As soon as Thursday, Virgin Galactic plans to fire the rocket motor of its spacecraft and fly to an altitude of more than 50 miles. If it accomplishes that goal, it would proclaim it has reached the edge of space, and that its pilots are the first astronauts to launch from United States soil since the last space shuttle mission in 2011.

But the test flight would also crystallize a long-simmering debate over where space begins. The Air Force and Federal Aviation Administration have awarded astronaut wings for pilots who have made it to 50 miles or above. But to many, the edge of space begins not at 50 miles, but at 62 miles, or 100 km, at the so-called Karman line, named for Theodore von Karman, one of the founders of NASA’s Jet Propulsion Laboratory.

As Virgin Galactic — the company founded by billionaire Richard Branson with a goal of taking tourists on suborbital trips high into the sky — prepares to eventually fly its first customers, the question remains: Where does space begin?

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“It’s an interesting question,” said Bill Barry, NASA’s chief historian. “There isn’t any agreed upon international definition.”

The Karman line is used by the Federation Aeronautique Internationale, the World Air Sports Federation, a record-keeping organization that promotes aeronautical activities around the world. And it was the measuring stick in the $10 million Ansari X Prize competition in 2004 — the first spacecraft to pass that altitude twice in two weeks won. As a result, it widely became accepted as the boundary of space.

Blue Origin, the space company founded by Jeffrey P. Bezos, also plans to fly its customers 62 miles or more. (Bezos owns The Washington Post.) Alan Shepard, the first American to reach space, hit an altitude of 116 miles during his 15-minute suborbital flight in 1961.

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The recent flight by astronaut NASA Nick Hague shows how tricky the issue is. In October, his flight to the International Space Station was aborted due to a rocket failure. Initially, NASA said in a statement to The Post that he is still considered to have made it to space because “he scraped the edge of [62 miles], which is the theoretical boundary of space.”

But then it backtracked, saying that Hague and Russian cosmonaut Alexey Ovchinin actually reached an altitude just short of the Karman line at approximately 93 km, or about 58 miles. It added that it considers him to be “a flown astronaut because he launched and landed in a spacecraft; he was fully trained and prepared for the launch and mission to the International Space Station.”

There is no precise definition of where space officially begins in international law. Unlike a body of water, the atmosphere doesn’t end at any precise point. Instead, the air gets progressively thinner the higher the altitude.

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Many countries prefer that ambiguity, allowing them to fly spacecraft, such as intelligence satellites, over a foreign country without crossing into another nation’s airspace.

“If you’re flying an aircraft, national sovereignty matters,” said Brian Weeden, the director of program planning at the Secure World Foundation. “But if you’re a satellite, they can over fly pretty much anywhere they want without getting permission.”

Weeden is in the camp that believe the lower threshold should suffice. “From a technical perspective, [50 miles] is a good working definition,” he said.

It’s the definition used by the Air Force, which in the 1960s awarded astronaut wings to the pilots in the X-15 program who flew the jet 50 miles or higher.

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The FAA’s Office of Commercial Space Transportation also uses 50 miles in awarding what it calls “Commercial Astronaut Wings.” That definition put it in line with the military, and the FAA has said it would also help it promote the commercial space industry, part of the agency’s mandate.

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In 2004, the agency awarded astronaut wings the pilots in the Ansari X Prize. And it also plans to honor Virgin Galactic’s pilots should they reach 50 miles or more, according to Gregory Martin, the FAA’s assistant administrator for communications.

Recently, there has been pushback against the 62-mile boundary. Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has argued for the 50-mile definition based on a mathematical analysis of objects flying through the upper layers of the atmosphere.

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He keeps a list of astronauts and said if Virgin Galactic’s pilots crest 50 miles, they’d earn the honor of astronaut.

“My plan is to count those people as astronauts and to include them on my list,” he said.

And the FAI has also said it would revisit its decision to use the 100 km definition, citing “many scientific and technical discussions around this demarcation line for the ‘edge of space’ and variance around this as a boundary condition for recognition of ‘astronaut’ status,'” it said in a recent statement. As a result, it called for an international workshop “to fully explore this issue with input and participation from the astrodynamics and astronautical community.”

Virgin would welcome that. If 50 miles was good enough for the Air Force, it is good enough for the company, George Whitesides, Virgin Galactic’s chief executive said. “Virgin Galactic has always respected this recognition and will follow the same,” he said.

Ultimately, the company’s goal is to ferry many more people to 50 miles, or higher. It has more than 600 people who have signed up for the trips on SpaceShipTwo, its spaceplane, which now cost $250,000.

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“As of today, only about 550 people have ever been to space,” he said. “That is a crazy low number in my opinion. I mean, 550 people fit on one airplane, if it’s a big plane.”

The long-term goal is to make it routine. “You can easily imagine a future in which there are four or five spaceports around the planet, and we’re operating multiple times a week at each one of those and enabling tens of thousands of people to experience space.”

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