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Episode 165 - Little Things in Space

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Recap: Three separate topics all tied together by a commonality: A little bit of something that you tend to only experience in space. First up is microgravity, then near-vacuum, and then what it means to have a temperature in space.

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Episode Summary

Preamble: The idea for this episode came from my last appearance on The Reality Check podcast, episodes 448 and 449, where I talked somewhat about microgravity and vacuums. They are topics with significant misconception, and I think they really deserve their own treatment on this podcast, and the latter is related to temperature. Also, honestly, based on my own podcast listening habits, I suspect that if you don't already listen to The Reality Check, you probably aren't going to go listen to individual episodes when I mention I was on, so this won't be a repeat for most of you.

Microgravity

First up is microgravity. What it is? We can guess from the name, micro meaning 10^-6 or just more generically, "small," and gravity being gravity. So, little gravity. We often say that people in space experience microgravity. But, all except for about a dozen humans have gone no more than a few hundred miles above Earth's surface, which creates a disconnect: Many people think that either just a few hundred miles up and all or almost all gravitational force from Earth goes away, or, they don't understand why people would experience a microgravity environment when they're just another 5-10% above the center of the planet than we are when on the surface. So, here's what's going on:

We can start by talking about gravity in general, in that gravitational force follows an inverse-square law. I've talked about that before on this podcast because light also follows an inverse-square law. It means that the power of that thing - light or gravity - changes by one over the square of the distance change. So if you double the distance, the pull of gravity on you from that object is one-quarter. If you halve the distance, it's four times as much. So, if you are 10% farther away from the center of the planet, then the gravitational pull is around 85% as much. So if you weigh 150 pounds, or 70 kg, if you were stationary above Earth at that distance then the scale would read about 125-130 lbs, or a bit under 60 kg. I'm using round numbers here, hopefully obviously.

So that is not microgravity. You could lose 20 lbs dieting and doing exercise for a few months if you needed to, it's not worth going into orbit. But right there, in that sentence, I gave the key to experiencing microgravity: Orbit.

What all satellites - artificial, occupied, or natural - are doing are traveling in an orbit. Or, they are falling. They are falling at just the right speed to cancel out the force of gravity. If that doesn't make sense, think about being on a roller coaster, or if you're like my mother and don't do roller coasters, think about traveling in a fast elevator and going down. There's that moment where it seems almost as though the floor is falling out from under you - which it literally is - and then you catch up and travel normally. That moment of when it's falling out from under you is when you experience less weight because the floor is no longer pushing up at you as it normally is. If the elevator were much faster than allowed, you could feel weightless for a few seconds.

It's the same thing with astronauts in a spaceship in orbit: They are falling at just that right amount to make it seem as though gravity isn't a factor. So to their point of view, they are weightless. You can experience the same thing on Earth in drop tower rides at amusement parks for a few seconds, or you can experience it if you pay a lot of money and go on special airplanes, often nicknamed "vomit comets." These planes do just what the elevator would do in the thought experiment: They fly up pretty high, and then they fly down at just that right speed so that the plane is falling -- err, flying DOWN -- at just the same rate that gravity is pulling you down so you experience up to about 30 seconds of weightlessness. Or, microgravity. These planes can also fly at variations on that to simulate different gravitational fields, like Mars or the moon, and hence you can also get on these flights to do actual scientific experiments that require a microgravity environment or gravitational environment similar to that on another planet. The catch is that those experiments must be able to be done in under a minute.

Anyway, at this point, if you're paying attention, you may be wondering how it makes sense that astronauts can keep falling without hitting the ground. After all, as I just said those planes can only do it for a minute or less. The key is the second component to an orbit: Flying across the surface. So not just falling towards it, but also traveling at a right angle across it. To achieve orbit, you have to travel across the surface at a fast enough speed that the surface falls away from you due to its curvature. Hence requiring a spherical body and hence why Flat Earthers refuse to accept any space travel, but that's a story for a different episode.

The analogy I like to explain what's going on in this scenario is to picture three people throwing a ball: A baby, you, and The Hulk or Superman, whichever is your own favorite brand of comic, and before the nerds e-mail me, I know that The Hulk is not the same as Superman.

Okay, so the three of you start out at the top of a tall building. The baby has a ball, and the baby throws it as fast as it can horizontally. It's not going to go very far before it goes splat on the ground below the building. Now you're up, and you throw it as hard as you can. You, hopefully, will throw it farther than the baby. But, while it travels parallel to the surface of the ground for a bit of time, soon gravity will dominate, pulling it much faster down than your throw pushed it across, and it will hit the ground. Or a person, in which case you should hide and let the baby take the blame.

Next up is Superman or The Hulk, and they also throw it horizontally. Really hard. They throw it so hard, so fast horizontally, that even though gravity is going to pull it and accelerate it down, it's traveling across the Earth's surface fast enough that the curvature of the Earth allows the ball to stay the same height above the surface. Hence, orbit is achieved, and a bit later the ball will come around and smack the sucker in the back of the head. Or, if your superhero-of-choice were showboating and threw harder than they needed to, then it would travel across the surface of the planet FASTER than the ground falls away and it would not be in orbit, it would leave the influence of our planet and go off into space.

So now, hopefully you know why orbits work, or how they work, and what the tie is to microgravity. To make it more explicit, the spacecraft still experiences most of Earths' gravity as we do on the ground, but it's falling at a rate to cancel gravity, and it doesn't hit the planet because it travels across the planet fast enough that its falling is cancelled out by the ground curving away.

Why is that called microgravity? Because it's never perfect. If, perhaps, everything were ideal: We existed with nothing else in the universe, and Earth were a perfect, PERFECT sphere, then that kind of orbit could exactly cancel out gravity and you would, for all intents and purpose, be in a zero-gravity environment. But the universe has other stuff in it, and Earth has things like mountains and oceans and rocks with different densities, and things called "mascons" which are concentrations of mass in different spots that all conspire to perturb the gravitational field a little bit, making it such that you never get a truly perfect zero-gravity environment in orbit.

So, could you do it if you escaped Earth's orbit and went way far away from stuff? Again, no. It's perhaps conceivable that you could find some spot in the universe where the combined pull of gravity from everything around you is zero, but it's unlikely. As soon as a star moves slightly in its orbit in a distant galaxy, the combined gravitational field shifts slightly and you're no longer in zero gravity. Still, very, very, VERY close, good enough for government contract work, but it's never perfect. And that's why this gets first billing in my "Little Things in Space" episode.

Vacuum

Next up, because I'm not good with transitions, is a vacuum, one of the very, very few words in the English language that has two letter "u" in a row. In fact, one of only two that are common, along with continuum.

Anyway, English lesson aside, the first definition of "vacuum" in my Apple dictionary widget is, "a space entirely devoid of matter." Which is how most people think of the term if they're not doing housework. But, the first sub-definition is, "a space or container from which the air has been completely or partially removed."

It is the merging of these two definitions that form what a practical vacuum is in real life. Ideally, a vacuum is that first definition: A True Vacuum is when all matter is removed from a certain volume of space. However, that's not technically possible. And creating something close to a true vacuum is really really hard on Earth with our current technology. More on that in a moment.

On Earth, if you're near sea level, we experience about 1 bar of pressure, or one atmosphere (by definition), or around 100 kilopascals, or 14.5 pounds per square inch. On Mars, the pressure of atmosphere at the surface is about 0.6% that of Earth. That's 0.6 millibars.

Creating that kind of vacuum on Earth is possible, but hard. And creating one that's more than 60 times better, to less than 1/100th of a millibar, and have it survive in a spacecraft and burrow into Mars is currently impossible, or at least for the French.

Some of you may remember about two years ago it was announced that the next probe to Mars called InSight was delayed from 2016 launch to 2018. The reason is that this probe had a ground penetrator that would have a seismometer, an instrument called the Seismic Experiment for Interior Structure or "SEIS" for short. The requirements are that the probe of the instrument must maintain a vacuum less than 0.01 mbar in order to detect what it was designed to: motions as small as the width of an atom, or 0.01 nm (10 pm).

Now, go to Pluto, where New Horizons measured a pressure of about 3–60 µbar, which is around the vacuum that the SEIS must maintain.

Now, go to the Moon. Yes, there is an atmosphere, or what terrestrial folks may call an "exosphere" due to the very low density. It has an atmospheric pressure around a femtobar, or 10^-15 that of Earth's surface. Really low pressure.

And then, there's interstellar space, where the numbers I found were around 10 zeptobar. Zepto is the SI prefix for 10^-21, so we're talking around 10^-20 bars. Crazy low pressure.

BUT, it's something. There is material in space. If there weren't material in space, then stars, planets, etc. couldn't form. Therefore, because there is material, and a true vacuum is the absence of material, space is not a perfect vacuum. But it's really close. Much, much closer than we can get in the lab on Earth.

With that said, as a side-note, if you're in a spacecraft and there is explosive decompression - or an airplane for that matter - you are not sucked out of the craft. You are blown out because of all the air rushing from inside the craft into the vacuum. You are blown out with that air. So, kids, keep that in mind: Science doesn't suck, it blows.

Temperature

Moving on, the last bit is temperature. I have a clip for this one, coming from Crrow777's podcast and while the original clip was 10 minutes long, I decided to spare you all and took just the first minute or so.

"And so, when you start thinking about the satellites and you start studying the thermosphere, uh, I-I made a couple videos about this. The temperature in the thermosphere is supposed to be around 2000° Celsius - gets upwards of 2000° Celsius - and that's uh, temperature is a measure basically of kinetic energy in a substance and, and uh, so there's supposed to be very few, uh, molecules, atoms - you know, very little matter up in the thermosphere. And so, the temperature's really hot that they tell us that everything is– all the particles of matter are so far apart you can't actually put a thermometer up there and measure it so it would actually show a very low temperature, so it gets really confusing, what's the temperature up there?"

The rest of it is more of the same, where the guest, Brian Mullin actually gives the real explanation for what's going on but says he doesn't believe it and therefore satellites are fake.

So, temperature. As he said, temperature can be thought of as the motion of molecules. If molecules are moving fast, they have more energy, and we experience that as heat, or an increase in temperature. Absolute zero is the temperature at which all molecular motion stops, and there is nothing that we know of or have created that gets to absolute zero, though a few years ago a group of scientists got a substance below absolute zero using a different technical definition of temperature so let's not get into that. Moving on ...

Temperature is motion of particles. So, what happens when you have very few particles? Those particles can still individually have a very high temperature, meaning they have a lot of energy and are moving really fast. But, would you fell it? Could you measure it? Could that temperature be conveyed to something?

To answer those questions, we have to go back to a vacuum. Or, not a vacuum. On Earth, at sea level, we have a pretty thick atmosphere, all things considered relative to other rocky solar system bodies. In one cubic meter of air or a little over a cubic yard of air, there are on the order of 10^25 molecules. That's a lot of molecules. Using some basic powers of 10 division, in every cubic centimeter, that's 10^19 molecules of air. That's a lot of molecules to be bouncing into things. And, by bouncing into things, they transfer some of their energy to those things, causing the molecules in those things to heat up, too. Until they have the same amount of molecular motion and have reached the same temperature.

That's how a basic analog thermometer works, like the old-school mercury thermometers, or the ones with the red dye. Some digital ones work that way, too, though the ones that are like a gun and shoot a laser at something as a guide and give you a temperature work a different way.

Anyway, back to the analog thermometer, the molecules in it equilibrate to the same temperature or amount of motion as the substance in which they are immersed, usually air. With that in mind, they also want to radiate away that heat. You may recall, if you've been a long-time listener or a recent one who's gotten there in the archives, from Episode 5 and 7 on the Apollo Moon Hoax that there are three ways to transfer heat: Convection, conduction, and radiation.

Convection here doesn't apply, it's physical mixing of stuff. Conduction is what I just described, where the objects are touching. Radiation is where every object that has any temperature above absolute zero will radiate away light. This is a very slow process - the slowest usually of the three methods - but it gets really important in space. On Earth, when we're talking about a thermometer in air, the speed that the thermometer will radiate away energy is much, MUCH slower than it gets energy by conduction - or loses energy by conduction if it's touching something colder than itself.

But now, let's go up in Earth's atmosphere. There's still stuff. The layer that is the top-most is the exosphere, a term I used before. An exosphere is defined as gas molecules that are gravitationally bound to a body, but they are so far apart that they no longer really behave like a gas and don't collide with each other, or do so very rarely. For Earth, exosphere starts around 600 km up, and it continues until it grades into whatever you'd like to consider the start of space. The layer of atmosphere below the exosphere is the thermosphere which starts around 90-120 km above the ground, meaning that it's where most satellites are, and hence why Crrow777 was having issues. The thermosphere does behave like a gas, but the molecules are still so far apart that they can't convey sound. The temperature of the thermosphere is around 500-2000°C, really hot.

But now we get to my questions from before: What does temperature even mean here, when the molecules are so far apart that you reach a point where radiative energy transfer can start to dominate?

In other words, yes, each individual gas molecule in the thermosphere is hot. "Hot" meaning that its blackbody radiation spectrum gets up to the temperature of a small star, or a bonfire. And, if one of those gas molecules collided with you, the energy transfer from that collision would be significant and a molecule of your skin may burn. But, you are still radiating away heat energy. A spacecraft is still radiating away heat energy. And, it is able to distribute and radiate away that energy, especially when engineers worked out what the thermal balance was.

What I mean by that is, take the International Space Station as my example. It gets heat from probably three main sources: (1) The sun through radiation and conduction from the solar wind, (2) conduction from gas in the thermosphere, and (3) electronic equipment running on it (and people!). Whenever any part of the ISS heats up, it is physically touching other parts which means that the heat is distributed. It also has an enormous thermal control system which pumps material throughout the station to distribute the temperature. The ISS absorbs and creates more heat than it can radiate away normally, so engineers designed an external thermal control system which are large white panels. That same pumping system that distributes heat around the space station goes to the external thermal control system and helps to radiate that heat away.

This is ALMOST the exact same as a radiator you might have seen on a car, on the back of a refrigerator, or even a heat sink on a computer CPU or other computer component. On Earth, those extra pieces of metal have a large surface area to exchange heat through conduction to the external environment. On the ISS, those extra pieces have a large surface area to allow for radiative heat transfer away from the station, and it was carefully designed to compensate for all those sources of heat.

The same goes for other satellites: We know how heat is transferred, and we know basic physics, and so we can design systems to compensate for this stuff. If I might go on a mini side rant, it always amazes me how stupid the pseudoscientists think scientists and engineers are. The pseudoscientists think that they are the only ones who ever have thought of these issues and no one else has and no one could possibly have designed a system to take it into account, therefore it's impossible. This goes for satellites being able to withstand the thermosphere as much as it does for compensating for the heat island effects of cities on temperature measurements or water vapor in climate models.

Anyway, it may seem like there have been a lot of digressions in this discussion about temperature, but it all gets back to what exactly it means and how you measure it. On Earth, it seems straight forward. But, once you get into regions of the universe where molecules are few and far between, a near vacuum, "temperature" takes on a different characteristic, and while individual molecules may be highly energetic and have a large "temperature," transferring that information and thus heating up anything else, is a much different story.

Wrap-Up

And so, that wraps up this episode on little things in space. Zero-G isn't really zero gravity, but it gets really close. There's no such thing in the physical universe as a true vacuum, and getting even close to it in a lab on Earth is ridiculously hard if not impossible. With near-vacuum comes the concept of temperature and what it really means, and while the molecules in space are hot, that doesn't mean that taking a thermometer from your medicine cabinet and sticking it in space would let it register above 2000°C.

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