To try to explain the Mir computer and power shutdowns in 1997 several things need to be understood:
1. First of all you must understand that Mir's power system consists of several solar arrays of varying efficiency and capacity. These are connected to a system which switches the arrays to and from charging batteries and the stations power bus. The batteries must provide power as Mir flies into night nearly every orbit. So, each time Mir goes into night, it relies on batteries for power, once daylight seen again the solar arrays are pointed to it again (provided the batteries still have enough power to turn them and the computer still is working) and begin either charging batteries or supplying power directly to the stations electrical apparatus. The ammount of power available from time to time depends on how much capacity the batteries have. Very simply, if there are a lot of batteries then how often they get recharged is not so important since they won't be depleted as fast. Unfortunately, the best batteries on Mir in 1997 were in the Spektr module which was damaged and disconnected from the power system.
2. The stations computer is very important to keeping the station working, batteries charging and solar arrays pointed in the right direction. If something causes the computer to reset maybe a cable sending signals to the computer was interrupted, when the computer sensed the signals come and go in an unprogrammed way, or a sensor has failed and provided bad data to the computer. Without being programmed how to react the computer may default to resetting. During the Progress collision the interruption most probably was caused by a power spike from the array being damaged. During normal operations, the simple effect of full sunlight suddenly hitting a solar array can cause a power spike which could cause the system to reset. The power system is complicated having been built up to tie all the modules, launched over several years and equipped with apparatus not entirely expected years ago, together to varying degrees of success. Batteries and solar arrays vary in all the modules and the wiring between them all must be difficult to trace. It's possible to leave equipment operating overnight by accident only to run down the batteries faster then they get charged and bring the station to a halt. This is what happened in a well known episode in 1994. Oops. This only proves cosmonauts and mission controllers are human.
3. Once the computer resets, the station is in an uncontrolled mode and slowly looses its position pointed at the sun as the station flies around the earth. This is a very slow process lasting many minutes and is not what one would reasonable interpret as a Mir 'spiraling out of control' as members of the western press would have everyone believe. Mir can easily go for 3 or 4 hours in this mode with no ill effects other than running down battery power (if there was enough at the time of the upset), someone inside the station wouldn't notice much without the master alarm sounding to alert them to it. Once the computer is off-line, the attitude control Gryodynes, electrical motors with big flywheels attached, also go off-line since they are no longer being commanded as to which way to point the station. The station is in free drift. Now, the crew can attempt to control the station manually, or use the Soyuz wichever is easiest, to charge the batteries if they were completely dead before the crew could manually control the station.
4. A couple of backup systems attempt to engage when the stations computer resets, they try to measure the attitude of the station and sun. Once that is done the solar arrays can be realigned with the sun to begin charging batteries and supplying power again, and the Gyrodynes can be spun up in a mater of hours while the station is under control of attitude control jets. But the backup systems are slow to start up (it can take a few orbits) and if there wasn't enough power in the batteries to start with, then there simply isn't time for this process to complete, the backups, sensors, attitude control system themselves require power and once it is depleted without reestablishing attitude control then the station is helpless. To extend this time the maximum amount the crew can shutdown everything non-critical like lights, all experiment apparatus, primary life support systems like Electron, Vozdukh, etc. and hope they have enough power left to avoid completely shutting down the station.
5. If the backups can't point the station and its arrays before the batteries run down to a certain level then the station is left with little charge on the batteries and no way of pointing itself correctly. Without cosmonauts on board the station would go dark and cold. Fortunately, since the cosmonauts always have a Soyuz spacecraft attached,it is activated and used to manually orient the station for a few orbits to get the Mir batteries charged. In times with a Progress is attached AND long term communications available from Altair, its possible the ground could provide this control for the Progress. The Altair links are expensive and satellites go in and out of service regularly for long periods of time.
6. All Mir's non-essential system are deactivated to get the batteries charged as fast as possible since the manual pointing takes quite a lot of attention over hours and expends propellant on the Soyuz or Progress. Once the batteries have a good charge, the station's computer is powered up again along with the other critical flight control systems. Once it determines the stations attitude the Mir attitude control jets (or perhapse the Progress) can be used the Soyuz can be shutdown. The most important item to be restarted next are the Gyrodynes which can take several hours to get spun-up. Once they are back on line the attitude jets can be shut down to preserve expensive propellant. After that, the crew decides what systems to power up as the state of battery power and solar array power allows. Lights, Electron, Vozdukh, experiment freezers, material processing furnaces, etc.
This is a long process which was made longer in 1997 due to the fact that Mir lost use of its best batteries and the solar arrays of Spektr. This means they were operating with less time to react to system problems like computer resets. Once new batteries are delivered on STS-86 and some of Spektr's power is restored. Crews then had more ability to deal with these incidents and they became much less common.
These kind of power problems happen routinely on Mir, there were occurrences in 1994, 1996 in addition to 1997's. There is NOTHING life threatening about it, they are just annoying, interrupt routine daily life and delay science work.
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