By Dr. Gary L. Deel, Ph.D., J.D.
Associate Professor, Wallace E. Boston School of Business
Here’s a question: How cold is space? In other words, if you were standing on the Moon right now, how cold would you be? It’s an interesting query, and it’s made even more interesting because most people tend to get it wrong – at least in certain contexts.
People tend to think that space is unfathomably cold, everywhere and all the time. If you thought this way up to now, you’d be forgiven.
None of us – save for a few hundred lucky astronauts – have ever been to space, so we don’t have the benefit of firsthand experience. We have only the information that the news media and pop culture provide to us.
And they often get the temperatures in space wrong – sometimes accidentally and sometimes for the sake of entertainment. For example, near-instant freezing of key characters exposed to outer space without a spacesuit is depicted in popular movies such as “2001: A Space Odyssey,” “Sunshine,” and even “Guardians of the Galaxy.”
Space Is Cold – But Not Everywhere
To be fair, people who think space is cold aren’t completely wrong. It is cold in some places some of the time.
The vacuum of space is actually at an average of approximately -455° Fahrenheit. But here’s the thing to remember: That number is an average across all of space in the unfathomable enormity of the universe, the vast majority of which is dark and empty.
But our little neighborhood in space is not empty. We have planets, moons, asteroids and the Sun. The Sun puts out a tremendous amount of heat energy – 384.6 septillion watts to be precise.
And in the big picture, we’re relatively close to the Sun. So although most of space might be over 400 degrees below zero, the temperature turns out to be considerably different in our immediate vicinity.
Temperature Fluctuations on the Moon
Let’s return to the question of the Moon again. How cold do you think it is there? Minus 455° F? Never.
No, the coldest the Moon ever gets is roughly -300° F. And that would be on the “dark side” of the Moon – or the side that is not facing the Sun at any given point in time.
But what about the “light side?” How much warmer is it there?
It turns out that the Moon can warm up to over 220° F in direct sunlight. That’s literally hotter than boiling water on the surface of the Earth. So the Moon’s temperatures fluctuate by about 500° Fahrenheit between its day and night cycles.
Temperatures around the International Space Station Also Vary
The same variations in temperature are observed in closer orbit around the Earth, such as at the altitudes that the International Space Station (ISS) occupies. Temperatures at the ISS range between 250° F in direct sunlight and -250° F at opposition to the Sun.
You might be surprised to learn that the average temperature outside the ISS is a balmy 50° F or so. This average temperature is above the halfway point between the two temperature extremes because objects in orbit obviously spend more time in partial sunlight exposure than in complete opposition to the Sun.
The wild fluctuations of 500° F around the ISS are due to the fact that there is no insulation in space to regulate temperature changes. By contrast, temperatures on Earth’s surface don’t fluctuate more than a few degrees between day and night. Fortunately, we have an atmosphere and an ozone layer to insulate the Earth, protect it from the Sun’s most powerful radiation and maintain relatively consistent temperatures.
At best, space is only cold some of the time – at least in the direct vicinity of Earth. So if you were to step out of the ISS without a spacesuit, you would not instantly turn into a popsicle as some blockbuster movies suggest, even at the coldest points in the orbital cycle.
It would be cold, for sure. But not that cold. And if you stepped out of the ISS while it was in direct sunlight, you’d find that it’s pretty darn hot outside – painfully so in fact.
Now, to be fair, you’d quickly be dead either way for other reasons as well. First, there is no oxygen in space, so you would quickly suffocate without air to breathe.
Second, there is no atmospheric pressure in space. As a result, your blood would instantly begin to boil as nitrogen and other elements turn to gas and try to escape your body, causing an embolism that either blocks or bursts your blood vessels, both of which would be fatal.
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The Heat in Space Poses a Problem for Spacecraft
But for spacecraft, the heat in space is a much more challenging problem than the cold. Spacecraft are equipped with different ways of generating heat to keep astronauts warm and cozy. There are solar panels, nuclear power generators and other engineering solutions for this problem. But too much heat is another issue entirely, because it is very difficult to get rid of excess heat in space.
There are three primary methods of heat transfer: convection, conduction and radiation. Conduction occurs when heat is transferred from one solid object to another through direct contact.
For example, conduction is how your hand becomes burned if you touch a hot stove. But since spacecraft don’t touch anything in space, conduction is not a possibility for dispersing excess heat.
Convection occurs when heat is transferred through a medium such as air or water. For example, you can cool down the temperature of your body by turning on a fan or jumping in a swimming pool. But this is not an option for spacecraft either – because space is a vacuum and there is no medium to allow heat to be transferred away from the spacecraft.
Finally, there is radiation – and this is really the only means of evacuating heat in space. Radiation occurs when heat energy is emitted away from an object in the form of electromagnetic or thermal energy through waves of photons. The Sun uses this method to emit its energy out into the solar system, and it is the least efficient means of heat transfer by far.
Consequently, spacecraft need to employ specialized systems with water-cooled heat exchangers and cold plates to radiate excess heat into space. It turns out keeping spacecraft cool is actually a lot harder than keeping them warm.
Interestingly, the inefficiency of radiation heat transfer is a double-edged sword. While this inefficiency complicates excess heat evacuation on spacecraft, it is also the reason why those spacecraft don’t melt and burn up in the first place.
For example, the ISS orbits in what is called the thermosphere, which is a region above the Earth where there is very little air. However, temperatures in the thermosphere can reach obscenely high levels – as high as 3,600° F.
However, there aren’t enough gas molecules to transfer the heat to solid objects at these altitudes through convection, which means that radiation is the only means of heat absorption by objects orbiting in this region. As a result, the actual temperatures experienced there are much lower by comparison, which is why the ISS only gets as hot as around 250° F in direct sunlight.
Skylab Served as an Example of the Danger That Heat Poses to Spacecraft
The danger that heat in space poses to spacecraft was on full display in 1973 when the United States launched Skylab 1, a new space station for space research. On launch, Skylab 1 experienced problems from micrometeorite debris that prevented its solar array from deploying properly. The energy from those solar panels was needed to power the complicated cooling system for Skylab 1, and as a result of the failure, temperatures inside the lab skyrocketed to 126° F.
Fortunately, there was no crew onboard at the time of launch or they surely would have perished. In the end, NASA was able to rectify the problem and save Skylab before it succumbed to the circumstances – but it was a powerful example of how dangerous heat in space can be.
So if you’re ever privileged enough to go to space and are worried about the local temperature if you’re sucked out of an airlock without a spacesuit, you might be tempted to reach for a warm coat and scarf. But depending on where you are, you may actually be better off putting on some SPF2000 sunblock and making yourself a tall glass of ice-cold lemonade – because it’s very possible that the space environment around you is hot and not cold.
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