How cold is space, and does it drop to absolute zero? What would happen if you fell in without a spacesuit? While science fiction movies would have us believe that space is very cold — even very cold — space itself is not entirely cold. In fact, it actually has no temperature at all.
(Photo source: Getty Images)
Temperature is a measurement of the speed of particle movement, and heat is the energy possessed by particles of an object. So in a truly empty space, there would be no particles and no radiation, which means there would be no temperature either. Of course, space is full of particles and radiation that create heat and temperature. So how cold is space, are there truly empty areas, are there any places where the temperature drops to absolute zero?
How Stars Heat Space
The hottest regions in space are right next to stars, which contain all the conditions to start nuclear fusion. When radiation from a star reaches a point in space that contains a lot of particles, things really get warmer. This gives the radiation from stars like the Sun something to actually act on. This is why Earth is much warmer than the region between our planet and the star. The heat comes from particles in our atmosphere that vibrate with solar energy and then collide with each other, distributing this energy.
- False color composite image of surface temperature changes on Uranus taken by Voyager 2. (Image credit: NASA)
However, being close to our star and harboring particles does not guarantee warmth. Mercury - the planet closest to the sun - is very hot during the day and cold at night. Its temperature drops to a low of 95 Kelvin (-288 ⁰F/-178 ⁰Celsius). Temperatures on Uranus have dropped to -371 ⁰F (-224 ⁰C), making it colder than Neptune, the planet furthest from the sun, which still has a very cold surface temperature of -353 ⁰F (-214 ⁰C).
This is the result of a collision with an Earth-sized object early in Uranus' existence, causing Uranus to orbit the Sun at an extreme tilt, preventing it from capturing its internal heat. Particles moving away from the star are so dispersed that transferring heat through anything but radiation is impossible, meaning the temperature drops dramatically. This region is called the interstellar medium. The coldest and densest molecular gas clouds in the interstellar medium may have temperatures of 10 K (-505 ⁰F/-263 ⁰C or -263 ⁰C), while less dense clouds can be as high as 100 K (-279 ⁰F/-173 ⁰C).
What is cosmic background radiation?
The universe is so vast and filled with so many objects, some hot and some unimaginably cold, that giving space a single temperature should be impossible. However, there is something that permeates our entire universe, evening out the temperature to one part in 100,000. In fact, the difference is so insignificant that the change between hot and cold spots is only 0.000018 K. This is called the Cosmic Microwave Background (CMB), and it has a uniform temperature of 2.7 K (-45⁰F/-270⁰C). Since 0 K is absolute zero, the temperature is only 2.725 degrees above absolute zero.
- Image of the cosmic microwave background radiation, which has a temperature of 2.725 K (-450°F/-270 ⁰C). (Image credit: European Space Agency)
The CMB is the remnant of an event that occurred just 400,000 years after the Big Bang, called the last scattering. This is the point at which the universe ceases to be opaque after electrons combine with the protons that form hydrogen atoms, which stops electrons from endlessly scattering light and allows photons to travel freely. This fossil relic, therefore "frozen" into the universe, represents the last point at which matter and photons aligned in terms of temperature. The photons that make up the CMB weren't always that cold and took about 13.8 billion years to reach us; the expansion of the universe has red-shifted these photons to lower energy levels. Originating when the universe was denser and hotter than it is now, the starting temperature of the radiation that makes up the CMB is estimated to be about 3,000 K (5,000°F/2,726⁰C). As the universe continues to expand, that means space is now colder than ever before, and getting colder.
What happens if you are exposed to space?
If an astronaut is drifting alone in space, exposure to the vacuum of space cannot freeze the astronaut as is often depicted in science fiction. There are three ways heat is transferred, conduction, which occurs through touch, convection, which occurs when fluids transfer heat, and radiation, which occurs through radiation.
Due to the lack of matter, conduction and convection do not occur in space, and heat is transferred slowly only through radiation processes. This means that heat does not travel quickly through space. Since freezing requires heat transfer, exposed astronauts - who lose heat only through the process of radiation - would die from decompression much faster than they would freeze to death due to the lack of atmosphere.
