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'''Venus''' is the second [[planet]] from the sun. Quality, price and users testimonials are those 3 factors that I took into consideration while choosing appropriate writing service to write my essay. I hold my eye on primewritings.com and have never regretted my choice. They provided me with [http://primewritings.com custom papers] of the highest quality.
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{{Infobox Planet
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| bgcolour = #EDD58B
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| name = Venus
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| symbol = [[Image:Venus symbol.svg|25px|Astronomical symbol of Venus]]
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| image = [[Image:Venus-real.jpg|240px|Venus]]<!--Licensing conditions: Image processing by R. Nunes http://www.astrosurf.com/nunes-->
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| caption = Venus in real color
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| epoch = [[J2000]]
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| aphelion = 108,942,109&nbsp;km<br />0.72823128 [[Astronomical unit|AU]]
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| perihelion = 107,476,259&nbsp;km<br />0.71843270 AU
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| semimajor = [[1 E11 m|108,208,930]] km<br />0.723332 AU
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| eccentricity = 0.0068
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| period = [[1 E7 s|224.70069]] [[day]]<br/>0.6151970 [[julian year (astronomy)|yr]]
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| synodic_period = 583.92 days
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| avg_speed = 35.02&nbsp;km/s
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| inclination = 3.39471°<br />3.86° to Sun's equator
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| asc_node = 76.67069°
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| arg_peri = 54.85229°
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| satellites = None
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| physical_characteristics = yes
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| mean_radius = 6051.8 ± 1.0&nbsp;km<ref name=Seidelmann2007>{{cite journal
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  | last= Seidelmann| first= P. Kenneth
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  | coauthors= Archinal, B. A.; A’hearn, M. F.; et.al.
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  | title= Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006
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  | journal= Celestial Mechanics and Dynamical Astronomy
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  | volume=90 | pages=155–180 | year=2007
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  | doi=10.1007/s10569-007-9072-y
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  | url=http://adsabs.harvard.edu/doi/10.1007/s10569-007-9072-y
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  | accessdate=2007-08-28 }}</ref><br />0.9499 Earths
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| flattening = < 0.0002<ref name=Seidelmann2007/> <!-- calculated from data in ref name=Seidelmann2007 -->
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| surface_area = [[1 E14 m²|4.60×10<sup>8</sup>]] km²<br />0.902 Earths
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| volume = 9.38{{e|11}}&nbsp;km³<br />0.857 Earths
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| mass = [[1 E24 kg|4.8685×10<sup>24</sup>]] kg<br />0.815 Earths
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| density = 5.204&nbsp;g/cm³
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| surface_grav = 8.87&nbsp;[[Acceleration|m/s<sup>2</sup>]]<br />0.904 ''[[g-force|g]]''
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| escape_velocity = 10.46&nbsp;km/s
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| sidereal_day = [[retrograde motion|−]]243.0185&nbsp;day
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| rot_velocity = 6.52&nbsp;km/h
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| axial_tilt = 177.36°
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| right_asc_north_pole = 18 h 11 min 2 s<br/>272.76°<ref>
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{{cite web
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  | title = Report on the IAU/IAG Working Group on cartographic coordinates and rotational elements of the planets and satellites
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  | publisher = International Astronomical Union
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  | date=  2000
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  | url = http://www.hnsky.org/iau-iag.htm
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  | accessdate =  2007-04-12}}</ref>
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| declination = 67.16°
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| albedo = 0.65
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| magnitude = up to -4.6<ref name=nasafact/>
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| angular_size = 9.7" — 66.0"<ref name=nasafact>{{cite web|url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html|title = Venus Fact Sheet|publisher = NASA|last = Williams|first = Dr. David R.|accessdate = 2007-10-12|date=  April 15, 2005}}</ref>
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| temperatures = yes
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| temp_name1 = [[Kelvin]]
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| mean_temp_1 = 735&nbsp;K<ref>
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{{cite web
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  | last =
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  | first =
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  | authorlink =
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  | coauthors =
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  | title = Venus: Facts & Figures
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  | work =
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  | publisher = NASA
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  | date =
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  | url = http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Venus&Display=Facts&System=Metric
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  | format = html
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  | accessdate = 2007-04-12}}</ref><ref>{{cite web
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  | last =
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  | first =
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  | authorlink =
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  | coauthors =
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  | title =Space Topics: Compare the Planets: Mercury, Venus, Earth, The Moon, and Mars
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  | work =
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  | publisher = Planetary Society
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  | date =
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  | url = http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html
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  | accessdate = 2007-04-12}}</ref>
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| temp_name2 = [[Celsius]]
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| mean_temp_2 = 461.85&nbsp;°C
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| adjectives = [[Venusian]] or (rarely) [[Cytherean]], Venerean
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| atmosphere = yes
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| surface_pressure = 9.3 [[Pascal (unit)|MPa]]
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| atmosphere_composition = ~96.5% [[Carbon dioxide]]<br />~3.5% [[Nitrogen]]<br />.015% [[Sulfur dioxide]]<br />.007% [[Argon]]<br />.002% [[Water vapor]]<br />.0017% [[Carbon monoxide]]<br />.0012% [[Helium]]<br />.0007% [[Neon]]<br />trace [[Carbonyl sulfide]]<br />trace [[Hydrogen chloride]]<br />trace [[Hydrogen fluoride]]
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}}
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'''Venus''' ({{pronEng|ˈvinəs}}) is the second-closest [[planet]] to the [[Sun]], orbiting it every 224.7 Earth days. It is the brightest natural object in the night sky, except for the [[Moon]], reaching an [[apparent magnitude]] of −4.6. Because Venus is an [[inferior planet]], from [[Earth]] it never appears to venture far from the Sun: its [[Elongation (astronomy)|elongation]] reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the ''Morning Star'' or the ''Evening Star''.
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Classified as a [[terrestrial planet]], it is sometimes called Earth's "sister planet", for the two are similar in size, gravity, and bulk composition. Venus is covered with an opaque layer of highly reflective [[cloud]]s of [[sulfuric acid]], preventing its surface from being seen from space in [[visible light]]; this was a subject of great speculation until some of its secrets were revealed by [[planetary science]] in the twentieth century. Venus has the densest [[atmosphere]] of all the terrestrial planets, consisting mostly of [[carbon dioxide]], as it has no [[carbon cycle]] to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. It has become so hot that the earth-like oceans the young Venus is believed to have possessed have totally evaporated, leaving a dusty dry desertscape with many slab-like rocks. The best hypothesis is that the evaporated water vapor has dissociated and, with the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind. The [[atmospheric pressure]] at the planet's surface is 92 times that of the Earth.
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[[Category:Planets]]
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Venus's surface has been mapped in detail only in the last 22 years, by Project [[Magellan_probe|Magellan]]. It shows evidence of extensive [[volcano|volcanism]], and the [[sulfur]] in the atmosphere is taken by some experts to show that there has been some recent volcanism, but it is an enigma as to why no evidence of [[lava]] flow accompanies any of the visible [[caldera]].  It is also noteworthy that there are a surprisingly low number of [[impact craters]].  This demonstrates that the surface is relatively young, approximately half a billion years old.  There is no evidence for [[plate tectonics]], possibly because its crust is too strong to [[subduction|subduct]] without water to make it less [[viscous]], and some suggest that instead Venus loses its internal heat in periodic massive resurfacing events.
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The adjective ''[[Venusian]]'' is commonly used for items related to Venus, though the [[Latin]] adjective is the rarely used ''Venerean''; the now-archaic ''[[Cytherean]]'' is still occasionally encountered. Venus is the only planet in the [[Solar System]] named after a female figure,<ref>Goddesses such as [[Gaia]] and [[Terra]] were named after the [[Earth]], and not vice versa.</ref> although two [[dwarf planet]]s&mdash;[[Ceres (dwarf planet)|Ceres]] and [[Eris (dwarf planet)|Eris]]&mdash;also have [[female]] names.
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==Physical characteristics==
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Venus is one of the four solar [[terrestrial planet]]s, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar to the Earth, and is often described as its 'twin'. The diameter of Venus is only 650&nbsp;km less than the Earth's, and its mass is 81.5% of the Earth's. However, conditions on the Venusian surface differ radically from those on Earth, due to its dense [[carbon dioxide]] [[atmosphere]]. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% composed of [[nitrogen]].<ref>
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{{cite web |url=http://www.daviddarling.info/encyclopedia/V/Venusatmos.html |title=The Encyclopedia of Astrobiology, Astronomy, and Spaceflight |accessdate=2007-04-29 |format= |work= }}
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</ref>
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===Internal structure===
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Though there is little direct information about its internal structure, the similarity in size and density between Venus and Earth suggests that it has a similar internal structure: a [[Planetary core|core]], [[Mantle (geology)|mantle]], and [[Crust (geology)|crust]]. Like that of Earth, the Venusian core is at least partially liquid. The slightly smaller size of Venus suggests that pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of [[plate tectonics]] on Venus, likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated [[magnetic field]].<ref>{{cite journal |author=Nimmo, F. |year=2002 |title=Crustal analysis of Venus from [[Magellan]] satellite observations at Atalanta Planitia, Beta Regio, and Thetis Regio |journal=Geology |volume=30 |pages=987–990}}</ref>
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===Geography===
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About 80% of Venus's surface consists of smooth volcanic plains. Two highland '[[continents]]' make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called [[Ishtar Terra]], after [[Ishtar]], the [[Babylon]]ian goddess of love, and is about the size of [[Australia]]. [[Maxwell Montes]], the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11&nbsp;km above Venus's average surface elevation. The southern continent is called [[Aphrodite Terra]], after the [[Greek]] goddess of love, and is the larger of the two highland regions at roughly the size of [[South America]]. Much of this continent is covered by a network of fractures and faults.<ref name="Kaufmann">Kaufmann W.J. (1994), ''Universe'', W.H. Freeman, New York, p. 204</ref>
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As well as the [[impact crater]]s, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called ''farra'', which look somewhat like pancakes and range in size from 20&ndash;50&nbsp;km across, and 100&ndash;1000&nbsp;m high; radial, star-like fracture systems called ''novae''; features with both radial and concentric fractures resembling spiders' webs, known as ''[[arachnoid (astrogeology)|arachnoids]]''; and ''coronae'', circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin.<ref name="Frankel">{{cite book | first=Charles | last=Frankel| year=1996 | title=Volcanoes of the Solar System | publisher=Cambridge University Press| ISBN=0521477700 }}</ref>
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Almost all Venusian surface features are named after historical and mythological [[women]].<ref>Batson R.M., Russell J.F. (1991), ''Naming the Newly Found Landforms on Venus'', Abstracts of the Lunar and Planetary Science Conference, v. 22, p. 65</ref> The only exceptions are Maxwell Montes, named after [[James Clerk Maxwell]], and two highland regions, [[Alpha Regio]] and [[Beta Regio]]. These three features were named before the current system was adopted by the [[International Astronomical Union]], the body that oversees planetary nomenclature.<ref name=jpl-magellan>{{cite book | last=Young C. (Editor)|url= http://www2.jpl.nasa.gov/magellan/guide.html| coauthors= | year=August 1990 | title=The Magellan Venus Explorer's Guide | edition=JPL Publication 90-24 | publisher=Jet Propulsion Laboratory| location=California | id= }}</ref>
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Cartesian coordinates of physical features on Venus are expressed relative to its [[prime meridian]], defined as the line of longitude passing through a radar-bright spot at the center of the oval feature Eve, which lies to the south of Alpha Regio.<ref>{{Citation |last = Davies, M.E ''et al.''| title = Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites| journal = Celestial Mechanics and Dynamical Astronomy| volume = 63
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| issue = 2| year = 1994| doi = 10.1007/BF00693410}}</ref><ref>{{cite web |title = Three-Dimensional Perspective View of Alpha Regio | publisher = Jet Propulsion Laboratory |url=http://photojournal.jpl.nasa.gov/catalog/PIA00481 |accessdate = 2007-12-27 }}</ref>
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===Surface geology===
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{{main|Geology of Venus}}
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[[Image:Map of Venus.jpg|thumb|left|250px|Map of Venus, showing the elevated 'continents' in yellow: Ishtar Terra at the top and Aphrodite Terra just below the equator to the right]]
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Much of Venus's surface appears to have been shaped by volcanic activity. Overall, Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 100&nbsp;km across. The only volcanic complex of this size on Earth is the [[Hawaii (island)|Big Island]] of [[Hawaii]].<ref name="Frankel"/> However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's crust is continually recycled by [[subduction]] at the boundaries of [[tectonic plate]]s, and has an average age of about 100 million years, while Venus's surface is estimated to be about 500 million years old.<ref name="Frankel" />
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Several lines of evidence point to ongoing volcanic activity on Venus. During the Soviet [[Venera program]], the [[Venera 11]] and [[Venera 12]] probes detected a constant stream of [[lightning]], and Venera 12 recorded a powerful clap of [[thunder]] soon after it landed. The European Space Agency's [[Venus Express]] recorded abundant lightning in the high atmosphere.<ref name="Venus Express">{{cite web |url=http://www.cnn.com/2007/TECH/space/11/28/venus.lightning.ap/index.html |title=Venus also zapped by lightning |publisher=CNN |date=November 29, 2007 |accessdate=2007-11-29}}</ref> While [[rainfall]] drives [[thunderstorm]]s on Earth, there is no rainfall on the surface of Venus (though it does rain [[sulfuric acid]] in the upper atmosphere that evaporates around 25 km above the surface) One possibility is that ash from a volcanic eruption was generating the lightning. Another intriguing piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which were found to drop by a factor of 10 between 1978 and 1986. This may imply that the levels had earlier been boosted by a large volcanic eruption.<ref>Glaze L.S. (1999), ''Transport of SO2 by explosive volcanism on Venus'', Journal of Geophysical Research, v. 104, p. 18899–18906</ref>
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[[Image:Mgn p39146.png|thumb|left|250px|Impact craters on the surface of Venus (image reconstructed from radar data)]]
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There are almost 1,000 impact craters on Venus, more or less evenly distributed across its surface. On other cratered bodies, such as the Earth and the Moon, craters show a range of states of erosion, indicating a continual process of degradation. On the Moon, degradation is caused by subsequent impacts, while on Earth, it is caused by wind and rain erosion. However, on Venus, about 85% of craters are in pristine condition. The number of craters together with their well-preserved condition indicates that the planet underwent a total resurfacing event about 500 million years ago.<ref>Strom R.G., Schaber G.G., Dawsow D.D. (1995), ''The global resurfacing of Venus'', Journal of Geophysical Research, vol. 99, p. 10,899–10,926</ref> Earth's crust is in continuous motion, but it is thought that Venus cannot sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.<ref name="Frankel" />
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Venusian craters range from 3&nbsp;km to 280&nbsp;km in diameter. There are no craters smaller than 3&nbsp;km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain [[kinetic energy]] are slowed down so much by the atmosphere that they do not create an impact crater.<ref>Herrick R.R., Phillips R.J. (1993), ''Effects of the Venusian atmosphere on incoming meteoroids and the impact crater population'', Icarus, v. 112, p. 253–281</ref>
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===Atmosphere===
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{{main|Atmosphere of Venus}}
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Venus has an extremely thick [[celestial body atmosphere|atmosphere]], which consists mainly of [[carbon dioxide]] and a small amount of [[nitrogen]]. The atmospheric mass is 93 times that of Earth's atmosphere while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1&nbsp;kilometer under Earth's [[ocean]]s. The density at the surface is 65&nbsp;kg/m³ (6.5% that of water). The enormously CO<sub>2</sub>-rich atmosphere, along with thick clouds of [[sulfur dioxide]], generates the strongest [[greenhouse effect]] in the solar system, creating surface temperatures of over 460&nbsp;°C.<ref>{{cite web |url=http://filer.case.edu/sjr16/advanced/venus.html |title=Venus |publisher=Case Western Reserve University |date=September 14, 2006 |accessdate=2007-07-16}}</ref> This makes Venus's surface much hotter than [[Mercury (planet)|Mercury]]'s which has a minimum surface temperature of -220&nbsp;°C and maximum surface temperature of 420&nbsp;°C, even though Venus is nearly twice Mercury's distance from the Sun and receives only 25% of Mercury's solar [[irradiance]]. Because of the lack of any moisture on Venus, there is almost no relative humidity (no more than 1%) on the surface, creating a heat index of 450&nbsp;°C to 480&nbsp;°C.
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[[Image:Venuspioneeruv.jpg|thumb|left|250px|Cloud structure in Venus's atmosphere, revealed by ultraviolet observations]]
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Studies have suggested that several billion years ago Venus's atmosphere was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.<ref>
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{{cite journal
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| title = Runaway and moist greenhouse atmospheres and the evolution of earth and Venus
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| author = Kasting J.F.
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| journal = Icarus
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| volume = 74
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| issue = 3
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| pages = 472–494
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| year = 1988
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| doi = 10.1016/0019-1035(88)90116-9
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}}</ref>
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[[Thermal inertia]] and the transfer of heat by winds in the lower atmosphere mean that the temperature of Venus's surface does not vary significantly between the night and day sides, despite the planet's extremely slow rotation. Winds at the surface are slow, moving at a few kilometers per hour, but because of the high density of the atmosphere at Venus's surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat were not a problem.<ref>
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{{cite journal
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| title = Dust on the surface of Venus
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| author = Moshkin B.E., Ekonomov A.P., Golovin Iu. M.
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| journal = Kosmicheskie Issledovaniia (Cosmic Research)
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| volume = 17
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| issue =
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| pages = 280–285
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| year = 1979
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}}</ref> Above the dense CO<sub>2</sub> layer are thick clouds consisting mainly of [[sulfur dioxide]] and [[sulfuric acid]] droplets.<ref>
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{{cite journal
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| title = Chemical composition of the atmosphere of Venus
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| author = Krasnopolsky V.A., Parshev V.A.
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| journal = Nature
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| volume = 292
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| issue =
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| pages = 610–613
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| year = 1981
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| doi =
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| doi = 10.1038/292610a0 <!--Retrieved from CrossRef by DOI bot-->
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}}</ref><ref>{{cite journal
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| title = Chemical composition of Venus atmosphere and clouds: Some unsolved problems
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|author = Vladimir A. Krasnopolsky
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|journal = Planetary and Space Science
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|volume = 54
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|issue = 13–14
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|pages = 1352–1359
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|year = 2006
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|doi = 10.1016/j.pss.2006.04.019 }}</ref> These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of Venus's surface in [[visible light]]. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well lit. In the absence of the greenhouse effect caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus would be quite similar to that on Earth. Strong 300&nbsp;km/h winds at the cloud tops circle the planet about every four to five earth days.<ref>{{cite journal
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| title = Cloud-tracked winds from Pioneer Venus OCPP images
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|author = Rossow W.B., del Genio A.D., Eichler T.
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|journal = Journal of the Atmospheric Sciences
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|volume = 47
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|issue = 17
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|pages = 2053–2084
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|year = 1990
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|doi = 10.1175/1520-0469(1990)047%3C2053:CTWFVO%3E2.0.CO;2
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|url = http://ams.allenpress.com/archive/1520-0469/47/17/pdf/i1520-0469-47-17-2053.pdf
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|format = PDF}}</ref>
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In 2007 the Venus Express probe discovered that a huge double [[Polar vortex|atmospheric vortex]] exists at the south pole of the planet.<ref>{{cite journal |last=Various authors |first= |authorlink= |coauthors= |year=2007 |month=November |title= |journal=Nature |volume= |issue=450 |pages=633 - 660 |id= |doi= 10.1038/news.2007.297 |accessdate= 2007-11-29 |quote= }}</ref><ref>{{cite web |url=http://news.bbc.co.uk/1/hi/sci/tech/7117303.stm |title=Venus offers Earth climate clues  |accessdate=2007-11-29 |format= |work=BBC News }}</ref>
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====Climate====
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The surface of Venus is effectively [[isothermal]]; it retains a constant temperature between day and night and between the equator and the poles.<ref>{{cite web|title=NASA Venus Fact Sheet|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html|year=2005|accessdate=2007-08-21}}</ref><ref>{{cite web|title = Titan, Mars and Earth: Entropy Production by Latitudinal Heat Transport|author=Ralph D Lorenz, Jonathan I Lunine, Paul G Withers, Christopher P. McKay|work=[[Ames Research Center]], University of Arizona Lunar and Planetary Laboratory|url=http://sirius.bu.edu/withers/pppp/pdf/mepgrl2001.pdf|year=2001|accessdate=2007-08-21}}</ref>  The planet's minute [[axial tilt]] (less than three degrees, compared with 23 degrees for Earth), also minimises [[season]]al temperature variation.<ref>{{cite web|title=Interplanetary Seasons|work=NASA|url=http://science.nasa.gov/headlines/y2000/interplanetaryseasons.html|accessdate=2007-08-21}}</ref> The only appreciable variation in temperature occurs with altitude. In 1995, the [[Magellan_probe|Magellan]] probe imaged a highly reflective substance at the tops of Venus's highest mountain peaks which bore a strong resemblance to terrestrial [[snow]]. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental [[tellurium]] to lead sulfide ([[galena]]).<ref>{{cite web|title='Heavy metal' snow on Venus is lead sulfide |author=Carolyn Jones Otten |work=Washington University in St Louis|url=http://news-info.wustl.edu/news/page/normal/633.html|year=2004|accessdate=2007-08-21}}</ref>
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The clouds of Venus are capable of producing [[lightning]] much like the clouds on Earth.<ref name=Russell2007/> The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. However in 2006&ndash;2007 [[Venus Express]] clearly detected [[Electromagnetic electron wave|whistler mode wave]]s, the signatures of lightning. Their [[intermittent]] appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.<ref name=Russell2007>{{cite journal|last=Russell|first=S.T.|coauthors=Zhang, T.L.; Delva, M.; et.al.|title=Lightning on Venus inferred from whistler-mode waves in the ionosphere| journal=Nature|year=2007|volume=450|pages=661-662|doi=10.1038/nature05930|url=}}</ref>
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===Magnetic field and core===
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In 1980, The ''[[Pioneer Venus project|Pioneer Venus Orbiter]]'' found that Venus's [[magnetic field]] is both weaker and smaller (i.e. closer to the planet) than Earth's. What small magnetic field is present is induced by an interaction between the [[ionosphere]] and the [[solar wind]],<ref>Kivelson G. M., Russell, C. T. ''Introduction to Space Physics'', Cambridge University Press, 1995.</ref> rather than by an internal [[dynamo theory|dynamo]] in the [[Planetary core|core]] like the one inside the Earth. Venus's [[magnetosphere]] is too weak to protect the atmosphere from cosmic radiation.
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This lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected to also contain a dynamo in its core. A dynamo requires three things: a [[Electrical conductor|conducting]] liquid, rotation, and [[convection]]. The core is thought to be electrically conductive, however. Also, while its rotation is often thought to be too slow, simulations show that it is quite adequate to produce a dynamo.<ref>Luhmann J. G., Russell C. T. ''[http://www-spc.igpp.ucla.edu/personnel/russell/papers/venus_mag/  Venus: Magnetic Field and Magnetosphere]'' in Encyclopedia of Planetary Sciences, ed. J. H. Shirley and R. W. Fainbridge,
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905–907, Chapman and Hall, New York, 1997.</ref><ref>Stevenson, D. J., (2003). ''[http://dx.doi.org/10.1016/S0012-821X(02)01126-3 Planetary magnetic fields]'', Earth and Planetary Science Letters, 208, 1–11.</ref> This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. Since Venus has no [[plate tectonics]] to let off heat, it is possible that it has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified.
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==Orbit and rotation==
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[[Image:Terrestrial planet size comparisons.jpg|thumb|right|300px|Size comparison of terrestrial planets (left to right): [[Mercury (planet)|Mercury]], Venus, [[Earth]], and [[Mars]]]]
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Venus orbits the Sun at an average distance of about 108 million km, and completes an orbit every 224.65 days. Although all [[planetary orbit]]s are [[ellipse|elliptical]], Venus is the closest to [[circle|circular]], with an [[eccentricity (orbit)|eccentricity]] of less than 0.01. When Venus lies between the Earth and the Sun, a position known as 'inferior conjunction', it makes the closest approach to Earth of any planet, lying at a distance of about 40&nbsp;million&nbsp;km. The planet reaches inferior conjunction every 584 days, on average.
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Venus rotates once every 243 days—by far the slowest rotation period of any of the major planets. A Venusian [[sidereal day]] thus lasts more than a Venusian year (243 versus 224.7 Earth days). However, the length of a [[solar day]] on Venus is significantly shorter than the sidereal day; to an observer on the surface of Venus the time from one sunrise to the next would be 116.75 days.<ref>{{cite web |url=http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |title=The Terrestrial Planets |publisher=The Planetary Society |accessdate=2007-08-03}}</ref> The Sun would appear to rise in the west and set in the east. At the equator, Venus's surface rotates at 6.5&nbsp;km/h; on Earth, the rotation speed at the equator is about 1,600&nbsp;km/h.
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If viewed from above the Sun's north pole, all of the planets are orbiting in a counter-clockwise direction; but while most planets also rotate counter-clockwise, Venus rotates clockwise in [[retrograde motion|"retrograde"]] rotation. The question of how Venus came to have a slow, retrograde rotation was a major puzzle for scientists when the planet's rotation period was first measured. When it formed from the [[solar nebula]], Venus would have had a much faster, prograde rotation, but calculations show that over billions of years, [[tide|tidal]] effects on its dense atmosphere could have slowed down its initial rotation to the value seen today.<ref>{{cite journal |author=Correia A.C.M., Laskar J., de Surgy O.N. |year=2003 |title=Long-term evolution of the spin of Venus: I. theory |journal=Icarus |volume=163 |pages=1–23 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf |format=PDF }}</ref><ref>{{cite journal |author=Correia A.C.M., Laskar J. |year=2003 |title=Long-term evolution of the spin of Venus: II. numerical simulations |journal=Icarus |volume=163 |pages=24–45 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf |format=PDF  | doi = 10.1016/S0019-1035(03)00043-5 <!--Retrieved from CrossRef by DOI bot-->}}</ref>
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A curious aspect of Venus's orbit and rotation periods is that the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days. Whether this relationship arose by chance or is the result of some kind of [[tidal locking]] with the Earth, is unknown.<ref>{{cite journal |author=Gold T., Soter S. |year=1969 |title=Atmospheric tides and the resonant rotation of Venus |journal=Icarus |volume=11 |pages=356–366 | doi = 10.1016/0019-1035(69)90068-2 <!--Retrieved from CrossRef by DOI bot-->}}</ref>
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Venus is currently moonless, though the [[asteroid]] [[2002 VE68|2002 VE<sub>68</sub>]] presently maintains a [[quasi-satellite|quasi-orbital]] relationship with it.<ref>{{cite journal |author=Mikkola S., Brasser R., Wiegert P., Innanen K. |title=Asteroid 2002 VE68, a quasi-satellite of Venus |journal=Monthly Notices of the Royal Astronomical Society |volume=351 |pages=L63 |date=July 2004 | doi = 10.1111/j.1365-2966.2004.07994.x <!--Retrieved from CrossRef by DOI bot-->}}</ref>
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According to Alex Alemi and David Stevenson of the [[California Institute of Technology]], their recent study of models of the early solar system shows that it is very likely that, billions of years ago, Venus had at least one moon, created by a huge [[impact event]].<ref>{{cite news |publisher=SCIENTIFIC AMERICAN.com |date=October 31, 1994 |title=Double Impact May Explain Why Venus Has No Moon |author=George Musser |url=http://www.sciam.com/article.cfm?articleID=0008DCD1-0A66-152C-8A6683414B7F0000&ref=sciam |accessdate=2007-08-03}}</ref><ref>{{cite news |publisher=SkyandTelescope.com |date=October 10, 2006 |title=Why Doesn't Venus Have a Moon? |author=David Tytell |url=http://www.skyandtelescope.com/news/home/4353026.html |accessdate=2007-08-03}}</ref> About 10 million years later, according to Alemi and Stevenson, another impact reversed the planet's spin direction. The reversed spin direction caused the Venusian moon to gradually [[Tidal acceleration#Tidal deceleration|spiral inward]]<ref>{{cite web |publisher=Aerospaceweb.org |title=Moon Motion & Tides |url=http://www.aerospaceweb.org/question/astronomy/q0262.shtml |author=Justine Whitman |date=February 19, 2006 |accessdate=2007-08-03}}</ref> until it collided and merged with Venus. If later impacts created moons, those moons also were absorbed the same way the first one was. The Alemi/Stevenson study is recent, and it remains to be seen what sort of acceptance it will achieve in the scientific community.
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==Observation==
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[[Image:Sunset at Grain Elevator 012 Cropped more and red circle around Venus.jpg|thumb|right|250px|Venus as the Evening Star, next to a crescent moon]]
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[[Image:Venus pentagram.png|right|thumb|240px|Successive [[inferior conjunction]]s of Venus occur about 1.6 Earth years apart and create a pattern of [[Precession|precessing]] [[pentagram]]s, due to a near 13:8 [[orbital resonance]] (the Earth orbits nearly 8 times for every 13 orbits of Venus).]]
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Venus is always brighter than the brightest stars, with its [[apparent magnitude]] ranging from −3.8 to −4.6. This is bright enough to be seen even in the middle of the day, and the planet can be easy to see when the Sun is low on the horizon. As an [[inferior planet]], it always lies within about 47° of the [[Sun]].<ref>{{cite web| last = Espenak| first = Fred| year = 1996| url = http://sunearth.gsfc.nasa.gov/eclipse/TYPE/venus2.html| title = NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995–2006| work = Twelve Year Planetary Ephemeris Directory| publisher = NASA| accessdate = 2006-06-20}},</ref>
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Venus 'overtakes' the Earth every 584 days as it orbits the Sun. As it does so, it goes from being the 'Evening star', visible after sunset, to being the '[[Morning star]]', visible before sunrise. While [[Mercury (planet)|Mercury]], the other inferior planet, reaches a maximum [[Elongation (astronomy)|elongation]] of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported '[[unidentified flying object]]'. [[U.S. President]] [[Jimmy Carter]] [[Jimmy Carter UFO Incident|reported having seen a UFO]] in 1969, which later analysis suggested was probably the planet, and countless other people have mistaken Venus for something more exotic.<ref>{{cite web| last = Krystek| first = Lee| url = http://www.unmuseum.org/ifonat.htm| title = Natural Identified Flying Objects| publisher = The Unngatural Museum| accessdate = 2006-06-20}}</ref>
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As it moves around its orbit, Venus displays [[planetary phase|phases]] like those of the [[Moon]]: it is new when it passes between the Earth and the Sun, full when it is on the opposite side of the Sun, and a half-phase when it is at its maximum elongations from the Sun. Venus is brightest when it is a thin crescent; it is much closer to Earth when a thin crescent than when [[gibbous]], or full.
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Venus's orbit is slightly inclined relative to the Earth's orbit; thus, when the planet passes between the Earth and the Sun, it usually does not cross the face of the Sun. However, [[transits of Venus]] do occur in pairs separated by eight years, at intervals of about 120 years, when the planet's [[inferior conjunction]] coincides with its presence in the plane of the Earth's orbit. The most recent transit was in 2004; the next will be in 2012. Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the [[astronomical unit]], and hence of the solar system. [[Captain Cook]]'s exploration of the east coast of Australia came after he had sailed to [[Tahiti]] in 1768 to observe a transit of Venus.<ref>{{cite journal
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| author = Hornsby, T.
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| title = The quantity of the Sun's parallax, as deduced from the observations of the transit of Venus on June 3 1769.
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| journal = Phil. Trans. R. Soc.
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| volume = 61
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| pages =  574-579
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| year = 1771
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| url = http://gallica.bnf.fr/ark:/12148/bpt6k55866b/f617.chemindefer | doi = 10.1098/rstl.1771.0054 <!--Retrieved from CrossRef by DOI bot-->
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}}</ref><ref>{{cite journal
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| author = Richard Woolley
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| title = Captain Cook and the Transit of Venus of 1769
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| journal = Notes and Records of the Royal Society of London
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| volume = 24
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| issue = 1
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| pages =  19-32
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| year = 1969
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| url = http://links.jstor.org/sici?sici=0035-9149%28196906%2924%3A1%3C19%3ACCATTO%3E2.0.CO%3B2-K}}</ref>
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A long-standing mystery of Venus observations is the so-called [[Ashen light]]—an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object.<ref>{{cite journal |author=Baum, R. M. |year=2000 |title=The enigmatic ashen light of Venus: an overview |journal=Journal of the British Astronomical Association |volume=110 |pages=325}}</ref>
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<gallery>
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Image:Sunset_Venus.JPG|Venus and pelicans at twilight
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Image:Venustransit 2004-06-08 07-44.jpg|Venus transits the face of the Sun on [[June 8]] [[2004]].<br />The [[black drop effect]] is visible.
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Image:Moon 18-Jun-2007 8-05-13 PM.JPG|Venus, seen from [[Sharjah (emirate)|Sharjah]], before being eclipsed by the Moon on [[June 18]] [[2007]]
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</gallery>
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==Studies of Venus==
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===Early studies===
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[[Image:Phases-of-Venus.svg|thumb|right|250px|Galileo's discovery that Venus showed phases proved that it orbits the Sun and not the Earth]]
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Venus was known in the Hindu [[Jyotisha]] since early times as the [[navagraha|planet]] [[Shukra]]. In the West, before the advent of the [[telescope]], Venus was known only as a '[[wandering star]]'. Several cultures historically held its appearances as a morning and evening star to be those of two separate bodies. [[Pythagoras]] is usually credited with recognizing in the sixth century BC that the morning and evening stars were a single body, though he thought that Venus orbited the Earth. When [[Galileo]] first observed the planet in the early 17th century, he found that it showed [[planetary phase|phases]] like the Moon's, varying from crescent to gibbous to full and vice versa. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic [[geocentric model]] that the solar system was concentric and centered on the Earth.<ref>{{cite web| url = http://csep10.phys.utk.edu/astr161/lect/history/galileo.html| title = Galileo: the Telescope & the Laws of Dynamics| work = Astronomy 161; The Solar System| publisher = Dept. Physics & Astronomy, University of Tennessee| accessdate = 2006-06-20}}</ref>
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Venus's atmosphere was discovered as early as 1790 by [[Johann Schröter]]. Schröter found that when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised that this was due to [[scattering]] of sunlight in a dense atmosphere. Later, [[Chester Smith Lyman]] observed a complete ring around the dark side of the planet when it was at [[inferior conjunction]], providing further evidence for an atmosphere.<ref>{{cite journal|author =Russell H.N.|year= 1899|title = The Atmosphere of Venus| journal=Astrophysical Journal|volume= 9|pages =284 | doi = 10.1086/140593 <!--Retrieved from CrossRef by DOI bot-->}}</ref> The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as [[Giovanni Cassini]] and Schröter incorrectly estimated periods of about 24 hours from the motions of markings on the planet's apparent surface.<ref>{{cite journal|author = Hussey T.|year= 1832 | title = On the rotation of Venus| journal= Monthly Notices of the Royal Astronomical Society|volume= 2|pages =78|url= http://adsabs.harvard.edu/abs/1832MNRAS...2...78H}}</ref>
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===Ground-based research===
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Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint as to what its surface might be like, and it was only with the development of [[astronomical spectroscopy|spectroscopic]], [[radar]] and [[ultraviolet]] observations that more of its secrets were revealed. The first UV observations were carried out in the 1920s, when [[Frank E. Ross]] found that UV photographs revealed considerable detail that was absent in visible and [[infrared]] radiation. He suggested that this was due to a very dense yellow lower atmosphere with high [[cirrus]] clouds above it.<ref>{{cite journal|author = Ross F.E. |year = 1928 | title = Photographs of Venus |journal =Astrophysical Journal | volume = 68 | pages = 57|doi =10.1086/143130}}</ref>
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90% of of the surface Venus appears to be recently solid basalt lava.
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Spectroscopic observations in the 1900s gave the first clues about Venus's rotation. [[Vesto Slipher]] tried to measure the [[Doppler shift]] of light from Venus, but found that he could not detect any rotation. He surmised that the planet must have a much longer rotation period than had previously been thought.<ref>{{cite journal|author =Slipher V.M. |year =1903|title =A Spectrographic Investigation of the Rotation Velocity of Venus|journal =Astronomische Nachrichten|volume= 163|pages=35|url=http://adsabs.harvard.edu/abs/1903AN....163...35S | doi = 10.1002/asna.19031630303 <!--Retrieved from CrossRef by DOI bot-->}}</ref>  Later work in the 1950s showed that the rotation was retrograde. Radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period which were close to the modern value.<ref>{{cite journal|author = Goldstein R.M., Carpenter R.L. |year = 1963|title = Rotation of Venus: Period Estimated from Radar Measurements|journal = Science |volume= 139 |pages= 910–911|doi= 10.1126/science.139.3558.910}}</ref>
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Radar observations in the 1970s revealed details of Venus's surface for the first time. Pulses of radio waves were beamed at the planet using the 300&nbsp;m radio telescope at [[Arecibo Observatory]], and the echoes revealed two highly reflective regions, designated the Alpha and Beta regions. The observations also revealed a bright region attributed to mountains, which was called Maxwell Montes.<ref>{{cite journal|author =Campbell D.B., Dyce R.B., Pettengill G.H.|year =1976)|title =New radar image of Venus|journal =Science|volume=193|pages= 1123|doi=10.1126/science.193.4258.1123}}</ref> These three features are now the only ones on Venus which do not have female names.
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The best radar images obtainable from Earth revealed features no smaller than about 5&nbsp;km across. More detailed exploration of the planet could only be carried out from space.
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==Exploration of Venus==
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{{main|Observations and explorations of Venus}}
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===Early efforts===
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[[Image:Mariner 2 in space.jpg|right|thumb|200px|Mariner 2, launched in 1962]]
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The first [[Robotic spacecraft|robotic]] [[space probe]] mission to Venus, and the first to any planet, began on [[February 12]] [[1961]] with the launch of the [[Venera 1]] probe. The first craft of the otherwise highly successful Soviet [[Venera program]], Venera 1 was launched on a direct impact trajectory, but contact was lost seven days into the mission, when the probe was about 2&nbsp;million&nbsp;km from Earth. It was estimated to have passed within 100,000&nbsp;km from Venus in mid-May.<ref name=mitchell_1>
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{{Citation
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| first = Don | last = Mitchell
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| work = The Soviet Exploration of Venus
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| title = Inventing The Interplanetary Probe
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| year = 2003
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| url = http://www.mentallandscape.com/V_OKB1.htm
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| accessdate = 2007-12-27 }}</ref>
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The [[United States]] exploration of Venus also started badly with the loss of the [[Mariner 1]] probe on launch. The subsequent [[Mariner 2]] mission enjoyed greater success, and after a 109-day [[transfer orbit]] on [[December 14]] [[1962]] it became the world's first successful interplanetary mission, passing 34,833&nbsp;km above the surface of Venus. Its [[microwave]] and [[infrared]] [[radiometer]]s revealed that while Venus's cloud tops were cool, the surface was extremely hot—at least 425&nbsp;°C, finally ending any hopes that the planet might harbor ground-based life. Mariner 2 also obtained improved estimates of Venus's mass and of the [[astronomical unit]], but was unable to detect either a [[magnetic field]] or [[radiation belt]]s.<ref>{{Cite paper | author=Jet Propulsion Laboratory | title=Mariner-Venus 1962 Final Project Report | publisher=NASA | date=1962 | version=SP-59 | url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660005413_1966005413.pdf |format=PDF}}</ref>
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===Atmospheric entry===
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The [[Venera 3]] probe crash-landed on Venus on [[March 1]] [[1966]]. It was the first man-made object to enter the atmosphere and strike the surface of another planet, though its communication system failed before it was able to return any planetary data.<ref name=mitchell_2>
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{{Citation
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| first = Don | last = Mitchell
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| work = The Soviet Exploration of Venus
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| title = Plumbing the Atmosphere of Venus
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| year = 2003
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| url = http://www.mentallandscape.com/V_Lavochkin1.htm
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| accessdate = 2007-12-27 }}</ref> Venus's next encounter with an unmanned probe came on [[October 18]] [[1967]] when [[Venera 4]] successfully entered the atmosphere and deployed a number of science experiments. Venera 4 showed that the surface temperature was even hotter than Mariner 2 had measured at almost 500&nbsp;°C, and that the atmosphere was about 90 to 95% carbon dioxide. The Venusian atmosphere was considerably denser than Venera 4's designers had anticipated, and its slower than intended parachute descent meant that its batteries ran down before the probe reached the surface. After returning descent data for 93&nbsp;minutes, Venera 4's last pressure reading was 18&nbsp;bar at an altitude of 24.96&nbsp;km.<ref name=mitchell_2/>
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Another probe arrived at Venus one day later on [[October 19]] [[1967]] when [[Mariner 5]] conducted a flyby at a distance of less than 4,000&nbsp;km above the cloud tops. Mariner 5 was originally built as backup for the [[Mars]]-bound [[Mariner 4]], but when that mission was successful, the probe was refitted for a Venus mission. A suite of instruments more sensitive than those on Mariner 2, in particular its radio [[occultation]] experiment, returned data on the composition, pressure and density of Venus's atmosphere.<ref>{{Cite paper | author=Eshleman V, Fjeldbo G| title=The atmosphere of Venus as studied with the Mariner 5 dual radio-frequency occultation experiment | publisher=NASA | date=1969| version=SU-SEL-69-003 | url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690011426_1969011426.pdf |format=PDF}}</ref> The joint Venera 4&ndash;Mariner 5 data were analyzed by a combined Soviet-American science team in a series of colloquia over the following year, in an early example of space cooperation.
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Armed with the lessons and data learned from Venera 4, the Soviet Union launched the twin probes [[Venera 5]] and [[Venera 6]] five days apart in January 1969; they encountered Venus a day apart on [[May 16]] and [[May 17]] that year. The probes were strengthened to improve their [[crush depth]] to 25&nbsp;atmospheres and were equipped with smaller parachutes to achieve a faster descent. Since then-current atmospheric models of Venus suggested a surface pressure of between 75 and 100&nbsp;atmospheres, neither was expected to survive to the surface. After returning atmospheric data for a little over fifty minutes, they both were crushed at altitudes of approximately 20&nbsp;km before going on to strike the surface on the night side of Venus.<ref name=mitchell_2/>
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===Surface science===
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[[Venera 7]] represented a concerted effort to return data from the planet's surface, and was constructed with a reinforced descent module capable of withstanding a pressure of 180&nbsp;bar. The module was pre-cooled prior to entry and equipped with a specially [[reefing|reefed]] parachute for a rapid 35-minute descent. Entering the atmosphere on [[December 15]] [[1970]], the parachute is believed to have partially torn during the descent, and the probe struck the surface with a hard, yet not fatal, impact. Probably tilted onto its side, it returned a weak signal supplying temperature data for 23&nbsp;minutes, the first [[telemetry]] received from the surface of another planet.<ref name=mitchell_2/>
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The Venera program continued with [[Venera 8]] sending data from the surface for 50&nbsp;minutes, and [[Venera 9]] and [[Venera 10]] sending the first images of the Venusian landscape. The two landing sites presented very different visages in the immediate vicinities of the landers: Venera 9 had landed on a 20 degree slope scattered with boulders around 30–40&nbsp;cm across; Venera 10 showed [[basalt]]-like rock slabs interspersed with [[weathering|weathered]] material.<ref name=mitchell_3>
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{{Citation
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| first = Don | last = Mitchell
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| work = The Soviet Exploration of Venus
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| title = First Pictures of the Surface of Venus
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| year = 2003
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| url = http://www.mentallandscape.com/V_Lavochkin2.htm
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| accessdate = 2007-12-27 }}</ref>
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[[Image:Pioneer Venus orbiter.jpg|thumb|left|250px|The Pioneer Venus orbiter]]
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In the meantime, the United States had sent the [[Mariner 10]] probe on a [[gravitational slingshot]] trajectory past Venus on its way to [[Mercury (planet)|Mercury]]. On [[February 5]], [[1974]], Mariner 10 passed within 5790&nbsp;km of Venus, returning over 4,000 photographs as it did so. The images, the best then achieved, showed the planet to be almost featureless in visible light, but [[ultraviolet]] light revealed details in the clouds that had never been seen in Earth-bound observations.<ref>{{Cite paper | author=Dunne, J & Burgess E| title=The Voyage of Mariner 10 | publisher=NASA | date=1978 | version=SP-424 | url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780019203_1978019203.pdf |format=PDF}}</ref>
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The American [[Pioneer Venus project]] consisted of two separate missions.<ref>{{cite journal | author=Colin L, Hall C | title= The Pioneer Venus Program | journal=Space Science Reviews | year=1977 | volume=20 | issue=|pages=|format=PDF |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1977SSRv...20..283C&amp;data_type=PDF_HIGH&amp;type=PRINTER&amp;filetype=.pdf  |doi = 10.1007/BF02186467 | pages = 283}}</ref> The [[Pioneer Venus Orbiter]] was inserted into an elliptical orbit around Venus on [[December 4]] [[1978]], and remained there for over thirteen years studying the atmosphere and mapping the surface with [[radar]]. The [[Pioneer Venus Multiprobe]] released a total of five probes which entered the atmosphere on [[December 9]] [[1978]], returning data on its composition, winds and heat fluxes.
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Four more Venera lander missions took place over the next four years, with [[Venera 11]] and [[Venera 12]] detecting Venusian [[electrical storm]]s;<ref name=mitchell_4>
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{{Citation
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| first = Don | last = Mitchell
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| work = The Soviet Exploration of Venus
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| title = Drilling into the Surface of Venus
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| year = 2003
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| url = http://www.mentallandscape.com/V_Venera11.htm
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| accessdate = 2007-12-27 }}</ref> and [[Venera 13]] and [[Venera 14]], landing four days apart on [[March 1]] and [[March 5]] [[1982]], returning the first color photographs of the surface. All four missions deployed parachutes for braking in the upper atmosphere, but released them at altitudes of 50&nbsp;km, the dense lower atmosphere providing enough friction to allow for an unaided soft landing. Both Venera 13 and 14 analyzed soil samples with an on-board [[X-ray fluorescence]] [[spectrometer]], and attempted to measure the compressibility of the soil with an impact probe.<ref name=mitchell_4/> Venera 14, though, had the misfortune to strike its own ejected camera lens cap and its probe failed to make contact with the soil.<ref name=mitchell_4/> The Venera program came to a close in October 1983 when [[Venera 15]] and [[Venera 16]] were placed in orbit to conduct mapping of the Venusian terrain with [[synthetic aperture radar]].
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The Soviet Union had not finished with Venus, and in 1985 it took advantage of the opportunity to combine missions to Venus and [[Comet Halley]], which passed through the inner solar system that year. En route to Halley, on [[June 11]] and [[June 15]] [[1985]] the two spacecraft of the [[Vega program]] each dropped a Venera-style probe (of which Vega 1's partially failed) and released a balloon-supported [[aerobot]] into the upper atmosphere. The balloons achieved an equilibrium altitude of around 53&nbsp;km, where pressure and temperature are comparable to those at Earth's surface. They remained operational for around 46&nbsp;hours, and discovered that the Venusian atmosphere was more turbulent than previously believed, and subject to high winds and powerful [[convection cell]]s.<ref>{{cite journal | author=Linkin V, Blamont J, Preston R | title= The Vega Venus Balloon experiment | journal=Bulletin of the American Astronomical Society | year=1985 | volume=17 | issue= | pages= 722 | url= }}</ref><ref>{{cite journal |title = The VEGA Venus Balloon Experiment | author = Sagdeev, R. Z.; Linkin, V. M.; Blamont, J. E.; Preston, R. A.| journal = Science| volume = 231 | pages = 1407–1408|year = 1986|url = http://links.jstor.org/sici?sici=0036-8075%2819860321%293%3A231%3A4744%3C1407%3ATVVBE%3E2.0.CO%3B2-E |doi = 10.1126/science.231.4744.1407}}</ref>
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===Manned Venus flyby===
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{{main|Manned Venus Flyby}}
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A manned Venus flyby mission, using [[Apollo program]] hardware, was proposed in the late 1960s.<ref>{{Citation | last = Feldman ''et al.'' | title = Manned Venus Flyby | publisher = Bellcomm | url = http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790072165_1979072165.pdf | year = 1967}}</ref> The mission was planned to launch in late October or early November of 1973, and would have used a [[Saturn V]] to send three men to fly past Venus in a flight lasting approximately one year. The spacecraft would have passed approximately 5,000 kilometres from the surface of Venus about four months later.
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===Radar mapping===
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[[Image:Venus2 mag big.png|thumb|250px|right|[[Magellan_probe|Magellan]] topographical map of Venus]]
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The United States' [[Magellan probe]] was launched on [[May 4]], [[1989]] with a mission to map the surface of Venus with radar.<ref name=jpl-magellan/> The high-resolution images it obtained during its 4½ years of operation far surpassed all prior maps and were comparable to visible-light photographs of other planets. [[Magellan_probe|Magellan]] imaged over 98% of Venus's surface by radar and mapped 95% of its gravity field. In 1994, at the end of its mission, [[Magellan_probe|Magellan]] was deliberately sent to its destruction into the atmosphere of Venus in an effort to quantify its density. Venus was observed by the [[Galileo spacecraft|Galileo]] and [[Cassini-Huygens|Cassini]] spacecraft during flybys on their respective missions to the [[Solar system#Outer planets|outer planets]], but [[Magellan_probe|Magellan]] would otherwise be the last dedicated mission to Venus for over a decade.
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=== Current and future missions ===
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The [[Venus Express]] probe was designed and built by the [[European Space Agency]]. Launched on [[November 9]] [[2005]] by a Russian [[Soyuz-Fregat]] rocket procured through [[Starsem]], it successfully assumed a [[polar orbit]] around Venus on [[April 11]], [[2006]].<ref name="venus express">{{cite web | title=Venus Express| work=ESA Portal | url=http://www.esa.int/SPECIALS/Venus_Express/index.html | accessdaymonth=09 Feb | accessyear=2008}}</ref> The probe is undertaking a detailed study of the Venusian atmosphere and clouds, and will also map the planet's [[Plasma (physics)|plasma]] environment and surface characteristics, particularly temperatures. Its mission is intended to last a nominal 500&nbsp;Earth days, or around two Venusian years.<ref name="venus express"/> One of the first results emerging from Venus Express is the discovery that a huge double [[polar vortex|atmospheric vortex]] exists at the south pole of the planet.<ref name="venus express"/>
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NASA's [[MESSENGER]] mission to Mercury performed two flybys of Venus in October 2006 and June 2007, in order to slow its trajectory for an eventual orbital insertion of Mercury in 2011. MESSENGER collected scientific data on both those flybys.<ref>
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{{cite web | title=Timeline| work=MESSENGER | url=http://messenger.jhuapl.edu/the_mission/MESSENGERTimeline/TimeLine_content.html | accessdaymonth=09 Feb | accessyear=2008}}</ref> The [[European Space Agency]] (ESA) will also launch a mission to Mercury, called [[BepiColombo]], which will perform two flybys of Venus in August 2013 before it reaches Mercury orbit in 2019.<ref>{{cite web | title=BepiColombo | work=ESA Spacecraft Operations| url=http://www.esa.int/SPECIALS/Operations/SEMYRMQJNVE_0.html | accessdaymonth=09 Feb | accessyear=2008}}</ref>
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Future dedicated missions to Venus are planned. Japan's aerospace body [[JAXA]] is planning to launch its Venus climate orbiter, the [[PLANET-C]], in 2010.<ref>{{cite web | title=Venus Climate Orbiter "PLANET-C"| work=JAXA| url=http://www.jaxa.jp/projects/sat/planet_c/index_e.html | accessdaymonth=09 Feb | accessyear=2008}}</ref> Under its [[New Frontiers Program]], NASA has proposed a lander mission called the [[Venus In-Situ Explorer]] to land on Venus to study surface conditions and investigate the elemental and mineralogical features of the [[regolith]], equipped with a core sampler to drill into the surface to study pristine rock samples not weathered by the very harsh surface conditions of the planet.  Other proposed Venus exploration concepts include [[Observations and explorations of Venus#Future missions | rovers]], [[Aerobot#JPL aerobot mission concepts | balloons]], and [http://gltrs.grc.nasa.gov/Citations.aspx?id=1568 Venus airplane concepts].
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==Venus in human culture==
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===Historic understanding===
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[[Image:Dresden Codex p09.jpg|thumb|left|130px|The Maya [[Maya codices#Dresden Codex|''Dresden Codex]], which calculates Venus's appearances]]
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As one of the brightest objects in the sky, Venus has been known since prehistoric times and as such has gained an entrenched position in human culture. It is described in [[Babylon]]ian [[Cuneiform script|cuneiformic]] texts such as the [[Venus tablet of Ammisaduqa]], which relates observations that possibly date from 1600&nbsp;BC.<ref>{{Citation | last = Sachs | first = A. | title = Babylonian Observational Astronomy | journal = Phil. Trans. Royal Soc. Lond. | volume = 276 | issue = No. 1257 | pages = pp.43&ndash;50 | year = 1974}}</ref> The Babylonians named the planet ''[[Ishtar]]'' ([[Mesopotamian mythology|Sumerian]] ''[[Inanna]]''), the personification of womanhood, and goddess of love. The [[Ancient Egypt]]ians believed Venus to be two separate bodies and knew the morning star as ''Tioumoutiri'' and the evening star as ''Ouaiti''. Likewise believing Venus to be two bodies, the [[Ancient Greece|Ancient Greeks]] called the morning star {{Polytonic|Φωσφόρος}}, ''{{lang|grc-Latn|Phosphoros}}'' (Latinized ''[[Phosphorus]]''), the "Bringer of Light" or {{Polytonic|Ἐωσφόρος}}, ''{{lang|grc-Latn|Eosphoros}}'' (Latinized ''[[Eosphorus]]''), the "Bringer of Dawn". The evening star they called ''{{lang|grc-Latn|Hesperos}}'' (Latinized ''[[Hesperus]]'') ({{Polytonic|Ἓσπερος}}, the "star of the evening"), but by [[Hellenistic]] times, they realized the two were the same planet. Hesperos would be translated into [[Latin]] as [[Vesper]] and Phosphoros as [[Lucifer]] ("Light Bearer"), a poetic term later used to refer to the fallen angel cast out of heaven.<ref>[[Jerome]] translated [[Septuagint]] ''heosphoros'' and Hebrew ''helel'' as ''lucifer'', in [[Book of Isaiah|Isaiah]] 14:12.</ref> The [[Ancient Rome|Romans]] would later name the planet in honor of their goddess of love, [[Venus (mythology)|Venus]], whereas the Greeks used the name of her Greek counterpart, [[Aphrodite]] (Phoenician [[Astarte]]).
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To the [[Hebrews]] it was known as ''{{lang|hbo-Latn|Noga}}'' ("shining"), ''{{lang|hbo-Latn|Helel}}'' ("bright"), ''{{lang|hbo-Latn|Ayeleth-ha-Shakhar}}'' ("deer of the dawn") and ''{{lang|hbo-Latn|Kochav-ha-'Erev}}'' ("star of the evening").
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Venus was important to the [[Maya civilization]], who developed a [[Maya calendar|religious calendar]] based in part upon its motions, and held the motions of Venus to determine the propitious time for events such as war.
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The [[Maasai]] people named the planet ''[[Kileken]]'', and have an [[oral tradition]] about it called ''The Orphan Boy''.<ref>{{cite journal |last=Verhaag |first=G. |authorlink= |coauthors= |year=2000 |month= |title=Letters to the Editor: Cross-cultural astronomy |journal=Journal of the British Astronomical Association |volume=110 |issue=1 |pages=49 |id= |url=http://adsabs.harvard.edu/abs/2000JBAA..110...49V |accessdate= |quote= }}</ref>
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Venus is important in many [[Australian Aboriginal astronomy|Australian aboriginal cultures]], such as that of the [[Yolngu]] people in Northern Australia. The Yolngu gather after sunset to await the rising of Venus, which they call ''[[Barnumbirr]]''. As she approaches, in the early hours before dawn, she draws behind her a rope of light attached to the Earth, and along this rope, with the aid of a richly decorated "Morning Star Pole", the people are able to communicate with their dead loved ones, showing that they still love and remember them. Barnumbirr is also an important creator-spirit in the [[Dreaming]], and "sang" much of the country into life.
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[[Image:Venus symbol.svg|50px|left|♀]]
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In western [[astrology]], derived from its historical connotation with goddesses of femininity and love, Venus is held to influence those aspects of human life. In Indian [[Vedic astrology]], Venus is known as ''[[Shukra]]'', meaning "clear, pure" or "brightness, clearness" in [[Sanskrit]]. One of the nine [[Navagraha]], it is held to affect wealth, pleasure and reproduction; it was the son of [[Bhrgu]] and [[Ushana]], preceptor of the Daityas, and guru of the Asuras. Early Chinese astronomers called the planet ''Tai-pe'', or the "beautiful white one". Modern [[China|Chinese]], [[Korea]]n, [[Japan]]ese and [[Vietnam]]ese cultures refer to the planet literally as the ''gold star'' ([[Chinese language|Chinese]]: 金星), based on the [[Five elements (Chinese philosophy)|Five elements]]. [[Lakota people|Lakotan]] spirituality refers to Venus as the daybreak star, and associates it with the last stage of life and wisdom.
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The [[Astronomical symbols|astronomical symbol]] for Venus is the same as that used in biology for the [[female]] [[sex]]: a circle with a small cross beneath.<ref name=stearn>
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{{Citation
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| first = William | last = Stearn
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| title = The Origin of the Male and Female Symbols of Biology
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| journal = Taxon
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| volume = 11
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| issue = No. 4, May 1962
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| pages = 109&ndash;113
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| year = 1968}}</ref> The Venus symbol also represents [[femininity]], and in Western [[alchemy]] stood for the metal [[copper]].<ref name=stearn/> Polished copper has been used for mirrors from antiquity, and the symbol for Venus has sometimes been understood to stand for the mirror of the goddess.<ref name=stearn/><ref>
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{{Citation
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| first = Jennifer | last = Emick
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| title = Venus (alchemical copper)
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| url = http://altreligion.about.com/library/glossary/symbols/bldefsvenus.htm
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| accessdate = [[2007-12-27]] }}</ref>
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Perhaps the strangest appearance of Venus in literature is as the harbinger of destruction in Immanuel Velikovsky's ''Worlds in Collision'' (1950). In this intensely controversial book, Velikovsky argued that ancient references to Venus indicated that the planet had played a role in catastrophic events in the solar system within the past few thousand years. Scientists rejected Velikovsky's wild hypothesis, though his books were popular for a number of years.<ref>[[Worlds in Collision]]</ref>
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===In fiction===
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{{Main|Venus in fiction}}
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Venus's impenetrable cloud cover gave [[science fiction]] writers free rein to speculate on conditions at its surface; all the more so when early observations showed that it was very similar in size to Earth and possessed a substantial atmosphere. The planet was frequently depicted as warmer than Earth, but still [[Planetary habitability|habitable]] by humans.<ref name="miller">
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{{Citation
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| first = Ron | last = Miller
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| title = Venus
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| publisher = Twenty-First Century Books
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| page = 12
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| year = 2003
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| isbn = 0-7613-2359-7}}
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</ref> The [[genre]] reached its peak between the 1930s and 1950s, at a time when science had revealed some aspects of Venus, but not yet the harsh reality of its surface conditions. [[Robert A. Heinlein|Robert Heinlein]]'s ''[[Future History]]'' series had a Venus inspired by the chemist [[Svante Arrhenius]]'s prediction of a steamy [[carboniferous]] [[swamp]] upon which the rain dripped incessantly. It probably inspired [[Henry Kuttner]] to the subsequent depiction given in his novel ''Fury''. [[Ray Bradbury]]'s short stories ''[[The Long Rain]]'' and ''[[All Summer in a Day]]'' also depicted Venus as a habitable planet with incessant rain. Other works, such as [[C. S. Lewis]]'s 1943 ''[[Perelandra]]'' or [[Isaac Asimov]]'s 1954 ''[[Lucky Starr and the Oceans of Venus]]'', drew from a vision of a [[Cambrian]]-like Venus covered by a near planet-wide [[ocean]] filled with exotic aquatic life.<ref name=miller/>
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Findings from the first missions to Venus showed the reality to be very different, and brought this particular genre to an end,<ref>
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{{Citation
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| first = Steven | last = Dick
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| title = Life on Other Worlds: The 20th-Century Extraterrestrial Life Debate
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| publisher = Cambridge University Press
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| page = 43
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| year = 2001
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| isbn = 0521799120}}
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</ref> a passing which [[Brian Aldiss]] and [[Harry Harrison]] marked with their 1968 anthology ''[[Farewell Fantastic Venus]]''. However, as scientific knowledge of Venus advanced, so science fiction authors endeavored to keep pace, particularly by conjecturing human attempts to [[Terraforming of Venus|terraform Venus]].<ref>
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{{Citation
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| first = David | last = Seed
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| title = A Companion to Science Fiction
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| publisher = Blackwell Publishing
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| pages = 134-135
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| year = 2005
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| isbn = 1405112182}}
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</ref> [[Arthur C. Clarke]]'s 1997 novel ''[[3001: The Final Odyssey]]'', for example, postulates humans lowering Venus's temperature by steering [[comet]]ary fragments to impact its surface. A [[terraform]]ed Venus is the setting for a number of diverse works of fiction that have included ''[[Star Trek]]'', ''[[Exosquad]]'' and the manga ''[[Venus Wars]]''.
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==See also==
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{{portal|Solar System|Solar system.jpg}}
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*[[Colonization of Venus]]
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==References==
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{{reflist|2}}
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==External links==
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{{commons}}
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* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus Venus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
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* [http://www.mentallandscape.com/V_Venus.htm The Soviet Exploration of Venus], [http://www.mentallandscape.com/C_CatalogVenus.htm Image catalog]
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* [http://www.nineplanets.org/venus.html The Nine Planets: Venus]
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* [http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/venera1112.html NASA page about the Venera missions]
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* [http://www2.jpl.nasa.gov/magellan/ Magellan mission home page]
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* [http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/pvo.html Pioneer Venus information from NASA]
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* [http://sunearth.gsfc.nasa.gov/eclipse/transit/venus0412.html Detailed information about transits of Venus]
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* [http://www.geody.com/?world=venus Geody Venus], a search engine for surface features
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* [http://www.worldwindcentral.com/wiki/Venus Maps of Venus in NASA World Wind]
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* [http://www.sil.si.edu/exhibitions/chasing-venus/intro.htm Chasing Venus, Observing the Transits of Venus] Smithsonian Institution Libraries
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* [http://www.lpi.usra.edu/resources/vc/vchome.shtml Venus Crater Database] Lunar and Planetary Institute
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* [http://aa.usno.navy.mil/data/docs/diskmap.php  Calculate/show the current phase of Venus] (U.S. Naval Observatory)
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* [http://www.astronomycast.com/astronomy/episode-50-venus/ Astronomy Cast: Venus]
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==Cartographic resources==
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* [http://pdsmaps.wr.usgs.gov/PDS/public/explorer/html/fmappick.htm  PDS Map-a-Planet]
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* [http://pdsmaps.wr.usgs.gov/PDS/public/explorer/html/fmappick.htm  Venus Nomenclature]
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* [http://planetologia.elte.hu/venusz-terkep-elte-ttk-kavucs.pdf  Map of Venus]
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Latest revision as of 10:12, 19 February 2013

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