At orbital speed, spacecraft circle the Earth every 90 minutes, with a sunrise or sunset every 45 minutes, making it difficult to keep track of time as usual. Time in space is tracked more than one way. Astronauts sometimes wear more than one wristwatch, one set to Houston time and another set to mission time. This is a mission timer that is started at launch and stopped at landing. It shows how much time has elapsed since the mission began. Mission elapsed timers like this are located in the spacecraft and Mission Control Center, and also in the Press Center, so everyone knows how far into the mission they are, when scheduled events will happen, and when it's time to sleep and wake. Events that happen in space are typically recorded in mission elapsed time (MET). NASA gave this timer to the Museum when it was no longer needed on the Space Shuttle.