#explainer
Working produces body heat that a spacesuit must remove. In many EVA suits, water flows through fine tubes in an undergarment. It collects heat from the body and carries it to the cooling system.
Why cold space is not enough
A vacuum cannot remove heat through convection in surrounding air. Heat exchange with the environment occurs mainly through radiation, and the suit is also insulated. Sunlight and physical exertion can contribute to overheating; simply calling space cold does not explain how cooling works.
Where the heat goes
The backpack of American EMU suits uses a sublimator, where expendable water helps reject heat into the vacuum. Other designs can use different equipment. Circulating breathing gas also removes moisture. Oxygen alone is therefore insufficient: a spacewalk can become dangerous if cooling fails.
Two different water loops
In an EMU, water circulating through the cooling garment must be distinguished from water consumed by sublimator heat rejection. The first transports heat from the body; the second helps remove it from the system. Saying an astronaut carries water does not identify whether it is for drinking, heat transport or expendable cooling supply.
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Key points
- A vacuum cannot remove heat through convection in surrounding air.
- The backpack of American EMU suits uses a sublimator, where expendable water helps reject heat into the vacuum.
- In an EMU, water circulating through the cooling garment must be distinguished from water consumed by sublimator heat rejection.
Sources & further reading
- EVA suit construction, pressure garment and life support.
- Sublimation heat rejection in EMU and lunar-module systems.
- Technical requirements for spacesuit life support and operation.
Spaceflightpedia. (n.d.). How does an astronaut stay cool in a spacesuit?. Retrieved 17 October 2026, from https://spaceflightpedia.com/article/spacesuit-cooling
article text — CC-BY-SA · ◉ NASA photography — public domain · ✦ AI illustration — marked, generated from sources
More questions on this topic
- How does a spacesuit work?
- Why are spacesuits so bulky?
- How long can an astronaut stay in a spacesuit?
- What happens if a spacesuit is punctured?
