Venus

:This article is about the planet. For the Roman mythological figure, see Venus (mythology); for other meanings, see Venus (disambiguation).

Observations and explorations of Venus

Historical observations

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. One of the oldest surviving astronomical documents, from the Babylonian library of Ashurbanipal around 1600 BC, is a 21-year record of the appearances of Venus (which the early Babylonians called Nindaranna). The ancient Sumerians and Babylonians called Venus Dil-bat or Dil-i-pat; in Akkadia 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.

Related Topics:
Astronomical - Astronomical documents - Babylon - Ashurbanipal - 1600 BC - Sumeria - Akkad - Ishtar - Chinese - Metal element

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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 cultures 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).

Related Topics:
Maya - Ancient Mesoamerican culture - Dresden Codex - Synodic period

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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.

Related Topics:
Perihelion - Aphelion - Mercury

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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.

Related Topics:
Greeks - Pythagoras - 4th century BC - Heraclides Ponticus

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Because its orbit takes it between the Earth and the Sun, Venus as seen from Earth exhibits visible phases 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.

Related Topics:
Orbit - Phase - Galileo Galilei - 1610 - Copernicus - Heliocentric - Superior conjunction

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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.

Related Topics:
Inferior conjunction - Schröter

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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.

Related Topics:
Ecliptic - March 29 - 2001 - August 19 - 1999

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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.

Related Topics:
Transits of Venus - Transit - December 4 - 1639 - Jeremiah Horrocks - William Crabtree - 1761 - Mikhail Lomonosov - Parallax - June - December - 1874 - 1882 - 2004 - 2012

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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 Evpatoriia. 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.

Related Topics:
19th century - Giovanni Schiaparelli - Tidally locked - 1961 - Goldstone, California - Jodrell Bank - UK - Soviet - Evpatoriia - 1964

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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.

Related Topics:
1960s - Petroleum - 1956

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Observation by spacecraft

There have been numerous unmanned missions to Venus. Several Russian probes have included a soft landing on the surface, with up to 110 minutes of communication from the surface, all without return.

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Early flybys

On February 12, 1961, the Soviet spacecraft Venera 1 was the first probe launched to another planet. An overheated orientation sensor caused it to malfunction, but Venera-1 was first to combine all the necessary features of an interplanetary spacecraft: solar panels, parabolic telemetry antenna, 3-axis stabilization, course-correction engine, and the first launch from parking orbit.

Related Topics:
February 12 - 1961 - Soviet spacecraft - Venera 1 - Probe

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The first successful Venus probe was the American Mariner 2 spacecraft, which flew past Venus in 1962. A modified Ranger Moon probe, it established that Venus has no magnetic field and measured the planet's thermal microwave emissions.

Related Topics:
American - Mariner 2 - 1962 - Ranger - Magnetic field

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The Soviet Union launched the Zond 1 probe to Venus on April 2, 1964, but it malfunctioned sometime after its May 16 telemetry session.

Related Topics:
Soviet Union - Zond 1 - April 2 - 1964

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Early landings

On March 1, 1966 the Venera 3 Soviet space probe crash-landed on Venus, becoming the first spacecraft to reach the planet's surface. Its sister craft Venera 2 had failed from overheating shortly before completing its flyby mission.

Related Topics:
March 1 - 1966 - Venera 3 - Space probe - Venera 2

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The descent capsule of Venera 4 entered the atmosphere of Venus on October 18, 1967. The first probe to return direct measurements from another planet, the capsule measured temperature, pressure, density and performed 11 automatic chemical experiments to analyze the atmosphere. It showed 95% carbon dioxide, and in combination with radio occultation data from the Mariner 5 probe, it showed that surface pressures were far greater than expected (75 to 100 atmospheres).

Related Topics:
Venera 4 - October 18 - 1967 - Mariner 5

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These results were verified and refined by the Venera 5 and Venera 6 missions on May 16 and 17 of 1969. But thus far, none of these missions had reached the surface while still transmitting. Venera 4's battery ran out while still slowly floating through the massive atmosphere, and Venera 5 and 6 were crushed by high pressure 18 km (60,000 ft) above the surface.

Related Topics:
Venera 5 - Venera 6 - May 16 - 17 - 1969

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The first successful landing on Venus was by Venera 7 on December 15, 1970. It relayed surface temperatures of 455°C to 475°C (855°F to 885°F). Venera 8 landed on July 22, 1972. In addition to pressure and temperature profiles, a photometer showed that the clouds of Venus formed a layer, ending over 22 miles above the surface. A gamma ray spectrometer analyzed the chemical composition of the crust.

Related Topics:
Venera 7 - December 15 - 1970 - Venera 8 - July 22 - 1972 - Gamma ray spectrometer

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Early orbiters

The Soviet probe Venera 9 entered orbit on October 22, 1975, becoming the first artificial satellite of Venus. A battery of cameras and spectrometers returned information about the planet's clouds, ionosphere and magnetosphere, as well as performing bistatic radar measurements of the surface.

Related Topics:
Venera 9 - October 22 - 1975

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The 300 lb descent vehicle separated from Venera 9 and landed, taking the first pictures of the surface and analyzing the crust with a gamma ray spectrometer and a densitometer. During descent, pressure, temperature and photometric measurements were made, as well as backscattering and multi-angle scattering (nephelometer) measurements of cloud density. It was discovered that the clouds of Venus are formed in three distinct layers. On October 25, Venera 10 arrived and carried out a similar program of study.

Related Topics:
Nephelometer - October 25 - Venera 10

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Pioneer Venus

In 1978, NASA sent two Pioneer spacecraft to Venus. The Pioneer mission consisted of two components, launched separately: an Orbiter and a Multiprobe. The Pioneer Venus Multiprobe carried one large and three small atmospheric probes. The large probe was released on November 16, 1978 and the three small probes on November 20. All four probes entered the Venus atmosphere on December 9, followed by the delivery vehicle. Although not expected to survive the descent through the atmosphere, one probe continued to operate for 45 minutes after reaching the surface. The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978. It carried 17 experiments and operated until the fuel used to maintain its orbit was exhausted and atmospheric entry destroyed the spacecraft in August 1992.

Related Topics:
1978 - NASA - Pioneer - Pioneer Venus Multiprobe - November 16 - November 20 - December 9 - Pioneer Venus Orbiter - December 4 - 1992

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Further Soviet successes

Also in 1978, Venera 11 and Venera 12 flew past Venus, dropping descent vehicles on December 21 and December 25 respectively. The landers carried colour cameras and a soil drill and analyzer, which unfortunately malfunctioned. Each lander made measurements with a nephelometer, mass spectrometer, gas chromatograph, and a cloud-droplet chemical analyzer using X-ray fluorescence that unexpectedly discovered a large proportion of chlorine in the clouds, in addition to sulfur. Strong lightning activity was also detected.

Related Topics:
1978 - Venera 11 - Venera 12 - December 21 - December 25 - Mass spectrometer - Gas chromatograph - X-ray fluorescence - Lightning

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Venera 13 and Venera 14 carried out essentially the same mission, arriving at Venus on March 1 and March 5, 1982.

Related Topics:
Venera 13 - Venera 14 - March 1 - March 5 - 1982

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This time, color camera and soil-drilling/analysis experiments were successful. X-ray fluorescence analysis of soil samples showed results similar to potassium-rich basalt rock.

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On October 10 and October 11, 1983, Venera 15 and Venera 16 entered polar orbits around Venus. Venera 15 analyzed and mapped the upper atmosphere with an infrared Fourier spectrometer. From November 11 to July 10, both satellites mapped the northern third of the planet with synthetic aperture radar. These results provided the first detailed understanding of the surface geology of Venus, including the discovery of unusual massive shield volcanoes such as coronae and arachnoids. Venus had no evidence of plate tectonics, unless the northern third of the planet happened to be a single plate.

Related Topics:
October 10 - October 11 - 1983 - Venera 15 - Venera 16 - Fourier spectrometer - November 11 - July 10 - Synthetic aperture radar - Arachnoids

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The Soviet Vega 1 and Vega 2 probes encountered Venus on June 11 and June 15 of 1985. Landing vehicles carried experiments focusing on cloud aerosol composition and structure. Each carried an ultraviolet absorption spectrometer, aerosol particle-size analyzers, and devices for collecting aerosol material and analyzing it with a mass spectrometer, a gas chromatograph, and an X-ray fluorescence spectrometer. The upper two layers of the clouds were found to be sulfuric acid droplets, but the lower layer is probably composed of phosphoric acid solution. The crust of Venus was analyzed with the soil drill experiment and a gamma ray spectrometer. As the landers carried no cameras on board, no images were returned from the surface.

Related Topics:
Vega 1 - Vega 2 - June 11 - June 15 - 1985

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The Vega missions also deployed balloon-borne aerostat probes that floated at about 53 km altitude respectively for 46 and 60 hours, traveling about 1/3 of the way around the planet. These measured wind speed, temperature, pressure and cloud density. More turbulence and convection activity than expected was discovered, including occasional plunges of 1 to 3 km in downdrafts. The Vega spacecraft continued to rendezvous with Halley's Comet nine months later, bringing an additional 14 instruments and cameras for that mission.

Related Topics:
Aerostat - Halley's Comet

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Magellan

On August 10, 1990, the US Magellan probe arrived at its orbit around the planet and started a mission of detailed radar mapping. 98% of the surface was mapped with a resolution of approximately 100 m. After a four-year mission, Magellan, as planned, plunged into the atmosphere on October 11, 1994, and partly vaporized; some sections are thought to have hit the planet's surface.

Related Topics:
August 10 - 1990 - Magellan probe - Radar - October 11 - 1994

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Recent flybys

Several space probes en route to other destinations have used flybys of Venus to increase their speed via the gravitational slingshot method. These include the Galileo mission to Jupiter and the Cassini-Huygens Mission to Saturn (two flybys). Rather curiously, during Cassini's examination of the radiofrequency emissions of Venus with its radio and plasma wave science instrument during both the 1998 and 1999 flybys, it saw absolutely no high-frequency radio waves (0.125 to 16 MHz), which are commonly associated with lightning. This is in direct opposition to the findings of the Soviet Venera missions 20 years earlier. It is postulated that perhaps if Venus does have lightning, it might be some type of low-frequency electrical activity, due to the fact that radio signals cannot penetrate the ionosphere at frequencies below about 1 megahertz. An examination by physicist Donald Gurnett of the University of Iowa of radio emissions of Venus by the Galileo spacecraft during its gravity assist flyby in 1990 did reveal what were interpreted at the time to be indicative of lightning. However the Galileo probe was over 60 times as distant to Venus as was Cassini during its flyby, making its observations substantially less significant. To this day it remains a mystery as to whether or not Venus does in fact have lightning in its atmosphere.

Related Topics:
Gravitational slingshot - Galileo - Jupiter - Cassini-Huygens Mission - Saturn - Radiofrequency

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Future missions

Venus Express is a mission prepared by the European Space Agency which will study the atmosphere and surface characteristics of Venus from orbit. The nominal mapping mission is planned to start in 2006 and is expected to last for two Venusian days (about 500 Earth days).

Related Topics:
Venus Express - European Space Agency - 2006

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Future flybys en route to other destinations include the MESSENGER and BepiColombo missions to Mercury.

Related Topics:
MESSENGER - BepiColombo

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Proposals

To overcome the hellish surface conditions, a team led by Geoffrey Landis of NASA's Glenn Research Center in Ohio has proposed http://www.newscientist.com/article.ns?id=dn7354 a Venus Rover mission that includes a tough surface rover in communication with a solar-powered aircraft. The aircraft would carry the mission's sensitive electronics in the relatively mild temperatures of Venus' upper atmosphere.

Related Topics:
Geoffrey Landis - NASA - Glenn Research Center - Ohio

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Landis also makes a case for Venus as a target for human colonization. At 50km above the surface, the temperature range is 0-50°C, the air pressure drops to 1 atmosphere, the gravity is 0.9 that of Earth, and the resources for life are plentiful (see Venus Related Papers at http://www.sff.net/people/Geoffrey.Landis/papers.html).

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