 Images of the surface of Venus taken from the Venera 13 mission. (credit: NASA) |
Access to Venus
by John Strickland
Monday, July 24, 2023
Venus is the opposite of Mars in regard to terraforming. In fact, you would practically have to terraform Venus before you could land humans on it. It has a planetary surface almost as large as the Earth’s. However, removing the 90 atmospheres of carbon dioxide, even at the very high volatile transfer rates proposed for terraforming Mars, would probably take many millennia and an enormous amount of energy. A low energy, faster alternative would be to build a 15,000-kilometer-wide sunshade for Venus which would cause the carbon dioxide atmosphere to collapse into a liquid carbon dioxide ocean or frozen dry ice layer, at Antarctic-like temperatures. After the collapse, enough sunlight would be allowed to keep the land surface from reaching low cryogenic temperatures and pressures, and provide good visibility. The remaining nitrogen and traces of argon would not be a problem. This would open most of the surface to scientific observation and potential mining operations for high-value materials. Insulated surface or below-ground settlements on such a partly sun-shaded Venus would also not be out of the question as there would be no GCR flux and almost a 1-G gravity environment.
Purpose
Note: This article is not about terraforming Venus, although the method described could eventually be used, as a first step, to start doing just that, given access to a staggeringly large amount of energy. Instead, here is a method to provide humans with permanent access to the surface of Venus, without requiring massive amounts of energy and thousands of years of time. Such a method can and would only be implemented when humanity has a space-based industrial civilization with the sufficient industrial strength, finances, technical knowhow, and energy output to support such an effort, and when we want access to the surface of Venus strongly enough to mount the effort. If such an effort were made, the level of effort would greatly decrease once the sunshade was built and the methods to keep it in position were verified. At that point, there would be a long wait, lasting over a century, for the cooldown to proceed. I should note that human efforts in the past which required a continuing large effort for over a century have occurred, including the building of huge gothic cathedrals.
| Lots of articles have been written about terraforming Venus, but most simply do not recognize the sheer scale of such a task. |
The basics of the method are reasonably well known to many space advocates—using a huge sunshade to block essentially all sunlight from reaching the surface of Venus—but differs in that we would be attempting only to provide relatively easy access to the surface, not supporting a full terraforming attempt following the sunshade step. The reason for the difference is that performing just the first step in what could be the start of a terraforming effort requires vastly less energy and effort.
Scale
Lots of articles have been written about terraforming Venus, but most simply do not recognize the sheer scale of such a task. In order to actually terraform Venus, the effort (step 2) would need to “deal with” the roughly 465 quadrillion metric tons of carbon dioxide, (that’s 465,000,000,000,000,000 tons) assuming that the 3.5 atmospheres (3.5 bar) of nitrogen have a mass of only about 15 quadrillion tons. This is also about 200 times more mass than the amount of gas needed to give Mars a full oxygen-nitrogen atmosphere: only about 2.8 quadrillion tons. The Venusian carbon dioxide is about one-third the mass of all of Earth’s ocean water. The carbon dioxide also contains several hundred septillion joules of heat. Any way you deal with it, such as physical removal or chemical interaction, though not impossible, would be incredibly energy intensive and probably take an extremely very long time to effect. By not attempting the terraforming step (at least for now), humanity could still get easy and rapid access to much of the surface of Venus.
There are clearly a lot of questions that need to be answered before such an effort could be attempted, primarily how to create a giant planetary sunshade and how to stably maintain its position at the Venus-Sun L1 point (VS-L1) for at least several centuries. While a similar sunshade proposed to reduce global warming for the Earth would be much smaller, I do not support using a sunshade for that purpose since it would still be a long time before we can build one. Instead, I support much nearer-term geoengineering methods whose effects end automatically when the method is suspended or stopped.
Why
Why should we want or need access to the surface of Venus? It is the closest planet we know to the Earth in terms of its size and probably its internal geology, so it would be the best planetary analog for the Earth. With the current hellish conditions on the surface (about 480 degrees Celsius), any scientific investigations of its internal geology will be massively hampered by the temperature, pressure, and acidic chemical environment. It is probable that there are active volcanoes on Venus, and geologists would love to know if there are any active plate tectonics or if the volcanoes are instead the result of multiple hot spots or mantle plumes. Radar maps from orbit do not tell us everything we need to know about geological activity on Venus. We know nothing about any seismic activity on Venus and, after probably a billion years of very high temperatures, the surface minerals must be very different from those on the Earth. There are thus multiple possible and practical reasons we would want such access, even including commercial mining and potential sub-surface human settlements. In addition, at least in the space community, there seems to be a strong desire to “do something” with and at Venus, when most of the realistic options seem currently impossible.
Sunshade
We can assume that the sunshade would be made of solar sail-like material, as there would be a large amount of light pressure trying to move it from the desired position, and it would face the same type of problems that very large solar sails would face. The typical design has a polymer layer that supports a thinner layer of metal film that provides the reflectance and, in this case, primarily blocks the sunlight. I have seen numbers indicating that solar sails may eventually be able to be fabricated in space at thicknesses of between 0.2 and 5 microns, where a micron is one-thousandth of a millimeter. This thickness of just the metal layer is in the range of the size of bacteria and will require very advanced equipment to fabricate it in place. It could weigh as little as 0.1 gram per square meter, up to about 1.0 grams per square meter with the plastic support. All of these thin layers must be fully ultraviolet-resistant.
The material should probably be reinforced with heavier fibers at intervals to prevent tearing from meteoroid impacts, and possibly with heavier stiffening, open truss booms to keep the whole shade relatively flat and oriented correctly. Other options have the shade rotating to keep it relatively flat. The metal film layer needs to be thick enough to keep significant amounts of sunlight from penetrating through the ultra-thin foil. We do not yet have such technology but test solar sails with two layers have been successfully deployed in orbit.
Venus is about 108,000,000 kilometers from the Sun and the VS-L1 point is about 1.6 million kilometers sunward of Venus. The center of mass of the shade cannot stay exactly at the L1 point, since the orbit of Venus is not perfectly circular, so the shade might need to be oval in shape. However, Venus’s orbit is more circular than any other planet, so perturbations to the position of the sunshade might be affected more by the gravity of other planets. The need for the shade’s center of mass to “orbit” the L1 point makes its actual required size somewhat hard to estimate. Venus’s gravity pulls on any object at VS-L1 enough to balance the lesser solar orbital velocity, but this does not take into account the light pressure from the Sun, which is about 1.2 kilograms of pressure per square kilometer. Thus the actual position must balance gravity against light pressure. Estimating areas for the sunshade would give us both the total mass and the total light pressure. Note that the sunshade can have large, tilted segments which can slow or speed it up in its solar orbit, greatly reducing any need for station keeping propellants.
Venus has a diameter of about 12,100 kilometers. A full sunshade must be wider than the planet it is shading since the sun is about 1.4 million kilometers. A guestimate of a reasonable sunshade diameter would be about 15,000 kilometers wide, with close to 176,725,000 square kilometers of sunshade material needed. This would provide about 1,500 kilometers of shade margin for each side of the planet.
How heavy would such a sunshade be? Such a shade would be manufactured in place by in-space industry, probably using materials obtained from off-Earth sources. The point is to keep the energy requirements low. There are a range of thickness and mass estimates, ranging from about 0.1 gram per square meter up to about 10 grams. The thinner the material is, the harder it is to fabricate and deploy. Using the middle (logarithmic) value of 1 gram, the total sunshade mass would be 176,725,000 metric tons, while the lowest value would make it 17,672,500 tons, about the mass of 20 very large supertankers. Remember that this mass does not need to be launched from the ground since the sunshade would be made from in-space materials. Such a shade might have about 261,500 metric tons of light pressure against it, a tiny component compared to the gravitational forces.
Cooldown process
Let us assume that such a full sunshade can be built at a cost reasonable for the time it is built, and that it can also be relatively easily maintained at the Venus-Sun L1 point (VS-L1). Since Rome cannot be built in a day, the construction would probably start in the center and extend outwards in balanced segments or in a spiral. Thus there would be no sudden cutoff of light to Venus, but we could assume that it might be completed in about five years based on the total surface area of fabric able to be created and attached per month. As the amount of light reaching the cloud tops of Venus and the top air layers diminishes, the air there would cool off, eventually creating convection cells at random locations due to density differences. It is unclear how wide each convection cell would be, and whether the cells would extend all the way to the surface.
The physics of fluids, especially carbon dioxide, which has some remarkable properties, are crucial in this scenario. Since all of the carbon dioxide below a certain point in the atmosphere is a supercritical fluid, acting in some ways like a liquid, it may transfer heat faster than plain gases. In combination with the high wind speeds aloft and the thermally opaque sulfuric acid clouds, this keeps the temperature very similar all over the planet in spite of its slow rotation.
| If we remove part of the sunshade and keep the surface pressure just above five bars and at about –56 to –60 degrees Celsius, boat and ship traffic would be possible on the carbon dioxide ocean. |
Once the sunlight is fully cut off, the top air layers would start to strongly cool, as they radiate heat into space. The cloud tops radiate heat more strongly than the air itself, since it has no surface to radiate from. As the convection cells grow larger and stronger, they would start to interfere with the circum-Venus wind pattern, which moves the upper atmosphere around the planet in about four days at about 100 meters per second, a continuous blast of wind comparable to our strongest hurricane. The energy to sustain this massive movement ultimately comes from the solar heating, so at some point it would start to subside. Just like stirring a pot of boiling soup, the wind could temporarily disrupt the convection cells, more strongly towards the equator.
Eventually the convection cells would win out and surface heat would start to be rapidly moved to the top of the atmosphere. At altitudes above 120 kilometers, the thermosphere is the coldest place on Venus, cooling during the night due to radiation of infrared to as low as −173 degrees Celsius. This shows how strong the cooling effect is when there is no sunlight hitting the cloud tops. The convection cells would greatly enhance this effect if they extend down to the surface.
Assuming that the planetary wind pattern would be dispersed within about a year, the convection cells would then be the main factor in heat movement. Calculations in fluid dynamics or those who have planetary circulation models might be able to show how quickly the heat content of the atmosphere would be reduced. As temperature and pressure is reduced, the level where the carbon dioxide is supercritical would slowly fall towards the surface. At the point where about 17% of the carbon dioxide has been liquefied, the top of the supercritical layer would reach the surface and the remaining 83% would all be a regular gas.
Once the air temperature had been significantly reduced, the surface rock temperature would also start to slowly cool off. Heat moves through a solid more slowly since conduction is the only internal factor. There would be some radiation of heat from rock into the air right at the surface and some from conduction due to local circulation of the convection cells.
At some point, the air temperature at the surface would pass through the temperature range where humans could exist but the massive, crushing pressure would still be there. During this range of temperatures, the carbon dioxide would begin turning into actual liquid, since the high pressure would remain until a large portion of the carbon dioxide had condensed. As the temperature near the surface continued to fall, the liquid layer would start to accumulate, eventually reducing the pressure. The temperature would continue to drop below the freezing point of water, but the liquid carbon dioxide would probably boil continuously from the hot rock layers below it.
Cooldown time logic
This scenario is based on the simple fact that the same amount of heat as the daily heat input of the sunlight, in the form of Infrared radiation into the Venus atmosphere, leaves Venus every day, with the input and output of heat energy always in balance. If the solar input is removed, the entire surface of Venus will be left radiating the heat away from the top layer of the atmosphere.
I used a relatively simple calculation method to estimate the time it would take to collapse the atmosphere into a liquid state. I calculated the total global estimated heat flow, using both global heat flow, and that of single air columns: the roughly 1,000 tons of all the air directly above 1 square meter and the roughly 1 billion tons over each 1 square kilometer. The cross section area of sunlight in space that hits Venus is equivalent in area to the cross section area of Venus itself, or 115,066,000 square kilometers. Sunlight intensity in space is 2,603 watts/square meter near Venus. However, the clouds reflect away three quarters of the sun’s heat before it can be absorbed. So the total heat actually absorbed by the Venus atmosphere is one fourth of that, or only 651 watts/square meter from the cross-section “beam”. Even so, the total amount of energy absorbed is 76 quadrillion watts of heat per second.
Since the sunlit side of Venus is a hemisphere, twice the area of the cross section, the instantaneous absorption is spread (very unevenly) over an area twice the cross section size, and averages about 325 watts/square meter. Much of the heat is then distributed relatively evenly around the planet, to both hemispheres, so the resulting global infrared heat emission from Venus, which is continuous, is emitted over an area four times larger than the arrival cross section, averaging only 163 watts/square meter. If it did not radiate the heat away, Venus would continuously heat up until it glowed like a star. So the planet Venus simultaneously must radiate away heat at the same rate it arrives, or of about 76 quadrillion watts per second. If the sunlight is cut off, the heat will continue radiating away at the same high rate.
The part of the solar heat (sunlight) that hits Venus arrives in a “beam” with the cross section of Venus, while it actually hits the sunward hemisphere which has twice the area of the cross section, and departs from the whole globe with an area 4 times that of the cross section “beam”.
With a total mass of about 465–480 quadrillion metric tons, the atmospheric “surface” would have an area similar to the entire surface of Venus, (totaling 460,230,000 square kilometers), to cool the planet, not just the night side. Notice that this allocates roughly 1 square kilometer of surface (and slightly more for the atmosphere) to cool each billion tons of air, or 1 square meter for each 1,000 tons.
In summary, incoming sunlight in space is 2,603 watts per square meter. Three quarters of this, or 1,982 watts per square, is reflected away as light from the cloud tops, leaving 651 watts per square to be absorbed by the atmosphere. This adds up to the total of 76 quadrillion watts/second of heat. Divided by the area of Venus as a globe, each square meter must on average radiate away heat at a rate of about 163 watts/square meter. So each square meter of the atmosphere radiates away about one sixteenth of the energy per square meter that arrives from the sun in space. Since there are 31,557,600 seconds in a year, Venus should radiate away about 2.4 septillion watts in a year.
Cooldown time calculations
Several values are needed to determine the cooldown time:
- The specific heat for carbon dioxide gas. (This is the amount of heat needed to change the material’s temperature by 1 degree Celsius).
- The amount of material to be heated or cooled.
- The number of degrees of heating or cooling needed (and thus the amount of joules needed to be removed).
- The cooldown rates per second and per year.
Values 1, 2, and 4 have already been determined, while value 3 is determined by the intent of the operation (the desired target temperature). There are values for the specific heat of carbon dioxide on a number of websites, but they are inconsistent in how they are presented, and the values given vary a lot. I am using the value given by the Engineering Edge site, since they do specify a gas and give values for a range of temperatures. So this calculation would look like:
(1) Specific heat for carbon dioxide: 1,170 joules per kilogram (1,170,000 joules per ton) of carbon dioxide gas at about 800 kelvins. This number changes with the temperature of the carbon dioxide gas, which means the cooldown time result is approximate.
(2) Mass of all carbon dioxide: 480,148,000,000,000,000 tons or 480.15 quadrillion tons (using the larger number).
(4) Cooling rate: same as the solar input or 76 quadrillion watts per second or 2.4 septillion watts in a year.
The time question now shifts to how much heat is actually stored in all that hot carbon dioxide or rather how much heat we need to get rid of per ton of carbon dioxide. The two different values considered are:
(3a) The initial cooling target or milestone is –56 degrees Celsius, near the triple point for carbon dioxide. This initial temperature level might be used for a short time to allow runoff of condensing carbon dioxide liquid from areas above the new “ocean” level. The total temperature drop is determined by the Celsius temperature drop: adding the current temperature at the surface (where the gas is densest): 462 degrees Celsius plus the desired temperature (below zero): –56 degrees Celsius. So the temperature reduction (in absolute degrees) is: 462 °C + 56 °C = 518 °C.
1,170,000 J/ton times 480,148,000,000,000,000 tons times 518 °C = 290,998,496,880,000,000,000,000,000 total joules “in air” or 290 septillion, where the target temperature is –56 °C. Dividing the joules of heat to be removed by the amount of cooling per year gives the approximate cooling time:
290,998,496,880,000,000,000,000,000 joules/2,400,000.000,000,000,000,000,000 (2.4 septillion) watts in a year. Result: 121.28 years - time for global cooldown to reach a state at or just below the carbon dioxide triple point (assuming a steady cooling rate of air only). This may or may not allow the carbon dioxide to remain a liquid if the pressure drops below 5 bars.
(3b) The final target temperature should probably be about –80 degrees Celsius, which is where the carbon dioxide would stay frozen, except for over volcanic areas. This is a few degrees below the sublimation temperature, where with just 3.5 bars of nitrogen pressure left after condensation, (with essentially no carbon dioxide gas), the dry ice would be able to sublimate back into gas. Here the temperature reduction (again in absolute degrees) is: 462 °C + 80 °C = 542 °C:
1,170,000 J/ton times 480,148,000,000,000,000 tons times 542 °C = 304,481,052,720,000,000,000,000,000 total joules “in air” or 304.8 septillion where the target temp is –80 °C. Next, divide joules by heat removal per year:
304,481,052,720,000,000,000,000,000 (304 septillion) joules / 2,400,000,000,000,000,000,000,000 (2.4 septillion) watts in a year. Result: 126.9 years - time for global cooldown to reach a state at or just below the carbon dioxide sublimation point (assuming a steady cooling rate of air only).
The cooling times will be longer than the calculated values due to Newton’s law of cooling. However, since calculations using that law (which shows how cooling rates slow as temperature differences lessen), only apply accurately to relatively small masses and small temperature differences, it cannot be used to accurately calculate cooldown times for very large, very hot objects, like a planet’s entire atmosphere! So it is reasonable to expect that the time to effective full condensation would be more than 127 years and probably less than 200 years.
The convection cells should allow the cooldown of the atmosphere to occur at a rate not limited by conduction of heat. Since the top of the atmosphere will radiate heat as quickly as it is delivered by the convection cells, the cells themselves might be a somewhat limiting factor, since the supercritical fluid layer is probably more viscous than a true gas. Once the near surface layer of such gas disappears due to the lowering pressure, convection may speed up.
| For humans, the conditions would be similar to the Antarctic interior. They would need to wear insulated clothing similar to that worn outdoors at the South Pole Station, but would wear just a helmet supplied with an oxygen-helium mix. |
In addition to the joules of heat energy in the carbon dioxide gas, we would normally have to account for the heat released once the carbon dioxide gets cold enough to condense, but since the lowest layer of carbon dioxide is initially a supercritical fluid, there is probably no condensation point and thus no sudden release of extra heat at that point. Initially the weight of the carbon dioxide gas column would provide enough pressure to maintain supercriticality at and near the surface, but that should vanish after about one-fifth of the carbon dioxide mass condenses. After that point, some heat of condensation would be released.
As the last amounts of carbon dioxide condense, the remaining nitrogen would provide only about 3.5 atmospheres of pressure. This is below the 5 atmospheres needed to keep carbon dioxide as a liquid at room temperature, but the actual temperature at that point would be closer to the initially desired –56 degrees Celsius. It is not clear exactly what would happen during this phase, but conversion from what would then be just a gas to a liquid would probably occur gradually as the temperature drops closer to the desired temperature. Presumably the temperature would then stabilize at the triple point until all of the gas had condensed.
Hot rock heat conduction and carbon dioxide rivers
Heat from the hot surface rock would continue to flow by conduction into the layer of liquid carbon dioxide, and carbon dioxide that condensed at higher elevations might be flowing down into the new carbon dioxide global ocean in temporary streams and rivers. It may be desirable to maintain the temperature high enough to allow time for the carbon dioxide condensing over land areas to finish running off into the ocean, but pressure must be about five bars for the carbon dioxide to stay as a liquid. Some remaining carbon dioxide gas might keep it above that level for a while.
Any hot areas around active volcanic features would create constant boiling of the carbon dioxide ocean in those areas. Gas from these features would later probably fall as carbon dioxide rain nearby. This would provide a source of heat that would keep some small portion of the carbon dioxide as a gas. Like lava pillows that extrude into cold water, any fresh lava coming into contact with the –56 degree Celsius ocean would quickly freeze. The top several meters of rock would also cool to an intermediate temperature, since conduction of heat through rock is not fast.
Finally, if we let the planet cool down about another 25 degrees, the liquid carbon dioxide would begin freezing into dry ice. There is only a narrow zone of temperatures and pressures where carbon dioxide can be a liquid. This process would probably begin at the bottom, since dry ice would sink in the liquid. It is not clear how long it would take for the entire ocean to freeze all the way to the surface, but eventually this would provide access to the entire planet’s surface via drilling through up to about one kilometer of dry ice. This would allow access to much more of the planet even though three-fourths of it would then be covered by a layer of dry ice. This layer could probably be driven on. If we remove part of the sunshade and keep the surface pressure just above five bars and at about –56 to –60 degrees Celsius, boat and ship traffic would be possible on the carbon dioxide ocean.
The full sunshade status would nominally be maintained until the temperature near the surface reached about –80 degrees Celsius. At that point, most carbon dioxide boiling would have stopped except directly over volcanic vents, and enough sunlight would then be admitted to stabilize the temperature at about –80 degrees Celsius and to have enough sunlight to see with. The amount of sunlight let through would probably be comparable to about 10% of full sun at Venus, about half the sunlight received at Mars and similar to what is received by main belt asteroids. There would remain about 3.5 atmospheres of nitrogen and traces of argon, neither of which would liquefy at these temperatures.
Land areas above the new “sea level”, which would be about one kilometer above the average lowland level, would be at a lower pressure, and the temperature might also be lower, so some of the condensed carbon dioxide may freeze out on the land surface. Hopefully, most of the carbon dioxide as a liquid will have already drained off the higher areas before that happens. If some of these areas are covered with frozen white dry ice, the amount of sunlight let in can be increased, or if the dark rock absorbs too much sunlight, the level could be decreased.
Surface access for humans and robots
Once this stage is reached, humans and robots could land on at least 20-25% of the surface, and establish monitoring stations and even science bases. For humans, the conditions would be similar to the Antarctic interior. They would need to wear insulated clothing similar to that worn outdoors at the South Pole Station, but would wear just a helmet supplied with an oxygen-helium mix, and thus would not need to wear a pressure suit. Bases would probably be built underground, using standard Earth pressure, to take advantage of the planet’s massive internal heat.
While the surface materials seem to be primarily derived from volcanism, there are two or more continent-like areas. Of the largest two, one (Ishtar Terra) is closer to the North Pole and the other (Aphrodite Terra) is equatorial. Maxwell Montes, a volcanic massif about 11 kilometers high, is on Ishtar Terra. These do not seem to have been accreted by plate tectonics. There are also a number of large pancake “domes” that are really rather flat but very circular, as well as three other named highland regions. There are lots of landforms that seem to have no counterpart on Earth. Indications are that there are different rock types in some of the different areas, such as those that are highly radar-reflective, which could be extensive deposits of iron pyrite. There are also multiple indications that there are lot of active volcanoes on Venus, which do not show up as well on the topographic radar maps. One of the top geological mysteries is whether there are any areas of granitic (continent-like) rock, or even sedimentary rock from a possible ancient wet Venus.
Significantly lower air pressures than 3.5 bars could be found by focusing human landing sites on areas at higher elevations, but none of the locations would tax robotic landers. “Sea level” landings would be easier and takeoffs would be harder due to the thicker nitrogen atmosphere. Some of the surface mineralogy would be very different from the Earth, and there would be no limestone or marble, as all of the carbonates that might have existed would have been converted into carbon dioxide. Access to the surface would allow geological investigations to see if Venus had a moderate climate in the past, as well as direct investigations of its interior structure with seismograph networks.
| Once the stability of the sunshade and its temperature level had been proved, it would be possible to create human settlements on most of the parts of Venus above the “sea” level, whether liquid or solid. Such settlements would have the advantage of near 1G gravity and no galactic radiation. |
With solar input to the surface limited to an amount comparable to what asteroids get, the amount of wind and kind of weather patterns generated in the remaining nitrogen atmosphere could be difficult to predict. Due to the 25-fold reduction in atmospheric mass, the amount of heat transferred in the wind from the day to the night side, and the wind pattern that would carry it, would probably be very different. Heat might be carried by winds from the subsolar point to the center of the night hemisphere, with wind returning closer to the surface. This wind pattern would probably shift in time to match the slow Venus rotation rate. The amount of solar heat admitted to the day side might have to be adjusted in real time once we see what kind of temperature differences and wind patterns emerge. With 3.5 bars of air pressure, a gentle breeze could be the same as a strong wind.
Once the stability of the sunshade and its temperature level had been proved, it would be possible to create human settlements on most of the parts of Venus above the “sea” level, whether liquid or solid. Such settlements would have the advantage of near 1G gravity and no galactic radiation. Settlements built in tunnels would be naturally warmer than the surface, while surface installations would need very efficient insulation from the cold. Habitation tunnels could be drilled at the most convenient temperature level. Most of these would probably exist to support scientific and commercial outposts. Mining could provide sources of materials for orbital installations. Boats could be used to move along coastlines at the higher of the two target temperatures, but swimming in the near-cryogenic carbon dioxide ocean would not be a good idea without a special insulated dry suit and sealed helmet.
Tourism could be expected to develop at some point once transport costs are reduced to affordable levels. Surface temperatures would be higher than on most asteroids, but with zero galactic radiation. Outdoor hiking trips on Venusian mountain terrain could be very interesting, as the sky would probably have few clouds and the sunlight admitted would illuminate the surface well. The temperature difference caused by the slow rotation rate could create high winds along the terminator line dividing the “day” side from the night side.
During the cooldown period, there would probably be an orbital monitoring station to keep track of this gigantic physics experiment, with a human crew and satellites in various orbits to observe both the changing dynamics of the atmosphere and the surface. It would be difficult to predict what will happen to the cloud layer for improved observation from orbit. If our technology has advanced far enough to have surface landers or active rovers that could survive the pre-condensation surface conditions for long periods, they could be positioned to capture the effects of the period of carbon dioxide condensation and runoff, which could be quite spectacular.
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