In early September 2016, a Falcon 9 exploded during a propellant fill operation for a standard pre-launch static fire test.[79][80] The payload, the Amos-6 communications satellite valued at US$200 million, was destroyed.[81] Musk described the event as the "most difficult and complex failure" in SpaceX's history; SpaceX reviewed nearly 3,000 channels of telemetry and video data covering a period of 35–55 milliseconds for the postmortem.[82] Musk reported that the explosion was caused by the liquid oxygen that is used as propellant turning so cold that it solidified and ignited with carbon composite helium vessels.[83] Though not considered an unsuccessful flight, the rocket explosion sent the company into a four-month launch hiatus while it worked out what went wrong.