On July 3 we were preparing for the main event -- the command load that takes us through the flyby, which starts on the 7th. [A command load is a sequence of commands for the spacecraft, a program that it will execute autonomously.] All the observations from the 7th of July through the encounter on the 14th and on through the 16th are in a single command load. On the 3rd we loaded it onto the backup computer, and on the 4th we were loading it on to the primary computer.

At the same time, we were taking all the data we wanted to get down to the ground but wanted to save, and were compressing it into a single location on the [solid-state] data recorder. We were doing multiple things on the spacecraft at the same time, and what occurred is as we were doing the compression of all the data to save it for later download, we were burning to the flash memory the command load into the primary computer.

The computer was trying to do two things at the same time, and the two were more than the processor could handle at the same time, so the processor overloaded. The system autonomously switched to the backup computer and since it did that and we were not in encounter mode, we went to safe mode. At that point we lost the downlink from the primary side of the spacecraft because it had switched to the secondary side. [So note that while it was compressing data and loading software, New Horizons was in the middle of doing a third thing, downlinking data.]

We knew it would take about an hour for the spacecraft to transmit to Earth from the backup computer. We started looking for signal on backup side, and found it when expected. We looked at data, figured out what was happening, and started to put a plan in place to recover. The command load has now been put on the primary side so that when the 7th rolls around, we will go into the core load as planned.