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    Quote Originally Posted by factotum View Post
    Venus doesn't have plate tectonics, though, so the crust will tend to stay in one place rather than moving around as it does on Earth--that gives the heat more chance to seep down into the depths.
    I wonder if this is, in fact, what's happening on Venus?

    Our own core is very hot due to a mix of residual heat from our planet's formation and the decay of radioactive elements. If Venus also has the same, which I'd find very believable, I'd expect the planet's interior to be much hotter than the surface. That'd mean heat was traveling from rather than into the depths of our sister planet.

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    Quote Originally Posted by Leewei View Post
    I wonder if this is, in fact, what's happening on Venus?

    Our own core is very hot due to a mix of residual heat from our planet's formation and the decay of radioactive elements. If Venus also has the same, which I'd find very believable, I'd expect the planet's interior to be much hotter than the surface. That'd mean heat was traveling from rather than into the depths of our sister planet.
    Venus does not seem to have plate tectonics, per se. However, the entire surface of Venus is much younger than the age of the solar system. I don't remember if it is 100,000 or 1,000,000 years old, but the entire surface is about the same age. Furthermore, there are what appear to the extinct volcanoes on its surface. I am willing to bet that the interior of the planet is hotter than the surface.

    Edit: So I hunted down some numbers on the heat capacity of carbon-dioxide and the mass of the Venusian atmosphere. It looks like the atmosphere is about 4.8*10^20 kg. The specific heat of carbon-dioxide at 750 K is about 1.148. Assuming that the specific heat is piecewise linear (i.e. it is constant from 700 to 750 at the 750 value), it would take 127 hours for the surface temperature of Venus to drop 50 K. That number has all kinds of bad assumptions built in to it, but I think they all cancel out. It does assume that solar irradiation is completely blocked. If we are willing to wait a couple months for that temperature drop (i.e. wait 1270 hours), we would only need to block 10% of solar irradiation. Even that is very much beyond our current tech. I doubt we could block 1% of solar irradiation of Venus with any kind of reliability at the moment.
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    Quote Originally Posted by Rockphed View Post
    Venus does not seem to have plate tectonics, per se. However, the entire surface of Venus is much younger than the age of the solar system. I don't remember if it is 100,000 or 1,000,000 years old, but the entire surface is about the same age. Furthermore, there are what appear to the extinct volcanoes on its surface. I am willing to bet that the interior of the planet is hotter than the surface.

    Edit: So I hunted down some numbers on the heat capacity of carbon-dioxide and the mass of the Venusian atmosphere. It looks like the atmosphere is about 4.8*10^20 kg. The specific heat of carbon-dioxide at 750 K is about 1.148. Assuming that the specific heat is piecewise linear (i.e. it is constant from 700 to 750 at the 750 value), it would take 127 hours for the surface temperature of Venus to drop 50 K. That number has all kinds of bad assumptions built in to it, but I think they all cancel out. It does assume that solar irradiation is completely blocked. If we are willing to wait a couple months for that temperature drop (i.e. wait 1270 hours), we would only need to block 10% of solar irradiation. Even that is very much beyond our current tech. I doubt we could block 1% of solar irradiation of Venus with any kind of reliability at the moment.
    Well, solar materials aren't beyond our capacity, it's just the sheer scale of manufacture that's a problem. If you could shuttle a particularly aluminium-rich asteroid to the lagrange point and introduce some van-neumann factory-machines to handle construction, it doesn't strike me as physically infeasible. Technologically that's on an order of decades-to-centuries, not hundreds of thousands of years.

    I think Leewei's original estimate included waiting for the crust to cool down as well? How does that work out?


    Oh, since SpaceX and re-usable rockets came up earlier, this might be of some interest:
    http://www.sciencealert.com/nasa-fir...pace-x-mission
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    Quote Originally Posted by Lacuna Caster View Post
    Oh, since SpaceX and re-usable rockets came up earlier, this might be of some interest:
    http://www.sciencealert.com/nasa-fir...pace-x-mission
    Do you know if they recovered the boosters for a third run? I scanned the article, and it doesn't seem to say (may have missed it, I admit it, sorry if I did). I saw there was a video, but I'm in a bit of a hurry, and couldn't sit to watch it.

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    Looks like a smooth touchdown, so yeah, in principle the boosters should be good for another run.

    EDIT: The long-term cost Musk is talking about here is closer to 50$ per kilo, but maybe Rakaydos is more familiar with the price structure there?
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    What about tipping asteroids into Venus and using the impacts to jettison gas?

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    Quote Originally Posted by Lacuna Caster View Post
    Well, solar materials aren't beyond our capacity, it's just the sheer scale of manufacture that's a problem. If you could shuttle a particularly aluminium-rich asteroid to the lagrange point and introduce some van-neumann factory-machines to handle construction, it doesn't strike me as physically infeasible. Technologically that's on an order of decades-to-centuries, not hundreds of thousands of years.

    I think Leewei's original estimate included waiting for the crust to cool down as well? How does that work out?
    My "1% on a good day" estimate was in regards to the sheer scale of blocking out the sun. I imagine that within a couple centuries we could figure out how to do it. I'm less certain that we will ever want to.

    I used earth's crust mass to approximate the mass of Venus's crust and I am assuming that it is all quartz. Earth's crust is about 2.8 x 10^24 kg. Quartz has a heat capacity of 730 J/(kg K). In that case, the time to reduce the temperature of all of Venus's crust and all of its atmosphere by 50 K is 541 years. I am fairly certain that that is not actually a useful number.
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    Just take a chunk of the atmosphere to Mars and elsewhere with a fleet of self replicating exosphere skimmers equipped with collapsible solar sails, then sprinkle pulverized comets across the molten sucker.
    Last edited by WhatThePhysics; 2017-12-17 at 05:42 AM.

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    Quote Originally Posted by WhatThePhysics View Post
    Just take a chunk of the atmosphere to Mars and elsewhere with a fleet of self replicating exosphere skimmers equipped with collapsible solar sails, then sprinkle pulverized comets across the molten sucker.
    I'm pretty sure I just showed that the atmosphere is not the problem vis-a-vis cooling Venus. The problem is the 700K rocks that need to cool down. I could probably construct a model for a reasonable heat gradient and use that to calculate the total heat that needs to be removed.

    Also, unless we slow the comets down first, I am pretty sure that comets have more kinetic energy in their orbits than they would remove by being cold.
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    Quote Originally Posted by Rockphed View Post
    I'm pretty sure I just showed that the atmosphere is not the problem vis-a-vis cooling Venus. The problem is the 700K rocks that need to cool down. I could probably construct a model for a reasonable heat gradient and use that to calculate the total heat that needs to be removed.

    Also, unless we slow the comets down first, I am pretty sure that comets have more kinetic energy in their orbits than they would remove by being cold.
    Transporting most of the atmosphere elsewhere is a multipurpose aspect to Venusian terraforming. Not only do you supply other locations with useful chemicals, you also reduce friction based heating from cometary material sprinkling, minimize the amount of toxic compounds that'll form alongside the hydrosphere, and lower the final surface pressure to a more survivable and useful figure. Ideally, the comets would be placed into orbit around Venus after the L1 solar shade has been built, and their powdered contents would be used to make it snow across the entire planet until the surface supports liquid water.

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    I was actually thinking the other day about how science fiction has traditionally depicted a Mars colony. It either ends up as a total craphole or succeeds from Earth and we end up in an interplanetary war.
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    With neither atmosphere or sun, a black body with surface temperature 300K emits about 460 Watts per square meter.
    Rock conducts heat at a rate of about (1.5 Watts per square meter) per (Kelvin per meter), therefore supporting a temperature gradient of about 300 Kelvin per meter. 700K rocks need only be a few meters down below a comfortable surface temperature, if the only heat source is internal, and there is no atmosphere to shift the radiation to a cooler effective surface.

    Please check these numbers. I was surprised.


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    Quote Originally Posted by WhatThePhysics View Post
    Transporting most of the atmosphere elsewhere is a multipurpose aspect to Venusian terraforming. Not only do you supply other locations with useful chemicals, you also reduce friction based heating from cometary material sprinkling, minimize the amount of toxic compounds that'll form alongside the hydrosphere, and lower the final surface pressure to a more survivable and useful figure. Ideally, the comets would be placed into orbit around Venus after the L1 solar shade has been built, and their powdered contents would be used to make it snow across the entire planet until the surface supports liquid water.
    (emphasis mine)

    The amount of atmosphere doesn't hugely affect the amount of energy transferred when you crash a comet into a planet1. High atmospheric friction2 with the comet means there is less kinetic energy in the comet when it hits the ground. A thinner atmosphere means that more of the kinetic energy is transferred directly to the ground. In either case, the same amount of energy is added to the planet's heat budget: energy equal to the comet's mass times the square of its speed relative to the planet.

    1. The atmosphere can affect how much of the impact energy stays on the planet. A thicker atmosphere will make it harder for debris (and thus energy) from a large impact to escape the planet. No real difference for small impacts, though.

    2. Friction between the atmosphere and the comet/meteoroid itself is pretty negligible. Most of the heating is caused by the compression of gas at the normal shock generated by the comet moving faster than the speed of sound in the atmosphere it's moving through.
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    Quote Originally Posted by DavidSh View Post
    With neither atmosphere or sun, a black body with surface temperature 300K emits about 460 Watts per square meter.
    Rock conducts heat at a rate of about (1.5 Watts per square meter) per (Kelvin per meter), therefore supporting a temperature gradient of about 300 Kelvin per meter. 700K rocks need only be a few meters down below a comfortable surface temperature, if the only heat source is internal, and there is no atmosphere to shift the radiation to a cooler effective surface.

    Please check these numbers. I was surprised.


    -G.G. -- Aye, I just finished up two novels by PKD where the Mars colonies were pretty unpleasant places to live.
    That does sound a little low. I am going to assume we need to cool the equivalent of 20 meters of Venusian rock to achieve our aims. On the other hand, people don't need multiple meter thick walls on their ovens to be able to support 400 degrees higher temperatures inside. I'll see what I come up with with that new information. If anybody has a better material for the properties of rock than quartz, I would love to hear it. Maybe I should look up basalt.

    Edit: Using Basalt instead of quartz, 20 meters of cooled rock, and assuming that the rate of radiation will be constant over the time frame, getting Venus down to earth temperatures (i.e. 16 C) will take 27.24 years. That seems almost doable if we can build a sun-shade big enough to blanket all of Venus in shadow and stick it at the Venus L1 point. Now to go figure out how big it would need to be.

    Edit2: I did some math, and it looks like a total shade would be 1.8 times the diameter of Venus. It would have an area about 80% of Venus's surface area.
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    Quote Originally Posted by DavidSh View Post
    Rock conducts heat at a rate of about (1.5 Watts per square meter) per (Kelvin per meter), therefore supporting a temperature gradient of about 300 Kelvin per meter. 700K rocks need only be a few meters down below a comfortable surface temperature, if the only heat source is internal, and there is no atmosphere to shift the radiation to a cooler effective surface.

    Please check these numbers. I was surprised.
    I don't have time to check them now, but I will note that one dimensional radial heat transfer looks pretty different than one dimensional linear heat transfer, with cylindrical and spherical systems also being pretty dramatically different. This looks like a linear heat transfer calculation that you've done.

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    Quote Originally Posted by Lord Torath View Post
    (emphasis mine)

    The amount of atmosphere doesn't hugely affect the amount of energy transferred when you crash a comet into a planet1. High atmospheric friction2 with the comet means there is less kinetic energy in the comet when it hits the ground. A thinner atmosphere means that more of the kinetic energy is transferred directly to the ground. In either case, the same amount of energy is added to the planet's heat budget: energy equal to the comet's mass times the square of its speed relative to the planet.

    1. The atmosphere can affect how much of the impact energy stays on the planet. A thicker atmosphere will make it harder for debris (and thus energy) from a large impact to escape the planet. No real difference for small impacts, though.

    2. Friction between the atmosphere and the comet/meteoroid itself is pretty negligible. Most of the heating is caused by the compression of gas at the normal shock generated by the comet moving faster than the speed of sound in the atmosphere it's moving through.
    Thanks for the correction and elaboration.

    Unless I'm mistaken, the most optimal impact trajectory for the cometary snow would be straight down with no acceleration beyond that provided by Venusian gravity, right? Maybe snowmakers could detach from the towed comet, release their ground up payloads as they descend at 90 angles, then reenter stable orbits to rendezvous with the comet via hydrogen oxide rockets. If most of the atmosphere has been siphoned by exospheric skimmers, there'd be less exothermic reactions between imported water and local acids. With enough comets performing recursive Venus flybys, we could even shorten the Venusian day.

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    Quote Originally Posted by WhatThePhysics View Post
    Thanks for the correction and elaboration.

    Unless I'm mistaken, the most optimal impact trajectory for the cometary snow would be straight down with no acceleration beyond that provided by Venusian gravity, right? Maybe snowmakers could detach from the towed comet, release their ground up payloads as they descend at 90 angles, then reenter stable orbits to rendezvous with the comet via hydrogen oxide rockets. If most of the atmosphere has been siphoned by exospheric skimmers, there'd be less exothermic reactions between imported water and local acids. With enough comets performing recursive Venus flybys, we could even shorten the Venusian day.
    The venusian day is currently negative five thousand some odd hours. That is right, it rotates backwards. I'm not sure if the proper adjective for what we want to do is shortening or lengthening at this point. Venus also has a 177.36 degree orbital obliquity (earth has 23 degrees). Maybe what we really want is to try to fix that orbital inclination to something closer to 20 degrees with the day going the right way before we start trying to change the length of the day.
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    Quote Originally Posted by Rockphed View Post
    The venusian day is currently negative five thousand some odd hours. That is right, it rotates backwards. I'm not sure if the proper adjective for what we want to do is shortening or lengthening at this point. Venus also has a 177.36 degree orbital obliquity (earth has 23 degrees). Maybe what we really want is to try to fix that orbital inclination to something closer to 20 degrees with the day going the right way before we start trying to change the length of the day.
    Yeah, I'm aware of the retrograde rotation, and I think we can also alter the axial tilt with the recursive comet flybys.

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    Quote Originally Posted by Rockphed View Post
    The venusian day is currently negative five thousand some odd hours. That is right, it rotates backwards. I'm not sure if the proper adjective for what we want to do is shortening or lengthening at this point. Venus also has a 177.36 degree orbital obliquity (earth has 23 degrees). Maybe what we really want is to try to fix that orbital inclination to something closer to 20 degrees with the day going the right way before we start trying to change the length of the day.
    While the temperature is way too hot that stuff doesn't matter, IMO, and I'm not sure it would matter when the temperature was survivable either, it's the sort of thing that life can adapt to fairly quickly.

    Let me be clear, in my opinion living in space (in habitats in space in the first place) is important, living on other planets or bodies is less so, but of the planets Venus is available to us for long term use with less effort than any other planet would be. It would be possible to live on the Moon or Mars as if you were under the sea or something, and we couldn't do that on Venus, but if you want to be able to walk around without a spacesuit one day, getting to that stage will be easier on Venus.
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    I'mma rain on all your parades... I may be championing all kinds of cutting edge rockets like SpaceX, but throwing asteroids around? Altering the spin of planets? You people have no idea the magnitude of the forces required.

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    Flinging an asteroid directly at its target takes enormous energy. Slightly altering an asteroid so its course eventually brings it to orbit a planet is a different matter. It still takes enormous amounts of energy, but it's far, far less than the direct route.

    Changing rotational direction of Venus is pretty mind-boggling.

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    Quote Originally Posted by Leewei View Post
    Changing rotational direction of Venus is pretty mind-boggling.
    Also, unnecessary, AFAICT. Other than the sun rising in the "west", what exactly is the problem that would be solved by flipping Venus' rotation axis?

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    Quote Originally Posted by Grey_Wolf_c View Post
    Also, unnecessary, AFAICT. Other than the sun rising in the "west", what exactly is the problem that would be solved by flipping Venus' rotation axis?

    GW
    I agree, except for the notation, for me west is where the sun sets, that overrules the other language issues with Venus's rotation. As I read wikipedia, Venus actually has a fairly normal rotation that's just backward compared to Earth with very little axial tilt, which would mean there would be much less in the way of seasonal change.
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    Quote Originally Posted by halfeye View Post
    I agree, except for the notation, for me west is where the sun sets, that overrules the other language issues with Venus's rotation. As I read wikipedia, Venus actually has a fairly normal rotation that's just backward compared to Earth with very little axial tilt, which would mean there would be much less in the way of seasonal change.
    Um, the venusian sidereal day is five-thousand some hours. The apparent day is two-thousand some. If you ignore the spinning backwards bit, the axial tilt is 3 degrees (my previous post was under the assumption that the axial tilt was about 90 degrees). My previous post was advocating pushing the tilt somewhere other than "pointed at the sun for a good chunk of the year", not trying to flip the thing over. If anything, simply changing the spin rate is probably cheaper and faster than trying to flip the planet over.
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    Quote Originally Posted by Rockphed View Post
    If anything, simply changing the spin rate is probably cheaper and faster than trying to flip the planet over.
    I'd argue otherwise? The actual moment of inertia of the planet is going to be roughly the same regardless of which direction you try to spin it in, and spinning it perpendicular to its axis of rotation can be done very slowly--yes, it might take years for the planet to finish turning over, but it's still a lot easier than making a major change to its axial spin rate.

    Having said that, both ideas are so far beyond anything we can reasonably expect to be doing in the next thousand years that I don't think they're worth discussing--the energy requirements are colossal.

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    Quote Originally Posted by Leewei View Post
    Flinging an asteroid directly at its target takes enormous energy. Slightly altering an asteroid so its course eventually brings it to orbit a planet is a different matter. It still takes enormous amounts of energy, but it's far, far less than the direct route.

    Changing rotational direction of Venus is pretty mind-boggling.
    Quote Originally Posted by factotum View Post
    I'd argue otherwise? The actual moment of inertia of the planet is going to be roughly the same regardless of which direction you try to spin it in, and spinning it perpendicular to its axis of rotation can be done very slowly--yes, it might take years for the planet to finish turning over, but it's still a lot easier than making a major change to its axial spin rate.

    Having said that, both ideas are so far beyond anything we can reasonably expect to be doing in the next thousand years that I don't think they're worth discussing--the energy requirements are colossal.
    Right, we should be using comets, not asteroids. Comets can be contained in stress resistant polymer shells composed of local chemicals, then redirected with a combination of gravity tractors, thrusters fueled by local chemicals, reflective coatings, and gravitational slingshots assisted by asteroids, planetoids, and other comets. By exploiting cyclical Hohmann transfer orbits, we can set up a comet "railroad" that goes between Venus and Mars, which will provide both planets with useful volatiles, shipping access, and rotation modification. Sure, it'll cost a lot of energy to set this up, but it's a multipurpose system that can help accelerate the development of a stable, interplanetary civilization.

    As for whether we can reasonably expect to be doing this in the next 1000 years, I think it's quite possible and worth discussing. Ideas that seem unrealistic today can seem obvious tomorrow, and even the most incredible concepts can add flavor to science fiction, at the very least.
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    Quote Originally Posted by WhatThePhysics View Post
    Right, we should be using comets, not asteroids. Comets can be contained in stress resistant polymer shells composed of local chemicals, then redirected with a combination of gravity tractors, thrusters fueled by local chemicals, reflective coatings, and gravitational slingshots assisted by asteroids, planetoids, and other comets. By exploiting cyclical Hohmann transfer orbits, we can set up a comet "railroad" that goes between Venus and Mars, which will provide both planets with useful volatiles, shipping access, and rotation modification. Sure, it'll cost a lot of energy to set this up, but it's a multipurpose system that can help accelerate the development of a stable, interplanetary civilization.

    As for whether we can reasonably expect to be doing this in the next 1000 years, I think it's quite possible and worth discussing. Ideas that seem unrealistic today can seem obvious tomorrow, and even the most incredible concepts can add flavor to science fiction, at the very least.
    I'm pretty agnostic to where the mass comes from, but aren't comets inaccessible and rare? If you're using comets to change Venus's rotation, it's going to take a heck of a lot of them.

    For that matter, it isn't at all clear to me why Venus's rotational direction needs to change.

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    Default Re: Living on Mars?

    Quote Originally Posted by Leewei View Post
    I'm pretty agnostic to where the mass comes from, but aren't comets inaccessible and rare? If you're using comets to change Venus's rotation, it's going to take a heck of a lot of them.

    For that matter, it isn't at all clear to me why Venus's rotational direction needs to change.
    Assuming the Kuiper Belt has a total mass between 0.04 to 30 Earth masses, and follows the same mass distribution as the Main Belt with respect to object size, we can estimate that the Kuiper Belt has somewhere between 56 million to 103 billion objects with diameters of 1 km or more. If just 1% of these objects are primarily composed of light chemicals, which may be pessimistic, that gives us anywhere between 560 thousand to 1 billion comets. Assuming each object has a roughly spherical shape, diameter of 1 km, and water ice density, this gives us about 268 exagrams to 480 zettagrams worth of cometary material.

    I'm not a physicist, so I can't tell you how much time it'd take to apply all of this mass towards changing Venus's rotation. All I know is that we have a lot of comets to work with, and it'd be worth fetching them even if we don't use them for this purpose.

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    Default Re: Living on Mars?

    Quote Originally Posted by WhatThePhysics View Post
    I'm not a physicist, so I can't tell you how much time it'd take to apply all of this mass towards changing Venus's rotation. All I know is that we have a lot of comets to work with, and it'd be worth fetching them even if we don't use them for this purpose.
    The problem you have is energy, again. Changing the orbits of Kuiper belt material to bring it down into the inner Solar System wouldn't be too hard because they're not travelling very fast when they're out there, but pulling them into a useful orbit when they're actually *in* the inner Solar System and travelling at ridiculous velocities (something like 1.4x Venus' orbital velocity when they're at the same orbital distance, I think) would be an incredibly difficult thing to do.

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    Default Re: Living on Mars?

    Quote Originally Posted by factotum View Post
    The problem you have is energy, again. Changing the orbits of Kuiper belt material to bring it down into the inner Solar System wouldn't be too hard because they're not travelling very fast when they're out there, but pulling them into a useful orbit when they're actually *in* the inner Solar System and travelling at ridiculous velocities (something like 1.4x Venus' orbital velocity when they're at the same orbital distance, I think) would be an incredibly difficult thing to do.
    The Kuiper belt is also mind-bogglingly huge. Detecting useful matter in this region is a staggering challenge which we'd need to overcome before putting it to use. Distances are between 30AU and 50AU from the sun, compared to a bit more than 3AU for the asteroid belt past Mars (which is at 1.5AU from Sol, or 0.5AU from Earth's orbital path).

    A simple, 2D comparison of the Kuiper belt to the rest of the solar system suggests you'd need to survey a region nearly twice as large as the rest of the system. Matter in the Kuiper belt would be far more diffuse and poorly-lit than it is closer to the sun. It may well be that there is a lot of stuff out there worth putting to use, but the challenge of doing so is an order of magnitude harder compared to using other sources.

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