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Jupiter is the fifth planet from the Sun and by far the largest within our solar system; some have described the solar system as consisting of the Sun, Jupiter, and assorted debris. It and the other gas giants Saturn, Uranus, and Neptune are sometimes referred to as "Jovian planets." The Romans named the planet after the Roman god Jupiter. The astronomical symbol for the planet is a stylized representation of the god's lightning bolt. The Chinese, Korean, and Japanese cultures refer to the planet as the Wood Star, based on the Five Elements.

Overview

Jupiter has been known since ancient times and is visible to the naked eye in the night sky. In 1610, Galileo Galilei discovered the four largest moons of Jupiter using a telescope, the first observation of moons other than Earth's.

Jupiter is 2.5 times more massive than all the other planets combined, so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). It is 318 times more massive than Earth, with a diameter 11 times that of Earth, and with a volume 1300 times that of Earth. It has been termed by many a "failed star", even though the comparison would be akin to calling an asteroid "a failed Earth". As impressive as it is, extrasolar planets have been discovered with much greater masses. However, it is thought to have about as large a diameter as a planet of its composition can, as adding extra mass would only result in further gravitational compression (until ignition occurs). There is no clear-cut definition of what distinguishes a large and massive planet such as Jupiter from a brown dwarf, although the latter possesses rather specific spectral lines, but in any case Jupiter would need to be about seventy times as massive if it were to become a star.

Jupiter also has the fastest rotation rate of any planet within the solar system, making a complete revolution on its axis in slightly less than ten hours, which results in a flattening easily seen through an Earth-based amateur telescope. Its best known feature is probably the Great Red Spot, a storm larger than Earth. The planet is perpetually covered with a layer of clouds.

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus; however at times Mars appears brighter than Jupiter, while at others Jupiter appears brighter than Venus). It has been known since ancient times. Galileo Galilei's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) was the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition.

Physical Characteristics

Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, surrounded by liquid hydrogen, which is surrounded by gaseous hydrogen. There is no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.

The atmosphere contains trace amounts of methane, water vapour, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.This atmospheric composition is very close to the composition of the solar nebula. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Jupiter's upper atmosphere undergoes differential rotation, an effect first noticed by Giovanni Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes flow in opposing directions on the prevailing winds. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot.



The Great Red Spot is an anticyclonic storm on the planet Jupiter, 22° south of the equator; which has lasted at least 300 years. The storm is large enough to be visible through Earth-based telescopes. It was first observed either by Cassini or Hooke around 1665.
This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 5.7 million miles (9.2 million kilometers) from Jupiter. Cloud details as small as 100 miles (160 kilometers) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.

Storms such as this are not uncommon within the atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last hours or centuries.

It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the colour may be caused by any of "complex organic molecules, red phosphorus, or yet another sulfur compound", but a consensus has yet to be reached.

The Great Red Spot is remarkably stable, having first been spotted over 300 years ago. Several factors may be responsible for its longevity, such as the fact that it never encounters solid surfaces over which to dissipate its energy and that its motion is driven by Jupiter's internal heat. Simulations suggest that the Spot tends to absorb smaller atmospheric disturbances.

At the start of 2004, the Great Red Spot is approximately half as large as it was 100 years ago. It is not known how long the Great Red Spot will last, or whether this is a result of normal fluctuations.

The Great Red Spot should not be confused with the Great Dark Spot, famously seen in the atmosphere of Neptune by Voyager 2 in 1989. The Great Dark Spot was an atmospheric hole, not a storm, and was no longer present as of 1994 (although another, similar spot had appeared farther to the north).

On October 19, 2003 a black spot was photographed on Jupiter by Belgian astronomer Olivier Meeckers. Although not an unusual occurrence, this one caught the fantasy of some science fiction fans and conspiracy theorists, who went as far as speculating that the spot was evidence of nuclear activity on Jupiter, caused by Galileo's crash into the planet a month prior. Galileo carried about 15.6 kg of plutonium-238 as its power source, in the form of 144 pellets of plutonium dioxide, a ceramic. The individual pellets (which would be expected to separate during entry) initially contained about 108 grams of 238Pu each (about 10% would have decayed away by the time Galileo entered Jupiter), and are short of the required critical mass by a factor of about 100.

Planetary Rings

Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked from its moons by meteor impacts. The main ring is made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. There is also an extremely tenuous and distant outer ring that circles Jupiter backwards. Its origin is uncertain, but this outer ring might be made of captured interplanetary dust.

Magnetosphere

Jupiter has a very large and powerful magnetosphere. In fact, if you could see Jupiter's magnetic field from Earth, it would appear five times as large as the full moon in the sky despite being so much farther away. This magnetic field collects a large flux of particle radiation in Jupiter's radiation belts, as well as producing a dramatic gas torus and flux tube associated with Io. Jupiter's magnetosphere is the largest planetary structure in the solar system.

The Pioneer probes confirmed the existence that Jupiter's enormous magnetic field is 10 times stronger than Earth's and contains 20,000 times as much energy. The sensitive instruments aboard found that the Jovian magnetic field's "north" magnetic pole is at the planet¹s geographic south pole, with the axis of the magnetic field tilted 11 degrees from the Jovian rotation axis and offset from the center of Jupiter in a manner similar to the axis of the Earth's field. The Pioneers measured the bow shock of the Jovian magnetosphere to the width of 26 million kilometres (16 million miles), with the magnetic tail extending beyond Saturn¹s orbit.

The data showed that the magnetic field fluctuates rapidly in size on the sunward side of Jupiter because of pressure variations in the solar wind, an effect studied in further detail by the two Voyager spacecraft. It was also discovered that streams of high-energy atomic particles are ejected from the Jovian magnetosphere and travel as far as the orbit of the Earth. Energetic protons were found and measured in the Jovian radiation belt and electric currents were detected flowing between Jupiter and some of its moons, particularly Io.

Exploration of Jupiter

Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. They provided important new data about Jupiter's magnetosphere, and took some low resolution photographs of the planet.

Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved our understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close up images of the planet's atmosphere.

In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of 900,000 km (6.3 Jovian radii). The flyby was required to attain a polar orbit around the Sun. The probe conducted studies on Jupiter's magnetosphere. Since there are no cameras onboard the probe, no images were taken. In February 2004, the probe came again in the vicinity of Jupiter. This time distance was much greater, about 240 million km.

So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter in December 7, 1995. It orbited the planet for over seven years and conducted multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event. However, the information gained about the Jovian system from the Galileo mission was limited by the failed deployment of its high-gain radio transmitting antenna.

An atmospheric probe was released from the spacecraft in July, 1995. The probe entered the planet's atmosphere in December 7, 1995. It parachuted through 150 km of the atmosphere, collecting data for 58 minutes, before being crushed by the extreme pressure to which it was subjected. It would have then quickly melted and vaporized. The Galileo orbiter itself underwent a more rapid version of the same fate when it was deliberately crashed into the planet on September 21, 2003 at a speed of over 50 km/s, in order to avoid any possibility of it crashing into and possibly contaminating Europa, one of the Jovian moons.

In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet.

NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2010.After the discovery of a liquid ocean on Jupiter's moon Europa, there has been great interest to study the icy moons in detail.

A mission proposed by NASA was dedicated to study them. The JIMO (Jupiter Icy Moons Orbiter) was expected to be launched sometime after 2012. However, the mission was deemed too ambitious and its funding was cancelled.

In 2007, Jupiter will also be briefly visited by the New Horizons probe, en route to Pluto.

Jupiter's Moons

Jupiter has at least 63 moons. For a complete listing of these moons, please see Jupiter's natural satellites. For a timeline of their discovery dates, see Timeline of natural satellites.The four large moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. Without this resonance, tidal forces would tend to circularize the moons' orbits over time.

The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretches the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.

Classification of Jupiter's moons

It used to be thought that Jupiter's moons were arranged neatly into four groups of four, but recent discoveries of many new small outer moons have complicated the division; there are now thought to be six main groups, although some are more distinct than others.

1. The inner group of four small moons all have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.
2. The four Galilean moons were all discovered by Galileo Galilei, orbit between 400,000 and 2,000,000 km, and include some of the largest moons in the solar system.

3. Themisto is in a group of its own, orbiting halfway between the Galilean moons and the next group.

4. The Himalia group is a tightly clustered group of moons with orbits around 11-12,000,000 km from Jupiter.

5. Carpo is another isolated case; at the inner edge of the Ananke group, it revolves in the direct sense.

6. The Ananke group is a group with rather indistinct borders, averaging 21,276,000 km from Jupiter with an average inclination of 149 degrees.

7. The Carme group is a fairly distinct group that averages 23,404,000 km from Jupiter with an average inclination of 165 degrees.

8. The Pasiphaë group is a disperse and only vaguely distinct group that covers all the outermost moons.

It is thought that the groups of smaller moons may each have a common origin, perhaps as a larger moon or captured body that broke up into the existing moons of each group.

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Venus 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 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.

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 clouds of carbon dioxide, 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. The atmospheric pressure at the planet's surface is 90 times that of the Earth.

Venus' surface has been mapped in detail only in the last 20 years. It shows evidence of extensive volcanism, and some of its volcanoes may still be active today. Venus is thought to undergo periodic episodes of plate tectonics, in which the crust is subducted rapidly within a few million years, separated by stable periods of a few hundred million years.

The planet is named after Venus, the Roman goddess of love; most of its surface features are named after famous and mythological women. 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, although two dwarf planets - Ceres and Eris - also have female names.

Structure

Venus is one of the four solar terrestrial planets, 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 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.

Internal structure

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 core, mantle, and 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.

Geography

About 80% of Venus' 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 Babylonian 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 km above Venus' 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.

As well as the impact craters, 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 50 km across, and 100 1000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin.

Almost all Venusian surface features are named after historical and mythological women. 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.

Surface Geology

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. The only volcanic complex of this size on Earth is the Big Island of Hawaii. 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 plates, and has an average age of about 100 million years, while Venus' surface is estimated to be about 500 million years old.

Several lines of evidence point to ongoing volcanic activity on Venus. During the Russian 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. While rainfall drives thunderstorms on Earth, there is no rainfall on Venus. 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.

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. 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.

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. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater.

Atmosphere

Venus has an extremely thick atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The pressure at the planet's surface is about 90 times that at Earth's surface - a pressure equivalent to that at a depth of 1 kilometer under Earth's oceans. The enormously CO2-rich atmosphere generates a strong greenhouse effect that raises the surface temperature to over 400 °C (752°F). This makes Venus' surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and receives only 25% of the solar irradiance.

Studies have suggested that several billion years ago Venus' 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. Venus is thus an extreme example of climate change, making it a useful tool in climate change studies.

Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of Venus' 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' surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface.

Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets. These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of Venus' 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 heated or 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 km/h winds at the cloud tops circle the planet about every four to five earth days.

Magnetic Field and Core

In 1980, The Pioneer Venus Orbiter found that Venus' 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, 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.

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 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. This implies that the dynamo is missing because of a lack of convection in Venus' 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.

Orbit and Rotation

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, with an eccentricity of less than 1%. 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. 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. The Sun would appear to rise in the west and set in the east. At the equator, Venus' surface rotates at 6.5 km/h; on Earth, the rotation speed at the equator is about 1,600 km/h.

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 anticlockwise, Venus rotates clockwise in "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, tidal effects on its dense atmosphere could have slowed down its initial rotation to the value seen today.

A curious aspect of Venus' 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.

Venus is currently moonless, though the asteroid 2002 VE68 presently maintains a quasi-orbital relationship with it. 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.

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 spiral inward 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.

Observation

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, Venus is a commonly misreported 'unidentified flying object'. In 1973, future U.S. President Jimmy Carter 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.

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 is brightest when it is a thin crescent; it is much closer to Earth when a thin crescent than when gibbous, or full.

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. 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.

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.

Studies of Venus
Early Studies

Venus was known in the Hindu Jyotisha since early times as the 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 espoused the view that Venus orbited the Earth. When Galileo first observed the planet in the early 17th century, he found that it showed 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.

Venus' 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. The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Giovanni Cassini and Schroter incorrectly estimated periods of about 24 hours from the motions of markings on the planet's apparent surface.

Ground-based Research

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 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.

Spectroscopic observations in the 1900s gave the first clues about Venus' 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. 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.

Radar observations in the 1970s revealed details of Venus' surface for the first time. Pulses of radio waves were beamed at the planet using the 300 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. These three features are now the only ones on Venus which do not have female names.

The best radar images obtainable from Earth revealed features no smaller than about 5 km across. More detailed exploration of the planet could only be carried out from space.

Exploration of Venus
Early Efforts

The first robotic space probe mission to Venus, and the first to any planet, began on 12 February 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 14 December 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.

Atmospheric Entry

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.[40] The joint Venera 4 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.

Surface Science

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 15 December 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 30-40 cm across; Venera 10 showed basalt-like rock slabs interspersed with weathered material.

In 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.

Radar mapping

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 4 and a half 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.

Current and Future Missions

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.

Japan's aerospace body JAXA is planning to launch its Venus climate orbiter, the PLANET-C, in 2010. Future flybys en route to other destinations include the MESSENGER and BepiColombo missions to Mercury.

Venus in Human Culture
Historic Connections

Babylonians:
One of the brightest objects in the sky, Venus has been known since prehistoric times and has had a significant impact on human culture from the earliest days. It is described in Babylonian cuneiformic texts such as the Venus tablet of Ammisaduqa, which relates observations that possibly date from 1600 BC. The Babylonians named the planet Ishtar (Sumerian Inanna), the personification of womanhood, and goddess of love. The Ancient Egyptians 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 Greeks called the morning star, Phosphoros (Latinized Phosphorus), the "Bringer of Light" or Eosphoros (Latinized Eosphorus), the "Bringer of Dawn". The evening star they called Hesperos (Latinized Hesperus) (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. The Romans would later name the planet in honor of their goddess of love, Venus, whereas the Greeks used the name of her Greek counterpart, Aphrodite (Phoenician Astarte). 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 in Akkadia it was the special star of the mother-god Ishtar; and in Chinese it is Jin-xing, the planet of the metal element. In India, Venus is called Shukra Graha (the planet Shukra) which is named after a powerful saint Shukra. The word 'Shukra' is also associated with semen, or generation.

Hebrews:
To the Hebrews it was known as Noga ("shining"), Helel ("bright"), Ayeleth-ha-Shakhar ("deer of the dawn") and Kochav-ha-'Erev ("star of the evening").

Maya:
Venus was important to the Maya civilization, who developed a religious calendar based in part upon its motions, and held the motions of Venus to determine the propitious time for events such as war. 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). 2012

Maasai:
The Maasai people named the planet Kileken, and have an oral tradition about it called The Orphan Boy. In western astrology, derived from its historical connotation with goddesses of femininity and love, Venus is held to influence those aspects of human life.

India:
In Indian Vedic astrology, Venus is known as Shukra (Hindi:, 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.

China:
Early Chinese astronomers called the planet Tai-pe, or the "beautiful white one". Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the metal star (Chinese), based on the Five elements. Lakotan spirituality refers to Venus as the daybreak star, and associates it with the last stage of life and wisdom.

Australia:
Venus is important in many 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

Greece:
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.

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Saturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. Saturn has large rings consisting of mostly ice particles with a smaller amount of rocky debris. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle.

Physical Characteristics

Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water.

Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made. Saturn exhibits long-lived ovals and other features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994 another, smaller storm was observed. Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only planet in the solar system known to do so.

Rotational Behaviour

Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it, like in Jupiter's case: System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close in value to System II, it has largely superseded it.

While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). [2] The cause of the change is unknown.

Saturn's Rings

Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system. See rings of Saturn for a list of the planet's rings.

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one [Saturn itself] is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo.

The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time.

In 1675 Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Edouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces. A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.

While the largest gaps in the rings, such as the Cassini division and Encke division, could be seen from Earth, the Voyagers discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways.

Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be undiscovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings actually consists of spiral waves raised by the moons' periodic gravitational perturbations.

Data from the Cassini space probe indicates that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen.

The side of Saturn's rings that is lit by the Sun looks very different to the backlit side, which is darker overall and appears almost black in the thick B ring. From Earth, we cannot appreciate this because the Earth cannot view Saturn from an angle that displays the backlit side of the rings, and our only views of it are from spacecraft. In 2004, the Cassini spacecraft revealed the first views of the backlit side in 25 years.

Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found dark radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism behind the spokes is still unknown.

As of February 2005, the Cassini spacecraft has not observed any spokes in the rings, despite possessing imaging equipment of higher quality than the Voyagers'. It is possible that the spokes appear and disappear seasonally.

Saturn's moons

Saturn has a large number of moons, 49 are currently confirmed, 34 of which have names. The precise figure will never be certain as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere.

Due to the tidal forces of Saturn, the moons are currently not at the same position as they were when they were first formed.

Exploration of Saturn

Saturn was first visited by Pioneer 11 in 1979. It flew within 20,000 km the planet's cloudtops. Low-resolution images were acquired of the planet and few of its moons. Resolution was not good enough to discern surface features, however. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. It also measured the temperature of Titan.

In November, 1980, Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and the satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths, so no surface details were seen. The flyby also changed spacecraft's trajectory out from the plane of the solar system.

Almost a year later, in August, 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's camera stuck and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell and Keeler gaps.

On July 1, 2004 the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI Cassini had already studied the system extensively. In June, 2004, it had conducted a close flyby of Phoebe sending back high-resolution images and data. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005 sending flood of data during the atmospheric descent and after the landing. As of 2005, Cassini is conducting multiple flybys of Titan and icy satellites. The primary mission ends in 2008 when the spacecraft has completed 74 orbits around the planet.

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