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SESK: Stuff Everyone Should Know · sesk.info/aero-deep-space-background-temperature

About how warm is deep space's background radiation?

About 2.725 kelvins, just above absolute zero.

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  1. 273 kelvins
  2. 2.725 kelvins ✔
  3. 2,725 kelvins

Why: Far from nearby heat sources, the cosmic microwave background sets a temperature only a few degrees above absolute zero. Objects in sunlight can be much hotter.

Huh, didn't know that: Absolute zero is 0 kelvin, equal to minus 273.15 degrees Celsius.

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Deep space brought a sweater and called it scientific equipment.
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References

Answer: 2.725 kelvins. Far from nearby heat sources, the cosmic microwave background sets a temperature only a few degrees above absolute zero. Objects in sunlight can be much hotter.

  1. Aerospace Dimensions Module 5, Space Environment akwg.cap.gov/media/cms/Module_5__Space_Environment_C640727DEF762_83c5b.pdf
    the temperature in space is not quite at absolute zero, but rather about 2.725 K
    Checked 2026-10-10.

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