Where Does Space Begin?

Space launches are exciting to watch and feel. A rocket leaps off the pad to space, roaring its way up and creating a shockwave of sound that rattles your bones (if you're within a few miles). Within a few minutes, it has entered space, ready to deliver payloads (and sometimes people) to space. 

But, when does that rocket actually enter space? It's a good question that doesn't have a definite answer.

There is no specific boundary that defines where space begins. There isn't a line in the atmosphere with a sign that says, "Space is Thataway!"  

The Boundary between Earth and Space

The line between space and "not space" is really determined by our atmosphere. Down here on the surface of the planet, it's thick enough to support life. Rising up through the atmosphere, the air gradually gets thinner. There are 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. All satellites 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, scientists had to come up with an official "boundary" between atmosphere and space.

Today, the commonly agreed-upon definition of where space begins is around 100 kilometers (62 miles). It's also called the von Kármán line. Anyone who flies above 80 km (50 miles) in altitude is usually considered an astronaut, according to NASA.

Exploring Atmospheric Layers

To see why it's difficult to define where space begins, 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 at 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 into regions where the air is quite thin. The tallest mountains rise up to between 4,200 and 9,144 meters (14,000 to nearly 30,000 feet). 

Most passenger jets fly at around up around 10 kilometers (or 6 miles) up. Even the best military jets rarely climb above 30 km  (98,425 feet). Weather balloons can get up to 40 kilometers (about 25 miles) in altitude. Meteors flare about 12 kilometers up. The northern or southern lights (auroral displays) 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. It is well above the dividing line that indicates the beginning of space. 

Types of Space

Astronomers and planetary scientists often divide the "near-Earth" space environment into different regions. There is "geospace," which is that area of space nearest Earth, but basically outside the dividing line.

Then, there's "cislunar" space, which is the region that extends out beyond the Moon and encompasses both Earth and the Moon. Beyond that is interplanetary space, which extends around the Sun and planets, out to the limits of the Oort Cloud. The next area is 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. Those regions usually contain gas molecules and dust and effectively make up a vacuum.

Legal Space

For purposes of law and record-keeping, most experts consider space to begin at an altitude of 100 km (62 miles), 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. 

There are some very straightforward reasons why such a division exists. It reflects an environment where rockets are able to fly. 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. This was still well above the von Kármán line, but in reality, there were good engineering reasons to define for the shuttles, which carried human lives and had a higher requirement for safety. 

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. What that means is that no country can stake a claim in space and keep others out of it.

Thus, it became important to define "outer space" for geopolitical reasons having nothing to do with safety or engineering. The treaties that invoke the boundaries of space govern what governments can do at or near other bodies in space. It also provides guidelines for the development of human colonies and other research missions on the planets, moons, and asteroids.