The Galactic Census Bureau was some form of Nexus-wide organization that tracked populations of the many planets of the Nexus. They visited Athos once a year, spending approximately a week each time; they also brought the mail on these visits. The particular ship that visited Athos had an all-male crew because women didn't care to have 4 months of time on a ship with no planetary leave. At the time Ethan Urquhart rode outbound from Athos as a passenger on the ship, it had a crew of eight; on his return trip, there were five people (a statistician, his assistant, and a crew of three).