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Why defining the boundary of space may be crucial for the future of spaceflight

It could change how we regulate the vehicles in the sky

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A view of Earth from the International Space Station
A view of Earth from the International Space Station
Image: NASA

On Thursday morning, just after 8AM PT, the two pilots on board Virgin Galactic’s spaceplane ignited the vehicle’s rocket engine high above the Mojave Desert for a total of 60 seconds, soaring to an altitude of 82.7 kilometers. When the vehicle reached its maximum height, the Virgin Galactic team celebrated: they had finally reached space — a new record for the company.

At least, Virgin Galactic argues it went to space. But for many, that’s not where space is.

At some point, the higher you travel through the sky, Earth’s dense atmosphere starts to thin, eventually giving way to the vacuum of space. But the exact height at which airspace ends and outer space begins has never been totally agreed upon, with many different groups offering up varying answers. Now, one international organization is considering changing its definition for where space “starts,” potentially bringing the world closer to a consensus on a complicated and surprisingly tense topic.

“Where does someone’s airspace stop? That’s clearly of interest.”

The World Air Sports Federation — or the FAI, for Fédération Aéronautique Internationale — announced on November 30th that it is considering modifying the Karman line — the altitude that is considered the boundary to space. The line is named after Theodore von Karman, a Hungarian engineer and mathematician who first calculated where Earth’s atmosphere becomes too thin for airplanes to achieve flight. For decades, the FAI has set the Karman line at 100 kilometers, or 62 miles high. But now the organization, which is responsible for cataloguing air and spaceflight records, is considering moving it to 80 kilometers, or 50 miles high.

In the short term, altering the definition for the boundary of space seems inconsequential. It might change around a few historical statistics about who has actually been beyond Earth (and the pilots who just flew Virgin Galactic’s spaceplane can boast that they were astronauts today.) But having an internationally agreed upon legal definition for space could have some big implications for the future of the spaceflight industry. For instance, it could complicate which vehicles we consider to be spacecraft and aircraft, changing how we regulate these vehicles in the future. “Where does someone’s airspace stop? That’s clearly of interest,” Jonathan McDowell, an astrophysicist at Harvard and spaceflight expert, tells The Verge.

Let’s get technical

The FAI is considering a change thanks to new research presented by McDowell. As someone who avidly records spaceflight statistics, McDowell wanted to figure out how many rockets had actually gone to space, and how many people could be considered astronauts. And to truly answer these questions, he had to know where space began. So he went through the entirety of spaceflight history to see if he could do a better job of defining this.

After combing through numerous sets of orbital statistics for spacecraft over the years, he came up with an estimate that he says is more precise than the one currently used by the FAI: 80 kilometers, plus or minus 10 kilometers. In easy-to-understand terms, this is the lowest altitude a satellite can go and still complete orbits around the Earth. To stay in orbit, and also reach such a low altitude, the vehicle has to be in an elliptical orbit. That’s one where the spacecraft swings out far away from Earth most of the time and comes in close to 80 kilometers for just a brief part of the trip. In this configuration, a spacecraft can stay in orbit for days or weeks, according to McDowell.

A long exposure photo of a SpaceX rocket, putting a spacecraft into orbit
A long exposure photo of a SpaceX rocket, putting a spacecraft into orbit
Image: SpaceX

McDowell says that 80 kilometers is the point at which gravity becomes more important than the atmosphere. “You’re in space if you can basically ignore the atmosphere,” he says. “And that doesn’t mean it has no effect, but gravity is the dominant thing you have to worry about.”

To really wrap our heads around this, we have to understand what it takes to get into orbit in the first place. The most important thing is speed. At the altitude of the International Space Station (about 254 miles / 408 kilometers), spacecraft are moving at more than 17,000 miles per hour. You need to be going that fast in order to get into a constant state of revolution around our planet. Unfortunately, our atmosphere is too thick to allow an object to orbit the Earth lower in the sky at similar speeds. “You would very quickly burst into flames, because you’d be trying to go through the air way too fast,” says McDowell. That’s what he means by the atmosphere being important.

Even above 80 kilometers, Earth’s atmosphere still exists — it’s just super thin. Satellites that orbit much higher than 80 kilometers are still interacting with the particles from our atmosphere. The air is just so thin that it’s not detrimental to a spacecraft’s orbit. “So then the question is, where do you draw a boundary where you’re no longer in space? It’s when you can’t even dip through the atmosphere briefly at orbital speed and keep on going.” says McDowell.

Virgin Galactic’s VSS Unity spaceplane, which is now considered a spacefaring vehicle by some
Virgin Galactic’s VSS Unity spaceplane, which is now considered a spacefaring vehicle by some
Image: Virgin Galactic

So if this is the most technical answer, how did the FAI’s formal definition end up set at 100 kilometers? Karman himself set his own limit at 83.8 kilometers in 1956 — however he wasn’t even trying to find the boundary of space. He was mostly trying to define how high a plane could fly and still achieve lift. Ultimately this limit was misinterpreted as the boundary of space, according to Thomas Gangale, an expert on space law and executive director of OPS-Alaska, a global research network. He and McDowell suppose that we creeped up to 100 kilometers because people simply started rounding up to a nice easy-to-remember number. “Around 1960, the FAI decided to set the limit at 100 kilometers, just for the purpose of record setting flights — that any flight above that would be considered to be a spaceflight.”

However, not everyone adheres to the FAI’s definition of space. The US Air Force, for instance, already sets the limit at 50 miles, or roughly 80 kilometers, and will give badges to any of its personnel that fly above this height. NASA does the same. And while the Federal Aviation Administration does not have an official definition, it usually gives out astronaut badges to those who have gone above 50 miles. It’s something that may become more defined as more commercial actors go to space. “Plans to issue and how to issue astronaut wings are under consideration,” an FAA spokesperson tells The Verge.

Space law reaches new heights

But while different organizations have their own definitions, there is no universal agreement. In fact, the US maintains that defining space through international law just isn’t necessary. At a United Nations meeting in Vienna in 2001, the US delegation had this to say:

With respect to the question of the definition and delimitation of outer space, we have examined this issue carefully and have listened to the various statements delivered at this session. Our position continues to be that defining or delimiting outer space is not necessary. No legal or practical problems have arisen in the absence of such a definition. On the contrary, the differing legal regimes applicable in respect of airspace and outer space have operated well in their respective spheres. The lack of a definition or delimitation of outer space has not impeded the development of activities in either sphere.

The truth is, defining space could make international relations and regulations a little murkier. If there is an internationally agreed upon solid line for where space begins, then other countries could cry foul if the US flies at a super-high altitude above a foreign territory, but below the space line. Technically, the US could be flying through another country’s airspace. As it is, there’s nothing to stop the US from doing that. “They want to keep it undefined, primarily because the US military feels it gives them the flexibility to do things at any altitude,” says Gangale. “You can’t break a law that doesn’t exist.”

“You can’t break a law that doesn’t exist.”

Ultimately, setting a line could help countries parse out what types of laws should apply to each area of the sky: the laws for aircraft or the laws for spacecraft. But things get even more complicated when you consider the fact that there’s a big gap between where the highest aircraft can go and where spacecraft can go. The highest balloons can only reach about 50 kilometers or 31 miles high, and most airplanes operate much lower than that. So in reality, there’s about a 18-mile zone between the highest an aircraft can go and where space begins. So should laws for aircraft apply to this region?

This “gray area” of the sky is really only used by spacecraft — either going to space or leaving it. Plus, some spacecraft may fly even lower than this gray area to get back to Earth. The Space Shuttle, for example, used to fly as low as 34 kilometers over Cuba’s restricted airspace when it returned from space, according to Gangale. That’s why Gangale argues that a spacecraft should be defined by where it’s meant to go. “Any flight that takes off with the intention of reaching [the 80 kilometer] altitude should be covered by space law,” he says. “It’s really the intention of a flight that decides what legal regime applies.”

Sierra Nevada Corporation’s Dream Chaser
Sierra Nevada Corporation’s Dream Chaser
Image: NASA

Gangale believes that now is the time to define what space truly is, because more and more commercial companies have ambitious plans to go to space. And some of them may need to fly over parts of the Earth that might be below the space line, too. Sierra Nevada Corporation is working on a small winged vehicle that glides back to Earth like the Space Shuttle did. It’s possible future companies may want to avoid any legal problems that might arise if they have to fly low over another country on the way back home from space. And they might want to know what laws apply in case there is some kind of mishap in the air. “In the long-term, as we see more commercial operations in those altitudes, especially getting up into orbit and coming back, these private companies are going to want the legal certainty that would be provided by having a set altitude or limit,” says Gangale.

It seems that the US is in no hurry, though. But in case more entities express a desire for a clear definition, Gangale has written a draft convention that the United Nations can adopt if necessary. And given the interest in his work, he thinks that having a clear definition may have to happen soon. “As spaceflight becomes more and more commercialized, I think this idea that we don’t need a definition becomes less and less tenable.”