Where Does Outer Space Begin?

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Sometimes really interesting questions can be asked very simply. For example, where does outer space begin?

This seems like a straightforward question. We live on the surface of Earth, shielded from the vacuum of space by a blanket of air. But we know that our planet’s atmosphere gets thinner, less dense, with height. So it makes sense that at some altitude, the air becomes so tenuous that you’d essentially be in space. How high up is that?

The thing is, it depends on what you mean by “space,” which is a term that’s surprisingly hard to define. Currently, the generally accepted demarcation line is 100 kilometers above Earth’s surface, but that value hasn’t been rigorously defined mathematically or physically. Moreover, when rigor is applied, a “space starts here” demarcation of 80 km is arguably a better height to use.

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To understand why, let’s start with some basics. Air is technically a fluid—something that can flow. The molecules in a gas are free to slip past one another, so they can move around to fill any container. In the case of a planet like Earth, though, there’s no physical container around it, and it doesn’t have a lid like a jar or box does. Instead gravity and pressure play the role of containing the atmosphere.

Think of Earth’s atmosphere as a well-defined layer of gas. A small volume of that gas on the surface—say, one cubic centimeter—feels the pressure of all those cubic centimeters of gas above it (close to a kilogram, all told), which compresses it and makes it denser. The higher up you go, the less overlying gas there is pushing down from above, decreasing the ambient air pressure around you.

This is why air is less dense on a mountaintop, for example, than at sea level. For a high enough mountain, such as Mount Everest, climbers need to bring pressurized air because the ambient air is almost too thin to breathe.

Thin air is also a problem for airplanes. They fly because of lift, a force generated by creating a region of lower pressure above the wing. The higher pressure under the wing pushes upward, lifting the aircraft into the sky. The strength of lift depends on many factors, including the shape of the wing, the speed of the airplane and, crucially, the density of the surrounding air. At a high enough altitude, there’s simply not enough air to provide the force necessary to keep the plane aloft, and that’s as high as a plane can fly.

In the late 1950s Hungarian polymath Theodore von Kármán calculated how much lift was generated by air versus altitude and aircraft speed, given the engineering limits of the time. One way for an aircraft to generate more lift is to move faster, but von Kármán found that at an altitude of approximately 84 km, an odd limit is reached: to generate enough lift above that height, an aircraft would have to move so rapidly that it would burn up. Compressing a gas heats it, and at those speeds the high temperatures created would turn an aircraft into a fiery meteor. For historical reasons, this limit is now called the Kármán line.

Mind you, von Kármán wasn’t trying to determine what the edge of space was; he was investigating how high an aircraft could fly. Not long after von Kármán’s work, astronomer Robert Jastrow took the (literally) top-down approach and suggested that 160 km be accepted as the transition line between Earth and space because that was the approximate lower limit for a satellite’s orbital height. Although many other studies across the decades have promoted different altitudes, a 100-km limit (higher than von Kármán’s original calculation and lower than Jastrow’s proposal ) is now as close to official as it can be after it was adopted by World Air Sports Federation, which certifies records for aeronautic and astronautic travel.

There’s merit to this approach. Below this notional air-space demarcation zone, a vehicle would have to travel at nearly orbital speeds to generate any lift. An object is in a stable orbit when the force of gravity pulling it down is balanced by the centrifugal force outward that is created by its curving path, which, for Earth, is about eight kilometers per second. That’s fast enough to generate fierce and even destructive heating if a satellite gets too low. But how low?

Astrophysicist and space historian Jonathan McDowell has long researched this issue, and he has published his conclusions in Acta Astronautica. (Full disclosure: McDowell is also my friend.)

Given that spaceflight is the most common activity of relevance for the demarcation line, McDowell established its value by studying the viability of various orbits for satellites. For example, on a very low circular orbit around Earth, a satellite will always be in the atmosphere’s upper fringes. Such a satellite would need to stay above a certain height—McDowell calculates about 125 km—to minimize the drag forces it continuously feels and maintain a stable orbit.

But some satellites are on elliptical paths, in which they can dip lower toward Earth at their perigee (closest approach). Because of orbital mechanics, this is when a satellite would travel fastest, so it wouldn’t linger at these lower reaches. Even so, the cumulative drag from repeated low-altitude dips can cause the satellite to fall out of the sky and burn up.

Examining more than 40,000 satellites, McDowell found 50 with perigees lower than 100 km that survived for at least two complete revolutions around our planet. This indicates that the popular 100-km demarcation line may be too high. He performed a mathematical analysis of the physics of lift, drag, and orbits and concluded that an altitude of 80 km is a better fit for all the data.

McDowell’s calculated 80-km limit also corresponds to the top of the atmospheric layer called the mesosphere, which is often regarded as the edge of Earth’s “proper” atmosphere. But the thermosphere and exosphere reach well past the mesosphere, and satellites orbit happily in those layers all the time. The air there is thinner than a ghost’s whisper, but it’s still extant and measurable, so these regions can be considered as Earth’s extended atmosphere despite harboring many spacecraft.

Why should we worry about all this? There are some actual concerns. Where does a country’s airspace stop? You can’t easily prevent a satellite from passing over a given parcel of Earth’s surface, which can have legal ramifications. Moreover, laws governing air flight are different from those covering spaceflight. At what point should one transition to the other? Also, different governing bodies, such as civilian and military authorities, award astronaut “wings” to travelers who reach different altitudes. This can be confusing, especially when military personnel and civilians are on the same flight, and one group gets wings while the other doesn’t. Standardizing the rules dictating the boundaries of space would help prevent confusion.

At some level this argument is semantic because a universal, rigid definition of what is literally a fuzzy border is not possible. Notably, McDowell suggests that there isn’t one universal definition that works for all contexts; the legal definition of where space begins may differ from those used by historians or engineers. This strikes me as eminently practical and consistent with the fact that many concepts in science—planets, colors, biological sex—defy a single, simple definition.

In the end, it may be best to consider that Earth’s atmosphere and “space” overlap, and where you draw your line in the air depends on the context. Nature rarely, if ever, provides sharp boundaries for discerning one concept from another. Context matters, and that’s a science lesson we should apply to a great many human endeavors.

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