Why Space Is Cold If the Sun Is Hot

The question might sound simple, but it has a complex answer.
Kashyap Vyas

The Sun is about 150 million kilometers away from the earth, but we can feel its warmth every single day. It is astonishing how a burning object from far away can cast its heat over such a large distance.

We aren’t talking about temperatures that barely register its presence. In 2019, Kuwait’s temperature hit 63°C under direct sunlight. If you were to stand for an extended period in such temperatures, you risk dying from heatstroke.


But what puzzles the most is that outer space remains cold even when the earth scorches millions of kilometers away. So, why is space so cold if the sun is so hot?

To understand this puzzling phenomenon, it is important to first recognize the difference between the two terms that are often used interchangeably: heat and temperature.

The role of heat and temperature

In simple terms, heat is the energy stored inside an object, while the hotness or coldness of that object is measured by temperature. So, when the heat is transferred to an object, its temperature rises. And, there’s a decline in temperature value when the heat is extracted from the object.

This heat transfer can happen through three modes: conduction, convection, and radiation.

Heat transfer through conduction occurs in solids. As the solid particles are heated, they begin to vibrate and collide with one another, transferring heat in the process from hotter particles to colder ones.

Heat transfer through convection is a phenomenon observed within liquids and gases. This mode of heat transfer also occurs at the surface between solids and fluids.

When the fluid is heated, the molecules rise upwards and carry the heat energy along with them. A room heater is the best example demonstrating convective heat transfer.

When the heater heats the surrounding air, the temperature of the air will increase and the air will rise to the top of the room. The cool air present at the top is forced to move down and get heated, creating a convection current.

Heat transfer through radiation is a process wherein the object releases heat in the form of light. All materials radiate some amount of thermal energy based on their temperature.

At room temperature, all objects including us humans radiate heat as infrared waves. It is due to radiation that thermal imaging cameras can detect objects even during the night.

The hotter the object, the more it will radiate. The sun is an excellent example of heat radiation that transfers heat across the solar system.

Now that you know the difference between heat and temperature, we’re very close to answering the question posed in the title of this article.

We know now that temperature can only affect matter. However, space does not have enough particles in it, and it is almost a complete vacuum and endless space.

This means that transferring heat is ineffective. It is impossible to transfer the heat through conduction or convection.

Radiation remains the only possibility.

When the sun’s heat in the form of radiation falls on an object, the atoms that make up the object will start absorbing energy. This energy starts to move the atoms vibrate and make them produce heat in the process.

However, with this phenomenon, something interesting happens. Since there is no way to conduct heat, the temperature of the objects in the space will remain the same for a long time.

Hot objects stay hot and cold things stay cold.

But, when the sun’s radiations enter the earth’s atmosphere, there is a lot of matter to energize. Hence, we feel the radiation of the sun as heat.

This naturally begs the question: what would happen if we place something outside the earth's atmosphere?

space can freeze or burn you with ease

When an object is placed outside of the earth’s atmosphere and in direct sunlight, it would be heated to around 120°C. Objects around the earth, and in outer space that do not receive direct sunlight are at around 10°C.

The 10°C temperature is because of the heating of some molecules that escape the earth’s atmosphere. However, if we measure the temperature of the empty space between the celestial bodies in space, it is just 3 Kelvin above absolute zero.

So, the main takeaway here is that the temperature of the sun can only be felt if there is matter to absorb it. space has almost no matter in it; hence the coldness.

The two sides of the sun’s heat

We know that shaded regions get cold. The best example is night time where the temperatures decline since there is no radiation hitting that part of the earth.

However, in space, things are a little different. Yes, objects that are hidden from the sun’s radiation will be colder than the spots that receive sunlight, but the difference is quite drastic.

The object in space will face two temperature extremes on its two sides.

Let us take the moon for example. The areas that get the sunlight are heated to 127°C and the dark side of the moon will be at a freezing -173°C.

But why doesn't the earth have the same effects? Thanks to our atmosphere, the infrared waves from the sun are reflected and the ones that do enter the earth’s atmosphere are evenly distributed.

This is why we feel a gradual temperature change rather than extreme hotness or coldness.

Another example that shows the temperature polarity in space is the effects of the sun on the Parker Solar Probe. The Parker Solar Probe is a NASA program where a probe was sent to space to study the sun.

In April 2019, the probe was just 15 million miles away from the sun. To protect itself, it used a heat shield.


The temperature of the heat shield when it was bombarded with the sun’s radiation was 121°C while the rest of the probe sat at -150°C.

Space is the ultimate thermos

When there is nothing to heat up, the temperature of a system remains the same. This is the case with space. The sun's radiation may travel through it, but there are no molecules or atoms to absorb that heat.

Even when a rock is heated above 100°C by the sun’s radiation, the space around it will not absorb any temperature because of the same reason. When there is no matter, temperature transfer does not take place.

Hence, even when the sun is hot, space remains cold as ice!

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