Far outside our solar system and out past the distant reachers of our galaxy—in the vast nothingness of space—the distance between gas and dust particles grows, limiting their ability to transfer heat. Temperatures in these vacuous regions can plummet to about -455 degrees Fahrenheit (2.7 kelvin). Are you shivering yet?
But why is the vacuum of space this cold? Well, it's complicated.
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For physicists, temperature is all about velocity and motion. “When we talk about the temperature in a room, that’s not the way a scientist would talk about it," astronomer Jim Sowell, of the Georgia Institute of Technology, tells Popular Mechanics. “We would use the expression ‘heat’ to define the speeds of all the particles in a given volume.”
⚠️Most scientists use the kelvin instead of Fahrenheit to describe extremely cold temperatures, so we'll be doing that here, too.
Most, if not all of the heat in the universe comes from stars like our sun. Inside the sun, where nuclear fusion occurs, temperatures can swell to 15 million kelvin. (On the surface, they only reach up to about 5,800 kelvin.)
The heat that leaves the sun and other stars travels across space as infrared waves of energy called solar radiation. These solar rays only heat the particles in their path, so anything not directly in view of the sun stays cool. Like, really cool.
At night, the surface of even the closest planet to the sun, Mercury, drops to about 95 kelvin. Pluto’s surface temperature reaches about 40 kelvin. Coincidentally, the lowest temperature ever recorded in our solar system was clocked much closer to home. Last year, scientists measured the depths of a dark crater on the surface of our moon and found that temperatures dropped to about 33 kelvin, according to New Scientist.
That’s super cold, as in -400 Fahrenheit degrees.
But our universe is vast—unimaginably vast. (And possibly a loop?) What about the vacuum of space?
Well, that’s where things get tricky. Within near and distant galaxies, the mesh of dust and clouds that weaves between the stars has been observed at temperatures between between 10 and 20 kelvin. The sparse pockets of space that contain little but cosmic background radiation, leftover energy from the formation of the universe, hover in at around 2.7 kelvin.
These temperatures dip perilously close to an elusive measurement: absolute zero. At absolute zero, which to -459.67 degrees Fahrenheit—no motion or heat is transferred between particles, even on the quantum level.
In the vacuum of space, gas particles are few and far between—about one atom per spoonful, or 10 cubic centimeters, according to Quartz—so they aren’t able to readily transfer heat to each other through conduction and convection. Heat in space can only be transferred through radiation, which regulates how particles of light, or photons, are absorbed or emitted, according to UniverseToday.
The farther you travel into interstellar space, the colder it gets. “I don’t know that you’ll ever get down to absolute zero,” Sowell says. “You’re always going to see some light and there’ll be some motion.”
There may be pockets of the universe where temperatures drop to 1 Kelvin above absolute zero, Sowell notes, but so far, the closest measurement to absolute zero has only been observed in laboratories here on Earth.
"Humans are actually pretty good at creating extreme temperatures," Alasdair Gent, a graduate student in astroparticle physics also of the Georgia Institute of Technology, tells Popular Mechanics. Scientists are able to recreate the same temperatures seen in the vacuum of space as well as inside the core of stars like our sun.
Our Protective Atmosphere
Back here on Earth, we have it easy. “You can have high-speed particles zipping by us outside the Earth's atmosphere, but if you took off your space suit, you would feel cold because there aren't that many particles hitting you,” says Sowell. “Here on the surface of the earth, particles aren't moving really fast, but there are zillions of them.”
Earth’s atmosphere does an excellent job of circulating the sun’s heat through conduction, convection and radiation. That’s why we feel temperature changes so acutely on Earth. “The particles are moving just a bit faster due to the sunlight or weather patterns,” says Sowell.
When we venture out past the safety and confines of our planet, we wear spacesuits and travel in spacecraft that help protect us from these extreme temperatures. Here, a large dose of creativity and a whole lot of insulation is critical.
The Apollo-era spacesuits, for example, had heating systems that included flexible coils and lithium batteries. Modern suits come equipped with tiny, microscopic balls of heat-reactant chemicals that helped protect astronauts from the frigid temps.
The Artemis spacesuits, which will take the next man and first woman to the moon in 2024, come equipped with a portable life support system that will help future moonwalkers regulate their temperature on the moon and beyond.
Were you to weave between galaxies in the vacuum of space without a spacesuit, the heat from your body—about 100 watts, according to Space.com—would start to radiate away from you because conduction and convection don't work in space. This would be a slow, frigid way to go, and, eventually, you'd freeze to death. But ... it's likely you'd asphyxiate first.
After all, space is all about extremes.
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