Sun Is Hot But Space Is Cold: Why?


Why is space so cold if the Sun is so hot? It may seem to be an elementary question from the school curriculum, but if you think about it, it turns out to be far from simple. Scientists have given an exhaustive answer to the cosmic riddle.

The Sun is the closest star to us, which is of type G2V, which is a yellow dwarf. Despite its relatively small size compared to many space giants, the Sun is very hot – it’s surface temperature is 9,941°F, and the core is 27 million degrees Fahrenheit.

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Meanwhile, if you move far enough from the Sun, for example, to the orbit of Neptune, then the temperature drops to -270°C. This is literally three degrees above absolute zero. How can this be?

space is cold
Why space is so cold? Image credit:

The heat in space is distributed in the form of radiation from an infrared wave of energy that migrates from hotter objects to colder ones.

The heat from the sun spreads in the form of radiation waves that excite molecules which make them heat up. But the bottom line is that radiation heats only molecules of a substance that is directly in its path. Everything else remains cold.

According to NASA, on Mercury, the temperature difference is more than 500 degrees Celsius – the dayside warms up to 427°C, while the night side drops to -193°C.

On Earth, nighttime temperatures still remain relatively warm even when the seasons change. This is because heat spreads across our blue planet through a dense gas atmosphere in three ways: conductivity, convection, and radiation. When the Sun’s rays heat molecules in our atmosphere, they transfer this additional energy to neighboring molecules. This heat transfer from one molecule to another one is called conductivity, and it is a chain reaction that heats areas outside of sunlight on the dark side.

Outer space is a vacuum. Space vacuum, of course, is not perfect, because even in interstellar space there are 5-10 hydrogen atoms per cubic centimeter. However, the gas molecules in deep space are too sparse and are far apart to regularly collide and transmit energy.

Thus, even when the Sun heats them with infrared radiation, heat transfer through conduction is not possible. Similarly, convection, which is a form of heat transfer that occurs in the presence of gravity, is important for heat dissipation throughout the Earth but does not occur in weightless space.

How To Protect From The Hot Sun And The Cold Space?

Thermal engineer Elizabeth Abel of the DART project from NASA develops thermal control systems for spacecraft, designed for long journeys in space. One of her projects was the Parker solar probe, which collects data through the outer atmosphere of the star, called the corona.

parker probe
Parker Probe: Image credit NASA

In April 2019, the probe passed close to the surface of the Sun – at a distance of 15 million kilometers. A heat shield placed on one side of the probe allowed for a risky maneuver.

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Elizabeth Abel explained that the heat shield of the probe was designed in such a way so that no solar radiation would melt the spacecraft. While the shield warms up to 250°C, the device itself remains cold with a temperature of -238°C.

The huge temperature difference between cold space and the Sun creates serious problems. Certain parts of the spacecraft need a comfortable temperature to stay cool enough to avoid short circuits, while others need heating elements to keep them warm enough to function.

Preparation for temperature differences of hundreds of degrees may seem wild, but that is how things really are in space. The same thing applies to Earth, which maintains the internal temperature in such a way that life is formed and exists on it, in contrast to the vast cold space.


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