Venus

 Image Courtesy NASA/JPL-Caltech

Venus is the second-closest planet to the Sun and is classified as a terrestrial planet. It is sometimes called Earth's "sister planet", for the two planets are very similar in size and gravity. Venus is covered with an layer of highly reflective clouds of sulfuric acid. This makes it practically impossible to view the surface of venus.
Venus has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide. The atmospheric pressure at the planet's surface is about 92 times that of the Earth. Venus' surface has been mapped in detail in the last 20 years by Project Magellan. Most of the surface consists of smooth volcanic plains. Two 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 highest mountain on Venus, lies on the northern hemisphere. It's peak is 11 km above Venus' average surface elevation. The southern continent is covered by a network of fractures and faults.
Much of Venus' 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 km across.
There are almost 1,000 impact craters on Venus, more or less evenly distributed across its surface about 85% of craters are in pristine condition. Venusian craters range from 3 km to 280 km in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. In 1980, The Pioneer Venus Orbiter found that Venus' magnetic field is both weaker and smaller than Earth's. What small magnetic field is present is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo in the core like the one inside the Earth.
Venus' magnetosphere is too weak to protect the atmosphere from cosmic radiation. A dynamo like core requires three things: a conducting liquid, rotation, and convection. Either one of these conditions must be true to generate an intrinsic magnetic field. Venus is believed to lack one or more of those conditions

Venus orbits the Sun at an average distance of about 108 million km, and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus is the closest to circular. 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 million km. The planet reaches inferior conjunction every 584 days, on average. Venus rotates once every 243 days by far the slowest rotation period of any of the major planets. 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" rotation.


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.
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, the other inferior planet, reaches a maximum 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. As it moves around its orbit, Venus displays 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 crescent when it is at its maximum elongations from the Sun.

Venus' 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.


Mariner 2, launched in 1962The first 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 million km from Earth. It was estimated to have passed within 100,000 km from Venus in mid-May.

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 km above the surface of Venus. Its microwave and infrared radiometers revealed that while Venus' cloud tops were cool, the surface was extremely hot at least 425 °C, finally ending any hopes that the planet might harbor ground-based life. Mariner 2 also obtained improved estimates of Venus' mass and of the astronomical unit, but was unable to detect either a magnetic field or radiation belts.

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. Venus' 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 °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 minutes, Venera 4's last pressure reading was 18 bar at an altitude of 24.96 km.

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 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' atmosphere. The joint Venera 4 and 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.

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 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 atmospheres, neither were 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 km before going on to strike the surface on the night side of Venus.


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 bar. The module was pre-cooled prior to entry and equipped with a specially 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 minutes, the first telemetry received from the surface of another planet.

The Venera program continued with Venera 8 sending data from the surface for 50 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 to 40 cm across; Venera 10 showed basalt-like rock slabs interspersed with weathered material.


The Pioneer Venus orbiterIn the meantime, the United States had sent the Mariner 10 probe on a gravitational slingshot trajectory past Venus on its way to Mercury. On February 5, 1974, Mariner 10 passed within 5790 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.

The American Pioneer Venus project consisted of two separate missions. 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.

Four more Venera lander missions took place over the next four years, with Venera 11 and Venera 12 detecting Venusian electrical storms; 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 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. Venera 14, though, had the misfortune to strike its own ejected camera lens cap and its probe failed to make contact with the soil. 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.

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 km, where pressure and temperature are comparable to those at Earth's surface. They remained operational for around 46 hours, and discovered that the Venusian atmosphere was more turbulent than previously believed, and subject to high winds and powerful convection cells.

A manned Venus flyby mission, using Apollo program hardware, was proposed in the late 1960s. 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.

The United States' Magellan probe was launched on May 4, 1989 with a mission to map the surface of Venus with radar. 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 imaged over 98% of Venus' surface by radar and mapped 95% of its gravity field. In 1994, at the end of its mission, 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 and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan would otherwise be the last dedicated mission to Venus for over a decade.

The Venus Express probe was designed and built by the European Space Agency. Launched by the Russian Federal Space Agency on November 9, 2005, it successfully assumed a polar orbit around Venus on April 11, 2006.

The probe is undertaking a detailed study of the Venusian atmosphere and clouds, and will also map the planet's plasma environment and surface characteristics, particularly temperatures. Its mission is intended to last a nominal 500 Earth days, or around two Venusian years. One of the first results emerging from Venus Express is the discovery that a huge double atmospheric vortex exists at the south pole of the planet.

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. The European Space Agency (ESA) will also launch a mission to Mercury, called BepiColombo, in August 2013 that will perform two flybys of Venus before it reaches Mercury orbit in 2019.

Future dedicated missions to Venus are also being planned. Japan's aerospace body JAXA is planning to launch its Venus climate orbiter, the PLANET-C, in 2010. Under its New Frontiers Program, NASA has proposed a lander mission called the Venus In-Situ Explorer, which would launch in 2013 and land on Venus to study surface conditions and investigate the elemental and mineralogical features of the regolith. The lander will also be 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.

The diameter of Venus is only 650 km less than the Earth's, and its mass is 81.5% of the Earth's.  The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% composed of nitrogen.

Facts:

Discovered By
  Known by the Ancients
 
  
Date of Discovery
  Unknown
 
  
Average Distance from the Sun
  Metric: 108,208,930 km
English: 67,237,910 miles
Scientific Notation: 1.0820893 x 108 km (.723332 A.U.) 
By Comparison: 0.723 x Earth
 
  
Perihelion (closest)
  Metric: 107,476,000 km
English: 66,782,000 miles
Scientific Notation: 1.07476 x 108 km (0.718 A.U.) 
By Comparison: 0.730 x Earth
 
  
Aphelion (farthest)
  Metric: 108,942,000 km
English: 67,693,000 miles
Scientific Notation: 1.08942 x 108 km (0.728 A.U.) 
By Comparison: 0.716 x Earth
 
  
Equatorial Radius
  Metric: 6,051.8 km
English: 3,760.4 miles
Scientific Notation: 6.0518 x 103 km
By Comparison: 0.9488 x Earth
 
  
Equatorial Circumference
  Metric: 38,025 km
English: 23,627 miles
Scientific Notation: 3.8025 x 104 km
 
  
Volume
  Metric: 928,400,000,000 km3
Scientific Notation: 9.284 x 1011 km3
By Comparison: 0.88 x Earth's
 
  
Mass
  Metric: 4,868,500,000,000,000,000,000,000 kg
Scientific Notation: 4.8685 x 1024 kg
By Comparison: 0.815 x Earth
 
  
Density
  Metric: 5.24 g/cm3
By Comparison: Comparable to the average density of the Earth.
 
  
Surface Area
  Metric: 460,200,000 km2
English: 177,700,000 square miles
Scientific Notation: 4.602 x 108 km2
By Comparison: 0.902 x Earth
 
  
Equatorial Surface Gravity
  Metric: 8.87 m/s2
English: 29.1 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 91 pounds on Venus.
 
  
Escape Velocity
  Metric: 37,300 km/h
English: 23,200 mph
Scientific Notation: 1.036 x 104 m/s
By Comparison: 0.927 x Earth
 
  
Sidereal Rotation Period (Length of Day)
  -243 Earth days (retrograde)
-5832 hours (retrograde)
By Comparison: 244 x Earth
 
  
Sidereal Orbit Period (Length of Year)
  0.615 Earth years 
224.7 Earth days 
By Comparison: 0.615 x Earth
 
  
Mean Orbit Velocity
  Metric: 126,077 km/h
English: 78,341 mph
Scientific Notation: 35,021.4 m/s
By Comparison: 1.176 x Earth
 
  
Orbital Eccentricity
  .0068
By Comparison: 0.405 x Earth
 
  
Orbital Inclination to Ecliptic
  3.39 degrees
 
  
Equatorial Inclination to Orbit
  177.3 degrees
By Comparison: 7.56 x Earth
 
  
Orbital Circumference
  Metric: 675,300,000 km
English: 419,600,000 miles
Scientific Notation: 6.753 x 108 km
By Comparison: 0.731 x Earth
 
  
Minimum/Maximum Surface Temperature
  Metric: 462 °C
English: 864 °F
Scientific Notation: 735 K
 
  
Atmospheric Constituents
  Carbon Dioxide, Nitrogen
Scientific Notation: CO2, N2 

 

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