Curious Kids: how does heat travel through space if space is a vacuum?

Venus feels the sun’s heat – but how? NASA, SDO, AIA/Flickr. , CC BY
CC BY-ND

Curious Kids is a series by The Conversation, which gives children of all ages the chance to have their questions about the world answered by experts. All questions are welcome: you or an adult can send them – along with your name, age and town or city where you live – to curiouskids@theconversation.com. We won’t be able to answer every question, but we’ll do our best.


How does heat travel through space if space is a vacuum? – Katerina, age ten, Norwich, UK.

What a great question!

First off, to understand what heat is, you need to know that everything you can touch or see is made up of tiny building blocks called atoms. Atoms are so small that you can’t even see them (except with some very special equipment) – yet they make up all the matter in the universe.

If something is hot, it means that its atoms have lots of energy and are bouncing around. If something is cold, its atoms have much less energy and they stay quite still.

It’s true that space is a vacuum, which means that there isn’t much matter floating around out there. Space isn’t a perfect vacuum though. Even if we ignore the big stuff like stars, planets and comets, space is not completely empty.

In fact, the sun is constantly blowing matter, known as the solar wind, out into our solar system. This is part of what causes the beautiful light display we call the aurora.


Read more: Curious Kids: what causes the northern lights?


But the solar wind isn’t very dense - it has much, much fewer atoms in it than air, for example. This means it can’t carry much heat in it and so it can’t explain how the warmth from the sun reaches Earth.

There are three ways heat can be shared: conduction, convection and radiation. Let’s think about each of these in turn, to discover which one allows heat to travel through space.

Conduction

Conduction is what scientists call the transfer of heat through touching. If you touch something warm, heat goes from it to you. If you touch something cold, heat goes from you to it.

Some materials, such as metals, are good conductors. Other materials, such as glass, are poor conductors, and are called insulators.

Heat can also be conducted in more than one step. For example, if you hold a metal spoon in a mug of hot tea, heat will be transferred from the tea to the spoon, and then from the spoon to your hand.

From tea to you. Rawpixel/Unsplash., FAL

But we’re not touching the sun (and that’s a good thing too - its surface temperature is over 5,000°C!) and space is a vacuum so there isn’t anything to act as a spoon and conduct the heat. So we can rule out conduction.

Convection

Convection is the transfer of heat through the flow of fluids. Both liquids and gases can convect heat. Atoms will flow away from hot regions toward cooler regions, carrying their heat and energy with them.

If you’ve ever been in a bath that has started to go cold, and then turn the hot tap on, you’ll feel the hot water convect from the tap further into the bath.

The hot atoms will then bump into colder atoms, sharing their heat through conduction, until the bath becomes an even temperature.

But because space is a vacuum, there are no liquids or gases to convect heat away from the sun, all the way to Earth. So we can rule out convection.

Radiation

Hot bodies of matter such as the sun – and even our own human bodies – give off heat. As the matter’s atoms move and vibrate they give off, or “radiate”, electromagnetic energy – this is called “thermal radiation”.

Electromagnetic energy comes in a range, or spectrum, of types - some of these we can see: they make up the rainbow of “visible light”. Other types that we cannot see exist too, such as the infrared energy our hot bodies radiate and microwave energy we use to cook food.

A thermal camera lets you ‘see’ heat, by picking up thermal radiation. Shutterstock.

Unlike conduction and convection, radiation does not need matter to transfer heat. Energy is radiated from the sun, through the vacuum of space at the speed of light. When this energy arrives at Earth, some of it is transferred to the gases in our atmosphere.

Some of it passes through and heats up the atoms on the earth’s surface. Some will even be absorbed by your skin.

The ground soaks up the energy from the sun’s radiation, and this causes it to give off heat, too. Some of this heat is conducted – like when the hot sand on the beach burns your feet in the summer. Some is convected through wind and ocean currents, and some of it is radiated back into the atmosphere, or even outer space.


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