What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky

Isn’t Space Really Cold?

by Ken Graun

The simple answer is NO, but over a period of time, an object in space will cool off and get very cold. There are a lot of misconceptions about the temperature of space. Basically, there is nothing in space—it is a very good vacuum—so it has no temperature. We produce vacuums on Earth and these vacuums are not “cold spots.” Anyhow, we use vacuums around thermoses to keep their contents hot or cold—a vacuum has insulating value. 

The immediate danger of space to humans is not freezing, but the vacuum. We, as human beings, need pressure, the pressure of air to keep all the stuff inside our bodies intact and working properly. Although no one has ever been thrown out an airlock to observe the direct consequences of being “spaced,” it is believed that the eardrums would immediately rupture, then the body, in general, would greatly swell (including the brain), so you would be in extreme pain as you suffocated to death from lack of air! However, you would not immediately freeze or get cold as this was happening. 

When spacecraft are sent into space, especially around or near Earth, a major problem can be the buildup of heat, caused by sunlight—the energy from the Sun warming the spacecraft.

So, spacecraft are designed, according to their mission, to maintain operating temperatures of its systems—this includes strategies to warm or cool the spacecraft and/or electronic components, as needed. For example, when the Lunar Prospector spacecraft, which orbited the Moon in ’98 and ’99 (this was the spacecraft that first found signs of water at the poles) went into the Moon’s shadow, heaters automatically turned on to keep equipment within specified temperature operating range. Now, the heaters are not needed because space is cold! When the spacecraft is being shaded from the Sun, it will radiate heat (read about “radiate” in the next paragraph) and cool down. When Lunar Prospector was in sunlight, the heaters were not needed because the spacecraft was designed to use sunlight to maintain correct operating temperatures for all equipment

Now, if an object is left in deep space, away from any “direct” starlight or sunlight (let’s say between galaxies), it will reach the very low temperature of just a few degrees above absolute zero (the lowest temperature possible in the Universe is –459.7° F, so a few degrees above this), but this will take time and not happen immediately. How does an object in space lose its heat? Through radiation! 

There are three ways heat or energy is transferred. They are conduction, convection and radiation. Conduction is when two objects are touching, like when someone touches you with a cold hand. This doesn’t happen in empty space. Convection is the transfer of heat or energy from a moving gas or fluid. Hot or cold air certainly warms or chills but there are no breezes in space, either. Radiation is different (this has nothing to do with radioactivity). Almost all matter in the universe has some temperature and emits this “heat” in the form of electromagnetic radiation, often infrared radiation. Electromagnetic radiation (which includes visible light) will easily and naturally travel through empty space. So, an object in space will shed heat by radiating it into space. Objects in deep space cannot reach absolute zero, which is the lowest possible temperature because there is a small amount of electromagnetic radiation that permeates all of space, a leftover from the Big Bang. This remnant energy “heats” up anything in deep space a few degrees above absolute zero.