Venus or Mars for terraforming and colonisation?

[factotum]

Even if you were able to split all the carbon dioxide you would still be left with ~70 bar atmosphere of pure oxygen, which isn't much better (and good luck managing any coal fires that break out).

I don't know if this is at all practical, but couldn't you react the atmospheric oxygen with iron (sourced from asteroids again) to form iron oxide? AFAIK all iron oxides have very high melting points and would thus all be solid even at Venus' current temperatures, so if you can somehow split the CO2 into its constituents and then reduce the oxygen by reacting it with the iron, it might be possible to achieve a good result.

I have absolutely no idea just how much iron you'd need to do this, though!

[Forum Explorer]

You don't need to convert the CO₂ to oxygen and carbon, and you actually don't really want to. What you want to do is cool the crust enough that the rock can weather. The reason there is so much CO₂ is that rocks that would be carbonate at reasonable temperatures break into carbon dioxide and oxide rocks at higher temperatures. Even if you were able to split all the carbon dioxide you would still be left with ~70 bar atmosphere of pure oxygen, which isn't much better (and good luck managing any coal fires that break out). To call it terraformed you need to react the carbon dioxide with rock instead, which requires you to cool the whole planet crust down first. The same process gets rid of the SO₂.

Doing this is hard. Even with no sunlight at all the surface would stay hot for an absurd length of time so a sun shade of some form would be the easy part. Getting the atmosphere to stop behaving like a blanket is not easy. There might be a way though. The first step would be to seed the clouds to cause much larger droplet size than normal in the sulphuric acid clouds. You might even have to artificially capture large quantities of sulphuric acid and drop it through a pipe, so droplet size is in the cm range. The goal is to cause a massive storm in a single location, using the sulphuric acid as both a pump and heat exchanger. The deeper down you can deliver the sulphuric acid the more powerful the convection you drive, hence wanting large droplet size. Once you can start to get some sulphuric acid all the way down to the ground it will start to react with rock to form sulphates, slowly pulling the SO₂ out of the atmosphere. You don't need to cool down the whole crust to do this. Ideally you want to localise rainfall over a single area that you cool down first as a sink for sulphur. If you can get rid of the sulphur dioxide the greenhouse effect is not nearly so extreme, and you can then continue the storm cooling with water. If you choose your location right to start with you might be able to keep the majority of the water on the planet in the same area constantly powering your storm. That means that water vapour will not be a significant greenhouse gas everywhere, but would still drive a powerful convective heat transfer.

Hitting it with a couple of large icy comets might be an idea, because the more liquid phase components to the atmosphere the faster it can cool down, and I can't find one of those that doesn't need hydrogen. 10 comets the size of Haley's comet would ~double the amount of hydrogen available, so those are viable numbers. Regular bombardment with comets throwing dust into the upper atmosphere could even be how you shade from the sun. The life of a terraformer would be interesting, to say the least. You would be constantly living in the heart of a storm on a planet that people are deliberately throwing things at!

Even if you do manage to get the surface cool, the timescales and amount of work to get all the carbon dioxide to weather is extreme. You would need to turn the entire surface into gravel to a depth in the tens to hundreds of meters if you want it to go at any pace. Fracking type techniques might work, but cooling subsurface rock is much harder too.

If you do all that, you should be left with a liveable planet. It should even be stable, because I think it is the hydrogen that caused the runaway in the first place. As long as you can get it cool enough that carbon and sulphur stay in rocks you should be fine.

Small quibble, not breaking CO2 into O2 and Carbon, but changing it to H2O and Graphite. Also Venus lacks hydrogen. There may be a bunch stored underneath the crust, but as far as I know, that's theoretical. But hydrogen certainly isn't to blame for the runaway greenhouse effect in the first place.

I don't know if this is at all practical, but couldn't you react the atmospheric oxygen with iron (sourced from asteroids again) to form iron oxide? AFAIK all iron oxides have very high melting points and would thus all be solid even at Venus' current temperatures, so if you can somehow split the CO2 into its constituents and then reduce the oxygen by reacting it with the iron, it might be possible to achieve a good result.

I have absolutely no idea just how much iron you'd need to do this, though!

Yes you could! Asteroids are a good source of this (bombarding the planet actually can blow away approximately 0.001% of the atmosphere at a time) and can reduce the atmosphere at the same time. Alternatively, Mercury has a ton of iron you can access.

[Fat Rooster]

Small quibble, not breaking CO2 into O2 and Carbon, but changing it to H2O and Graphite. Also Venus lacks hydrogen. There may be a bunch stored underneath the crust, but as far as I know, that's theoretical. But hydrogen certainly isn't to blame for the runaway greenhouse effect in the first place.

Sorry, where are you getting the hydrogen to make water from? There is enough to be used catalytically, but not as a reactant (20ppm water). Even hitting it with a lot of quite major comets isn't going to provide it with enough hydrogen for that.

How do you believe the thermal runaway happened if you don't think hydrogen played an important part? Carbonate and sulphate rocks don't decompose until they get to several hundred degrees, so it wasn't carbon dioxide or sulphur dioxide that set it off. Methane or other hydrocarbons could do it, but no mechanism is known that would continue to produce vast quantities above 200' and they still require hydrogen. The only believable theory I've seen has water oceans that boiled off, with water vapour driving it until the rock started to outgas. The hydrogen required for that ocean is no longer there, so it couldn't happen again (though it could happen here, if we screw up really badly).

I should clarify, when I say hydrogen I don't just mean molecular hydrogen. I mean all hydrogen, whether it is in water, sulphuric acid, methane, or anything else.

Yes you could! Asteroids are a good source of this (bombarding the planet actually can blow away approximately 0.001% of the atmosphere at a time) and can reduce the atmosphere at the same time. Alternatively, Mercury has a ton of iron you can access.

Not enough. 1kg of iron can lock up about ~300g of oxygen, which comes from ~400g of CO₂. You would need a larger mass of iron than Ceres, and M type asteroids are not common. We could probably find a better use for ones that do exist too.

As for getting Iron from Mercury, the energy requirements are actually higher than simply striping off the atmosphere and accelerating it above escape velocity. There is no point bothering with chemistry at those energy levels, or anything fancy. You can just brute force remove the excess atmosphere.

[Forum Explorer]

Sorry, where are you getting the hydrogen to make water from? There is enough to be used catalytically, but not as a reactant (20ppm water). Even hitting it with a lot of quite major comets isn't going to provide it with enough hydrogen for that.

How do you believe the thermal runaway happened if you don't think hydrogen played an important part? Carbonate and sulphate rocks don't decompose until they get to several hundred degrees, so it wasn't carbon dioxide or sulphur dioxide that set it off. Methane or other hydrocarbons could do it, but no mechanism is known that would continue to produce vast quantities above 200' and they still require hydrogen. The only believable theory I've seen has water oceans that boiled off, with water vapour driving it until the rock started to outgas. The hydrogen required for that ocean is no longer there, so it couldn't happen again (though it could happen here, if we screw up really badly).

I should clarify, when I say hydrogen I don't just mean molecular hydrogen. I mean all hydrogen, whether it is in water, sulphuric acid, methane, or anything else.


Not enough. 1kg of iron can lock up about ~300g of oxygen, which comes from ~400g of CO₂. You would need a larger mass of iron than Ceres, and M type asteroids are not common. We could probably find a better use for ones that do exist too.

As for getting Iron from Mercury, the energy requirements are actually higher than simply striping off the atmosphere and accelerating it above escape velocity. There is no point bothering with chemistry at those energy levels, or anything fancy. You can just brute force remove the excess atmosphere.

That's right, I had forgotten that it was theorized that Venus used to have a bunch of hydrogen in its atmosphere that was slowly stripped away by the sun.

I haven't been using throw an ice moon as an exaggeration. I think it would take pretty much that much matter to make a difference. I mean, I'm pretty sure you'd need to throw dozens of Oumuamuas in order to get enough hydrogen. Theoretically these could be found in the Oorts cloud. Alternatively you could slowly mine hydrogen from a gas giant and build your own Oumuamuas to launch.

You could also throw Mimas or Lapetus at Venus, though those are mostly frozen water, and I'm not sure if just dumping a bunch of water on Venus would be all that useful.

Speaking of brute force, if you hit Venus with something big enough to create a moon from the impact (like what happened with Earth) would that moon have an atmosphere of its own?

[Yora]

I have absolutely no idea just how much iron you'd need to do this, though!

Even if the chemistry works out, I think it's the whole scale of the required operation that I think makes the entire idea unfeasible.
Given that human civilization only had 12,000 years so far and Earth should remain habitable for humans for another 500,000,000 years, saying with certainty that something will never be done seems rationally unsound.
But even in a best case scenario where all the required asteroids and comets can be collected and dumped onto Venus, how long will it take for all the chemical reactions to convert all of the atmosphere? A thousand years? Ten thousand years? I find it hard to believe that a massively expensive project would be run for hundreds of years so that someone will benefit from it thousands of years in the future. For something with very marginal benefit, as a civilization that has the technology and resources for such an undertaking should have no problems with maining human habitats in space.

(And for people who feel they need to figure out how humanity will survive in half a billion years, space habitats have the advantage of being possible to move into any orbit around the sun that has the optimal intensity of solar radiation. Even when it's a white dwarf. Space cities are both the short and long term solution. )

[halfeye]

Even if the chemistry works out, I think it's the whole scale of the required operation that I think makes the entire idea unfeasible.
Given that human civilization only had 12,000 years so far and Earth should remain habitable for humans for another 500,000,000 years, saying with certainty that something will never be done seems rationally unsound.
But even in a best case scenario where all the required asteroids and comets can be collected and dumped onto Venus, how long will it take for all the chemical reactions to convert all of the atmosphere? A thousand years? Ten thousand years? I find it hard to believe that a massively expensive project would be run for hundreds of years so that someone will benefit from it thousands of years in the future. For something with very marginal benefit, as a civilization that has the technology and resources for such an undertaking should have no problems with maining human habitats in space.

(And for people who feel they need to figure out how humanity will survive in half a billion years, space habitats have the advantage of being possible to move into any orbit around the sun that has the optimal intensity of solar radiation. Even when it's a white dwarf. Space cities are both the short and long term solution. )

I like space habitats a lot, I think they are the long term solution for people. My personal preference would be to put the tigers, aardvarks and penguins etc. on Venus and never let humans down there at all, because humans are going to take all the land and most of the sea on Earth.

Reducing the temperature will mean putting some solar sails between Venus and the Sun, we'll do that for Earth first, to stop global warming sinking the coastal cities, but once that's cheap (and really it will be, solar sails are thin and should be cheap to make) it'll be the obvious thing to do to use them for Venus. Bacteria can double their population in a couple of hours, if the environment suits them, Venus with an earthlike temperature should suit some of them fine.

Water will be necessary, but most of Earth's water is in the seas, we really don't need that much for land life at all, though seas would be nice.

[factotum]

I like space habitats a lot, I think they are the long term solution for people.

There are advantages to building on planets, though. You get gravity for free, and if there's any sort of decent atmosphere you also get free protection from small and medium sized rocks, not to mention quite a bit of hard radiation. You can also dig underground tunnels to get even more protection from stray rocks and the like. Doesn't apply so much to Venus because of the heat and the acid in the atmosphere, but would definitely be some advantages to living on Mars.

[Radar]

Reducing the temperature will mean putting some solar sails between Venus and the Sun, we'll do that for Earth first, to stop global warming sinking the coastal cities, but once that's cheap (and really it will be, solar sails are thin and should be cheap to make) it'll be the obvious thing to do to use them for Venus. Bacteria can double their population in a couple of hours, if the environment suits them, Venus with an earthlike temperature should suit some of them fine.

Using gigantic nightshade to cool down Earth might be a last resort at some point, but I would not use it, if there are alternatives. Less sunlight on Earth would have far reaching consequences for the climate and plant life.

Water will be necessary, but most of Earth's water is in the seas, we really don't need that much for land life at all, though seas would be nice.

Without oceans the water circulation would be very different. There might be no rain or natural rivers and that would make cultivating any plant life that much more difficult. A self-sustaining ecosystem based on Earth would most likely require vast oceans.

[Yora]

There are advantages to building on planets, though. You get gravity for free, and if there's any sort of decent atmosphere you also get free protection from small and medium sized rocks, not to mention quite a bit of hard radiation. You can also dig underground tunnels to get even more protection from stray rocks and the like. Doesn't apply so much to Venus because of the heat and the acid in the atmosphere, but would definitely be some advantages to living on Mars.

The problem with planetary gravity is that you can't adjust it. On Venus, you have 91% Earth Gravity, which I guess the human body probably could adjust to. For Mars and Mercury it's only 38%, which I seriously doubt is enough for the healthy development of bones, circulatory system, and nervous system in children. Next one is Io with a measly 18% and from there it's only downhill.
Which is why I think that everything but Venus can be ruled out for permanent habitation right from the start. (Doing 6 month tours in shipyards and fuel production facilities on the Moon is a totally different story. No children and elderly there, though.)

If we had a planet with the size of Venus and the surface conditions of Mars, I'd say permanent colonization would absolutely be doable, possibly within the next 100 years.
But as it is, we only have two possible candidates in the solar system and both of them happen to have two huge, but completely different dealbreakers.

Simulated gravity in rotating space habitats can be adjusted to whatever strength you want, provided the whole thing is big enough. (100m radius seems to roughly be where tidal forces on head and feet become small enough to not cause disorientation, which is why we haven't build anything like it yet.) And the cool thing about weightless objects in a vacuum is that once you get them spinning, they will keep spinning forever without any energy input. Which makes that approach actually quite cheap, compared to building the habitat in the first place.

[factotum]

Thing is, though, we've never actually studied what the long-term effects are of living in simulated gravity via rotation environments. Until we *do* have such a thing (whenever we get round to building it), we can't say for sure that the "fake" gravity in a rotating station would be better for us than lower gravity on a planet.

[Fat Rooster]

Thing is, though, we've never actually studied what the long-term effects are of living in simulated gravity via rotation environments. Until we *do* have such a thing (whenever we get round to building it), we can't say for sure that the "fake" gravity in a rotating station would be better for us than lower gravity on a planet.

If they turn out to be different the physicists are going to be even more interested than the biologists. That would break relativity. People not having significant issue with 1G centrifuges is one of the few possibilities that even the most conservative test programs would take on trust (getting 1G centrifuges right so they don't break is a far bigger concern).

It is certainly true that we have never studied 'sub G' properly. We know things go wrong at 0G, and work well at 1G, but we really have no idea how large life will behave at .1G or .3G. Impossible to study on earth, hard to study in space. All we have are theories at this point. We can be pretty sure it will be far harder to adjust to Earth from Mars than the other way around, but that doesn't say anything about the viability of life on Mars, with or without therapies (exercises, drugs, diet, whatever is required). Studying it in space would require a whole new station; one that is structural (ISS is flimsy enough that one of it's tiny accelerations almost shook it apart). That means heavier and more expensive. There are other hurdles too, though some have been overcome. The new dragon could probably manage to dock at a spinning hub, but berthing is sort of out of the question. All the designs that include a rotating seal are probably no-go, so docking ports have to be rotating too. Solar panels could maybe be static, as a transformer can be integrated into a bearing just fine, but radiators are much harder. Many of them probably need to be on the rotor.

Such a station would have micro-G regions in the hub, but it would largely be unsuitable for studying micro-G effects. You could have it attached to another station that wasn't spinning, but getting between them would require either another 'craft' (could be more like a lift than a craft, but it would essentially have to undock from one, match spin with the other, then dock). It gets complicated.

[Radar]

If they turn out to be different the physicists are going to be even more interested than the biologists. That would break relativity. People not having significant issue with 1G centrifuges is one of the few possibilities that even the most conservative test programs would take on trust (getting 1G centrifuges right so they don't break is a far bigger concern).

It is certainly true that we have never studied 'sub G' properly. We know things go wrong at 0G, and work well at 1G, but we really have no idea how large life will behave at .1G or .3G. Impossible to study on earth, hard to study in space. All we have are theories at this point. We can be pretty sure it will be far harder to adjust to Earth from Mars than the other way around, but that doesn't say anything about the viability of life on Mars, with or without therapies (exercises, drugs, diet, whatever is required). Studying it in space would require a whole new station; one that is structural (ISS is flimsy enough that one of it's tiny accelerations almost shook it apart). That means heavier and more expensive. There are other hurdles too, though some have been overcome. The new dragon could probably manage to dock at a spinning hub, but berthing is sort of out of the question. All the designs that include a rotating seal are probably no-go, so docking ports have to be rotating too. Solar panels could maybe be static, as a transformer can be integrated into a bearing just fine, but radiators are much harder. Many of them probably need to be on the rotor.

Such a station would have micro-G regions in the hub, but it would largely be unsuitable for studying micro-G effects. You could have it attached to another station that wasn't spinning, but getting between them would require either another 'craft' (could be more like a lift than a craft, but it would essentially have to undock from one, match spin with the other, then dock). It gets complicated.

Depending on the size of the spinning habitat, Coriolis effect might also be important, but I doubt it will do anything that people did not experience on ships for centuries.

[factotum]

If they turn out to be different the physicists are going to be even more interested than the biologists. That would break relativity.

Why? All relativity states is that it's not possible to distinguish between standing on a planet in a 1G gravity field and accelerating in a straight line at a constant 1G. If you're going in a constant rotary motion then the vector of your acceleration is constantly changing, so it's not the same situation.

[Tyndmyr]

Would a superfast potato have enough (relative) time to interact with regular matter in a meaningful way?
Wouldnt its atoms just move unhindered through the Earth? Subatomic space is mostly empty. Before meaningful forces could act, most particles would be past each other.
Shooting a bullet at a paper sheet doesn't disintegrate the whole papersheet. It just punches holes where it hits.

Ballistics begins to work oddly as you approach the speed of light. Even at relatively small fractions of it(getting anything of any decent size accelerated to that speed is hard), you start seeing density and size dominate over speed.

Given objects of equal density, you'll generally only get penetration equal to the projectile's length.

A potato not being particularly dense, it would be unlikely to penetrate very deep into the earth no matter how fast its going, and would instead essentially detonate. Assuming an earth with an atmosphere, this'll probably happen there, as despite the vastly lower density of the atmosphere, there's enough of it to stop the potato long before it gets to ground.

This'd result in a pretty impressive boom as all that energy gets converted to light, heat and sound. This would still probably be pretty bad for any unfortunate souls actually on the rock we're popping potatoes at.

I'm honestly not sure how this would work for specifically *at* light speed, but this approximation should work as you approach light speed, so it probably won't change dramatically by going a little bit faster. A potato is only so tough, after all.

[erikun]

I'll throw my hat into the "Why not a space station?" group.

At this point, Mars does not have an appreciable atmosphere, and does not have an appreciable magnetic field to protect such an atmosphere (not to mention land-bound residents) from solar winds. Plus, there is not a large amount of gravity on the planet, certainly less than is believed healthy for living humans. Combine this with the length of the journey, and it seems like any habitation on Mars would involve figuring out how to produce an acceptable atmosphere and interstellar protection for an extended journey, just to end up in a location which also needs to produce an acceptable atmosphere and interstellar protection for an extended stay. At that point, I'm not sure why there is a desire to place oneself onto a planet - with the whole difficulty of propulsion and getting back off again - instead of just making the travel vehicle itself the thing with the atmosphere and the protection and the whole habitation thing going on. Certainly, docking two spacecraft would be easier to work out than attempting to land/leave a planet repeatedly.

I can certainly understand a corporation or organization's greater interest in planetary habitation: there's a lot more valuable stuff on Mars than a random chunk of rock in the asteroid belt, if only because they can just set up one base in one location (Marsside landing pad) to establish transport routes. But I'm not sure how living on Mars would be superior for the people there than just living on a ship out in space. People have mentioned digging under the surface for protection from solar radiation, but rock not being completely-impermeable just means putting nearly as much work into sealing the rock faces underground as it would sealing a structure above ground. Plus, it wouldn't help any with any sort of mining work for resources on the planet itself.

[Tyndmyr]

Why? All relativity states is that it's not possible to distinguish between standing on a planet in a 1G gravity field and accelerating in a straight line at a constant 1G. If you're going in a constant rotary motion then the vector of your acceleration is constantly changing, so it's not the same situation.

You get a couple of interesting effects inside of a massive centifruge.

For one thing, depending on how big it is, there might be a noticeably different rotational speed between your head and your feet. That might be a little weird.

Certainly the effective gravity would vary far more rapidly than it does on earth. If you live in a two story house, upstairs might feel noticeably different than downstairs. Likewise, a ball thrown in the air would probably experience significant gravity variance, and would probably fly differently than we'd expect on earth.

If you have no structure in that space, and instead it's just open air, well, you might get some interesting wind forces. Certainly they'd be very different than earth weather patterns, but given a large enough of a structure, you *would* have weather. After all, extremely large buildings on earth do. To the best of my knowledge, nobody's built a centrifuge big enough to mess with this.

I don't see any showstoppers in here, but I would expect there to be a lot of little oddities cropping up in the first testbed.

[Forum Explorer]

The problem with planetary gravity is that you can't adjust it. On Venus, you have 91% Earth Gravity, which I guess the human body probably could adjust to. For Mars and Mercury it's only 38%, which I seriously doubt is enough for the healthy development of bones, circulatory system, and nervous system in children. Next one is Io with a measly 18% and from there it's only downhill.
Which is why I think that everything but Venus can be ruled out for permanent habitation right from the start. (Doing 6 month tours in shipyards and fuel production facilities on the Moon is a totally different story. No children and elderly there, though.)

If we had a planet with the size of Venus and the surface conditions of Mars, I'd say permanent colonization would absolutely be doable, possibly within the next 100 years.
But as it is, we only have two possible candidates in the solar system and both of them happen to have two huge, but completely different dealbreakers.

Simulated gravity in rotating space habitats can be adjusted to whatever strength you want, provided the whole thing is big enough. (100m radius seems to roughly be where tidal forces on head and feet become small enough to not cause disorientation, which is why we haven't build anything like it yet.) And the cool thing about weightless objects in a vacuum is that once you get them spinning, they will keep spinning forever without any energy input. Which makes that approach actually quite cheap, compared to building the habitat in the first place.

I'm curious, how deep would you have to dig on Mars to experience a higher level of gravity? It would go up as you got deeper wouldn't it?

I'll throw my hat into the "Why not a space station?" group.

At this point, Mars does not have an appreciable atmosphere, and does not have an appreciable magnetic field to protect such an atmosphere (not to mention land-bound residents) from solar winds. Plus, there is not a large amount of gravity on the planet, certainly less than is believed healthy for living humans. Combine this with the length of the journey, and it seems like any habitation on Mars would involve figuring out how to produce an acceptable atmosphere and interstellar protection for an extended journey, just to end up in a location which also needs to produce an acceptable atmosphere and interstellar protection for an extended stay. At that point, I'm not sure why there is a desire to place oneself onto a planet - with the whole difficulty of propulsion and getting back off again - instead of just making the travel vehicle itself the thing with the atmosphere and the protection and the whole habitation thing going on. Certainly, docking two spacecraft would be easier to work out than attempting to land/leave a planet repeatedly.

I can certainly understand a corporation or organization's greater interest in planetary habitation: there's a lot more valuable stuff on Mars than a random chunk of rock in the asteroid belt, if only because they can just set up one base in one location (Marsside landing pad) to establish transport routes. But I'm not sure how living on Mars would be superior for the people there than just living on a ship out in space. People have mentioned digging under the surface for protection from solar radiation, but rock not being completely-impermeable just means putting nearly as much work into sealing the rock faces underground as it would sealing a structure above ground. Plus, it wouldn't help any with any sort of mining work for resources on the planet itself.

A space station has the problems of no durability, and effectively being stuck in a submarine for your entire life. There a huge host of psychological problems that comes from that, which would be hard to manage. Not to mention a space station is basically human only, and cannot be allowed to break down at all, ever.

[Tyndmyr]

I'm curious, how deep would you have to dig on Mars to experience a higher level of gravity? It would go up as you got deeper wouldn't it?

You'd have to dig a lot in order to experience any significant change, and it wouldn't really solve this for you. If you dug all the way to the core, the mass would be roughly evenly distributed around you, so you'd experience almost no gravity.

Digging does help with radiation shielding, though.

A space station has the problems of no durability, and effectively being stuck in a submarine for your entire life. There a huge host of psychological problems that comes from that, which would be hard to manage. Not to mention a space station is basically human only, and cannot be allowed to break down at all, ever.

Those are unfortunately also problems on planets. It's a question of if you prefer hard vacuum outside or abrasive dust.

[Forum Explorer]

You'd have to dig a lot in order to experience any significant change, and it wouldn't really solve this for you. If you dug all the way to the core, the mass would be roughly evenly distributed around you, so you'd experience almost no gravity.

Digging does help with radiation shielding, though.



Those are unfortunately also problems on planets. It's a question of if you prefer hard vacuum outside or abrasive dust.

Wait really? I thought being surrounded by all that mass would effectively put the pressure of the entire planet on you.


Well yes. That's my problem with terraforming Mars. You aren't really. You're just putting a space station on Mars' surface. Venus, the end goal is to have a planet you can take a walk on.

[Yora]

I'm curious, how deep would you have to dig on Mars to experience a higher level of gravity? It would go up as you got deeper wouldn't it?

I did take a little look around on that topic, and all the sources seem to conclude that on a body of uniform density, gravity actually decreases as you go below the surface. As you descend down, there is less Earth beneath you to pull you down, but you keep getting more and more Earth above you that pulls you up. At the very core, you would be pulled into all directions equally and be completely weightless.

The Earth is of course not homogeneous and of uniform density, and the heavy metal core much denser than the silicon oxide and aluminium oxide that makes up the crust. So as you do get closer to the metal core, you actually get somewhat of an increase in gravity. But according to this graph the increase is very small. Gravity goes from 9.8 m/s² to maybe 10.8 m/s², at an incredible depth of 3,000 km.
I don't know about the composition of Mars, but if the effects there are similar, you'd have to go hundreds of km down to get from 0.18g to maybe 0.22g. It would be negligible, and also practically impossible.

A space station has the problems of no durability, and effectively being stuck in a submarine for your entire life. There a huge host of psychological problems that comes from that, which would be hard to manage. Not to mention a space station is basically human only, and cannot be allowed to break down at all, ever.

The same problems apply to a planetary station as well. Unless you can turn the planet into a copy of Earth. Which is the premise of this thread, but probably way too optimistic even if terraforming gets actually attempted at a large scale.
Yes, objects in space can be hit by flying debris that can cause catastrophic damage. But even in Low Earth Orbit, which is already full of high speed junk, I've only heard of that happening once.
Impacts by tiny fragments on a large station might punch a hole into some walls, but the overall structural damage would be limited. Larger pieces that could cause significant destruction are much easier to spot from large distances out in space, and since the station would be in space, it would only take a slight push to get it out of the way.
A real concern, but something I consider very much manageable by a civilization that can build cities in space.

[erikun]

A space station has the problems of no durability, and effectively being stuck in a submarine for your entire life. There a huge host of psychological problems that comes from that, which would be hard to manage. Not to mention a space station is basically human only, and cannot be allowed to break down at all, ever.

To add to what others have said: Living space on Mars would have the same restrictions as living space in a permanent extraterrestrial location. A person on Mars would be stuck living inside a bunch of submarine-like tubes just to provide habitable space. And if the Mars station can expand outwards to accommodate more living space while not leaving itself open to getting hit by a stray meteorite, then I'm not sure why a space station could not.

[Tyndmyr]

Wait really? I thought being surrounded by all that mass would effectively put the pressure of the entire planet on you.

Yeah, gravity in different directions effectively cancels out.*

For instance, if you are directly between the moon and the earth, there's a point where the gravity of the two cancels(much closer to the moon), and you'll experience zero g. This is one type of Langrage point.

There may be some slight variations, because gravity in practice is never exactly even, so there may be local areas where you can get marginally higher gravity than what Mars normally has, but this would be a very small effect, it's not going to be noticeable on a human level.

*in most cases. If the gradient is extremely high, such as while falling into a black hole, there are...severe effects. Mostly this won't matter on a planetary scale, though, and can be ignored.

Well yes. That's my problem with terraforming Mars. You aren't really. You're just putting a space station on Mars' surface. Venus, the end goal is to have a planet you can take a walk on.

Assuming such terraforming is possible, that would be cool, but it is probably not an easier problem than building a reliable space station. We've actually built space stations, scaling those up to bigger and better ones is a far smaller jump than going to planet scale terraforming.

Honestly, we haven't even gotten down management of earth's greenhouse effect. If we can't handle that, Venus's seems like hard mode.

[erikun]

Assuming such terraforming is possible, that would be cool, but it is probably not an easier problem than building a reliable space station. We've actually built space stations, scaling those up to bigger and better ones is a far smaller jump than going to planet scale terraforming.

Our current space stations actually reside within the Earth's magnetosphere, meaning that they are protected from a large amount of cosmic radiation.* This means that, if you were to take the ISS up into deep space, nobody on board would end up surviving - certainly not for the entire month-long journey to Mars. We definitely have some other concerns to work out before even long-term visitation well away from the Earth is reasonable.

* Wikipedia

[Tyndmyr]

Indeed.

And still, problems of such a magnitude are tiny in comparison to fixing an entire world such as Venus.

The effort required to nudge climate even a degree or two lower on a planetary scale is truly immense. Difficult to contemplate doing successfully even on the planet we're currently on. Even stopping the warming is a pretty big challenge, let alone reversing it.

[halfeye]

Ballistics begins to work oddly as you approach the speed of light. Even at relatively small fractions of it(getting anything of any decent size accelerated to that speed is hard), you start seeing density and size dominate over speed.

This has to be wrong. The mass rises as an object nears the speed of light and time dilates. This makes the velocity more important not less.

Given objects of equal density, you'll generally only get penetration equal to the projectile's length.

That works for sabot rounds, but not apparently for rail guns, if they could make them.

A potato not being particularly dense, it would be unlikely to penetrate very deep into the earth no matter how fast its going, and would instead essentially detonate. Assuming an earth with an atmosphere, this'll probably happen there, as despite the vastly lower density of the atmosphere, there's enough of it to stop the potato long before it gets to ground.

This is mistaken, the time for the potato to evaporate goes down as it's speed rises, but the distance it travels in that time goes up. At a high enough speed (probably something impossible like 0.9999c) it goes all the way through the Earth, what comes out the other side is a plasma, but it does come out and there's a big exit wound before the Earth flies appart. It's almost certainly not going to happen, big lumps of matter don't generally go that fast and if they did anything they hit would destroy them. I certainly don't want it to happen, but I'm pretty sure that's how things go relatively speaking.

This'd result in a pretty impressive boom as all that energy gets converted to light, heat and sound. This would still probably be pretty bad for any unfortunate souls actually on the rock we're popping potatoes at.

I'm honestly not sure how this would work for specifically *at* light speed, but this approximation should work as you approach light speed, so it probably won't change dramatically by going a little bit faster. A potato is only so tough, after all.

Light speed for matter is effectively impossible, but in theory at least we can work out what happens arbitrarily close to c.

[Fat Rooster]

Why? All relativity states is that it's not possible to distinguish between standing on a planet in a 1G gravity field and accelerating in a straight line at a constant 1G. If you're going in a constant rotary motion then the vector of your acceleration is constantly changing, so it's not the same situation.

There are some weird effects that come about from the rotating reference frame, but even for a small centrifuge they will be dwarfed by the forces put on your body from moving around, or even just standing differently. It is sort of comparable to being on a boat in high seas, and as far as I am aware nobody has noticed any physiological effects from that. You might expect similar symptoms while people adjust, but the differences in terms of magnitude or character of forces on parts of you are far smaller than what we already experience day to day. If lying down doesn't completely screw with your digestive system then it is extremely unlikely a tiny Coriolis force will.

I'll throw my hat into the "Why not a space station?" group.

Lack of local elements, or lack of local energy. Nitrogen is critical for a colony to grow, but is very rare inside the frost line (and even near it). Outside the frost line sunlight is dim, making energy scarce. After that, you lack geology. Geology isn't just stuff; it is also processes, some of which are extremely useful. The rare earth metals are not the rarest ones, they are the ones that do not have concentrating processes, and hence have no high quality ores. Without geology there is no reason to expect any ores, just a mish mash of everything. M type asteroids are the exception, but they lack all the elements required for a colony, and will always be a mining outpost entirely dependent on imports. Between the Oberth effect and aerobraking it is not even much easier to get materials between asteroids than from the surfaces of planets.

The question "why not" is also not really the one you want to ask. If we take it as a given that we are shipping people off earth, and looking for somewhere to put them, then it is a relevant question, but it is not a given that we set up a colony. Even if it was, the question would be about finding somewhere with everything needed, and no asteroids exist that have everything. We need a positive reason to set up a colony, and space station colonies don't really have one, while also being perpetually dependent on imported materials. That is not good. A Martian colony also struggles to find positive reasons (closest I can find is that gold might provide some returns, discussed earlier), but can at least be self sustaining after a while and even expand.

[Yora]

Honestly, we haven't even gotten down management of earth's greenhouse effect. If we can't handle that, Venus's seems like hard mode.

The whole concept of terraforming comes out of the mid 20th century that the exponential growth in the understanding of physics and the development of technology in the preceding one hundred year would continue forever.
It's at home in the same place that brought us Faster Than Light Travel, Dyson Spheres, the Kardachev Scale, and interstellar empires with thousands of planets inhabited by trillions of people. (Also our current economic and financial model.)
Smart educated people in the 50s and 60s might actually have believed these things. And it ended up in science fiction of that period to become staples of our cultural "space fantasy". It's a modern mythology that keeps being perpetuated not because our growing understanding of the subject makes them more likely, but because we want to belief that this fantasy world of our childhood will be real one day.
But the assumption that the industrialization and resulting population growth seen in their lifetimes and the lifetimes of their grandparents is not a temporary aberration, but an eternal exponential trend is really quite misguided.

While I see permanent human populations on space habitats as technicall doable, I don't see them actually becoming a thing within any time span tha could reasonably be called a "prediction". With the Earth still having half a billion years of complex life on the surface (it's roughly halway through this stage now), it's impossible to say if something will never happen.
But as actual predictions and extrapolations go for the next few centuries, I think asteroid mining and lunar shipyards (for building and fueling mining ships) will be all we'll get. And maybe some research stations on Mars and Europa as we have in the Antarctic.

[Tyndmyr]

This has to be wrong. The mass rises as an object nears the speed of light and time dilates. This makes the velocity more important not less.

In terms of potential energy, not in terms of penetration. As you exceed structural cohesiveness, things start behaving as liquids. This even happens in explosives, where metal becomes essentially liquid due to the energies involved.

The potato's just not going to stay together.

That works for sabot rounds, but not apparently for rail guns, if they could make them.

Rail guns exist, the US navy has even had one mounted on a ship for some time. However, projectile physics work the same regardless of how you accelerate the projectile to that speed.

For things where a lot of penetration is desired, you generally use an extremely dense core for this reason. Tungsten, DU. High speed, dense core, longer round, those are the hallmarks of extremely high penetration weapons.

Honestly, even those materials wouldn't cope well with impacts at a substantial fraction of light speed, but a potato would do a good deal worse.

As for Yora's post...I agree. The grandiose sci fi ideas of the 50s and 60s are turning out to be a good deal harder than they were considered at the time. That said, I'll be quite happy to see us putting humans anywhere beyond earth's orbit, even if it's a relatively small scientific research station. Progress is progress, after all.

[Radar]

Ballistics begins to work oddly as you approach the speed of light. Even at relatively small fractions of it(getting anything of any decent size accelerated to that speed is hard), you start seeing density and size dominate over speed.

Given objects of equal density, you'll generally only get penetration equal to the projectile's length.

A potato not being particularly dense, it would be unlikely to penetrate very deep into the earth no matter how fast its going, and would instead essentially detonate. Assuming an earth with an atmosphere, this'll probably happen there, as despite the vastly lower density of the atmosphere, there's enough of it to stop the potato long before it gets to ground.

This'd result in a pretty impressive boom as all that energy gets converted to light, heat and sound. This would still probably be pretty bad for any unfortunate souls actually on the rock we're popping potatoes at.

I'm honestly not sure how this would work for specifically *at* light speed, but this approximation should work as you approach light speed, so it probably won't change dramatically by going a little bit faster. A potato is only so tough, after all.

Actually those penetration estimations do not work properly at relativistic speeds as they assume that objects cannot phase into each other. If you fling anything fast enough, the atomic forces will not be enough to keep the potato and the ground separate. So relativistic projectiles would penetrate the target deeper than it would be estimated from the Newtonian model (he was actually the one, who gave this simple estimation of penetration depth based on densities and length of the bullet).

[halfeye]

The potato's just not going to stay together.

It's not, but it will exist for a time, and in that time, due to its extreme speed, it will travel a very long way.

Rail guns exist, the US navy has even had one mounted on a ship for some time. However, projectile physics work the same regardless of how you accelerate the projectile to that speed.

Projectiles do work the same given the same speed. Sabot rounds are much slower than rail gun rounds would be. Last I knew rail guns were still experimental, if they aren't being fitted as standard in new vessels, I'd say they're still in the prototype stage of development.

For things where a lot of penetration is desired, you generally use an extremely dense core for this reason. Tungsten, DU. High speed, dense core, longer round, those are the hallmarks of extremely high penetration weapons.

That's tanks, but it wasn't WW1/2 battleships, they achieved penetration with very heavy shells, long yes, but wide too.