K^2

The problem is that I can't find any citations of albedo for the ash. Now, volcanic ash can have albedo reaching 0.7. So I can see how that can cause a significant cooling effect. I'm going to try and track down the original articles for the publications Wikipedia cites, and see if I can find the values they've used. I can also put together a very simple model for both the heating and cooling effects and just run it for a range of albedo values to see what the break-even point would be. A simple model wouldn't be able to give me a very precise prediction on how strong the effect would be, but it should be fairly close on the break-even point, so it'd be possible to tell, at least, whether the net effect would be heating or cooling for a particular albedo.

Yes, but keep in mind that gravity between two moving objects is not Newtonian. At very least, you need to make a correction for gravitomagnetic effect. If relative speeds are really close to speed of light, then you have to use General Relativity.

Do you have some sort of a source that suggests that nuclear ash cloud would have an albedo that is significantly higher than Earth albedo, which averages to 0.3. In other words, Earth already reflects 30% of incoming sunlight, and I have hard time believing that ash from nuclear war would reflect more than 30% of sunlight.

They are the same. Without atmospheric circulation, ground simply reaches equilibrium with the clouds. And if the clouds are opaque, they reach the same temperature that ground would without the clouds. That's the whole point. It's only when you take air circulation into consideration that you start picking up a temperature differential between clouds and ground. And it's always towards warmer ground. Sure. If all that Venus had was perfectly transparent in visible light atmosphere of CO2. But it doesn't. It's atmosphere is pretty opaque. So light doesn't reach the ground, so you can't use conventional greenhouse effect to explain the absurdly high temperature on the ground. Try again. Demagoguery. And a false analogy to boot. Planets don't heat from heat transfer. It's not like Sun is blowing hot air at us. Planets heat from solar radiation. Now take your blanket, go outside on a warm sunny day, and stand under direct sunlight wrapped in a blanket. Then tell me how it makes you cooler. And it's not even just that. As I have explained above, if it weren't for pressure differential and air circulation, the surface temperature would be merely the same. But we have a refrigeration cycle running. Unless you have an alternative explanation for freezing temperatures at altitude. And it's not because "space is cold". Space is a vacuum. It doesn't have a temperature in any way that's meaningful here. The purpose of false-proof examples is to make you re-check the work. If you can't check 6th grade math in four lines of equations, that doesn't mean you can simply dismiss the argument. "I don't understand it, so you're wrong," should have stopped working in kindergarten. If you can't follow that simple math, the adult thing to do is ask questions or wait for somebody who does understand the argument to weigh in.

At high enough energy, a plasma beam behaves more like, well, a beam. The quantity of matter in it is actually tiny, but it's traveling damn near the speed of light. About the only difference between that and a powerful laser is going to be how it interacts with magnetic fields. So the most plausible explanation for suit's propulsion/weaponry is a VASIMR with an absurd ISP range. If we push physics to the absurd, we can picture suit's reactor as a D-D fusion reactor that produces He + ~3MeV of useful power. (I don't know how it fudges the branch fractions, shut up.) If we use that energy to accelerate He to optimize ISP without drawing external gas, we get ISP of 3.9% c/g. The Iron Man suit is said to be about 200kg with armor. So we're looking at about 280kg with the pilot. (I'm estimating/rounding like mad here, get off my back.) Putting it all together, I'm getting 32 grams of D2 for 1 hour of hover time. So in terms of absolute limits of pure physics, can do. Actually engineering a reactor and propulsion system that can actually deliver these figures? That's where we hit fantasy land.

I'm more bummed about the Energia II / Hurricane development coming to an end. That seemed like the sort of launcher we could have really used.

Something that has very different albedo in visible and IR could. But dust/ash tends to have pretty low albedo in both. Certainly, albedo of dust/ash isn't higher than that of Earth itself. I'm going to throw a bit of math at you to illustrate this a little better. Lets start with a world with perfectly translucent atmosphere. We'll further assume that temperatures equalize around the world. That's not strictly true, but it's not too far off for a simple estimate. Lets say that planet's visible light albedo is α. In other words. So for a total incoming solar energy flux of W, αW gets reflected away. That means, (1-α)W gets absorbed. At the same time, planet looses hit via IR radiation. Albedo for IR tends to be pretty close to zero, so we'll take it at that. In that case, total radiation flux leaving the planet is 4σT04, where T0 is temperature of the surface and σ is Stefan-Boltzmann constant. Factor of 4 comes from Earth being nearly a sphere. So we have the following. T04 = (1-α)W / (4σ) If you plug in values for Earth, you'll get in the ball park, but estimate will be a bit low, because we have not taken greenhouse effect and a few other corrections into account. Now, lets add a layer of ash with the same albedo to the top layer of atmosphere. We will further assume that atmosphere does not circulate vertically, allowing perfect insulation between that ash and ground. In other words, heat exchange is via IR only. (We'll augment that in a moment.) Lets call the new surface temperature T1 and ash cloud temperature Ta. The surface still radiates 4σT14, but all of that is now absorbed by the ash cloud. The ash cloud, in turn, radiates both with the top and bottom surface, so it radiates 8σTa4, of which 4σTa4 goes back to the surface, and rest leaves to space. In addition, ash cloud still receives (1-α)W of solar radiation. Putting it all together, we have two equations. 8σTa4 = (1-α)W + 4σT14 4σT14 = 4σTa4 After solving this, we have T14 = T04. In other words, ash in atmosphere has not changed surface temperature at all. Now, if ash happened to have significantly higher albedo, it would have caused temperature to drop. But that'd be no different from having that ash simply sit on the ground. Whether it's on the surface or up in the air makes no difference whatsoever. But we haven't talked about atmosphere dynamics yet, and that's a crucial element in climate. Imagine that we have the same situation. Perfectly opaque cloud of ash with roughly the same reflective properties as Earth's surface. But now, we also have air circulating between ground and upper atmosphere. The circulating air is at much higher pressure near the surface and at much lower pressure up above. The compression/decompression that air undergoes is roughly Adiabatic. So we have two heat sinks with a pressure gradient between them, at surface and cloud layer, and we have Adiabatic flow between them. What we really have is a giant, planet-sized refrigerator. Except, it's pointed with the hot side towards ground and cold side towards the sky. Now, if atmosphere is transparent and there are no clouds, it doesn't matter. All it does is make it cold at higher altitudes, which we do observe on this planet. But if your clouds are completely opaque, the clouds aren't going to get colder. They are at radiation equilibrium with Sun and space. The ash cloud will stay at the same Ta = T0 as before. What will change is the surface temperature. The atmosphere will keep pumping heat from the ash cloud down to the ground, establishing a much higher temperature on the surface. How much higher? Well, the temperature difference between ground and clouds is a starting point. It won't be quite as big of a gap, because there is still some radiation exchange, but that's the ballpark. So we aren't talking half a degree. We are talking about global temperatures rising by tens of degrees if this was to happen on Earth. For an example of this in action, take a look at Venus. Yes, it's closer by the Sun, and should be a little warmer, but it's not a little warmer. It's very, very hot. And the reason for it is the clouds. Opaque clouds at high altitude with a very high pressure differential between cloud layer and surface. The same exact atmospheric flows carry heat from clouds down to the surface heating it up. Whereas, if nuclear winter was a thing, you'd really expect Venus to be an icicle.

Again, we aren't talking about solar wind chipping away at it little by little. We are talking about thermodynamics. Atmosphere will be evaporating at a rate limited only by heat transfer between layers. In other words, we are talking years, maybe decades at best. Martian gravitational well is simply too shallow to provide a sufficient barrier against kinetic energy of gas under Earth-like conditions. It's that simple.

All else being equal, of course cooling is harder. But we don't have an "all else being equal" kind of scenario between Mars and Venus. Mars is barely large enough to support an atmosphere. It'll be even less capable of doing so if we warm it up and fill it with lighter, oxygen-rich air. Mars simply isn't massive enough to hold on to Earth-like atmosphere at Earth-like temperatures even if it had a magnetic field to protect upper atmosphere from Solar Wind. Yes, terraforming Venus is a Herculean task. But it's the only planet other than Earth on which it wouldn't be a Sisyphean labor. If you think cooling down Venus, importing water, and establishing day-night cycle is hard, what would you say about building a dome around an entire planet to keep in the atmosphere? Because that's what you'd have to do with Mars to call it terraformed. It's not going to happen.

We should all know by now how reliable a single data point is. The global average temperature has barely dropped. The impact on Europe was primarily due to a temporary climate change. Kind of how global warming can cause more severe winters in some regions. I'm not sure "Year Without Summer" even suggests global cooling as a result of debris in atmosphere, but even if it does, it's a single data point that's anything but conclusive. On the other hand, we have climate models that very clearly suggest that any opaque particulates in upper atmosphere contribute to greenhouse effect. So again, I ask if there is any modern evidence for nuclear winter.

This is simply rest frame. You take an object, and you move at the same speed as that object and right next to it. That's all. Fact that such a frame always exists should be trivial. And other than mathematical convenience, there is nothing special about that frame of reference. The other part of it is that it's only locally relevant. OP was talking about relative motion, and rest frame contains no information about that.

Do we even have any modern research that points to nuclear winter being a thing? Making upper atmosphere opaque causes environment heating, not cooling. This is well established, and is part of the greenhouse mechanism. All of the modern research seems to point at there being no such thing as nuclear winter. I wouldn't call result nuclear summer, because no sunlight, but it would hardly be a winter with rising temperatures.

As silly as it would be to try and terraform Mars, other suggestions are worse. I'm all for building cloud cities on Venus, but that's not terraforming. There is no terra to form there. And actually terraforming Venus? It might be a little easier than Mars, given enough time, (read, many millennia), but that still doesn't put it within reach. As for Ganymede and Titan, they are tiny little moons. The only reason Titan has atmosphere is because of cryogenic temperatures. Warm it up for human habitation, and all that atmo will be gone within decades. These things cannot be terraformed. Ever. There are precisely two bodies in the Solar System that can be terraformed. Earth and Venus. Earth is already naturally terraformed, and Venus is just a little too close to the Sun to make terraforming practical. Had Mars been actually the size of Venus, we'd be able to turn it into a lush green world within a few centuries. It would have been properly out-gassed, and actually contained most of the atmosphere we needed. It would be just a matter of heating it up and it'd be good to go. Cooling planets down is nowhere near as simple, though. And I'm using the word "simple" very liberally.

Helium leaks really well, because it is monoatomic and inert. Hydrogen's interactions are quite a bit stronger. It's still slightly more leaky than Oxygen and Nitrogen, but not to the point where it'd be causing trouble. And the abundance does make it easy to top off any of the lifting structures. Why would it make anything more brittle? It's not cryogenic LH2. It's just a gas. And not one that's particularly reactive, so it won't be changing the chemical composition of the walls. Any polymer that will withstand diluted sulfuric acid on the outside will be totally fine with Hydrogen on the inside. For maintenance, you can totally walk around the interiors of a hydrogen-filled balloon. Just suspend some catwalks inside. The only catch is that you couldn't use any open-cycle breathing system inside. Expelling used air into Hydrogen atmosphere would be very bad, since Oxygen content in that can still be pretty high, and it takes very little to set off Oxygen-Hydrogen mixture. But a rebreather solves this problem. It's a closed system that recycles air by scrubbing CO2 and adding O2, much like space ship's life support does. And yes, they make them portable enough for diving, so you can have maintenance crews going in with them without issues.

0.5 bar is perfectly fine. We can elevate Oxygen content to something like 30% without compromising the fire safety, and then the partial pressure ends up being comparable to elevation of, say Cusco, Peru. Which you adjust to in couple of days just fine. What I don't understand is why you are trying to get buoyancy out of habitat. I mean, it's nice that it contributes, but trying to make it entirely self-supporting by introducing a complicated and hard to support mixture is silly. It's far more efficient to have dedicated buoyancy sections filled with Hydrogen gas.

Anything that gets enough energy in one place will make a black hole. You could collide two electrons together if they are traveling fast enough. And the black hole will last as long as you like, so long as you feed it matter, which is how you'd use it for a ship. If you don't, it starts to output progressively more energy as it shrinks, until it eventually explodes, releasing significant fraction of its energy almost instantly. You really don't want to have equivalent of a few thousand tons of matter worth of energy released in a single burst on your ship. So you keep feeding the black hole and maintain constant energy output from it.

Yes, but it doesn't have to be a Kugelblitz. That just seems like the most reasonable way for us to create a black hole right now. Our ability to use either technology almost assumes that we'd have something way better in terms of ability to produce insane energy densities. And keep in mind that you don't have to have a way to make a black hole on the ship. I'm picturing this more as something that's built once, installed on the ship, and the ship constantly runs back and forward between a pair of stars.

Gee. If only there was a gas which is even more buoyant than Helium, is abundant in Venusian atmosphere, and isn't dangerous. It's a shame we can't use Hydrogen. Oh, wait, we can. There is no free Oxygen for Hydrogen to react with, so it's as safe as Helium, and we can make tons of it from water we condense out of the atmosphere. Erm. Why would you want to fix them? The entire city is carried by the winds, so there is no relative wind. Except for light turbulence, it will always be calm weather in the cloud city, so the balloons can be just balloons. Though, I would put them into some sort of semi-rigid grid, so we can attach solar panels on top, and have various service catwalks all over.

For the n-th time, no, it's more complicated than that. This kind of logic only works in space-time that is flat everywhere. Once you have objects with mass distorting space-time, this simplistic view doesn't work anymore. Relative velocity between two objects is still a matter of frame of reference.

Both have very serious drawbacks, but for a long push, assuming we can build either, black hole makes more sense. Rather than trying to figure out how to mass produce and then store anti-matter, once you have a black hole going, you can just feed it ordinary matter for as long as you like. Just make sure to refuel quickly at the other end and start a trip back, because these things shouldn't be left unfed for very long.

Except, there is no edge, and all of the room and chairs used to be a single point. Which makes the analogy somewhat weak.

Actually, equatorial winds on Venus can be in excess of 300m/s, and because Venus turns retrograde, the equatorial winds are actually prograde. So you get most of these 400m/s back. (Edit: Come to think of it, there isn't really a requirement to stick to prograde orbits, so direction of winds wouldn't have mattered. What's important is that they are comparable to Earth rotation speeds.) Mostly, I was thinking of it as launching from ~1bar and using the quoted value for scale height of ~15km. That got me an estimated dV requriements that were almost 500m/s higher than launch from Earth. But you are right, if your rocket is deployed from balloons anyways, there is no reason not to gain a lot more altitude before launching it. The scale height at altitude is probably lower as well. I just couldn't find figures for it. So in practice, you can probably get a launch with a smaller rocket.

The way space expands, every single point is at the center of the universe. It's more of an inflation, with space itself stretching, than "stuff flying apart."

You need some metals for plant life and, potentially, as supplements for colonists, but most of it will get recycled pretty efficiently, and you don't need much to begin with. This is nowhere near the imports you'd need to keep the heavy machinery working on Mars.

What did you forget on the surface? Everything you need for a colony can be extracted from atmosphere.