Temperature in Space
by Sam Atkins
When you’ve spent your whole life on Earth, it can be hard to appreciate how different the rest of the universe is. One of the more misunderstood elements of space is about temperature and heat. Does something exposed to the vacuum of space immediately become an ice cube? Which is the hottest planet? Or the coldest planet? How does heat propagate? Let’s walk through some of these questions and explore how hot or cold space is in different places!
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To better understand, let’s first consider what temperature actually is.
Video credit: Wikimedia Commons
Temperature is the measure of the average kinetic energy of particles. The atoms that make up everything are always vibrating, even when connected to a bunch of other atoms. How quickly and intensely they do this is measured as their temperature. Objects with faster and more intense-moving atoms are hotter while objects with slower and less intense-moving atoms are colder.
Depending on what an object is made of, its temperature can also affect the state of matter it exists in. A solid is made up of rigidly packed non-moving matter. A liquid is allowed more movement, but the matter is still close together and interacting. A gas is moving and much further away from each other and interact much less. If you increase the object’s temperature (make hotter) and make its atoms vibrate more intensely, you can change the object from a solid state to a liquid state to a gas state. Conversely, if you decrease the object’s temperature (make colder) and make its atoms vibrate less intensely, you can change the object from a gas state to a liquid state to a solid state.
The temperature at which an object turns from a liquid to a gas is known as its boiling point.
The temperature at which an object turns from a solid to a liquid is known as its melting point.
The temperature at which an object turns from a gas to a liquid is known as its condensing point.
The temperature at which an object turns from a liquid to a solid is known as its freezing point.
Again, remember that the temperature at which these changes in state occur depends on the composition of the object (what element or compound it is made of) and the surrounding air pressure.
We have numerous ways of measuring temperature such as thermometers, infrared sensors and spectrographs that can detect heat through direct contact with their surroundings or by catching light emitted from distant objects. The three most common units of measuring temperature are:
Fahrenheit: Commonplace in the United States and a few other countries. On this scale, 32°F is the freezing point of water and 212°F is the boiling point of water. Room temperature is 68°F.
Celsius: Commonplace in much of the rest of the world and in the scientific community. On this scale, 0°C is the freezing point of water and 100°C is the boiling point. Room temperature is 20°C.
Kelvin: Basically, identical to Celsius but 273.15° higher where 0 K represents absolute zero, the lowest theoretical temperature possible in space. Commonly used amongst astronomers.
Instead of thinking of cold as the opposite of hot, think of it as the absence of hot and that this state of being hot or cold can be transferred between objects or mediums of different temperatures in the form of heat.
There are three main ways that heat is transferred:
Conduction: the transfer of heat through direct contact between two objects or mediums.
Convection: the transfer of heat through the flowing motion within a non-solid medium.
Radiation: the transport of heat through the emission of infrared light. Doesn’t require a medium to propagate.
To illustrate how heat works, let’s start with a familiar environment and go from there.
The largest amount of heat the Earth receives is from the Sun via radiation (light and heat). When it reaches Earth, about 30% of the solar energy is reflected by the planet’s clouds and atmosphere, a layer of mostly nitrogen and oxygen gas that envelops the Earth’s surface, back into space. About 70% of the energy, however, becomes absorbed by the surface and atmosphere. When the ground is heated by radiation, the air near it becomes warmer and less dense, causing it to rise. As it rises, cooler air from above sinks down and swoops in to replace it. This is what causes wind and creates a continuous cycle of convection that distributes heat throughout the atmosphere. As the Earth rotates and the day side moves around to the night side, the air will gradually cool. However, because a lot of heat that was trapped by the atmosphere throughout the day remains, the planet is still able to stay relatively warm.
This is quite different from what happens on Mercury which has almost no atmosphere. This means that all the sunlight that reaches Mercury reaches the surface. However, with no atmosphere to trap and distribute heat around the planet, the night side of Mercury cools rapidly. Add on that Mercury is the closest planet to the Sun (about 40% the distance of Earth) and what you get is a temperature that fluctuates between a scorching 430°C (800°F) in the day and a freezing -180°C (-290°F) at night. In fact, there are even places deep within craters near Mercury’s poles that are permanently steeped in shadow and water ice has formed. This is the most extreme range of temperatures found on any terrestrial surface in the solar system.
Meanwhile, Venus has the opposite problem. The second closest planet from the Sun hosts an incredibly thick atmosphere (93 times denser than Earth’s) made up almost entirely of carbon dioxide which creates a runaway greenhouse effect. Although much more of the solar energy from the Sun is reflected by the thick clouds, pretty much all of the solar energy that does make it through is completely captured by the atmosphere. This not only heats up the day side beyond even what Mercury can reach but is able to maintain most of that heat across the night side. This is why, at 465°C (869°F), Venus is actually the hottest planet in the solar system (hot enough to melt lead), despite being further from the Sun than Mercury.
We go out further and arrive at Earth. We discussed how heat is distributed across the atmosphere, but we haven’t talked about the Earth’s placement in the solar system. Earth orbits around the Sun in what is known as the Goldilocks zone. This is a broadly defined region of space in which heat from the Sun is sufficient for liquid water to exist on a planet’s surface. Water closer than this region is boiled to a gas and water further than this region is frozen to a solid. Earth is generally considered to be at the inner regions of the Goldilocks zone while Mars orbits at the farthest end. Speaking of…
Mars has an atmosphere consisting almost entirely of carbon dioxide, just like Venus, yet is actually quite cold. How come it isn’t experiencing a runaway greenhouse effect? Well, not only is Mars twice as far from the Sun as Venus, but it also has a much thinner atmosphere. Not quite as thin as Mercury’s exosphere, but it is only 1/100th as dense as Earth’s and 1/9300th as dense as Venus’. The warmest it can get on Mars is on summer days at the equator where it can reach 20°C (68°F). While this is akin to a nice spring afternoon on Earth, this is not the case for most of the planet throughout most of its year. The median surface temperature on the red planet is -65°C (-85°F). That’s colder than Antarctica! If you really want to freeze your butt off, head to the polar ice caps on a long winter night where it can reach -153°C (-225°F).
Okay, so that’s how temperatures work on the various terrestrial planets in the inner solar system, but what about the giant planets in the outer solar system? Well, let’s come back to them. First, why not leave the planetary atmospheres and see how they contrast with objects reacting to heat and temperature in space?
Many people have this misconception that space is just uniformly cold. It is true that space is a near-perfect vacuum with very few particles of gas and dust floating around to conduct and convect heat. At sea level, Earth’s atmosphere has 27 quintillion particles per cubic cm. In space, there are only between 5 and 40 particles. That’s a colossal difference. Technically, this means that space doesn’t really even have a temperature to speak of. Without a significant presence of gas, the primary way for objects to receive and transmit heat in space is through radiation. In our solar system, that radiation will primarily come from the Sun. Therefore, an object’s temperature in space is not always going to be freezing but largely determined by its exposure to sunlight.
Image credit: NASA
For example, let’s look at the International Space Station orbiting 400 km above Earth’s surface, beyond the thickest regions of the atmosphere. In direct sunlight, its exterior can reach temperatures of up to 121°C (250°F) and when it passes into Earth’s shadow it can plunge to -157°C (-250°F). These are very general figures mind you as the ISS and other spacecraft are made of numerous different materials with their own capacity to absorb heat. Satellites and space probes must be equipped with a suite of thermal control systems to mitigate these environments such as insulation, radiators, heaters, sun shields, reflective paint and materials, etc.
As you can imagine, these temperatures are not what humans are used to on Earth’s surface where the atmosphere shields us from some of the most extreme elements of the Sun. Let’s imagine an astronaut leaves the safety of their spaceship in low Earth orbit (LEO) with no spacesuit and pushes into the vacuum of space. At 400 km above Earth’s surface, they would be met with a variety of terrible life-threatening problems (warning: this is a bit graphic). The lack of oxygen will cause suffocation, and the lack of air pressure will cause the boiling of body fluids, including blood. These would be the immediate concerns. Within fifteen seconds they would lose consciousness and within a few minutes they’d be dead. That is, however, besides the point of this thought experiment. Let’s put fatal conditions aside and focus on the hot and cold element.
If the astronaut enters the vacuum of space in direct sunlight, the astronaut will be taking nearly the full brunt of the Sun’s radiation. The 30% of the radiation blocked by Earth’s atmosphere comes largely in the form of dangerous ultraviolet light. Exposure to this would rapidly cause intense sunburns to whichever side of the astronaut is facing the Sun (depending on their complexion). The side of the astronaut facing away from the Sun would be largely in shadow, though would probably continue to receive a decent amount of reflected sunlight from the Earth behind them. The ability of the human body to radiate heat away is very slow, so in this situation they definitely wouldn’t be able to radiate that heat away faster than they are receiving it. In direct sunlight, a human exposed to the vacuum of space would become very hot, very quickly, and receive terrible burns within a matter of minutes and probably develop skin cancer. One thing that would help regulate the temperature between the astronaut’s two sides would be to rotate themselves like a rotisserie chicken. This would ensure each side would spend equal time warming up and cooling down, though would only delay the inevitable.
Now, let’s imagine the astronaut enters the vacuum of space while in Earth’s shadow. They will be shielded from direct sunlight by the planet’s looming size and will avoid severe ultraviolet radiation burns. But doesn’t that mean they will just freeze instead? Well, not quite. As mentioned before, without an atmosphere around them the transfer of heat through conduction and convection is basically non-existent. This actually makes space a really good insulator. The only way for heat to escape the astronaut’s body is through radiation. This would mean that the astronaut’s body temperature will gradually drop but it won’t happen in moments. It would actually take several hours for the body to become frozen solid (about a dozen depending on various factors). Rapid evaporative cooling of moisture on the skin may cause frosting somewhat quickly, especially in the mouth, but this isn’t overly-concerning. However, in low Earth orbit, the astronaut would spend 45 minutes in direct sunlight and 45 minutes in shadow. Even in shadow, the astronaut would be subjected to Earth’s radiative heat. This would likely prevent the body from ever freezing.
While in the vacuum of space, these temperature extremes are modified by one’s proximity to the Sun which radiates sunlight in all directions. Imagine this sunlight expanding outward with the Sun at its center. As the sphere gets bigger and bigger, the same amount of light is spread out across a broader area. This means that the light and heat radiated by the Sun weakens with a square of the distance (known as the inverse square law).
With that, let’s see how the temperatures change at different distances.
What would it be like really close to the Sun? NASA has a spacecraft currently in operation called the Parker Solar Probe which recently broke its own record for closest man-made object to the Sun. It reached within 6 million km from the solar surface. This is just 10% the distance of Mercury, 4% the distance of Earth and 0.1% the distance of Pluto. At this proximity, the PSP’s forward-facing heat shield endured 475 times the solar radiation that we experience on Earth and reached temperatures of up to 1,370°C (2,500°F), which is hotter than lava. However, even at this close proximity to the Sun, all the PSP needed to do was remain in the shield’s shadow to keep from being incinerated.
NOTE: Make no mistake in thinking this is remotely how hot the Sun can get. The Sun’s surface temperature is about 5,600°C (nearly five times hotter than lava) and the Sun’s core soars to a staggering 15,000,000°C, more than hot enough to trigger thermonuclear fusion.
As you get further and further from the Sun, the amount of solar radiation you can receive diminishes.
In the early solar system, there was a distance far enough from the fledgling Sun called the frost line (or sometimes the ice line or snow line) where the temperatures were low enough that volatile compounds like water, ammonia and methane can condense into solid ice, even in direct sunlight. It was located between the orbits of Mars and Jupiter in the asteroid belt at roughly 3 astronomical units (AU) from the Sun, or three times the distance between the Earth and Sun. Within this boundary, it was primarily the heavier elements which coalesced into the rocky and metallic terrestrial planets of the inner solar system. Beyond this boundary, the more abundant lighter elements were iced enough that they could accrete into the giant planets of the outer solar system. Today, with the Sun at its current temperature, the frost line has been pushed out to about 5 AUs, just shy of Jupiter’s orbit. Beyond the frost line allows for the stability and abundance of water ice which makes the outer solar system a good place to search for life.
NOTE: We have found many gas giants within the frost line around other stars, but they are believed to have likely formed beyond it and then migrated inward. Because these exoplanets, called “hot Jupiters,” are easier to find than other kinds, it’s possible that this phenomenon is not as common as the data suggests.
Next, we move on to the gas giants, Jupiter and Saturn. These are the largest and most massive planets in the solar system. In fact, Jupiter alone has 2.5 times the mass of all the other planets in the solar system combined. They are composed primarily of hydrogen and helium, much like the Sun, and have no solid surface to walk on. Instead, their outermost layers are made up of gas and get progressively denser as you descend into the planets towards their centers. For this reason, scientists measure a gas giant’s surface temperature at the point that its atmospheric pressure is equal to Earth at sea level.
For Jupiter, the mean surface temperature is -110°C (-166°F). As you can imagine, this is very cold. It’s colder than the lowest recorded temperature on Earth. After all, Jupiter is more than five times further from the Sun than Earth is. Unsurprisingly, Saturn is even colder. The ringed wonder is almost twice as far from the Sun as Jupiter and its lower mass means it produces less internal heat as well. All this being said, the gas giants are really only cold near the cloud tops. Their interiors are incredibly hot due to the incredible atmospheric pressure. The Galileo space probe which performed a death dive into Jupiter found hot temperatures only about 40 km below the cloud tops and stopped functioning at 156 km deep. The core of Jupiter is estimated to be over six times hotter than the surface of the Sun.
We then reach the most distant of the planets of the solar system. These are the ice giants, Uranus and Neptune. This is a relatively new classification adopted in the 1990’s that aims to distinguish them from their larger brethren based on their abundance of heavier elements like oxygen, carbon and nitrogen as well as icy volatiles like water, ammonia and methane. The latter are called as such due to their capacity to remain frozen at higher temperatures than the lighter elements. Despite the name, these compounds exist within the ice giants primarily in a supercritical fluid state. Imagine a great water-ammonia ocean raging beneath the hydrogen and helium cloud tops.
Uranus and Neptune are the coldest of the eight planets. Now, it would seem that Neptune is the colder of the two, based on its average temperature of -330°F (-200°C). This makes sense since Neptune is 50% further from the Sun than Uranus, which itself is 20 times further than Earth. However, the coldest temperature ever recorded in the solar system are on Uranus at -371.5°F (-224°C). Uranus has been observed to radiate about as much heat as it absorbs from the Sun, meaning it has a mysteriously low internal heat compared to the other giant planets. Its temperature is thus primarily driven by sunlight but that is a tenuous situation in and of itself. Uranus has the most extreme seasons of all the planets due to its ~98° axial tilt, essentially rotating on its side. Each of Uranus’ poles experience 42 years of continuous sunlight followed by 42 years of continuous darkness. These two things combined create the conditions for the coldest temperatures found on any planet.
At long last, we leave the planets of the solar system behind, crossing the icy expanse of the Kuiper belt and approach interstellar space. Our star recedes and dims until it is just another point of light among thousands. At this distance, the heat we feel from the Sun is just a small sliver of what it was in the inner solar system.
The average temperature of interstellar space hovers around just 2.7 K above absolute zero, the theoretical lowest temperature possible which is 0 Kelvin (-273.15°C or -460°F). This is based off temperature measurements of the cosmic microwave background radiation which permeates all of the space in the universe. This is what cosmologists believe is effectively the afterglow of the Big Bang some 13.8 billion years ago. The temperature of space can vary widely depending on the proximity to planets, stars and other celestial objects but the emptiest voids in the cosmos seem to remain stubbornly close to this background temperature.