Mission Elapsed Time (MET) is used by NASA during their space missions, most notably during their Space Shuttle missions. Due to the fact that timing is very consequential in a mission, all events after launch are scheduled on the Mission Elapsed Time. This avoids the constant rescheduling of events in case the launchtime slips. The MET-clock is set to zero at the moment of liftoff and counts forward in normal days, hours, minutes, and seconds. For example, 2:03:45:18 MET means it has been 2 days, 3 hours, 45 minutes, and 18 seconds since liftoff.[1][2] MET was formerly called Ground Elapsed Time (GET) prior to the Space Shuttle.[3]
The International Space Station (ISS) does not use an MET clock since it is a "permanent" and international mission. The ISS observes Greenwich Mean Time (UTC/GMT). The shuttles also had UTC clocks so that the astronauts could easily figure out what the "official" time aboard ISS was.[4]
In 2019, a test flight of the Boeing CST-100 Starliner spacecraft suffered a mission anomaly through an incorrectly set Mission Elapsed Time on the vehicle.[5]
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