Today seems to be temperature day!

My first indication was a post at Story Games where someone challenged the idea of needing heat dissipation mechanisms on spacecraft. Obviously, my first stop there is our core research point for spacecraft in the first place: Atomic Rocket. Atomic Rocket is the sanest and clearest and most awesome resource out there on space exploration. It pulls no punches with reality, and yet still frames exploration of space in plausible and heroic terms. Its use of vintage science-fiction images mingled with NASA “artists conceptions” sets a high water mark for this kind of content. Anyway, I won’t repeat the tons of material there, but suffice to say that if you are generating any kind of heat (by, say, storing and feeding animals, though firing trillion-joule rocket motors will be hot too) it needs to go somewhere and space just isn’t all that helpful.

Space is often described as cold, but temperature is a tricky beast. Space can’t reasonably be said to have a temperature — at least not such as is relevant to guy in a tin can stuck out in it — as it doesn’t have anything in it, and temperature is all about stuff banging into other stuff. Things sure heat up and cool down in space though — no question, if you stick your hand out sunside, with nothing protecting you from the naked radiation of that star, you will get burnt. Burnt by the energy of the sun, though, and not by the temperature of space. Similarly, if you stand outside in the shade, you will freeze solid. Again, not because of the chilliness of space, but because your energy will radiate away as fast as it can (which is slow in a space suit but fairly fast if you’re naked). The total lack of pressure helps with the freezing too, because at zero pressure water simply does not exist as a liquid.

Nice segué to the next one. Over at RPG.net, someone was asking why water is so hard to find on the moon, specifically wondering how it can “evaporate” with no atmosphere. It’s here that I get to break out a phase diagram, which gets me all hot:

A phase diagram for water

The interesting part is down where the pressure is very low. Actually there are several interesting parts, but the one related to the moon is the one down low. Note that if you draw a line across the chart at low pressure, no matter what the temperature is, water is either solid or gaseous. It’s never liquid down there. As the energy from the sun heats up the ice, it very suddenly sublimates into water vapour. Now, sadly, the moon is not massive enough to hold water vapour down, and so it leaves. Hence no water on the moon.

Except we did find water on the moon. A bunch of it. That means it must be hiding either in the shade or under a layer of insulating material or both. That means we can dig it up and make beer. On the moon.

There’s lots more stuff in that diagram up there that’s amazing. The triple point and the critical point, for example, where the phase of water is basically indeterminate — it could be any of the three. You can experience this sort of criticality yourself with a little luck and a bottle of beer. Put it in the freezer. The phase diagram for beer is a little different from that of water and, if you use the right beer at the right temperature, you will find your beer quite liquid after a few hours. Until you open it. And then it will — very and beautifully suddenly — freeze in the bottle. Corona is my preferred brand for this experiment because the bottle is clear and the beer is awful.

–BMurray