Mission Elapsed Time

{{short description|Time scale used by NASA during their space missions}}

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.{{cite web|url=https://science.nasa.gov/Realtime/Rocket_Sci/clocks/time-met.html |title=Mission Elapsed Time explained |accessdate=2007-11-09 |date=1995-09-13 |archiveurl=https://web.archive.org/web/20070718124711/https://science.nasa.gov/Realtime/Rocket_Sci/clocks/time-met.html |archivedate=2007-07-18 |url-status=dead}}{{cite web|url=http://spaceflight.nasa.gov/feedback/expert/answer/crew/sts-113/index_2.html|archive-url=https://web.archive.org/web/20030219212319/http://spaceflight.nasa.gov/feedback/expert/answer/crew/sts-113/index_2.html|url-status=dead|archive-date=2003-02-19|title=Ask the STS-113 crew: Question 14|accessdate=2007-11-09|date=2002-12-07}} MET was formerly called Ground Elapsed Time (GET) prior to the Space Shuttle.{{cite web |title= Apollo by the Numbers - Introduction |first=Richard |last=Orloff |url= https://history.nasa.gov/SP-4029/Apollo_00c_Introduction.htm}}

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.[https://web.archive.org/web/20030906153525/http://spaceflight.nasa.gov/station/crew/exp7/luletters/lu_letter9.html Eds Musings from Space] Expedition 7 astronaut Ed Lu, Updated: 09/08/2003 Accessed August 2007

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.{{Cite web|url=https://www.nasaspaceflight.com/2019/12/oft-starliner-landing-white-sands/|title=OFT Starliner Makes Early Landing at White Sands|date=2019-12-23|website=NASASpaceFlight.com|language=en-US|access-date=2019-12-24|first=Thomas |last=Burghardt }}