Why Space Is Cold If The Sun Is Hot

Outer space is most commonly described as cold, dark, and endless, but have you ever wondered why it is so cold when the sun is so hot?

Surely, with a huge ball of fire burning bright, space ought to be hotter than the Sahara on a summer’s day!

Why Space Is Cold If The Sun Is Hot

The sun is 150 kilometers away from our planet, and yet we feel its heat each morning and get sunburn if we stay out in its rays for too long.

So, is this the case in outer space too?

In this article, we explain exactly how temperature behaves in the vacuum of space, and how it differs from the temperatures and heat transfer that we experience here on earth.

Let’s go!

How Does Heat/Temperature Work?

Heat is the level of thermal energy stored inside an object, material, or matter.

Temperature is the measurement we give to the hotness or coldness of that object, material, or matter.

When heat (thermal energy) is transferred to an object, its temperature increases, and when heat is drawn away from an object, its temperature decreases.

How Does Heat Transfer Work?

Heat transfer can happen in three ways: through conduction, convection, or radiation.

  • Heat transfer through conduction occurs in solids. It is when the particles inside a solid heat up by vibrating more vigorously. The hot atoms bump into each other and transfer heat as they do so.
  • Heat transfer through convection happens in liquids and gasses. Here again, the liquid or gas particles move more and collide with each other. The hotter particles rise, which is what happens when you switch on your heater at home and see the hot air swirling about above it.
  • Heat transfer through radiation is when an object releases thermal energy in the form of light. The best example of this is sunlight. The sun’s infrared waves release heat as well as light, which raises the surface temperature of all the objects and materials it reaches.

How Does Temperature Work In Space?

In the vacuum of space, there are no air particles (which is why we can’t breathe in space).

When we talk about temperature, we usually mean the average kinetic energy of a particle in a substance.

On Earth, this is called the ‘absolute’ temperature. When talking about temperature in outer space, we need to take account of two factors:

  • Firstly, there are no air molecules to absorb our body heat. Outer space is a vacuum. There are no particles that can transfer heat via conduction or convection.
  • Secondly, there is no atmosphere to stop us getting too cold by trapping warmth like our atmosphere on earth does.

This means that the only way we can feel warm in space is by radiating our own body heat into outer space.

The problem is that we cannot radiate very much energy into space because there are no air molecules around to absorb it.

So instead of warming up, the human body would get colder and colder.

How Cold Is Outer Space?

How Cold Is Outer Space

Since there are no air molecules, this means that the temperature of space is equal to the average kinetic energy of electrons, protons, neutrons, and photons.

These are the four main types of particles found in the universe. They make up everything from atoms to stars.

As these particles move through space at different velocities, they collide with each other, giving them their average kinetic energy.

The average kinetic energy of an electron moving through space is 0.0027 eV.

That’s 27 millionths of a volt. If we measure the average temperature in space using this method, then we find that it is -270°C.

To put this number in context, this is equivalent to the temperature of liquid nitrogen (-196°C), which is used to cool superconductors!

This is way colder than anything that the human body can survive.

What Is Cosmic Background Temperature?

Another way of measuring temperature in space is to look at the background radiation emitted by the whole universe.

This is known as cosmic background radiation. It was discovered by Penzias and Wilson in 1965.

Since then, scientists have been trying to work out how old the universe is.

One way of doing this is to calculate how much radiation has been coming from the universe since its birth 13 billion years ago in the Big Bang.

By comparing this with the current level of radiation, we can work out how long it has taken for the universe to reach its present state.

The most recent measurements show that the universe is just over 14 billion years old. We now know that the universe started off extremely hot.

But after about 380 000 years, the temperature dropped dramatically.

CMB is currently at a temperature of 2.7 K (0.000273 degrees kelvin) which is minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius!

Is The Sun Hot In Space?

As there is no atmosphere in space, direct sunlight can cause unbelievable temperature extremes.

The sun transfers heat through radiation, and without a protective atmosphere to form a heat shield, a human body in space would experience intense and acute exposure to that radiation.

Never mind sunburn, the human body would be fried in the nothingness of space!

For this reason, outer space is a place of extreme temperatures.

Everything that direct sunlight reaches will become a very hot object, whereas anything in darkness will be very cold.

What Is Solar Radiation?

Solar radiation comes from the sun. It consists of electromagnetic waves, such as light, radio waves, microwaves, X-rays, and gamma rays.

When solar radiation hits the Earth, it causes heating effects on objects near the surface. This is why you need to wear sunscreen when outside.

At night, the upper layers of the atmosphere cool down and contract. This means less solar radiation passes through.

If the atmosphere were not there, the sun would burn us alive. However, the atmosphere protects us from the sun’s harmful radiation.

Without it, life on earth would be impossible.

Is It Hot On The Moon?

Is It Hot On The Moon

The dark side of the moon, which is facing away from the sun, is always cold.

However, when the moon passes between Earth and the sun, the part of the moon that faces us becomes extremely hot.

This is called the lunar day. During the lunar night, the opposite happens. Therefore, the temperature on the moon is a story of dramatic extremes.

The areas that are touched by direct sunlight become heated to 127°C, whilst the dark side of the moon will be at a freezing -173°C.

With no atmosphere to trap heat, and no particles to transfer heat, temperature in outer space is linked to light.

What Would Happen To The Human Body In Space?

If you were to go into space today, your chances of surviving are pretty slim. You’d need to wear special clothing, including a spacesuit.

Your suit would also need to protect you against the vacuum of space, which would suck all the air out of your lungs. Without any oxygen, you’d die within minutes.

Your skin would start to burn because it wouldn’t be able to breathe. The blood vessels in your eyes would burst due to lack of pressure.

And your brain would swell up like an over-filled balloon. It would not be a pleasant experience.

Final Thoughts

Outer space is extremely cold as it is a vacuum without air particles. However, it is also an environment of extreme variations.

Radiation (and a lack of any atmosphere to create a heat shield) means that anything touched by the sun’s light will heat to an extreme temperature.

Heat and light are inextricably linked in space.

Gordon Watts