There is no sign saying “Welcome to Outer Space” when a ship such as Virgin Galactic’s SpaceShipTwo arrives at its edge, as it reportedly did Thursday. The atmosphere doesn’t abruptly end, but thins out gradually. The craft’s maximum altitude of 82.7 km was lower than the 100-km limit set by the Ansari X prize back in 2004. (This was won by SpaceShipOne, then of Mojave Aerospace Ventures, which maxed out at 112.17 km.)

Is it OK to lower the bar on the edge of space? Who gets to decide where space begins? Virgin Galactic and its competitors in the space tourism trade would like to make it low enough to say their customers are getting there. (Though moving the line won’t affect Elon Musk if he pulls off his promised trek around the moon.)

The boundary shouldn’t be an arbitrary one. Astrophysicist and space historian Jonathan McDowell argues that the edge of space should be defined by physics. In keeping track of space history, it matters who gets official credit for getting there first. In the mid-20th century, scientists tried to set that limit at how low you can go and still sustain an orbit — an altitude known as the Karman line, named after aerospace engineer Theodore von Karman. At some point atmospheric drag becomes too big a factor to sustain even a highly elliptical orbit — one that swings in close and then out much farther.

For years, the official Karman line has been set at 100 km. But that was not the value Karman set for it. In a paper published earlier this year in the journal Acta Astronautica, McDowell recalculated the Karman line, and found it’s considerably closer — just close enough to make SpaceShipTwo a space-faring craft.

When I called McDowell, who works at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, he said that the U.S. government has long resisted creating an official legal boundary between air and space, even though there’s a need for one. Aircraft are subject to rules regarding airspace, and objects in space are not, but are subject to international treaties on the peaceful use of space.

When North Korea launched a missile last year, reportedly over Japanese airspace, it was actually higher than the International Space Station, McDowell said.

“Of course it’s in space, and it doesn’t make sense to say it’s in Japanese airspace,” he said, but without an international agreement about the boundary between air and space, such confusion is inevitable.

He said that people had previously tried to calculate the Karman line back in the 1950s and early 1960s, and they got values pretty close to his value, which was 80 km. But in the late 1960s, it was set at 100 km. It wasn’t clear why, except that people didn’t think it was necessary to do the exact calculations, and this was a nice round number. It’s clearly higher than airplanes can fly — which is only about 50 km. There’s a gap, he said, between where air-y things can fly, and space-y things can sustain an orbit.

The limit for space-y things isn’t absolute because denser objects can go through thicker atmosphere and stay in orbit; a feather has a higher Karman limit than a bowling ball. And there are seasonal and regional differences in atmospheric density. But 80 km looks like a much better approximation than 100 km. Which reopens an old question: Who got to space first?

The German V-2 rockets would have been the first to reach space, in the 1940s. And the first humans to get there? The pilots of the X-15 Space Plane, McDowell said. This joint NASA-Defense Department project looked like a rocket with little wings. It flew 200 times between 1959 and 1968, carried under a B-52 bomber and released over Nevada.

Despite the setting of the Karman limit at 100 km, the U.S. did decide to grant astronaut wings to all X-15 pilots who went over 80 km.

“This is one of the reasons astronaut wings are a thing,” McDowell said. They are granted by NASA and the U.S. Air Force to all pilots who reach that altitude. The astronaut wings are still a thing, and Virgin Galactic has reported that their pilots Mark Stucky and Frederick Sturckow will get theirs, though Sturckow, a NASA pilot, presumably already has astronaut wings.

Having an official, legal, science-based definition of space would get rid of any ambiguity surrounding the granting of wings and other facets of space history. According to a story in SpaceNews, international bodies are considering making 80 km official.

Farther out, there’s a boundary defined by the Earth’s magnetic field, where we’re shielded from the wind of charged particles coming off the sun. And farther still, there’s a boundary between our solar system’s bubble of solar wind and true interstellar space, into which the spacecraft Voyager 2 crossed earlier this month, a few years behind its predecessor, Voyager 1.

What we’re really talking about here is different realms of space, because, as a precocious 7-year-old recently pointed out to my editor, all inhabitants of Earth are traveling through space. The fact that there are different kinds of space is part of what makes it so interesting to explore.

Science writer Faye Flam is a Bloomberg Opinion columnist.

In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.