While reading the recent question 'Do exercise machines on the ISS generate electric energy?', I got thinking all of the exerciser machines generate heat (heat energy). So yes all exercise machine generate energy for the ISS. But is that energy a positive attribute or a negative attribute to the homeostasis?

We have an exceptional answer on 'Air temperature and humidity inside the ISS' about how many different air control systems there are on the ISS and how they all work. It is very complex, and it seems that as there is no central control they are probably working at cross purposes on occasion (heating vs cooling). Obviously being in orbit around the earth the ISS will alternately be exposed to the sun, or in the shadow of the earth. I would assume that heating and cooling requirements fluctuate during the 92 minute orbit.

Does the ISS require a net heat input, or does it need to shed more heat than it makes? If it needs to shed heat (cool) is there any attempt to limit heating producing activities to the period when the ISS is in earths shadow?

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    $\begingroup$ The two answers so far are very informative, but neither of them seem to directly address the question. $\endgroup$
    – Dan Hulme
    Commented Mar 5, 2014 at 22:59
  • $\begingroup$ @DanHulme you are welcome to write a better answer. $\endgroup$ Commented Mar 5, 2014 at 23:45
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    $\begingroup$ If I knew the answer I would! I'm here because I want to know :-) $\endgroup$
    – Dan Hulme
    Commented Mar 6, 2014 at 9:45
  • $\begingroup$ @DanHulme Given the answers so far, and related Q&A on the site, I am not sure we can say definitively at this point. Both heat and cooling are required, cooling is more complex (possibly more energy consuming also) but I am not sure if there is record kept of the BTU/therms up and down. $\endgroup$ Commented Mar 6, 2014 at 12:49
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    $\begingroup$ @DanHulme ISS is definitely "heat positive", meaning more effort is involved in removing excess heat than heating it up. Fastest way of determining that is by simply looking at the chosen colors for its truss that are mostly white or reflective, to keep heat absorption low. Same goes for EVA suits. So while both heating and cooling are required as the ISS orbits the Earth and is roughly half of the time exposed to the Sun, and the other half in Earth's shadow, it's inherently more difficult to lose heat, since in near vacuum convection doesn't work and needs to be done through radiation alone. $\endgroup$
    – TildalWave
    Commented Mar 12, 2014 at 3:35

3 Answers 3


The ISS has a number of thermal management systems. The most visible part are the radiators attached to the main truss (they are the two sets of 3 white panels just to the left and right of middle, and yes, there's a tear towards the end of the first one on the right side)

enter image description here

Those panels are part of the External Thermal Control System (ETCS), if I'm not mistaken, which deals with heat loads generated by equipment inside the station. You also see radiators below each set of solar panels. Those are part of the Photo Voltaic Thermal Control System (PVTCS), which is a separate system.

Ammonia runs through the panels on the ETCS, taking in heat from various equipment and heat collectors on the station, and transporting the heat to the panels, which then lose heat via radiation.

There was a serious problem with one of the pumps for this system in May 2013, which required the astronauts on board to power down some systems, before eventually donning their spacesuits and going outside to repair it. Here's a great article on that problem and repair: http://www.nasaspaceflight.com/2013/05/ammonia-leak-iss-contingency-spacewalks/


In general, spacecraft have two issues with temperature:

  1. Getting rid of heat
  2. Thermal cycling

In space, no one can hear you cry? Nor can they see you sweat. The equipment onboard the station has serious design constraints on cooling. As you can imagine, without gravity to let hotter air (which is lighter) float higher, and cooler air to settle, internal heat control is hard.

Thus they do a lot of work with fans, and heat sink systems.

Getting rid of the heat is the hard part, and as you can see from the size of the various radiator systems it takes a lot of work.

It is worth noting that the solar panels on Zarya (FGB Control Block) module on the Russian segment had to be folded up to allow room for the ETCS radiators to extend. That is a fairly large power loss for the Russian segment, almost half their power generation.

I have wondered, if they considered moving those panels to say the Nauka or Rassvet module later. (Of course they may not be designed for that, who knows).

Thermal cycling is a problem as well, since the temperature swings a lot, every 90 minutes or so in orbit. Materials issues are a concern from all the expanding/contracting. The radiators do not help much with that, since they focus on moving heat internal to the station out of the station.


Our distance from the sun causes all local bodies to be at a temperature that is below freezing. The Earth would be frozen if it weren’t for the greenhouse effect of water vapor trapping solar radiation. E. g. the moon’s average temperature at its equator is below freezing because of no greenhouse gas. The ISS has the problem of having to get rid of heat because of internal electrical loads that produce heat in a well insulated and confined space. Also required are fans to distribute the air since there is no convection possible in zero gravity.

  • $\begingroup$ In addition to the electronics, each crew member aboard the ISS is producing about 100 watts worth of heating. $\endgroup$ Commented Jul 17, 2018 at 18:49
  • $\begingroup$ @RussellBorogove: true, but negligible compared to 100 kW of electrical power off the solar panels and 3 (?) kW/m^2 of insolation. $\endgroup$
    – Hobbes
    Commented Jul 17, 2018 at 19:17
  • $\begingroup$ The heat produced by a crew member depends on activity, 80 to 100 W at rest, but when training about 500 W are possible. $\endgroup$
    – Uwe
    Commented Jul 17, 2018 at 19:22

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