Fun Fact - Keeping Cool on Venus and Mercury

I was thinking about a small house on Venus, but the heat is a huge problem. On an outer planet you can always generate heat, by radioactivity if need be, but you can't create cool, so what to do? Let's say you have a source of energy, maybe a nuclear reactor, so that's not a problem. Plenty of power to cool your house. Thing is, an air conditioner works by pumping heat out of your house and into coils in the back yard that lose their heat to the outside air. That means the coils have to be hotter than the outside air; otherwise the heat can't flow "downhill". The air conditioner on Venus has to heat the external tubing to more than 500 C (900 F) to work. With only modest changes in pressure as per the compressor, we need a coolant that is liquid / gass in a temperature range from 0 to 500 C. I don't know any substance like that, do you? I can't imagine how to build an air conditioner that would work on Venus, even in theory. I guess nothing will be landing on Venus any time soon.

In the 1960's and 70's, Russia successfully landed Venera probes on Venus, which sent back pictures of the surface. With no air conditioning, the probes lasted about an hour, more or less. Snap a few pictures, send them back to earth, then succumb to the heat. That's a lot of money, time, and effort for not much data. In contrast, Opportunity has been tooling around Mars for 13 years. One hour versus 13 years. Well we had to see the surface of Venus at least once, didn't we. To make sure the Three Stooges weren't there. So thank you, Russia, for doing that. Yes, there was a movie, Have Rocket Will Travel, released in 1959, wherein the Three Stooges went to Venus, but that's another story.

How about a house on Mercury? Surprisingly, air conditioning can be accomplished, and for not much energy. Daytime temperatures climb to 427 C, 800 F, but at night the temperature falls to -173 C, -280 F. construct a large, subterranean tank of water next to the house. At night, pipes carry pressurized ammonia (or some other liquid) through this reservoir, and out to tubes that are connected to radiator fins. Heat vents into space, whereupon the supercooled ammonia returns to the water, which then freezes into ice. During the day, the ammonia circulates between the tank of ice and your house. The ice melts, and keeps your house cool. You're taking advantage of the day / night cycle, averaging out the temperatures through the thermal inertia of the water. You need a big tank however, because a day on Mercury is 176 earth days long. That's a lot of heat to capture, and then radiate back into space. This trick doesn't work on Venus, because its thick atmosphere keeps the entire planet the same temperature, day and night, all year round.

NASA's Messenger probe used a similar cooling strategy, on a much smaller scale, as it orbited Mercury for four years. Despite a sunshade, it absorbed heat from the sun while it was on the day side of the planet, then it radiated that heat away when it was on the night side. An elliptical orbit gave it more time on the night side to cool off.

Let's say you have a nice little house on Mercury, equipped with the aforementioned air conditioning. As you look out the window at the blazing sun, perhaps you recall the words of Isaiah 38:7-8.

“This is the Lord's sign to you that the Lord will do what he has promised - I will make the shadow cast by the sun go back the ten steps it has gone down on the stairway of Ahaz.” So the sunlight went back the ten steps it had gone down.

This seems unlikely in a literal sense, the amount of energy needed to stop and reverse Earth's rotation is almost unimaginable, but, maybe the prophets were imagining life on Mercury. As mentioned earlier, a day on Mercury is 176 earth days. In other words, Mercury spins very slowly on its axis. That's 88 days of intense sunlight as the sun creeps across the sky, and 88 days of darkness. But watch what happens around noon. The sun has passed the zenith moving west, then it slows to a stop, backs up, now traveling west to east, and passes over your house once again. This happens for 8 earth days, then it stops and resumes its normal east to west trajectory, passing over your house once again. From here the day proceeds normally, as the sun creeps to the west and down to the horizon, where it quietly sets.

Now how can that be? The orbit of Mercury is elliptical, which means it zips around the sun faster when it is close to the sun, and slower when it is farthest from the sun. At its closest, it moves around the sun faster than it spins on its axis. The apparent eastern motion from revolution exceeds the apparent western motion from rotation. From the perspective of your house, the sun seems to back up. It doesn't go forward (westward) again until you are well passed perihelion. If your house is 90 degrees around, you will see the sun set in the west, then rise in the west, just for a couple earth days, then set in the west again. I think that's pretty cool, though I'm not planning to move to Mercury any time soon.

What about a future colony on Jupiter, well, in Jupiter, since there is no surface to stand on? There is a layer of atmosphere that is a comfortable pressure and temperature for humans, so air conditioning is not an issue. However, we have to build a city in the clouds, like Stratos on Star Trek. There's no such thing as antigravity, so our city has to float, hover, or fly. Hovering takes a lot more energy than flying, I don't know why, but I asked a hummingbird and he told me it does. Thus a helicopter can't fly all the way across an ocean. So hovering isn't feasible. How about a flying city, avoiding the Great Red Spot and other nasty storms. We have built solar powered drones that can fly for a long time, almost continuously, but these are light craft with no payload, and the sun is ten times as strong on earth as it is at Jupiter, so let's agree that solar power won't be sufficient. Chemical fuels would quickly run out, so it's nuclear or fusion. Both are heavy reactors, and they must be carried aloft along with the city. Fusion power is tempting, because the whole atmosphere is hydrogen. It's fuel everywhere you look! But fusion power is centuries away, if it can be done at all. In the 70's I read articles in Scientific American: fusion power was coming, and would solve all our energy needs. Now I read articles in the same magazine on why it is a profound engineering challenge, and it's far easier to harvest energy from the sun. Whether fusion or nuclear, there is risk in a flying city; if the reactor fails even for a few minutes, or if the engines fail, or if a wing breaks off, or if anything goes wrong, then you fall into the depths, until heat and pressure destroy the city. That won't work, so let's assume the city floats.

The city is a grid of modules connected by flexible tunnels, so people can move from module to module. Some are living quarters, some are kitchens, some are science stations, some have airlocks to the outside to perform maintenance as needed. One module is a movie theater so our residence can watch a movie. One module is sickbay, and one module has showers and most of the water facilities. Tunnels are flexible, so a sudden updraft jostles the city and moves the modules about, but doesn't snap the city in half. This is the same way we earthquake proof large buildings, they flex and bend. Ok, that was the easy part, now there are 3 almost unsolvable problems, even looking millennia into the future.

  1. No desert on earth, or Mars, or Venus, is as dry and barren as the atmosphere of Jupiter. It's hydrogen, and nothing but hydrogen; and unlike the space station in low earth orbit, there aren't any supply ships coming. Oh maybe one every 10 or 20 years, that's it. If you lose even a nanogram of carbon, it's going to be difficult to glean another nanogram of carbon (methane) out of the atmosphere. Even harder for the heavier elements. If a bolt comes loose and drops away, it might take ten thousand years to pull enough iron atoms out of the atmosphere to make another one. Supply ships will have to replace the metallic items, but meantime, every molecule has to be recycled. We don't vent or waste anything! Every metabolic byproduct is broken down and recycled, not just for the water, but for every atom of carbon and nitrogen and sulfur and oxygen. A machine that we can't even imagine today, something like Star Trek's replicator, takes our breath and our waste and turns it back into food we can eat, and products that we use (soap etc), so that the city is entirely closed. Ok that's nearly impossible; let's move on.

  2. Gravity is 2.5 times that of earth. If you're 200 pounds on earth, you are effectively 500 pounds on Jupiter. I can't think of any sex positions that will work. With practice and care you might learn to walk about, I suppose, but a slip & fall almost certainly leads to a broken arm, or broken ribs, or some other injury. Beyond this, months or years of this gravity probably strains the heart to the point of failure, pumping all that heavy blood around. I'm not a doctor, and this is just a guess, but I'd be surprised if a human could live for more than a year in that gravity.

  3. How does the city float? Each module is supported by a dozen large balloons, so if one balloon fails, or if the string breaks, the remaining balloons still support the city. We put a new balloon in place as time permits. Realize that this is a stable equilibrium. If the city drifts up, the "air" is thinner, the balloons less buoyant, and the city settles back down again. Similarly, the city cannot descend too far down, as the air is thicker and pushes the city back up. So there's time to repair or replace a broken balloon; you'll just be 100 meters lower for the interim. But I've skirted around the main problem, the elephant in the room; what is in the balloons? On earth, our blimps are filled with helium. That works because helium is lighter than air; but it's not lighter than hydrogen. Guess what, nothing is lighter than hydrogen! There is literally no substance we can put into a balloon that is lighter than the atmosphere of Jupiter. Well how bout a vacuum? Each balloon is a metal shell with nothing inside. Ok, but the metal shell weighs a lot more than a mylar balloon. We could make the shel bigger, so it encloses more and more vacuum, but the shell has to be stronger to stand up to more and more pressure. That makes it heavier. There's no way to win here. The last possibility is a hot air balloon, but now we need lots of energy pumped continuously into the system. Here comes that nuclear or fusion reactor again. Sure, it isn't as fragile as flying. We don't have wings and flaps and mechanical engines to fail, and we can store energy in case the reactor is off line, and the balloons have thermal inertia, so if we don't pump heat into them for a couple hours we'll probably survive, but still it's a complex system. If something goes wrong, and we don't fix it in 6 to 8 hours, we're probably drifting down to our deaths, into a hot thick soup of hydrogen.

With all that in mind, I predict that we will never, not in a billion years, and no matter how much technology we develop, establish a colony in the hydrogen atmosphere of Jupiter. the poles of mercury are more inviting. We could however build a science plane that flies through the Jovian atmosphere for a couple days, gathering data, until its fuel runs out. Its fuel is, of course, oxygen.

Further Reading

The Venera and Messenger space probes.
Day and night on Mercury.