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Virgin Galactic gets set for SpaceShipTwo flights that aim f…

SpaceShipTwo / VSS Unity

Virgin Galactic is getting VSS Unity ready for a crucial series of test flights. (Virgin Galactic Photo)

Virgin Galactic says it’s beginning a series of SpaceShipTwo test flights that could cross the edge of the space frontier as early as Thursday — amid a debate over where exactly that edge kicks in.

The company has been flight-testing its VSS Unity rocket plane for more than two years, with its most recent rocket-powered flight rising to a height of 32 miles (52 kilometers) in July. The plan for the next stage of testing at California’s Mojave Air and Space Port was laid out in a statement issued today.

“During this phase of the flight program we will be expanding the envelope for altitude, air speed, loads and thermal heating,” Virgin Galactic said. “We also plan to burn the rocket motor for durations which will see our pilots and spaceship reach space for the first time. Although this could happen as soon as Thursday morning, the nature of flight test means that it may take us a little longer to get to that milestone.”

50 vs. 62 miles

When Virgin Galactic was unveiled in 2004 by British billionaire Richard Branson, at the height of the $10 million Ansari X Prize for private spaceflight, the prize competition’s organizers defined the edge of space was typically defined as 100 kilometers (62 miles) in altitude.

That boundary, known as the Karman Line, is based on the standard adopted by recordkeeping groups such as the Fédération Aéronautique Internationale, or FAI, and the International Astronautical Federation, or IAF.

But Virgin Galactic has been focusing on a different space standard that’s energetically easier to achieve: 50 miles, or 80 kilometers, which was the bar set for U.S. military pilots to get their astronaut wings.

The 50-mile standard is the one that’s written into the contracts that Virgin Galactic has with its more than 600 “Future Astronauts,” CEO George T. Whitesides told me back in 2014. “Fifty miles has been in there from the start,” he said.

Nevertheless, there’s some magic about the 100-kilometer mark. Amazon billionaire Jeff Bezos’ Blue Origin space venture, for example, has used that altitude as a target for uncrewed flight tests of its own New Shepard suborbital spaceship. New Shepard is due to start carrying people next year.

The 50-mile vs. 100-kilometer debate has picked up in recent months, due in part to a research paper written by Jonathan McDowell, an astrophysicist and satellite expert at the Harvard-Smithsonian Center for Astrophysics.

The paper, published by Acta Astronautica in July, notes that an elliptical Earth orbit could remain stable even if a spacecraft dipped down to an altitude as low 80 kilometers. For that reason, plus other reasons drawn from history and atmospheric science, McDowell suggested that 80 kilometers, or 50 miles, would serve as a more appropriate space boundary.

Such arguments were compelling enough that FAI and IAF plan to reconsider their own 100-kilometer definition next year.