April 16, 2018 at 05:36AM

Today I Learned: 1) Read a few things on space suit tech today. The big one is on space suit thermoregulation. Long explanation of the physics of temperature control in space coming up; numbers two and three are much, much shorter, you can skip to those if you want. How do astronauts stay warm in space? Actually, the better question is, how do they stay *cool*? See, there are basically three ways to cool a human -- conduction, convection, and radiation. Okay, there's also sweating, which isn't really any of those three, so really there's conduction, convection, radiation, and sweating (evaporative cooling). Anyway, conduction is the direct flow of heat between objects touching each other, through the contact interface. That doesn't really happen in space, since there's nothing to touch. Convection is like conduction, but in a fluid media that's moving around and forcing a high temperature gradient to suck out heat, which makes convection much more efficient than conduction. Convection also doesn't really happen in space, because, again, there's nothing to convect *with*. It's space. Evaporative cooling by sweat *could* work in space, but only if you directly exposed that sweat to outside vacuum, which would be a bad idea for a number of reasons. That only leaves radiative cooling, which is loss of heat from spontaneous emission of light. All objects do this; humans emit something like 50 W under normal resting conditions, which represents about 50% of the energy we give off. In space, radiation... still works! But it's not tremendously effective -- only about half of your body heat can escape if all you have is radiation, so if you're stuck in space without a fancy way to actively cool off, then it's going to be hard to dump heat faster than your body naturally produces it. Now, as you heat up, you *will* radiate more efficiently... but the equilibrium temperature you'd reach isn't really human-friendly. Ergo, the problem in space isn't staying warm -- it's staying cool. ...well, that's what I thought. Today I learned that what I just described is... somewhat wrong. It's true that under "normal conditions", humans radiate about 50 W. However, part of "normal conditions" is being on Earth, immersed in air that's slightly cooler than we are, that's *also* radiating heat. That 50 W number is the *net* radiative emission of a human *on Earth*. In space, there's not a ton to radiate back to you (except the sun, which is a HUGE caveat!), so your *net* radiate emission is much, much higher than the usual 50 W -- more like 700 W, which is more than you can continuously generate for long stretches. So yeah, you're going to get pretty cold in space. But wait! What about the sun? Well... at Earth-orbit-distance, the sun provides about 1300 W per square meter (I use that number a lot. I should really memorize it one of these days so I don't have to keep looking it up). The human body, as a number of sources have kindly informed me, has about two square meters of surface area, about half of which can face the sun at once, so if you're floating naked in space, you should get about 1300 W from the sun. Now we're back to overheating. Except on the side that faces away from the sun, that side will cool pretty quickly. So yeah, you freeze on one side and heat to deadly levels on the other, unless you rotate nicely to keep yourself evenly insolated, in which case you just overheat. Oh, and if you happen to fall into Earth's shadow, then you immediately start to freeze instead. So, the truth is, staying safe in space isn't "all about staying warm" or "all about staying cool" -- you have to do *both*, *actively*, because whether or not you're in shadow makes all the difference between freezing solid and heat-stroking. You can see this in space suit design. One common feature of space suits is to provide a thermos-like layer of insulation between the astronaut and the outside of the spacesuit, which buffers them tremendously against *changes* in ambient radiation. Of course, then the astronaut has exactly the opposite problem as a naked person floating in orbit -- they *can't* radiate heat very effectively, and can only conduct and convect and evaporately cool with the rest of the suit. In practice, from what I understand, the astronaut usually needs more help cooling off than staying warm, *within the thermos layer of a space suit*, so space suits come with liquid cooling systems to draw heat from the body to the exterior of the suit, where it can be (radiatively) dumped into space. 2) Here's a smaller space suit fact -- space suits are specifically designed to keep a constant volume no matter how the wearer moves, because changing the volume of a pressurized gas-filled container takes work (in both the physical and colloquial senses of the word). There are a bunch of solutions to the constant-volume problem, but the most interesting nugget of this fact, to me, is that the constant-volume problem is a design consideration at all. 3) There's a horizontal white line in the mouth, right about where the teeth come together. It's called the "linea alba". I didn't read much about it, but apparently it's thought to be a thickening of the skin in response to friction, rubbing, grinding, etc. from the teeth. As far as I can tell, the linea alba provides no fitness benefits. In fact, it is now my go-to example of an evolutionary spandrel -- it's a total side-effect of whatever feedback system makes skin grow tougher where there's more wear and tear.