K^2

Diamond is even more brittle, and doesn't like heat. It'd be worse than graphite for engine components.

Certainly. The only part I'm not sure about is the actual engine. Combustion chamber, nozzle, and the bell. Parts where temperatures are really high. Graphite works well for smaller rockets, but might not be a suitable material for larger ones. In particular, it might prove too brittle, and there is no good carbon-only way to add a matrix and retain thermal resistance. At least, none that I know of. So engines for ascent rockets might need to be imported.

Actually, you do have to explain the issue with the rotating frame. The only "issue" I can think of is that it's non-inertial. Well, guess what? There is no such thing as inertial frame of reference in the real world. Thanks to gravity, we don't have flat space-time. No flat space-time, no inertial frame of reference. Which makes rotating frame of reference as good as any other. And once we extend this properly, with an arbitrary choice of a metric that matches the actual curvature of space time, we can assign to every point in space any velocity we like. Even to the point that, yes, two objects moving with respect to each other will be both at rest with respect to a third object. This is the reality of relative velocities. I know it's hard to accept, because you spend your entire life picturing all space as Eucledian, where naive notions of relativity work. But it's not. And relative velocities are a slightly more complciated concept than that. Not only do actual velocities of objects are a matter of coordinate system choice, but also the relative velocity between any set of objects is a matter of coordinate system choice. That's just how universe works.

Most of science you'd be doing on Venus is atmospheric. That's done primarily with aircraft even on Earth, so boots on ground is kind of a moot point here. Speaking of planes, yes, you can build a drone that will operate on Mars. The trouble is with making it practical for transportation. Low pressure means very high stall speeds. Which means there is absolutely zero way of landing or taking off from a runway. You need VTOL. Which you don't have air for, so you have to use rockets. Now, if you take a look at how much fuel is wasted on VTOL aircraft on Earth, and adjust for the fact that a rocket eats about 10x as much fuel for hover as something like Harrier jet engine, you should start seeing how it's not a good idea. You can definitely have a plane for emergencies. You should, in fact. Search and rescue, that sort of thing. But you can't use these routinely for transportation. You'll never mine enough fuel to keep them running. Expanding a cloud city is actually easier than doing the same work on Mars. Workers can work outside on Venus wearing a light hazmat suit. Construction can be done with polymer pipes. No heavy machinery needed. No cranes, either, as every block is literally designed to support itself and can be easily moved around. All of the materials would be made from plant by-products. So you don't need mining, or mining equipment. Have we even discussed mining in near-vacuum? Because it's not easy. Yes, we have a lot of experience of keeping a few people in a large tin can in space for a long time. We have some experience attaching modules together and doing minor repair work. We have nothing like the sort of experience we need to actually construct things from scratch in a space-like environment. And it's even worse on Mars. You don't have enough pressure to make oxygen mask a viable option, but you have enough pressure for dust storms. Fun! Gas giants aside, Mars' surface is about as inhospitable an environment as we have in the Solar Systems. We can build a colony that will survive there, but making it self-sufficient is outside of our reach right now. The only indisputable point is that it will be much easier to ascend from Mars. Venus ascent would require equivalent rocket to Earth ascent. Maybe even a tiny bit heftier. (If scale height of 15km holds, it can be as much as 500m/s of extra dV for ascent from a cloud city vs. ascent from Earth.) Mars, in contrast, is practically airless for the purpose of ascent, and we are looking at almost 3x less dV compared to Earth/Venus ascent. Which means that a mars ascent vehicle is certainly going to be an SSTO craft. Other than the size of the rocket, however, there are no additional difficulties on Venus. It would be straight forward enough to have a dedicated module for launches, which can carry the rocket away from main colony prior to launch, not unlike what you'd do with a tracked vehicle on Earth. Alternatively, air launch is an option. Deploying a large aircraft from Venus is as easy as dropping it off underneath. Disposable winged stage would be a bit more expense, but it might be worth it for the ease of the launch. All in all, all this is really saying to me that Mars is a much better place for a temporary outpost, supplied from Earth. So long as you stay marooned on the base, it's reasonably safe, and you can maintain the base with equipment from Earth. Then, if things go south, you just jump into an SSTO craft and wave the base goodbye as you ride to orbit. Venus, in contrast, makes much more sense as a permanent, nearly self-sufficient colony which, hopefully, can become entirely self-sufficient over time. Yes, it will be much more difficult to leave it, but the idea is that people wouldn't do that nearly as often. It wouldn't be a two year expedition trip, but more of a relocation for work. A place you'd go to for many years, perhaps even decades. And that's really what we want to build. A Jamestown of sorts. P.S. Why do you keep saying "An Mars..."? I can't picture any way to pronounce "Mars" to start with a vowel sound.

Try that on Mars. Or are you planning to never take the space suit off? That'll get old fast. On a Venus base, the entire structure needs to fail before you are actually in trouble. If you have to evacuate a section, worst case scenario, you can even make a short dash outside. You'll need medical attention afterwards, but you'll live. On Mars, a crack in just one of the panels is enough to kill you, and you simply cannot survive outside for any meaningful amount of time. If you were in a section that lost pressure, and didn't make it to the exit before automatic doors shut, you are dead.

As I've explained a number of times in this thread alone, this is not true.

Balloons supporting a Venusian colony would weigh almost nothing. To protect the same area from radiation exposure alone would require far more weight on Mars. Not to mention the fact that once you establish a greenhouse, you can start producing materials to expand the colony. And all that's required is a very short runway, since arresting cables will do all the work in actually stopping the lander. In contrast, you're suggesting airplanes on Mars? Good luck building runways for these monsters. I get the impression that you're picturing Martian colony like a camping trip. Put down the tent, and you're set! You have radiation. You have hurricane winds. You have inability to travel anywhere, because trying to fly there is insanity, and driving can be done at barely above pedestrian speeds. And all you get in return is piles, and piles of sand. Sand you'd probably be able to refine into something useful, if you can just keep the bloody solar farms from getting destroyed by constant sand-blasting.

He did not say "every frame". He said "everything". If we simply count every single particle, we can construct a frame in which every single one of these particles are at rest. Again, to within quantum fluctuations.

Other way around. If you give me any number of objects that you insist are already in relative motion, whatever that motion may be, I can construct a coordinate system in which all of these objects are at rest. Trivial example, picture a bunch of objects on a rotating table. They are all in motion with respect to each other from perspective of any inertial frame. But in a rotating frame of reference, they are all at rest with respect to each other. General Relativity allows me to generalize this notion for arbitrary relative motion.

... Until you get into General Relativity, where this could totally be the case in a particular coordinate system. In fact, all relative velocities become rather arbitrary. And yes, you could even define a coordinate system in which there exists a point that's at rest with respect to any arbitrary number of points. So to within a quantum fluctuation, you could have a point that's at rest with respect to everything. And it can basically be any point in the universe with suitable choice of coordinate systems.

Even in graduate mechanics, we've barely scratched the surface of the sort of dynamics that KSP does not describe. You have to take a senior/graduate level course dedicated to orbital mechanics before KSP stops being useful as a learning tool. If I was to teach an undergraduate intro course on orbital mechanics, you can bet I'd include KSP in necessary materials*, and there would be homework. * Required textbooks often cost in excess of $100, and requiring students to purchase software is nothing new in academia. At least, in the States. This seems like it'd actually be worth their money.

Well, you wouldn't build a base on an asteroid, because it'd be all sorts of painful to build centrifuges on it if said asteroid happens to tumble even the tiniest bit. And they pretty much all do. So we're really talking about a deep-space habitat in proximity of some asteroids. And probably not just one, because it's very unlikely to find just one that will have everything you need. Of course, you want your colony pretty close to asteroid for mining, so you'll really end up needing a whole bunch of colonies next to a whole bunch of asteroids, mining different resources and exchanging them. Long term, I think that's the way forward for a civilization. But it'd take enormous effort to make these self-sufficient. Yes, we'll probably start building mining outposts like that long before we can make them self-sufficient. They will rely on constant shipments of resources they can't mine locally, but it will be worth it for the resources they can mine. And it's all good for the march forward. But there is also value in establishing a fully self-sufficient outpost somewhere as soon as possible, because life on Earth is somewhat fragile. We can establish a colony on Venus that can survive the end of Earth faster than any other location. Yeah, it will still take a very long time, but we can do a lot of useful atmospheric science while we get there. So to reiterate, very long term, yes, gravity wells aren't the way forward. Medium-long term, we're stuck with them. And Venus makes far more sense than Mars if we want to build anything that doesn't rely on Earth.

Well, yeah. If we were talking about a planet composed of just one kind of material, say, hydrogen, we'd have fairly sharp boundaries. Give or take for circulation. But with mixtures, especially of varying compositions, all you are going to get is a gradients of slush. Still, that's a lot closer to being "surface", even if very fuzzy one, than what you get in terms of transition from gas to liquid. Supercriticality means you won't notice any kind of a difference, other than stuff getting denser and denser. As for transition from fluid hydrogen to metallic hydrogen, all theory points to a sharp phase transition. So while it might still be more of a slush than a surface, there are going to be entirely distinct liquid states and solid states.

Ever seen a fighter hit the deck of a carrier? It's not the only option, though. I would also consider an autogyro landing or even a rocket propelled descent. Either way, you aren't going to try and do docking in flight. You are looking for a landing on a deck, then transfer to a hangar where the pod would either be docked, or the entire hangar would get vented with clean air.

Rakaydos addressed winds adequately. Leaks aren't a problem, since hydrogen leaking from the buoyancy modules would simply rise even higher. I am not picturing any hab modules, with oxygenated environment, located above any buoyancy modules. There would be a need for all sorts of serviceable systems up top, like the landing decks, solar farms, etc, but it would be far safer just to send people in hazmats with air tanks up there than actually filling any of these walkways with breathable air. Fire is the last thing you want in a place like this. As for the ascent crew capsule, I see two options. Once we have heavy traffic between LVO and cloud city or cities, a reusable entry vehicle might be desirable. In that case, that would have to be the crew capsule for ascent ride as well. In orbit, it'd dock with an orbital station, where crew would transfer on, with fresh crews using the same vehicle for reentry. On the other hand, for early operation, this is a lot of extra weight and resources. Instead, the entry vehicle would be brought from Earth, it would land at the city, and have its aerodynamic and heat shielding parts stripped and recycled, leaving just the bare crew capsule. That crew capsule would be mated to the ascent rocket, saving dramatically on weight, while still reusing the life support, avionics, and docking components. In either case, yes, this is a pretty large rocket to air-launch. It might be a good idea to deploy it on its own balloons first, letting it drift away from the station before firing the engines. Won't be cheap. But that's just more reason to try and keep the city as self-sufficient as possible, to require minimum rotation.

There is no such state as a "solid gas", or "supercritical solid". Transition between solid and liquid is far more dramatic, since solids have much higher order. Whereas liquids and gases have the same amount of disorder at the same density. There is, however, a very interesting and very rare state of matter known as supersolid. There are some condensed matter theory groups that have suggested that metallic hydrogen in gas giants might exist in this supersolid state. However, even theoretical support for this is weak, and there is absolutely no experimental evidence for it. Of course, later doesn't say much, since we're still having trouble confirming metallic hydrogen in the lab. Another potential property of metallic hydrogen that's of great interest is metastability. If metallic hydrogen is, indeed, a metastable solid, then like diamond, while it would require immense amount of pressure to create, it could exist at much, much lower pressure. Probably not room temperature and pressure, but perhaps in a cryogenic, pressurized container. The hype over metastable metallic hydrogen stems from two predicted properties. First, there is some indication that it would be an exceptional superconductor at fairly high temperatures. Some people blame Jupiter's magnetic field on this fact. And second, that it would be the best chemical fuel known to man, allowing for as much as a factor of two in ISP gain over conventional LH2 fuel. On paper, at any rate. Coming up with a good design for an engine will be extremely challenging, even if we manage to produce it in sufficient quantities. Shame we couldn't just mine it from gas giants, huh?

Terminator winds are strongest on planets with rare atmosphere, and much weaker on planets with thick atmosphere. There are certainly high winds at altitudes we are talking about, but they have relatively low shear. This is evidenced by the cloud observation data. You wouldn't be able to hold the colony anchored in one place, but it will drift in relative calm. Furthermore, relatively minor steering will be necessary to keep it circulating around constant latitude. Which is all you really care about. Water vapor content in Venusian atmosphere is quite significant. More than enough to extract hydrogen for the support structures. And since, as I've pointed out, Venusian atmosphere contains no oxygen, or anything else for hydrogen gas to react with violently, hydrogen will be as safe as helium, so long as habitat modules and buoyancy modules are well separated. With exception of flammability, airship have fantastic safety record on Earth. Additionally, we have a lot of experience with huge floating structures in sea, including their ability to withstand weather. It's not quite the same thing, but in terms of being able to resist stress due to variations in buoyancy, a lot of this knowledge would carry over. That would be a good first step, yes. But the only reason we'd build one on Earth is to prepare for a mission to Venus, which I'm trying to convince you lot is a good idea. By no means do I suggest we go in half-cocked and just go directly for a build on Venus with no testing or preparation. Unfortunately, Earth's weather would prevent us from building a true full-size implementation at low altitude, and we don't have enough air to support something so massive at high altitude. But we can certainly build and test individual modules, floating them above oceans or some other large, open areas. It might even be a good idea to run a few mock landings to make sure we can land people and cargo on it safely from orbit. I'm picturing lifting body entry capsule landing on short carrier-style deck with arresting cables. But that's just off the top of my head.

Tell that to 3,000 people at WTC. Fact is, we already have countless people living in far more dangerous conditions. We have people living in flood areas, sides of volcanoes, tornado alleys, and poorly constructed buildings in seismologically active areas of the planet. Skyscrapers are safe in comparison, even when you consider that it's hard to escape from a 100th floor of a building if something's happening to it. A cloud colony would be safer yet. Because the atmosphere lacks oxygen, meaning you don't have to worry about fires nearly as much. The structure doesn't have the single point of support, like a foundation, because individual sections have their own flotation support. And if section you're in does happen to start falling, you do actually just walk out. You walk out into an adjacent section. Not by climbing a hundred flights of stairs, but by stepping through a few doors. All of the arguments you are making, are all of the same arguments people make about airplane flights. And it's still the safest mode of travel we have. Because "landism" is a fallacy. There is nothing inherently safe about being on land.

Correction. When the airlock blew, Mark thought, 'Hm, good thing there is no pressure differential. I should get around to fixing it some time, though."

There is no point at which gas becomes a liquid on gas giants. Their "oceans" are supercritical. If you were to descend from above, you'd encounter denser and denser gas, until there is no longer a physical distinction between gas and fluid, but you'd never hit a surface. In larger gas giants, like Jupiter, the first real surface you'd encounter would be metallic hydrogen. In a smaller one, like Neptune, you'll hit a mantle of water, ammonia, and methane ice. But in neither case is there an actual surface of the ocean, despite the fact that at the bottom, atmosphere behaves like a liquid in every way. P.S. I hate this new text editor with passion. A mod that allows us to edit bbcode instead would be greatly appreciated.

How many people do you know that still live with their feet on the ground? Most buildings have at least a basement or a crawl space bellow the first floor. Not to mention the high risers of modern cities. In largest ones, there are places where you can spend majority of the day walking from building to building without ever touching ground. People might think ground is important to them, but once placed in environment far removed from it, nobody thinks about it twice. Expansion is limited by construction materials anywhere. On Mars you need significant protection from environment. That means significant construction of concrete, metal, and glass above or bellow ground. On Venus, all you need is a light barrier between you and the atmosphere. Construction would be all carbon polymer, which will be plant-derived. While acquisition would be somewhat slower, the amount of material you need to expand would also be greatly reduced. If anything, there would be greater freedom of expansion, since two cloud cities at quite a large distance can be connected by air traffic. Whereas two colonies on Mars would either have to be close together, or you have to undertake the major investment in building covered roads or rail.

Why in the world would you want to? Not much useful on Martian surface. Venusian atmosphere is far more useful for a colony.

That IS the topic. Whether or not Venus is worth the exploration. Whether or not it's worth sending all the probes to. If we aren't talking about future use for colonization, or mining, or some other practical use, we are not talking about space exploration. And asking a question about why the probes were sent there without discussion of what we can do with the planet is absolutely ridiculous. This is what the discussion is about, and you selectively deleting stuff is not helping to have a constructive discussion.

Graphene makes for much better batteries. There are organic substitutes for silicone, and you really don't need metal for anything, except, maybe, a few high temperature applications here and there. These are the only things you'd really need shipped in from Earth and/or the belt. Again, you have a very Earthman attitude towards these things. Do a bit of research. Moon can't. Mars would be very difficult. Based on tech and materials we have. You need completely different materials to keep bases on these bodies pressurized and thermally regulated. The kind that will make the base always operate at a deficit, requiring constant, substantial influx of materials and parts from Earth. Note that I'm not saying belt here, because it's not just raw materials you are going to need anymore. I can make PLA from plants I grow in a greenhouse and 3D print it with a printer I can get at Best Buy. Steel requires a bit more love. And you are going to be stuck in a tin can going to Phobos for 5-7 months. And then you'll be stuck in a tin can for 5 months, before you take another 5-7 month trip back. I'll take two 3 month trips in a tin can vs nearly two years in one continuously. You can ask some ISS guys if they think 3 months vs 2 years is a big difference. Again, do your own math. Don't just trust something you've read on the internet. It's impossible to explore 90% of a planet in 5 months. Even with satellites buzzing about, it's tricky. Actually visiting, absolutely impossible, even with fast vehicles. Realistically, a Martian rover is going to crawl at pedestrian pace most of the time.

A leak would be dangerous if there was pressure differential. Without one, it's really more about diffusion. Thrown on a couple of layers of cellophane between you and the outside world, and even if one of them springs a leak, you'll have days before it gets to dangerous levels. So long as you have detectors installed in enough places, you'll know about it early enough to fix or evacuate at your leisure. Minerals. You're such an earthling. What do you need minerals for in atmosphere that consists of carbon dioxide, water vapor, nitrogen, and sulfuric oxide/acids? Sure, you'll need some tiny quantities for plants and humans to be healthy. And you might need to import a bit of metal for vehicles. But if you're building a cloud city out of metals/minerals, you're doing it wrong. Completely and utterly wrong. Carbon-based polymers are going to be your primary construction materials for absolutely everything. And these can be plant-derived. Not to mention that food and oxygen are going to be your byproducts. Seriously, if you can't make a cloud city self-sustaining in that nice of an environment, you don't deserve to call yourself a sentient species. Leaving for five months in a tin can. That's what every human being dreams about. Also, it's baloney either way. To consider the surface well-explored, you'll need to pass within at least 1km of every location. For 10 rovers to explore 90% of the surface in 5 months, they'd have to be traveling non-stop at a speed of 1km/s. Do the math before quoting somebody on internet. Or it can be a cheap, disposable probe you'll use one to collect the samples you need, then you manufacture another one back at the base. Most probes would be the size of the hockey puck, made from cheap plastics, and you'll drop it overboard to collect the data for five minutes, and forget it ever existed as it gets melted on the surface. The hell would you want to build expensive, multi-use probes for? This isn't Mars. You aren't sending the probes there from several light minutes away. You just fly out on your plane from the base, and drop them. They don't need to endure the environment for months. Hell, they don't even need to be reliable. If this one didn't work, drop another one. Again, you keep thinking of it in terms of exploration we've been doing. The beauty of Venus is that once we have a base there, we don't have to explore in these stupid, expensive, complicated ways. We can leverage environment and technology to our advantage for once. Also, please don't make three posts in a row. Just edit the posts you've already made.