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::''Main article: Venus '' Venus is the most prominent Astronomical feature in Earth's morning and evening sky (other than the Sun and Moon), and has been known since before recorded history. As Earth's closest neighbor, it was one of the first objects to be studied with scrutiny by the scientific community, and with the advent of Space Exploration , details about it are continuously being revealed. PRE-SPACEFLIGHT OBSERVATIONS Ancient cultures One of the oldest surviving Astronomical Documents , from the Babylon ian library of Ashurbanipal around 1600 BC , is a 21-year record of the appearances of Venus (which the early Babylonians called ''Nindaranna''). The ancient Sumeria ns and Babylonians called Venus ''Dil-bat'' or ''Dil-i-pat''; in Akkad ia it was the special star of the mother-god '' Ishtar ''; and in Chinese it is Jīn-xīng (金星), the planet of the Metal Element . Venus was considered the most important celestial body observed by the Maya , who called it ''Chak ek'', "the Great Star", possibly more important even than the Sun. The Mayans monitored the movements of Venus closely and observed it in daytime. The positions of Venus and other planets were thought to influence life on Earth, so Maya and other Ancient Mesoamerican Culture s timed wars and other important events based on their observations. In the Dresden Codex , the Maya included an almanac showing Venus's full cycle, in five sets of 584 days each (approximately eight years), after which the patterns repeated (since Venus has a Synodic Period of 583.92 days). At the half-full phase Venus is at greatest elongation — east of the Sun when an evening star and west of the Sun as a morning star. The precise angle the planet makes with the Sun at this time varies from approximately 45.0° to 47.8° depending on whether Earth and Venus are at Perihelion or Aphelion . This range is much smaller than that of Mercury because Venus's orbit is far less eccentric than Mercury's. Early Greeks thought that the evening and morning appearances of Venus represented two different objects, calling it ''Hesperus'' when it appeared in the western evening sky and ''Phosphorus'' when it appeared in the eastern morning sky. They eventually came to recognize that both objects were the same planet; Pythagoras is given credit for this realization. In the 4th Century BC , Heraclides Ponticus proposed that both Venus and Mercury orbited the Sun rather than Earth. Modern-era observations Because its Orbit takes it between the Earth and the Sun, Venus as seen from Earth exhibits visible Phase s in much the same manner as the Earth's Moon. Galileo Galilei was the first person to observe the phases of Venus in December 1610 , an observation which supported Copernicus 's then contentious Heliocentric description of the solar system. He also noted changes in the size of Venus's visible diameter when it was in different phases, suggesting that it was farther from Earth when it was full and nearer when it was a crescent. This observation strongly supported the heliocentric model. Venus (and also Mercury) is not visible from Earth when it is full, since at that time it is at Superior Conjunction , rising and setting concomitantly with the Sun and hence lost in the Sun's glare. Venus is brightest when approximately 25% of its disk is illuminated; this typically occurs 37 days both before (in the evening sky) and after (in the morning sky), its Inferior Conjunction . Its greatest elongations occur approximately 70 days before and after inferior conjunction, at which time it is half full; between these two intervals Venus is actually visible in broad daylight, if the observer knows specifically where to look for it. The planet's period of retrograde motion is 20 days on either side of the inferior conjunction. In fact through a telescope Venus at greatest elongation appears less than half full due to Schröter 's effect first noticed in 1793 and shown in 1996 as due to its thick atmosphere. On rare occasions, Venus can actually be seen in both the morning (before sunrise) and evening (after sunset) on the same day. This scenario arises when Venus is at its maximum separation from the Ecliptic and concomitantly at inferior conjunction; then one hemisphere (Northern or Southern) will be able to see it at both times. This opportunity presented itself most recently for Northern Hemisphere observers within a few days on either side of March 29 , 2001 , and for those in the Southern Hemisphere, on and around August 19 , 1999 . These respective events repeat themselves every eight years pursuant to the planet's synodic cycle. across the Sun .]] Transits Of Venus , when the planet crosses directly between the Earth and the Sun's visible disc, are rare astronomical events. The first time such a Transit was observed was on December 4 , 1639 by Jeremiah Horrocks and William Crabtree . A transit in 1761 observed by Mikhail Lomonosov provided the first evidence that Venus had an atmosphere, and the 19th-century observations of Parallax during its transits allowed the distance between the Earth and Sun to be accurately calculated for the first time. Transits can only occur either in early June or early December, these being the points at which Venus crosses the ecliptic (the orbital plane of the Earth), and occur in pairs at eight-year intervals, with each such pair more than a century apart. The previous pair of transits of Venus occurred in 1874 and 1882 , and the current pair is in 2004 and 2012 . In the 19th Century , many observers stated that Venus had a period of rotation of roughly 24 hours. Italian astronomer Giovanni Schiaparelli was the first to predict a significantly slower rotation, proposing that Venus was Tidally Locked with the Sun (as he had also proposed for Mercury). While not actually true for either body, this was still a reasonably accurate estimate. The near-resonance between its rotation and its closest approach to Earth helped to create this impression, as Venus always seemed to be facing the same direction when it was in the best location for observations to be made. The rotation rate of Venus was first measured during the 1961 conjunction, observed by radar from a 26 m antenna at Goldstone, California , the Jodrell Bank Radio Observatory in the UK , and the Soviet deep space facility in Eupatoria , Crimea . Accuracy was refined at each subsequent conjunction, primarily from measurements made from Goldstone and Evpatoriia. The fact that rotation was retrograde was not confirmed until 1964 . Before radio observations in the 1960s , many believed that Venus contained a lush, Earth-like environment. This was due to the planet's size and orbital radius, which suggested a fairly Earthlike situation as well as to the thick layer of clouds which prevented the surface from being seen. Among the speculations on Venus were that it had a junglelike environment or that it had oceans of either Petroleum or carbonated water. However, microwave observations in 1956 , by C. Mayer ''et al'', indicated a high-temperature source (600 K). Strangely, millimeter-band observations made by A. D. Kuzmin indicated much lower temperatures. Two competing theories explained the unusual radio spectrum, one suggesting the high temperatures originated in the ionosphere, and another suggesting a hot planetary surface. SPACE EXPLORATION Pre-''Magellan'' After the Moon, Venus was the second object in the Solar System to be explored by radar from the Earth. The first studies were carried out in 1961 at NASA 's Goldstone Observatory , part of the Deep Space Network . At successive Inferior Conjunctions , Venus was observed both by Goldstone and the National Astronomy And Ionosphere Center in Arecibo . The studies carried out were similar to the earlier measurement of transits of the Meridian , which had revealed in 1963 that the rotation of Venus was Retrograde (it rotates in the opposite direction to that in which it orbits the Sun). The radar observations also allowed astronomers to determine that the Rotation Period of Venus was 243.1 days, and that its Axis Of Rotation was almost perpendicular to its Orbital Plane . It was also established that the Radius of the planet was 6,052 kilometres (3761 mi), some 70 kilometres (43 mi) less than the best previous figure obtained with terrestrial telescopes. Interest in the Geological characteristics of Venus was stimulated by the refinement of imaging techniques between 1970 and 1985 . Early radar observations suggested merely that the surface of Venus was more compacted than the dusty surface of the Moon. The first radar images taken from the Earth showed very bright (radar-reflective) highlands christened Alpha Regio , Beta Regio, and Maxwell Montes ; improvements in radar techniques later achieved an image resolution of 1–2 kilometres. Since the beginning of the age of Space Exploration , Venus had been considered as a site for future landings. Launch Window s occur every 19 months, and from 1962 to 1985 , every window was utilized to launch reconnaissance probes. In 1962, '' Mariner 2 '' flew over Venus, becoming the first man-made object to visit another planet. In 1965, '' Venera 3 '' became the first space probe to actually Land on another world, albeit a crash-landing. In 1967, '' Venera 4 '' became the first probe to send data from the interior of Venus's atmosphere, while '' Mariner 5 '' measured the strength of Venus's Magnetic Field at the same time. Finally, in 1970, '' Venera 7 '' made the first controlled landing on Venus. In 1974, '' Mariner 10 '' swung by Venus on its way to Mercury and took ultraviolet photographs of the clouds, revealing the extraordinarily high wind speeds in the Venusian atmosphere. In 1975 , '' Venera 9 '' transmitted the first images of the surface of Venus and made Gamma Ray observations of rocks at the landing site. Later in that same year, '' Venera 10 '' would send further images of the surface. In 1978 , the '' Pioneer 12 '' probe (also known as ''Pioneer Venus 1'' or ''Pioneer Venus Orbiter'') circled Venus and completed the first Altimetry and Gravity maps of the planet, between 63 and 78 degrees of Latitude . The altimetry data had an accuracy of 100 metres (328 feet). That same year, ''Pioneer Venus 2'' launched four probes into Venus's atmosphere which determined, when combined with data from prior missions, that the surface temperature of the planet was approximately 460°C (860°F), and that the Atmospheric Pressure at the surface was 90 times that of Earth, confirming earlier radar observations. In 1981 , the Soviet '' Venera 13 '' sent the first colour image of Venus's surface and analysed the X-ray Fluorescence of an excavated soil sample. The probe operated for a record 127 minutes on the planet's hostile surface. Also in 1981, the '' Venera 14 '' lander detected possible Seismic activity in the planet's Crust . In 1983 , the '' Venera 15 '' and '' 16 '' probes further advanced the work of the ''Pioneer Venus Orbiter'', acquiring more precise radar images and altimetry data for the northern latitudes of the planet. The images had a 1–2 kilometre (0.6–1.2 mile) resolution, comparable to those obtained by the best Earth radars. The altimetry data obtained by the Venera missions had a resolution four times better than ''Pioneer'''s. In 1985 , during the euphoria of Halley's Comet , the Soviet Union launched two '' Vega '' probes to Venus. ''Vega 1'' and ''2'' each sent an instrumented helium balloon to a height of 50 kilometres (31 mi) above the surface, allowing scientists to study the Dynamics of the most active part of Venus's atmosphere. All of these probes gathered data critical for the success of the ''Magellan'' probe, which made the most detailed investigations of Venus's geology. ''Magellan'' Launched May 4 , 1989 aboard the space shuttle '' Atlantis '', the ''Magellan'' probe was first placed into low Earth orbit, before firing its upper stage motor to send it on a trajectory toward Venus. On August 10 , ''Magellan'' arrived at Venus and began to take images with radar. Each day it made 7.3 Venus orbits, imaging a strip 17–28 kilometres (11–17 mi) wide and 70,000 kilometres (43,496 mi) long. Covering the whole planet required 1,800 strips, which were combined into a single mosaic image. The first images of Venus were received on August 16 , 1990 , and routine mapping operations began on September 15 , 1990 . The first mapping cycle (Cycle 1) lasted 243 terrestrial days—the time it takes Venus to rotate on its own axis under the probe's orbital plane. Cycle 1 was completed successfully on May 15 , 1991 , mapping 84% of the Venusian surface. Cycle 2 began immediately afterwards and lasted until January 15 , 1992 . In each cycle, the probe was inclined at a different "look angle", producing stereoscopic data which enabled scientists to compile a three-dimensional map of the surface—a technique known as Synthetic Aperture Radar . Cycle 3 was due to finish on September 14 , 1992 , but was terminated a day early due to problems with onboard equipment. In total, radar coverage of 98% of the surface of Venus was obtained, with 22% of the images in stereo. ''Magellan'' produced surface images of unprecedented clarity and coverage, which are still unsurpassed. Cycles 4, 5 and 6 were devoted to collecting Gravimetric data, for which ''Magellan'' was Aerobrake d to its lowest possible stable orbit, with a Periapsis or closest approach of 180 kilometres (112 mi). At the end of Cycle 6 its orbit was reduced further, entering the outer reaches of the atmosphere. After carrying out a few final experiments, ''Magellan'' successfully completed its mission on October 11 , 1994 , and was de-orbited to burn up in Venus's atmosphere. Future missions LIST OF PROBES Successful missions
Failed missions
REFERENCES Planetary Missions at National Space Science Data Center (NASA) |
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