Today one test pilot died when Virgin Galactic's SpaceShipTwo crashed in the Mojave desert. A second pilot was evacuated to a hospital.

It's not clear why the crash occurred. SpaceShipTwo had been undergoing testing ahead of commercial flights. The spacecraft launches from the belly of the larger WhiteKnightTwo, once that cargo aircraft has reached cruising altitude similar to a normal airplane. Once SpaceShipTwo separates, it functions as a glider with a rocket motor. WhiteKnightTwo's takeoff occurred at 9:20 a.m. local time; SpaceShipTwo was released at 10:10. At 10:12, the ground crew became aware of an "inflight anomaly."

"From my eyes and my ears, I detected nothing abnormal," said Stuart Witt, the chief executive officer of the Mojave Air and Space Port, where Virgin Galactic operates. "I didn't detect anything other than a pause. It wasn't what did happen, it was what didn't I see or hear. If there was an explosion, I didn't see it." Parabolicarc.com, which had been livetweeting the flight, reports that SpaceShipTwo "came down in pieces" after exploding in mid-air.

Witt had been briefed to expect that the plume from the rocket engines would look slightly different, because the fuel had been changed, he said. In May, Virgin Galactic announced it was changing the type of solid fuel it was using to avoid engine instabilities. The required qualification tests for the switch continued until about a month before today and this was the first test using the new fuel. The engine had also been changed, and this was its fourth flight, according to Kevin Mickey, the president of Scaled Composites. Scaled Composites is the aerospace company that built the spacecraft and employed the test pilots.

The families of the test pilots have been notified, Mickey said. The names of the pilots have not yet been released. Richard Branson, founder of the Virgin Group, is scheduled to arrive in Mojave on Saturday morning, as will the National Transportation Safety Board.

This has been a bad week for aerospace companies. On Oct. 28, Orbital Sciences Corp.'s Antares rocket exploded shortly after liftoff during a resupply mission to the International Space Station. No one was aboard the ship, and no one was injured. Today's accident, however, was in a vehicle designed to carry people rather than just cargo.

Here's an earlier tweet, showing WhiteKnightTwo paired with SpaceShipTwo on the tarmac:

— Virgin Galactic (@virgingalactic) October 31, 2014

The last test of SpaceShipTwo was in January this year, when the spaceship went supersonic, according to CNET. The ship is built for short trips meant to offer a few moments of weightlessness. The wait list for a flight includes Ashton Kutcher and Lady Gaga, according to Space.com, as well as 700 other people.

Despite the grim week, Mojave's Witt enjoined the international community not to abandon space. "This is not easy. If it were easy, it would not be interesting," he said. "It hasn't been an easy week, it's been a challenge. But where I'm from, this is where you find out your true character."

Veteran space reporter Adam Rogers disagrees with this assessment. SpaceShipTwo is "not a vehicle for the exploration of frontiers. This would be true even if Virgin Galactic did more than barely brush up against the bottom of space," he writes at Wired, in an editorial entitled "Space Tourism Isn't Worth Dying For." "Virgin Galactic is building the world’s most expensive roller coaster, the aerospace version of Beluga caviar. It’s a thing for rich people to do: pay $250,000 to not feel the weight of the world."

Update 6:31 pm ET: Story updated with information from the Mojave Mojave Air and Space Port's press conference.