After a nine-month investigation, federal safety officials plan to announce Tuesday what they believe caused Virgin Galactic's SpaceShipTwo, an experimental rocket ship, to break apart high above the Mojave desert last fall, killing one of two pilots.
Investigators at the National Transportation Safety Board have said that error by a co-pilot was one of the potential causes of the Oct. 31 crash.
SpaceShipTwo was more than 10 miles high on a test flight when it disintegrated seconds after it fired its rocket engines.
The crash was a major setback for British billionaire Richard Branson's Virgin Galactic, which was created with the goal of eventually ferrying wealthy tourists to the edge of space.
Already investigators have revealed that video taken from inside the cockpit shows that co-pilot Michael Alsbury prematurely unlocked the spacecraft's movable tail, which appeared to set off the sequence of events that caused it to break apart seconds later.
Thirty-nine-year-old Alsbury died, while pilot Peter Siebold, 43, was thrown from the rocket ship, still harnessed in his seat.
Siebold told investigators that he somehow unbuckled himself from his seat before his parachute deployed automatically.
Last week, Virgin Galactic sent a letter to its 700 customers who have bought a ticket for as much as $250,000 to fly on SpaceShipTwo.
Branson's company told the customers – or its "astronauts" – that it would hold live briefings after the NTSB's meeting to explain the investigators' conclusions.
In the letter, a copy of which was obtained by the blog Parabolic Arc, Virgin Galactic continued to distance itself from Scaled Composites, a company that designed and built SpaceShipTwo. Mojave-based Scaled was operating the test flight when the spacecraft disintegrated.
The two test pilots worked for Scaled, which was founded by aviation engineer Burt Rutan and is now owned by Northrop Grumman, the aerospace giant.
The Virgin Galactic letter begins by telling customers that Scaled was "responsible for the flight."
The two companies split after the crash and Virgin Galactic is now building and testing a new version of SpaceShipTwo in a hangar at the Mojave Air and Space Port.
Even before the Oct. 31 crash, the companies were planning to soon transfer the license and full ownership of SpaceShipTwo to Branson's company, Virgin Galactic executives have said.
Both Virgin Galactic and Scaled have been cooperating with investigators.
SpaceShipTwo is designed to blast to the edge of Earth's atmosphere, about 60 miles high, where its engines burn out. Then the craft is supposed to begin a gradual descent back down, slowed by two innovative structures known as feathers.
On the way up, the feathers look like long tails of a regular aircraft. But on the way down they fold up perpendicular to the line of flight and create aerodynamic drag that slows the plane.
Co-pilot Alsbury was shown on video unlocking the feather too early by moving a lever as the rocket ship was still blasting upward.
Alsbury, the father of two young children, had worked for Scaled Composites for 14 years as a project engineer and test pilot.
In April 2013, he served as co-pilot on SpaceShipTwo's first rocket-powered flight. The aircraft broke the sound barrier, reaching Mach 1.2, before ending with a smooth landing in Mojave.
The Oct. 31 crash was the second fatal disaster at Scaled as it worked on SpaceShipTwo for Virgin Galactic. In 2007, three Scaled employees were killed when a tank of nitrous oxide ignited during a test of the spacecraft's propellant system.
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