Mission Elapsed Time

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Mission Elapsed Time (short MET , English for "expired mission time") describes a method of time measurement that was originally used by the US space agency NASA and later also by other organizations (such as the ESA ) for space flights.

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The expired mission time was used, for example, for a space flight mission of the NASA space transporter Space Shuttle .

At the Consultative Committee for Space Data Systems (CCSDS), the MET is set out in a blue book as the currently valid standard, along with other time formats. However, the CUC time format is usually used in data traffic .

The term replaces the older term Ground Elapsed Time (GET) since the beginning of space shuttle flights in order to avoid confusion with the then common GMT ( Greenwich Mean Time ).

At the time of a mission's liftoff, the shuttle's MET clocks and the astronauts' wristwatches are set to zero; the time is then recorded from the moment it is started in normal days, hours, minutes and seconds. 4/06: 48: 12 MET means, for example, 4 days, 6 hours, 48 ​​minutes and 12 seconds.

The Elapsed Time mission is used because it helps to plan experiments and time sequences on board the spaceship regardless of the actual start date of the mission, which often changes at short notice. However, it reaches its limits where it has to be synchronized with other time systems. Hence the additional use of the CUC and other time formats.

In contrast to time-limited space flights, in the permanent space station ISS, the time is measured according to Coordinated Universal Time (UTC).

The clock that measures the MET on board played a special role in the Gemini 6-A's mission . When attempting to start on December 12, 1965, the timer was triggered too early and began to run. When the main computer noticed this error, the engines were switched off again one second after ignition. In this situation, the astronauts would have had to save themselves using an ejector seat, since switching off the engines of a rocket that has already been launched (according to the current time measurement) must inevitably lead to the rocket falling back onto the launch site and exploding. However, since they did not feel the rocket moving, they mistrusted the clock and only reported that the engine was switched off. As a result, the start could be made up three days later and the rendezvous maneuver with Gemini 7 could be carried out.