What I remember most is that heart-stopping first human lunar orbit, where the crew of Apollo 8 were alone in deep space, hidden behind the moon. We waited, breathless with dread, until their voices came to us again. During that hiatus, the craft made a lunar orbit insertion burn to slow it down to enter lunar orbit, so ground control themselves didn't know whether it was successful until telemetry resumed and Lovell's voice came over the radio: "Houston, Apollo 8. Burn complete." It was Christmas Eve, and for the first time humans were orbiting another world.