[A third person has died. ]There's been a sad setback for Richard Branson's Virgin Galactic space tourism company yesterday: An explosion during a rocket motor test killed two people and injured another four.

It's not clear exactly what happened, but sometime earlier today the nitrous oxide motor exploded on a remote pad at the Mojave Air and Space port, mangling an 18-wheeler and spewing debris hundreds of feet. It's not known who was killed or what effect this will have on Virgin Galactic's future, but we'll keep you updated. [CNN]