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Where exactly is the edge of space?

As billionaires blast off in an effort to secure bragging rights or stroke their egos, an old question has been reignited: where does the atmosphere end and space begin?

Richard Branson's Virgin Galactic flight on July 12 rocketed up to 86km off the ground, while Jeff Bezos's Blue Origin flight this week reached just over 107km - but did either of them truly leave this planet?

According to Jonti Horner, an astronomer at the University of Southern Queensland, "It's one of those questions that's a bit like saying, 'When are you old enough to drink?' or 'When are you old enough to drive?' Every country has their own version of an answer."

Virgin Galactic's spaceship Unity in space on 12 July. Photo: Virgin Galactic

Virgin Galactic's spaceship Unity in space on 12 July. Photo: Virgin Galactic

Our age is a continuum, Horner says, and the threshold for driving or drinking is arbitrarily defined.

"Where space starts and the atmosphere ends is a little bit like that, in that the atmosphere doesn't just suddenly stop," he says. "The higher up you go, the thinner the atmosphere gets, and it keeps getting thinner and thinner and thinner, until eventually you can't tell that it's there anymore."

Blue Origins New Shepard Crew Capsule lands in West Texas after an April 2021 test flight. Photo: Blue Origin

Blue Origins New Shepard Crew Capsule lands in West Texas after an April 2021 test flight. Photo: Blue Origin

That "final" point is around 10,000km up above the surface - so it's unsurprising that some people want a line marking where the atmosphere ends and space begins a little closer to Earth.

Some have tried to define this line based on the various layers of the atmosphere, which is composed of the troposphere (up to 10km in altitude) through to stratosphere (10-50km) to the mesosphere (50-85km) to the thermosphere (85-500km) and the exosphere beyond.

But the real reason for defining an edge of space is not scientific. Instead, definitions differ between countries and companies.

The US military, the Federal Aviation Administration and NASA define the edge as 80 km off the ground, towards the upper part of the mesosphere; in the 1950s, the US Air Force awarded "astronaut wings" to anyone who flew above 50 miles (80 km).

NASA Mission Control uses a different altitude again for practical purposes - 122 km, since atmospheric drag affects objects below this.

The Fédération Aéronautique Internationale (FAI), an international record-keeping body for aeronautics, adopted their own definition in the 1960s. Called the "Krmn line", it marks the beginning of space at 100 km above Earth's mean sea level.

All of these definitions, Horner says, are "equally as good as each other and equally bad as each other.

"Now that we are in this era of commercial space tourism, suddenly people want to know where [the boundary] is because they want to know that what they did was really good enough."

Which is as easy to define as the edge of space.

  • This article is published in partnership with Cosmos Magazine. Cosmos is produced by The Royal Institution of Australia.