Colonization of Venus
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The colonization of Venus has been a subject of many works of science fiction since before the dawn of spaceflight, and is still discussed from both a fictional and a scientific standpoint. However, with the discovery of Venus' extremely hostile surface environment, attention has largely shifted towards the colonization of the Moon and Mars instead.
Reasons for colonization 
Space colonization is a step beyond space exploration, and implies the permanent or long-term presence of humans in an environment outside Earth. Colonization of space is claimed to be the best way to ensure the survival of humans as a species. Other reasons for colonizing space include economic interests, long-term scientific research best carried out by humans as opposed to robotic probes, and sheer curiosity. Venus is the second largest terrestrial planet and Earth's closest neighbour, which makes it a potential target.
Venus has certain similarities to Earth which, were it not for the hostile conditions, might make colonization easier in many respects in comparison with other possible destinations. These similarities, and its proximity, have led Venus to be called Earth's "sister planet".
At present it has not been established whether the gravity of Mars, 0.38 times that of the Earth, would be sufficient to avoid bone decalcification and loss of muscle tone experienced by astronauts living in an environment of microgravity. In contrast, Venus is close in size and mass to the Earth, resulting in a similar surface gravity (0.904 g) which would likely be sufficient to prevent the health problems associated with weightlessness. Most other space exploration and colonization plans face concerns about the damaging effect of long-term exposure to fractional g or zero gravity on the human musculoskeletal system.
Venus's relative proximity makes transportation and communications easier than for most other locations in the solar system. With current propulsion systems, launch windows to Venus occur every 584 days, compared to the 780 days for Mars. Flight time is also somewhat shorter; the Venus Express probe that arrived at Venus in April 2006 spent slightly over five months en route, compared to nearly six months for Mars Express.[original research?] This is because at closest approach, Venus is 40 million km from Earth (approximated by perihelion of Earth minus aphelion of Venus) compared to 55 million km for Mars (approximated by perihelion of Mars minus aphelion of Earth) making Venus the closest planet to the Earth.
Venus also presents several significant challenges to human colonization. Surface conditions on Venus are difficult to deal with: the temperature at the equator averages around 450 °C (842 °F), higher than the melting point of lead. The atmospheric pressure on the surface is also at least ninety times greater than on Earth, which is equivalent to the pressure experienced under a kilometer of water. These conditions have caused missions to the surface to be extremely brief: the Soviet Venera 5 and Venera 6 probes were crushed by high pressure while still 18 km above the surface. Following landers such as Venera 7 and Venera 8 succeeded in transmitting data after reaching the surface, but these missions were brief as well, surviving no more than a single hour on the surface.
Furthermore, water, in any form, is almost entirely absent from Venus. The atmosphere is devoid of molecular oxygen and is primarily carbon dioxide. In addition, the visible clouds are composed of corrosive sulfuric acid and sulfur dioxide vapor.
Exploration and research 
Due to the planet's hostile environment, Venus has not been studied as much as objects such as the Moon and Mars have, and it is extremely unlikely that research would be conducted with a view to a human mission to the planet. The probe Venus Express is currently in orbit around the planet, but other low-cost missions have been proposed to further explore the planet's atmosphere, as the area 50 kilometres above the surface where air pressure is at the same level as Earth has not yet been explored.
Aerostat habitats and floating cities 
Others suggest a different approach, however, claiming that rather than attempting to colonize Venus' hostile surface, humans might attempt to colonize the Venusian atmosphere. Geoffrey A. Landis of NASA's Glenn Research Center has summarized the perceived difficulties in colonizing Venus as being merely from the assumption that a colony would need to be based on the surface of a planet:
- “However, viewed in a different way, the problem with Venus is merely that the ground level is too far below the one atmosphere level. At cloud-top level, Venus is the paradise planet.”
Landis has proposed aerostat habitats followed by floating cities, based on the concept that breathable air (21:79 Oxygen-Nitrogen mixture) is a lifting gas in the dense carbon dioxide atmosphere, with over 60% of the lifting power that helium has on Earth. In effect, a balloon full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. At an altitude of 50 kilometres (31 mi) above Venusian surface, the environment is the most Earth-like in the solar system – a pressure of approximately 1 bar and temperatures in the 0°C–50°C range. Because there is not a significant pressure difference between the inside and the outside of the breathable-air balloon, any rips or tears would cause gases to diffuse at normal atmospheric mixing rates rather than an explosive decompression, giving time to repair any such damages. In addition, humans would not require pressurized suits when outside, merely air to breathe, protection from the acidic rain and on some occasions low level protection against heat. Alternatively, two-part domes could contain a lifting gas like hydrogen or helium (extractable from the atmosphere) to allow a higher mass density.
At the top of the clouds the wind speed on Venus reaches up to 95 m/s (approximately 212 mph), circling the planet approximately every four Earth days in a phenomenon known as "super-rotation". Colonies floating in this region could therefore have a much shorter day length by remaining untethered to the ground and moving with the atmosphere. Allowing a colony to move freely would also reduce structural stress from the wind.
Terraforming (literally, "Earth-shaping") is the theoretical process of modifying a planet, moon, or other body to a more human-habitable atmosphere, temperature, or ecology. Venus has been the subject of a number of terraforming proposals. The proposals seek to remove or convert the dense carbon dioxide atmosphere, reduce Venus's 450 °C (770 K) surface temperature, and establish a day/night light cycle closer to that of Earth.
Many proposals involve deployment of a solar shade and/or a system of orbital mirrors, for the purpose of reducing insolation and providing light to the dark side of Venus. Another common thread in most proposals involves some introduction of large quantities of hydrogen or water. Proposals also involve either freezing most of Venus's atmospheric CO2, or converting it to carbonates, urea or other forms.
- "Hawking says humans must go into space to survive". USA Today. 13 June 2006. Retrieved 20 March 2007.
- Landis, Geoffrey A. (Feb. 2-6 2003). "Colonization of Venus". Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM. doi:10.1063/1.1541418.; draft version of the full paper available at NASA Technical Reports Server (accessed 16 May 2012)
- Atkinson, Nancy (Jul. 16 2008). "Colonizing Venus With Floating Cities". Universe Today. Retrieved 4 July 2011.
- Birch, Paul (1991). "Terraforming Venus Quickly". Journal of the British Interplanetary Society 44: 157–167.
- Landis, Geoffrey A., Colozza, Anthony, and LaMarre, Christopher M., Atmospheric Flight on Venus (pdf), International Astronautical Federation Congress 2002, paper IAC-02-Q.4.2.03, AIAA-2002-0819, AIAA0, No. 5
- Fogg, Martin J. , Terraforming: Engineering Planetary Environments, SAE Press, 1995. ISBN 1560916095, ISBN 978-1560916093
- Landis, Geoffrey A., “Terraforming Venus: A Challenging Project for Future Colonization,” paper AIAA-2011-7215, AIAA Space 2011 Conference & Exposition, Long Beach CA, Sept. 26-29, 2011.