What time zone is used on ISS to define when a day starts? Is it the same as mission control? Do Russian cosmonauts use a different time zone respect to European and American astronauts?
Virtually all spacecraft use one of two time conventions, or more likely, both of these
- Universal Time Coordination (UTC)
- GPS Time
The difference between these two is slight, but changes periodically. The difference is currently 16 seconds, as can be seen on this clock. The reason for the difference is the failure for GPS time to keep track of leap seconds (This is deliberate, to not have to worry about missing seconds). The reason for this is that it is an international standard, established for things where a common time metric is needed across the world. Satellites move through 24 time zones in a very short period of time, the ISS less than 100 minutes, so it makes sense to have a common anchor point. Typically, navigation systems use GPS time, while other systems use UTC time, and that is exactly what is used on the spacecraft.
Typically, the spacecraft time is set to UTC even for non-Earth orbiting systems, where the speed of light delay is significant.
Like many armed forces, and Stack Exchange, they use UTC (or Zulu time)
Historically, Greenwich Mean Time was accepted as being the definitive time zone in 1884 to aid navigation (which then, as now, relied on accurate time) - and the Prime Meridian was defined as running through Greenwich at 0 degrees Longitude.
Coordinated Universal Time (UTC) succeeded Greenwich Mean Time in 1963 and is basically GMT with better (sub-second) accuracy.
While it is true that just about everything uses UTC on the ISS, the crew's schedule moves around. For example, if there is an upcoming EVA that must be done at a particular time, or a spacecraft arrival/departure scheduled, ground control will adjust the sleep cycle of the crew over several days. Thus the "day" as defined on the station by when the crew is awake can and does change.