosphere out to more than 800 kilometers (nearly 500 miles) away. They orbit well above our atmosphere and are officially considered "in space". Given that our atmosphere does thin so gradually and there is no clear-cut boundary, the commonly agreed-upon definition of where space begins is around 100 kilometers (62 miles). Anyone who flies above 80 km (50 miles) in altitude is usually considered an astronaut, according to NASA. 

Exploring Atmospheric Layers

To give you an idea of why it's difficult to define where space begins, just take a look at how our atmosphere works. Think of it as a layer cake made of gases.

It's thicker near the surface of our planet and thinner at the top. We live and work in the lowest level, and most humans live in the lower mile or so of the atmosphere. It's only when we travel by air or climb high mountains that we get higher.  The tallest mountains rise up to between 14,000 and nearly 30,000 feet.

 

Most passenger jets fly at around only 30,000-37,000  feet (nearly 10 kilometers (or 6 miles)) up. Even the best military jets rarely climb above 100,000 feet (a little less than 30 km (19 miles)).  

Weather balloons can get up to 40 kilometers (about 25 miles) in altitude. Meteors flare about 12 kilometers up. If you've ever seen the northern or southern lights (auroral displays), those are about 90 kilometers (~55 miles) high. 

The International Space Station orbits between 330 and 410 kilometers (205-255 miles) above Earth's surface and well above the atmosphere. Thus, it is also considered to be "in" outer space.

Types of Space

Astronomers and planetary scientists often break down the "near-Earth" space environment into different regions. These are "geospace", which is that area of space nearest Earth; "cislunar" space, which is the region that extends out beyond the Moon; interplanetary space, which extends around the Sun and planets, and interstellar space (which encompasses the space between the stars). Beyond that are galactic space and intergalactic space, which focus on the spaces within the galaxy and between galaxies, respectively. In most cases, the space between stars and ​the vast regions between ​galaxies are not really empty.

It usually contains gas molecules and dust and is effectively a vacuum.

For purposes of law and record-keeping, most experts consider space to begin at an altitude of 100 km (62 miles), a region called the von Kármán line. It's named after Theodore von Kármán, an engineer and physicist who worked heavily in aeronautics and astronautics. He was the first to determine that the atmosphere at this level is too thin to support aeronautical flight. However, there are other ways to determine where space begins that affect spacecraft and aircraft.

In very practical terms, engineers who design spacecraft need to make sure they can handle the rigors of space. Defining space in terms of atmospheric drag, temperature, and pressure (or lack of one in a vacuum) is important since vehicles and satellites have to be constructed to withstand extreme environments.

For purposes of landing safely on Earth, the designers and operators of the U.S. space shuttle fleet determined that the "boundary of outer space" for the shuttles was at an altitude of 122 km (76 miles). At that level, the shuttles could begin to "feel" atmospheric drag from Earth's blanket of air, and that affected how they were steered to their landings. 

Politics and the Definition of Outer Space

The idea of outer space is central to many treaties that govern the peaceful uses of space and the bodies in it. For example, the Outer Space Treaty (signed by 104 countries and first passed by the United Nations in 1967), keeps countries from claiming sovereign territory in outer space. Thus, it became important to define "outer space". It governs what governments can do at other bodies in space, and provides guidelines for the development of human colonies and other research missions on the planets, moons, and asteroids. 

Expanded and edited by Carolyn Collins Petersen.