Mir
Orbit control

Mir was maintained in a near circular orbit with an average perigee of 354 km (220 mi) and an average apogee of 374 km (232 mi), travelling at an average speed of 27,700 km/h (17,200 mph) and completing 15.7 orbits per day. As the station constantly lost altitude because of slight atmospheric drag, it needed to be boosted to a higher altitude several times each year. This boost was generally performed by Progress resupply vessels, although during the Shuttle- Mir programme the task was performed by US Space Shuttles, and, prior to the arrival of Kvant-1, the engines on the core module could also accomplish the task.

Attitude control was maintained by a combination of two mechanisms; in order to hold a set attitude, a system of twelve control moment gyroscopes (CMGs, or "gyrodynes") rotating at 10,000 rpm kept the station oriented, six CMGs being located in each of the Kvant-1 and Kvant-2 modules. When the attitude of the station needed to be changed, the gyrodynes were disengaged, thrusters (including those mounted directly to the modules, and the VDU thruster used for roll control mounted to the Sofora girder) were used to attain the new attitude and the CMGs were reengaged. This was done fairly regularly depending on experimental needs; for instance, Earth or astronomical observations required that the instrument recording images be continuously aimed at the target, and so the station was oriented to make this possible. Conversely, materials processing experiments required the minimisation of movement on board the station, and so Mir would be oriented in a gravity gradient attitude for stability. Prior to the arrival of the modules containing these gyrodynes, the station's attitude was controlled using thrusters located on the core module alone, and, in an emergency, the thrusters on docked Soyuz spacecraft could be used to maintain the station's orientation.
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