SpaceShipTwo undertook its 23rd glide flight on Dec. 19, 2012 in the pre-powered portion of its incremental test flight program. This was a significant flight as it was the first with rocket motor components installed, including tanks. It was also the first flight with thermal protection applied to the spaceship's leading edges.

SpaceShipTwo is a spacecraft that aims to take tourists on brief trips to suborbital space. Manufactured by The Spaceship Company, the vehicle is currently in the testing stage, with commercial operations expected to begin in the next few years. So far, the spacecraft's most notable milestone was flying beyond the boundary of the U.S. Air Force's definition of space on Dec. 3, 2018, reaching an altitude of 51.4 miles (82.7 kilometers).

The technology for SpaceShipTwo was forged in competition for the $10 million Ansari X Prize, which was created to spur the development of private spaceflight. After the space ship's predecessor, SpaceShipOne, won the prize in 2004, two companies — Scaled Composites and Virgin Galactic — merged to form The Spaceship Company. The goal of the new firm was to commercialize spaceflight technology. In 2012, Virgin Galactic bought out Scaled Composites, gaining full ownership of the venture.

While SpaceShipTwo prototypes have completed a number of engine-powered test flights (as opposed to the craft gliding from the larger carrier aircraft, WhiteKnightTwo), developing SpaceshipTwo has come with its share of setbacks.

X Prize ancestry

SpaceShipTwo's ancestor, SpaceShipOne, was designed with the goal of winning the $10 million Ansari X Prize by becoming the first privately-funded vehicle to fly to space and back twice within a two-week span. SpaceShipOne was manufactured by Mojave Aerospace Ventures, a joint venture of Scaled Composites — a firm led by famed aerospace designer Burt Rutan and Microsoft co-founder Paul Allen.

In 2004, SpaceShipOne launched from beneath a carrier aircraft, WhiteKnightOne, then separated and rocketed to 62.5 miles (100 km) in altitude to become the first privately funded crewed spacecraft to breach the Kármán line — the internationally accepted definition of the space boundary that sits at 62 miles (100 km) up.

Just before the spacecraft won the X Prize that year, Virgin Galactic's Richard Branson announced that he would work with Scaled Composites to manufacture a commercial version of the craft that would take tourists to suborbital space; development then began on SpaceShipTwo.

Tickets for tourists wanting a spot on the commercial spacecraft initially cost $200,000 each and later rose to $250,000.

SpaceShipTwo will carry six passengers up past 328,000 feet altitude (100 kilometers), the point where astronaut wings are awarded. See how Virgin Galactic's SpaceShipTwo works in this SPACE.com infographic (Image credit: Karl Tate, SPACE.com contributor)

Development milestones

As development of the spacecraft proceeded, the company focused on locating a launch area. In December 2005, the state of New Mexico offered Virgin Galactic Spaceport America, a taxpayer-funded, $225 million facility. Virgin Galactic accepted the offer and established its global headquarters and spaceflight operations there.

The company rolled out several construction milestones in the next year, but the momentum halted in 2007 after a fatal fire during a routine ground test killed three Scaled Composites employees. The accident sent shockwaves through the company and the greater "new space" community. In addition to the fatalities, multiple employees were injured during the test. As a result, work was suspended as the company investigated the cause of the accident.

In July 2008, Virgin Galactic held a major launch event to showcase WhiteKnightTwo, which would carry SpaceShipTwo into the sky. The first WhiteKnightTwo carrier aircraft, Eve, completed its first successful test flight before the end of 2008.

The first SpaceShipTwo, VSS Enterprise, was unveiled in December 2009; it was named after the famous "Star Trek" spacecraft. ( The first NASA space shuttle was also named Enterprise; it was only used for glide tests and never made it into space.)

(Image credit: Bill Deaver/Deaver-Wiggins and Associates)

Crewed test flights of VSS Enterprise began in July 2010. Two years later, in late 2012, the company successfully completed a major glide test. Then the first rocket-powered test flight took place in April 2013.

On Oct. 31, 2014, the vehicle broke apart during its fourth powered test flight, killing copilot Michael Alsbury and injuring pilot Peter Siebold. A crash investigation led by the National Transportation Safety Board indicated that the feathering re-entry system was deployed too early during the flight. The system's design was changed in the next SpaceShipTwo prototype, VSS Unity.

VSS Unity's first glide flight was on Dec. 3, 2016, and less than two years later, on April 5, 2018, the craft flew with engine power for the first time. Its highest altitude to date was achieved on Dec. 13, 2018, when it soared beyond the space boundary , at least according to the U.S. Air Force's definition. Unity has yet to reach above the Kármán line.

A number of Virgin Galactic's future astronauts gathered with Sir Richard Branson (center) for a group photo at Virgin Galactic's Final Assembly, Integration and Test Hangar (FAITH) in Mojave, Calif. Nearby is the WhiteKnightTwo mated with SpaceShipTwo. (Image credit: Virgin Galactic/Mark Greenberg)

A typical flight profile

When it begins commercial flights, SpaceShipTwo will carry six passengers and fly to 50,000 feet (15,240 meters) in altitude while attached to the bottom of WhiteKnightTwo. The craft will then separate for the trip into space by firing its engines for about 70 seconds and then shutting them off for the final coast up to 62 miles (100 km) above Earth. The passengers will feel weightless for about 5 minutes, Virgin Galactic representatives said. Next, SpaceShipTwo will turn back to Earth and "feather" its rudders, turning them up to 90 degrees to increase drag and control the movement of the spacecraft.

At 70,000 feet (21,336 m), the spaceship will have enough air around it to move the rudders back to a configuration that allows for gliding during its descent. It will then land on Earth on a regular aviation runway.

Branson aims to be onboard when SpaceShipTwo is ready to fly passengers to space. The billionaire is an amateur adventurer who is famous for his publicity stunts, and said he has been interested in space since at least the Apollo era.

The lengthy delay of the first commercial flights (Branson initially said the spacecraft would carry tourists in 2007) has caused some ticket holders to ask for refunds, but the vast majority still eagerly await the experience to visit space. Industry watchers have applauded Virgin's patient approach.

"They're taking the time necessary to make sure the vehicles are as safe as they possibly can be before they take paying customers up," John Gedmark, executive director of the Commercial Spaceflight Federation, told Space.com in 2011 . "There's never going to be such a thing as perfectly safe spaceflight, but they're going to get as safe as they possibly can."

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