Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 500 °C (770 K) surface temperature. These goals are closely interrelated, since Venus' extreme temperature is due to the greenhouse effect caused by its dense atmosphere.
In 1961 the astronomer and popularizer of science Carl Sagan proposed terraforming the planet Venus by seeding its atmosphere with algae, which would remove carbon dioxide and reduce the greenhouse effect until surface temperatures dropped to "comfortable" levels. Later discoveries about the conditions on Venus made this particular approach impossible, however: Venus simply has too much atmosphere to process and sequester. Even if atmospheric algae could thrive in the hostile and arid environment of Venus' upper atmosphere, any carbon that was fixed in organic form would be liberated as carbon dioxide again as soon as it fell into the hot lower regions.
Robert Zubrin has proposed a large solar shield, designed to protect Venus from the Sun and cool it down sufficently to permit sublimation of atmospheric carbon dioxide into dry ice, where it would "snow" onto the surface, after which it would be buried or shipped off-world (perhaps to Mars, which has the opposite of Venus' problem from a human perspective -- insufficient atmospheric pressure and greenhouse gasses, as opposed to too much). With the sun shielded and excess greenhouse gases removed, the problems of atmospheric pressure and heat are solved. Solar shades placed in the Sun-Venus L1 point or in a more closely-orbiting ring could be used to reduce the total insolation received by Venus, cooling the planet somewhat. This does not directly deal with the immense atmospheric density of Venus, but could make it easier to do so by other methods. They could also serve double duty as solar power generators.
Construction of a suitably large solar shade is a potentially daunting task. The sheer size of such a structure would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade at the Sun-Venus L1 point with the incoming radiation pressure which would tend to turn the shade into a huge solar sail.
Other proposed cooling solutions involve comets, or creating artificial rings. A comet at the Sun-Venus L1 point could produce a coma which could provide at least temporary shade for the planet, possibly allowing enough time for atmospheric processing to be done. Keeping a continuously decaying comet in a stable position could prove to be a difficult feat. Rings created by putting debris in orbit would provide some shade but to a lesser extent. The inclination of the rings would also need to be such that they present a significant amount of surface area to the Sun.
Space-based solar shade techniques are largely speculative due to the fact that they are beyond our current technological grasp. The vast sizes require material strengths and construction methods that have not even reached their infancy.
Cooling could be sustained by placing reflectors in the atmosphere or on the surface. Reflective balloons floating in the upper atmosphere could create shade. The number and/or size of the balloons would necessarily be great. Increasing the planet's albedo by deploying light color or reflective material on the surface could help keep the atmosphere cool. The amount would be large and would have to be put in place after the atmosphere had been modified already since Venus' surface is currently completely shrouded by clouds. The advantage of atmospheric and surface cooling solutions is that they take advantage of existing technology.
Moons have a high mass, low surface-area-to-mass ratio and maintain a stable orbit. They promote tectonic plate activity, tides, and volcanism. They're also essential for moonlight, which many nocturnal species rely on.
When a moon passes between the planet and a star it creates a shadow on the planet, an eclipse. An eclipse will, for a short time, shade the planet from direct sunlight. In the shadow of an eclipse the local temperature plunges. The amount of sunlight reaching the planet, as a whole, decreases slightly but regularly. By maximising eclipses a significant and permanent decrease in temperature can be achieved, and maintained with no additional cost.
Eclipses are often overlooked as a method of planetary cooling. Perhaps this is due to our own infrequent and brief eclipse experience with Luna. The orbital inclination of Luna is small, 5°, that's enough to keep its shadow off Earth most of the time. If the orbital inclination of Luna was 0°, there would be longer and regular total eclipses every month.
There are many options to maximise the decrease in temperature caused by an eclipse: 0° orbital inclination is essential! The slightest inclination will significantly reduce eclipse frequency and its shading effect. The orbit of any mass in a system is gravitationally effected by other masses, and will alter over time. Minimise this effect by ensuring all masses in the system, especially gas giants, have a 0° orbital inclination. Have multiple moons. Extra moons will have different orbits and thus different orbital periods. Careful calculations can maximise the number of moons and ensure multiple eclipses. Create a high volume, low density moon. The larger and lighter the moon, the closer it can move to the planet, and the greater the shading effect. Also, lighter moons mean more moons can fit in orbit of the planet. Albedo. Both high and low albedo moons create an eclipse when on the star side of the planet, but only high albedo moons create moonlight when on the other side. Moonlight will add an extra small amount of energy to a planet but the ability to watch multiple moons dash across the night sky might be worth it.
When lower temperatures are no longer required, a slight increase to the orbital inclination of each moon will significantly reduce the shading effect, and create a spectacular moon filled sky.
Removal of Venus' atmosphere could be attempted by a variety of methods, possibly in combination. Directly lifting atmospheric gas from Venus into space would likely prove very difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1993 that an impactor of 700 km diameter striking Venus at greater than 20 km/s, would eject all the atmosphere above the horizon as seen from the point of impact, but since this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreased a very great number of such giant impactors would be required. Smaller objects would not work as well, requiring even more. The violence of the bombardment could well result in significant outgassing that replaces removed atmosphere. Furthermore, most of the ejected atmosphere would go into solar orbit near Venus, eventually to fall right back onto Venus again.
Removal of atmospheric gas in a more controlled manner could also prove difficult. Venus' extremely slow rotation means that space elevators would be impossible to construct, and the very atmosphere to be removed makes mass drivers useless for removing payloads from the planet's surface. Possible workarounds include placing mass drivers on high-altitude balloons or balloon-supported towers extending above the bulk of the atmosphere, using space fountains, or rotovators. Such processes would take a great deal of technical sophistication and time, however, and may not be economically feasible without the use of extensive automation.
Alternatively, Venus' atmosphere could be converted into some other form in situ by reacting it with externally supplied elements. Bombardment of Venus with refined magnesium and calcium metal from Mercury or some other source, could sequester carbon dioxide in the form of calcium and magnesium carbonates.
Bombardment of Venus with hydrogen, possibly from some outer solar system source and reacting with carbon dioxide could produce elemental carbon (graphite) and water by the Bosch reaction. It would take about 4×1019 kg of hydrogen to convert the whole Venusian atmosphere, and the resulting water would cover about 80% of the surface compared to 70% for Earth. The amount of water produced would amount to around 10% of the water found on Earth. A solar shade or equivalent would also be necessary, as water vapor is itself a greenhouse gas. Oceans on Venus would increase the planet’s albedo and allow more incoming solar radiation to be reflected back into space.
An ingenious method that could convert the atmosphere involves the use of genetically engineered bacteria. Such organisms would be similar to those found in hotsprings or deep ocean vents which are able to survive harsh environments like that on Venus. The organisms could convert the CO2 and other elements in the atmosphere in order to transform it into one that is more amenable to terran life forms. Although such a technique might be slow, it would require no technology other than that which we currently posses, and would be very economical. Indeed, most scientists speculate this itself happened on an early earth, transforming a harsh atmosphere into one that allowed more complex eukaryotic photosynthetic life forms to develop.
Geoffrey A. Landis proposes colonising the cloud-tops of Venus. Initially, the image of floating cities may seem fanciful, but Landis' proposal points out that a Terran breathable air mixture (21:79 Oxygen-Nitrogen) is a lifting gas in the Venusian atmosphere. In effect, a gasbag full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. At an altitude of 50 kilometers above Venusian surface, the environment is the most Earthlike in the solar system - a pressure of approximately 1 bar and temperatures in the 0-50 Celsius range. Because there is not a significant pressure differential between the inside and the outside of the breathable-air balloon, any rips or tears would not result in an explosive decompression, but rather would only diffuse at normal atmospheric mixing rates, giving time to repair any such defects.
Such colonies could be constructed at any rate desired, allowing a dynamic approach instead of needing any 'fell swoop' solutions. They can be used to gradually transform the Venusian atmosphere, with their impact directly related to the amount of colonies in the atmosphere. The more colonies are constructed, the more they can use solar panels to absorb insolation and thus cool Venus, and they can also be used to grow plant matter that would reduce the amount of carbon dioxide in the air. In the beginning, any impact on Venus would be insignificant, but as the number of colonies grows, they can transform Venus more and more rapidly.
Venus' extremely slow rotation rate would result in extremely long days and nights, which could prove difficult for most Earth life to adapt to. Venus has a negligible magnetosphere. A magnetosphere protects the planet surface from low-energy cosmic radiation and coronal mass ejections (CME). The high atmospheric pressure on Venus has compressed and cooled the planetary core. The high atmospheric temperature has slowed the convection of heat from the core. These inhibitors have stopped the planetary dynamo from generating a magnetic field. Reducing atmospheric pressure and temperature will allow the core to expand, reliquify and for convection currents to start again. Speeding up Venus' rotation is a concept long debated by terraformers, and many theories have been proposed. All the theories can be divided into three camps: 1) series of asteroid impacts, 2) build a large moon, 3) electro-magnetic fields.
The Series of Asteroid Impacts theory holds that multiple impacts by redirected asteroids could alter the rotation of Venus’ crust, provided the trajectory of each asteroid could be precisely vectored. Obviously this method would require the planet’s atmosphere to have already been thinned sufficiently that the asteroids could reach the surface without burning up. With over 100,000 asteroids in the main belt, and over 2000 NEO asteroids in the inner system, there does not appear to me a shortage of ammunition, however, each asteroid impacted on Venus, would add it’s own material to the planet. Currently there is sufficient oxygen in the atmosphere to create an ocean covering ~80% of the planet, therefore any asteroid used should be devoid of additional oxygen, or the planet may end up becoming an oceanic world. The main problem with the SAI theory, is that the planet’s crust would speed up its spin, while the core would not. As the core has a higher mass than the crust, the friction between the two, within the mantle, would slowly slow down the crust’s spin, and therefore additional ongoing impacts would be required to maintain the day-night cycle. Clearly impacting a terraformed/colonized planet with an asteroid every few centuries is not an option.
The Build a Large Moon theory is based on the knowledge that the Earth’s moon regulates the Earth’s day-night cycle. Theoretically giving Venus a similarly sized and situated moon would cause Venus’s day-night cycle to slowly speed up to be similar to the Earth’s. The BLM theory has the benefit of speeding up the core of the planet along with the crust, and therefore is a better long term solution than the SAI theory, as it would also boost the planet’s geo-magnetic field which would protect the life there from radiation. The problem with the BLM theory is that there is not enough mass in the inner system to build this moon, without dismantling another planetary body. Current estimates of the total mass of the inner system, including the main asteroid belt, but sans Mercury, Venus, Earth, Luna, and Mars, at less than 4% of Luna’s mass. This means that the best option for building a large moon orbiting Venus is to move Mercury, or part of it, however, the energy requirements of such a project make it highly implausible.
The Electro-Magnetic Field theory is possibly the best currently being considered, as it relies only on what is available and could be used on a terraformed/colonized planet. The EMF theory would see a series of power generating plants build around the partially terraformed planet; given the planet’s geology, geo-electric plants would likely be the best. The plants would then be connected by a series of underground plasma tubes, to allow the electricity to be channelled from one plant to the next. The goal of the power-plants and plasma-tubes would be to create a massive electro-magnetic field that would repulse off of the core’s native magnetic field, and cause the crust’s spin to speed up in relation to the core’s. Given the mass differential, the reduction in the core’s spin should be negligible in comparison to the crust’s, meaning that once the system in shut off, the plasma tubes would serve as an ongoing source of electricity, as the core’s magnetic field would be constantly acting on the plasma. Naturally, the EMF theory has the same problem that the SAI theory has, in that over time the crust’s spin would slow, however, unlike the SAI theory, the EMF theory could be used on an inhabited planet without significantly affecting the life-forms. If the EMF theory were used, and the crust’s spin were regulated over the course of millennia, the spin of the core would also speed up, again due to mantle friction, and this would ultimately provide the same geo-magnetic field protection as the BLM theory, and possibly in a shorter period of time.
