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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 astronomer 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.

How the tests are run

In the meantime, Virgin Galactic is likely to continue with flight tests in California, in preparation for the start of commercial flights with paying passengers at Spaceport America in New Mexico. Those passengers have paid as much as $250,000 each for space tour packages, and some of them have been waiting for almost 15 years to take a ride.

SpaceShipTwo’s flight profile, for the tests as well as for commercial service, calls for the rocket plane to be carried up to an altitude of about 45,000 feet slung beneath a twin-fuselage carrier airplane known as White Knight Two.

From that height, the mothership releases the rocket plane, followed seconds later by ignition of SpaceShipTwo’s hybrid rocket motor. After firing for about a minute, the rocket cuts out, and the plane coasts to its maximum altitude. Up to six passengers aboard the plane would feel about four minutes of weightlessness and see the curving Earth beneath the black sky of space.

On the way down, SpaceShipTwo is designed to use a folding-wing system to slow its supersonic descent, and then glide down to a runway landing.

In today’s statement, Virgin Galactic said it would fire VSS Unity’s rocket motor during the next test flight “for longer than we ever have in flight before, but not to its full duration.”

The duration of the firing would depend on the craft’s supersonic handling qualities and its thermal dynamics, as observed by the plane’s two pilots and by Virgin Galactic’s mission control.

“If all goes to plan our pilots will experience an extended period of microgravity as VSS Unity coasts to apogee, although — being pilots — they will remain securely strapped in throughout,” Virgin Galactic said. “They should also have some pretty spectacular views, which we look forward to sharing as soon as possible post-flight.”

‘First passenger’ is waiting

The company said Unity will be carrying four research payloads that are part of NASA’s Flight Opportunities Program, in part to simulate the weight of passengers.

“Whether we complete all our objectives during the next flight or need to wait a little longer, we remain committed to completing the final stages of this extraordinary flight test program as quickly, but more importantly as safely, as possible,” Virgin Galactic said.

Getting to space, whether it’s defined as 50 miles or 100 kilometers, would be a huge milestone for a program that suffered a major blow in 2014 when the first SpaceShipTwo plane, VSS Enterprise, broke up during a rocket-powered test flight. The accident caused the death of one test pilot and left the other pilot seriously injured.

After a months-long federal investigation, Virgin Galactic made significant changes in the SpaceShipTwo design and in pilot training procedures. Some “Future Astronauts” canceled their reservations, but most stuck with Branson and his team.

In an interview last month, Branson told CNN that once the test program is finished, he would be the first passenger. “I’m not allowed up until the [test pilots] have broken it in a few times first,” he said.

Virgin Galactic’s sister venture, The Spaceship Company, is already building two more SpaceShipTwo models — reportedly nicknamed Etta and Artie in honor of Branson’s grandchildren.

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