<p>The fundamental problem is the heat capacity of the planet. If you keep it out of sunlight long enough it will eventually cool to the temperature of the background ratiation, bit it will take a <em>long</em> time. If we just treat it as a blackbody surrounded by a 0 K universe <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cootime.html" rel="nofollow noreferrer">the cooling time will roughly be</a> $t_{cool}\approx 2.5\cdot 10^{11}$ seconds if we want to reach 300 K. That is about 7900 years. Wait for about 29,000 years and you can start scooping up frozen carbon dioxide. And this assumes good heat conduction and that the interior is not hotter - I think there will be bad geological issues when trying to change the crust temperature.</p>

<p>Another issue is of course the total lack of sunlight. Martyn Fogg describes a neat solution in his book <em>Terraforming</em>: orbit a reflector in a polar orbit, angled at 45 degrees so it reflects sunlight to the planet. This would for the right orbit give a daily cycle despite Venus rotation issues and the shade shield. </p>

<p>Whether this is a sensible start for terraforming or just evidence that the technology does not help very much depends on your level of optimism and timescales. For exploration you may want a method that does not transform your exploration target as much.</p>

CC BY-SA 3.0
2017-12-18T03:31:12.723Z
We don't need to wait the cooling of the core, it is enough if its surface cools. On the Earth, the mornings are typically cooler as their previous evening, roughly with 10K - on this reason, the Venusian surface would cool down in weeks.
peterh
CC BY-SA 3.0
2017-12-17T23:09:23.130Z
2017-12-17T23:09:23.130Z
Anders Sandberg
<p>The Venus, like Earth, has 3 major heat sources or losses:</p>

<ol>
<li>Radiative heating from inside</li>
<li>Solar heating</li>
<li>Loss of heat due to thermal radiation to the space</li>
</ol>

<p>(1) is about the same as of the Earth - the planets were created from the same <a href="https://en.wikipedia.org/wiki/Protoplanetary_disk" rel="nofollow noreferrer">protoplanetary disk</a>.</p>

<p>(2) is significantly stronger, about the double of the Earth's.</p>

<p>Surprisingly, also (3) is the same as the Earth - the upper cloud layer what we can see (= which is enough high to be not covered by anything above) has about the same temperature as the Earth.</p>

<p>On the Venusian surface, there is very hot ($\approx 450 {}^\circ C$), but it is the result of the greenhouse effect and not of (1). The internal radiative heating is small, <a href="https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Global_internal_heat_flow" rel="nofollow noreferrer">around $70-100 \frac{mW}{m^2}$ on the Earth</a> and there is no reason to think that it would be on the Venus significantly different.</p>

<p>On an average night, the surface temperature loses 5-15 ${}^\circ C$ due to (3). It depends mainly on how cloudy is the sky. This happens only on the surface - some meters below the surface, there is no daily temperature fluctuation any more. The soil is a very good heat insulator, if it is enough thick, which is well visible also on that how small is (1).</p>

<p>Thus, if some space macroengineering project could cover the Venus with a large sunshade, then the Venusian temperature would decrease to a new equilibric state between (1), (2) and (3). This could be significantly smaller, as its current temperature, depending mainly on that, how big percent of the Venusian solar radiation is covered.</p>

<p>The time scale of the transition to the new equilibric temperature balance would happen in weeks, at most in months. It wouldn't have any effect to the deeper layers of the Venusian soil, thus <em>minig on the Venus</em> would be still problematic, but covering it with an enough large foil, could reach even a mean Earth-like temperature, too.</p>

<hr>

<p>There is another major problems. The largest is that the Venus practically doesn't rotate (more exactly, its rotation period is around a half year), thus an Earth-like mean temperature would mean a very hot sun-side and a very cold dark one. However, its tight athmosphere works as a strong heat balancer - on the current Venus, practically there is no difference between the day and night sides. Although a 90 atm CO2 + 4 atm N2 athmosphere isn't very humanfriendly, we would probably need to compansate it with spacesuits filled with 94 atm He + 0.2 atm O2.</p>

<p>The solar soil could be best positioned on the L1 Lagrange-point between the Venus and the Sun:</p>

<p><a href="https://i.sstatic.net/hFrl7.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/hFrl7.jpg" alt="enter image description here"></a></p>

<p><sub>This picture show the Earth-Sun Lagrange-points, but the Sun-Venusian Lagrange points are very similar</sub></p>

<p>The Sun-Venus L1 is around 2million km away from the Venus, and it is unstable for point-like mass points. However, in the case of a not point-like body (a planetary-sized foil isn't point-like), or by manipulating the <a href="https://en.wikipedia.org/wiki/Radiation_pressure" rel="nofollow noreferrer">radiation pressure</a> of the Sun, could be made statically stable, or could be stabilized dynamically.</p>

CC BY-SA 3.0
2017-12-18T05:48:05.377Z
2017-12-18T06:23:47.570Z
2017-12-18T06:23:47.570Z
peterh
<p>Lets say we had the technology to create a mini-Dyson structure, like a shutter style curtain between the Sun and Venus, blocking the sunlight in a controllable manner to cool down it's temperature. 
Could it be the first step to reverse the greenhouse effect and make Venus more "terraformable"? Or at least more explorable like Mars? Would there be any unintended consequences?
Would this be useful on other hot extra-solar earth-like planets?</p>

CC BY-SA 3.0
2017-12-17T22:58:35.893Z
Blocking the sun would help cool Venus, but this is only one of numerous problems with making it habitable - the atmosphere is not suitable, there are no essential life forms, what will you do for correct magnetic field, oxygen, atmospheric pressure, food, etc etc etc - I don't think this is on topic at all
Rory Alsop
CC BY-SA 3.0
2017-12-17T23:33:29.723Z
I'm voting to close this question as off-topic because it sounds more like a Worldbuilding question.
StephenG - Help Ukraine
CC BY-SA 3.0
2017-12-17T23:33:41.927Z
Venus' reflective atmosphere already reflects (essentially blocking) 80% of the sunlight that hits it.   In addition to the points already made, it has too little water.   (and shading an entire planet is nuts - just gonzo-crazy from an engineering standpoint).    Also, this is probably better for Worldbuilding.   This really isn't right for this site.
userLTK
CC BY-SA 3.0
2017-12-18T17:01:22.713Z
@userLTK I think the required hidrogen could be transfered there from the Sun. They would fly in its outer athmosphere, and with large ion cannons they would "fire" the ionized $H_2$ plasma to the Venus. It would be probably a much larger project as the solar sail.
peterh
<venus><earth-like-planet>
Would blocking sunlight on Venus help make it habitable?
tiago
# Would blocking sunlight on Venus help make it habitable?

(Asked by: username_0 on 2017-12-17)

Lets say we had the technology to create a mini-Dyson structure, like a shutter style curtain between the Sun and Venus, blocking the sunlight in a controllable manner to cool down it's temperature.
Could it be the first step to reverse the greenhouse effect and make Venus more "terraformable"? Or at least more explorable like Mars? Would there be any unintended consequences?
Would this be useful on other hot extra-solar earth-like planets?

**Tags:** venus, earth-like-planet
> **Comments:**
> **username_1**: Blocking the sun would help cool Venus, but this is only one of numerous problems with making it habitable - the atmosphere is not suitable, there are no essential life forms, what will you do for correct magnetic field, oxygen, atmospheric pressure, food, etc etc etc - I don't think this is on topic at all  
> **username_2**: I'm voting to close this question as off-topic because it sounds more like a Worldbuilding question.  
> **username_3**: Venus' reflective atmosphere already reflects (essentially blocking) 80% of the sunlight that hits it.   In addition to the points already made, it has too little water.   (and shading an entire planet is nuts - just gonzo-crazy from an engineering standpoint).    Also, this is probably better for Worldbuilding.   This really isn't right for this site.  
> **username_4**: @userLTK I think the required hidrogen could be transfered there from the Sun. They would fly in its outer athmosphere, and with large ion cannons they would "fire" the ionized $H_2$ plasma to the Venus. It would be probably a much larger project as the solar sail.  

---

## 2 answers

### Answer by username_5 on 2017-12-17. Score: 1

The fundamental problem is the heat capacity of the planet. If you keep it out of sunlight long enough it will eventually cool to the temperature of the background ratiation, bit it will take a *long* time. If we just treat it as a blackbody surrounded by a 0 K universe [the cooling time will roughly be](http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cootime.html) $t\_{cool}\approx 2.5\cdot 10^{11}$ seconds if we want to reach 300 K. That is about 7900 years. Wait for about 29,000 years and you can start scooping up frozen carbon dioxide. And this assumes good heat conduction and that the interior is not hotter - I think there will be bad geological issues when trying to change the crust temperature.

Another issue is of course the total lack of sunlight. Martyn Fogg describes a neat solution in his book *Terraforming*: orbit a reflector in a polar orbit, angled at 45 degrees so it reflects sunlight to the planet. This would for the right orbit give a daily cycle despite Venus rotation issues and the shade shield.

Whether this is a sensible start for terraforming or just evidence that the technology does not help very much depends on your level of optimism and timescales. For exploration you may want a method that does not transform your exploration target as much.

> **Comments:**
> **username_4**: We don't need to wait the cooling of the core, it is enough if its surface cools. On the Earth, the mornings are typically cooler as their previous evening, roughly with 10K - on this reason, the Venusian surface would cool down in weeks.  

---

### Answer by username_4 on 2017-12-18. Score: 1

The Venus, like Earth, has 3 major heat sources or losses:

1. Radiative heating from inside
2. Solar heating
3. Loss of heat due to thermal radiation to the space

(1) is about the same as of the Earth - the planets were created from the same [protoplanetary disk](https://en.wikipedia.org/wiki/Protoplanetary_disk).

(2) is significantly stronger, about the double of the Earth's.

Surprisingly, also (3) is the same as the Earth - the upper cloud layer what we can see (= which is enough high to be not covered by anything above) has about the same temperature as the Earth.

On the Venusian surface, there is very hot ($\approx 450 {}^\circ C$), but it is the result of the greenhouse effect and not of (1). The internal radiative heating is small, [around $70-100 \frac{mW}{m^2}$ on the Earth](https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Global_internal_heat_flow) and there is no reason to think that it would be on the Venus significantly different.

On an average night, the surface temperature loses 5-15 ${}^\circ C$ due to (3). It depends mainly on how cloudy is the sky. This happens only on the surface - some meters below the surface, there is no daily temperature fluctuation any more. The soil is a very good heat insulator, if it is enough thick, which is well visible also on that how small is (1).

Thus, if some space macroengineering project could cover the Venus with a large sunshade, then the Venusian temperature would decrease to a new equilibric state between (1), (2) and (3). This could be significantly smaller, as its current temperature, depending mainly on that, how big percent of the Venusian solar radiation is covered.

The time scale of the transition to the new equilibric temperature balance would happen in weeks, at most in months. It wouldn't have any effect to the deeper layers of the Venusian soil, thus *minig on the Venus* would be still problematic, but covering it with an enough large foil, could reach even a mean Earth-like temperature, too.

---

There is another major problems. The largest is that the Venus practically doesn't rotate (more exactly, its rotation period is around a half year), thus an Earth-like mean temperature would mean a very hot sun-side and a very cold dark one. However, its tight athmosphere works as a strong heat balancer - on the current Venus, practically there is no difference between the day and night sides. Although a 90 atm CO2 + 4 atm N2 athmosphere isn't very humanfriendly, we would probably need to compansate it with spacesuits filled with 94 atm He + 0.2 atm O2.

The solar soil could be best positioned on the L1 Lagrange-point between the Venus and the Sun:

[![enter image description here](https://i.sstatic.net/hFrl7.jpg)](https://i.sstatic.net/hFrl7.jpg)

This picture show the Earth-Sun Lagrange-points, but the Sun-Venusian Lagrange points are very similar

The Sun-Venus L1 is around 2million km away from the Venus, and it is unstable for point-like mass points. However, in the case of a not point-like body (a planetary-sized foil isn't point-like), or by manipulating the [radiation pressure](https://en.wikipedia.org/wiki/Radiation_pressure) of the Sun, could be made statically stable, or could be stabilized dynamically.


---

