Originally Posted by Larry Jacks

Why did the orbiting Salyut 7 get so cold when its electrical systems malfunctioned and its batteries drained empty?

A spacecraft's thermal control system is designed to handle normal heat loads, balancing heat generated by electrical systems inside (and humans in the case of crewed vehicles) with heat loss through the skin and heat gain from the sun. While I'm not familiar with the specifics of the Salyut 7 case you cited, if they lost electrical power then all of the heat generated by the onboard systems stopped while the heat loss continued. As the vehicle began to cool, electric heaters probably turned on to protect the propulsion lines from freezing. In short order, the batteries were drained (especially if the vehicle's attitude control system was also offline, meaning the solar arrays weren't correctly pointed towards the sun). This is very similar to the situation on Apollo 13. When they turned off so many electrical systems, the heat loss through the vehicle's skin was greater than the heat generated by body heat and heat gain from the sun. As a result, it got really cold inside.