What Makes Space 'Cold?'

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What Makes Space 'Cold?'
When people say space is cold, they’re actually talking about the cosmic microwave background radiation.

Temperature and heat aren’t the same. 

Scientists think of heat as how fast atoms are moving. Fast atoms are hot. Slow atoms are cold. 

Temperature is the average kinetic energy of those moving atoms. 

In space, there’s lots of, well, space. It’s a vacuum. 

In that vacuum, you can’t technically measure temperature. 

So, when people say space is cold, they’re actually talking about the cosmic microwave background radiation. 

That’s the remnants — or aftermath — of the earliest stages of the universe. It’s evidence of the big bang theory of how the universe started. 

And for a human, it's very cold. The temperature of the CMBR is 2.7 kelvin — or negative 455 degrees Fahrenheit.  

It's one of the many reasons space travelers need a pressurized ship: to stay warm enough to survive.