posted 04-21-2019 08:41 PM            

The orbital module was left at Salyut 6 (for five days) when the Soyuz T-4 crew undocked to return home in 1981. From the NASA "Mir Hardware Heritage" document:

Soyuz-T 4 leaves behind orbital module. This procedure was first followed on Soyuz-T 3, though then the module was left attached to the station for only a few hours.

On September 5 cosmonauts Alexandr Lyakhov and Abdul Ahad Mohmand undocked from Mir. They jettisoned the orbital module and made ready for deorbit burn to return to Earth ...

The deorbit burn did not occur at the appointed time because the infrared horizon sensor could not confirm proper attitude. Seven minutes after the scheduled time, the sensor determined that the correct attitude had been achieved. The main engine fired, but Lyakhov shut it down after 3 sec. A second firing 3 hr later lasted only 6 sec. Lyakhov immediately attempted to manually deorbit the craft, but the computer shut down the engine after 60 sec. The cosmonauts were forced to remain in orbit a further day ...

The cosmonauts were left for a day in the cramped quarters of the descent module with minimal food and water and no sanitary facilities. Reentry occurred as normal on September 7. After this the Soviets retained the orbital module until after deorbit burn, as they had done on the Soyuz Ferry flights.