How Did the Astronauts on Apollo 13 Survive the Cold?

I watched again the Apollo 13 movie and I am wondering how the astronauts survived the cold of space, returning home in the LEM? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by C Stuart Hardwick, Award-Winning Scifi Author, on Quora:

I’m not sure where this popular notion of space being super cold came from, but it’s not true.

Any object in orbit around the sun will reach an equilibrium temperature determined by the balance between energy arriving in the form of sunlight and energy radiating into the void as infrared light (heat).

The hotter an object is, the more energy it radiates away. The closer it is to the sun, the more energy it absorbs.

Near the sun, therefore, space is very hot. Far from the sun, it’s very cold. At our distance from the sun, things are just right (for us) hence the term “Goldilocks zone.”

That, however, is on average.

If you put a metal ball out in our orbit, it will have an average temperature somewhere in the neighborhood of the freezing point of water (depending on its color), but it might be 250 F on the sunny side and -250 F on the dark side.

For spacecraft, those extremes can be dangerous, plus the spacecraft and the people inside it generate a lot of heat. As a result, the Apollo spacecraft was covered with light and reflective surfaces and with multilayered insulation designed to slow the passage of heat in or out. That kept it from overheating in the sun, and radiator panels in the Service Module and a water ice sublimator in the LEM rejected excess heat generated by the machinery and crew.

When the command and service module was powered down, the LEM was powered up and drawing 25 and 37 amps. That’s as much as several coffee makers going full blast, so it stayed nice and warm inside.

After they powered down the LEM, it was only drawing 10 - 12 amps, so there was only that and body heat to keep things warm—with sunlight mostly bouncing off the reflective coatings. So it started getting cold.

Even then, they were still running the sublimator to keep the radio and other equipment from overheating. Remember that in space, there is no air to cool equipment, so heat was carried away using liquid coolant and eliminated by a sublimator. At first, they were using 4.8 pounds of water to cool LEM systems and were going to run out. They figured out a way to transfer water from the Command Module using the life support packs, and even considered using urine for coolant, but after the LEM was further powered down, none of that was needed.

So, while the crew may have been cold, the equipment was not, and the crew were never in danger of hypothermia. Lovell wrote in his autobiography that they considered putting on their suits, but were concerned that sweat would lead to wetness, and then they really might be in trouble. So they stayed dry and toughed it out.

That said, all the documentary evidence I’ve run across implies that the movie overstates the cold for dramatic effect. The Command Module got as cold as 38 F, while the LEM stayed between 49 and 55 F during powerdown. That would mean no ice on the windows and no frozen hotdogs, though that’s plenty cold if you are trying to sleep.

Lovell said they could fall asleep in their couches in the CM because in weightlessness, a layer of warm air would form around your body. But then something would disturb that warm layer and you’d bolt awake. He didn’t say it, but I suspect what was disturbing it was the body’s reflexive reaction to the buildup of CO2 around the face.

He also said he really did hug Fred Haise to warm him up, just like in the movie.

All this is easier to show on screen with icy windows.

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