According to a
statement on the Orbital Sciences website:
All systems appeared to be performing nominally until approximately T+15 seconds at which point the failure occurred. Evidence suggests the failure initiated in the first stage after which the vehicle lost its propulsive capability and fell back to the ground impacting near, but not on, the launch pad. Prior to impacting the ground, the rocket’s Flight Termination System was engaged by the designated official in the Wallops Range Control Center.
The official NASA video is on the
Wikipedia page; it's from near the launchpad up to a few seconds after launch, and then cuts to one from further away (possibly losing a few seconds). At around 0:45 on the video, the rocket exhaust flame gets brighter and bigger. Over the next few seconds, there looks to be a small explosion in the exhaust, the rocket stops accelerating, slows down and starts falling, and then there's another explosion or burst of flame. I would guess that the change in the flame at 0:45 is the "failure" reported by Orbital, and that the burst of flame after the rocket starts falling is the flight termination system.
How in the hell would they know that so soon?
I would guess that there's a prescribed safe path, dictating where the rocket should be at what time after the launch. If it deviates from that path, either in location or not accelerating correctly, then it's considered unsafe and is destroyed.
