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JUPITER

Astronomy
Jupiter Jupiter`s magnitude varies from about –2.8 to –1.8 between opposition and conjunction, which usually makes it the fourth brightest object in the sky after the Sun, the Moon and Venus. At its closest approaches to Earth, Mars is brighter at –2.9 magnitude. Jupiter is the fifth planet from the Sun and the largest in the solar system. It is essentially a giant ball of approximately 86% hydrogen and 14% helium gas with small amounts of other compounds.

The planet has no solid surface. The atmosphere slowly graduates, getting denser with depth, until it becomes part of an “ocean” composed of liquid metallic hydrogen. There is no actual surface on which you could, for example, float a boat. Knowledge of the interior of Jupiter (and the other gas planets) is currently scant. It is thought there may be a small core of rocky material at the centre of the ocean.

Jupiter is 1,400 times the volume of Earth and 318 times the mass. Jupiter`s mass is in fact more than twice that of all the other planets combined. It has a diameter of 142,800 kilometres (more than 11 times that of Earth) and a mean distance from the Sun of over 778 million kilometres. At aphelion it is almost 816 million kilometres from the Sun and at perihelion it is just less than 741 million kilometres from the Sun.

Jupiter`s day is equal to 9.8 Earth hours – the time it takes to revolve around its axis. Different parts of Jupiter`s gases and liquids rotate at different velocities. Jupiter`s year is equal to 11.86 Earth years – the time it takes to orbit the Sun. The planet`s axis is tilted at just 3, which is not enough to cause seasons.

Jupiter has high velocity winds confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the different coloured bands that dominate the planet's appearance. The light bands are called zones and the dark ones belts.

Jupiters Giant Red Spot

Jupiter`s best known feature, the Great Red Spot, is 28,000 kilometres in length and 14,000 kilometres in width – large enough to contain a couple of Earths. This anti-cyclonic storm was first discovered in 1664 and continues to rage like a hurricane because it never contacts over land, where hurricanes loose most of their energy.

Unseen from Earth is a faint ring system around Jupiter with a diameter of 250,000 kilometres. The system is composed of an inner halo, a main ring estimated to be less than 30 kilometres in width and two gossamer rings. The rings were detected first in 1979 by the Voyager 1 spacecraft and then the Galileo probe.

Jupiters Moons
Jupiter has at least 63 known satellites, 23 of which were discovered as recently as 2003 and 2004. The largest four moons (Io, Europa, Ganymede and Callisto) were discovered by Galileo in 1610 and are known as the Galilean satellites. It was the first discovery of a centre of motion that was not the Earth. The Galilean moons can be easily seen playing around the planet through a pair of binoculars.
Io, Europa, Ganymede and Callisto
At a mean distance of 421,600 kilometres, Io orbits Jupiter closer than the Moon to Earth. It is the most volcanic body known in the solar system with lava lakes and flows, and sulphurous geysers reaching 500 kilometres high.

Europa, with a diameter of 3,130 kilometres – about the same as Earth`s Moon, is the smallest of the Galilean satellites. It orbits Jupiter at a mean distance of 670,900 kilometres and appears to be the only body in the solar system, other than Earth, which potentially harbours a global ocean of liquid water (hidden under a frozen surface).

Ganymede is Jupiter`s largest moon and, with a diameter of 5,262 kilometres, is also the largest in the solar system. It orbits its parent planet at a mean distance of 1,070,000 kilometres. Part of Ganymede`s terrain is grooved with ridges and troughs which form complex patterns, several hundred metres high and running for thousands of kilometres.

Callisto orbits Jupiter at a mean distance of 1,883,000 kilometres. It is Jupiter`s second largest moon, about the same size as Mercury. Callisto is the most heavily cratered satellite in the solar system with a crust thought to be 4 billion years old (shortly after the solar system was formed).

Jupiter Missions
Jupiter was first visited by NASA`s Pioneer 10, launched in 1972, which flew past Jupiter in 1973 and later became the first spacecraft to leave the solar system. It lost final contact in 2003 and will take more than 2 million years to pass Aldebaran, the nearest star on its trajectory.

Later fly-bys included Pioneer 11 in 1974 which went on to be the first spacecraft to visit Saturn and with which contact was lost in 1995, Voyager 1 and 2 in 1979 taking the first close-up images of the planet`s atmosphere and continuing into interstellar space, the solar probe Ulysses in 1992 and the Cassini probe en route for Saturn in 2000 which provided some very high resolution images.

The only spacecraft to orbit Jupiter to date was Galileo in 1995. Galileo conducted multiple flybys of the Galilean moons and, during its approach, witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter in 1994. A probe was released from the Galileo spacecraft in July 1995, which parachuted into the atmosphere collecting data for nearly an hour before being crushed by extreme pressure. The Galileo orbiter was also deliberately steered into the planet in September 2003 to avoid any possibility of a crash into and contamination of Europa.

Future missions include NASA`s Juno orbiter planned to launch in 2010 to go into polar orbit and study Jupiter in detail. In 2007 the New Horizons probe will fly-by Jupiter en route to Pluto. A proposed NASA mission planned to launch in 2012 to study Europa has been cancelled, but other NASA proposed NASA missions include:
2015 - Europa Geophysical Explorer. To examine Europa`s subsurface oceans and search for possible future landing sites.
2020 - A Jupiter fly-by with probes to study properties including gravitational and magnetic fields, atmosphere and gross dynamic and structural properties.
2035 - Europa Astrobiology Lander. A mission to study elemental astrobiology and geology on Europa.

Mythology
Jupiter, also known as Jove, was the Roman sky god, the equivalent of the Greek god Zeus. The cult of the Jupiter Optimus Maximus ("the best and greatest") began under the Etrucan kings, who were expelled from Rome around 507 BC. At first, Jupiter was associated with the elements, especially storms, and lightning, but he later became the protector of the Roman people and their powerful ally in war. The games held in the Circus in Rome were dedicated to him. Jupiter was the son of Saturn and the brother of Neptune.

Astrology
In terms of astrology, Jupiter is the planet of happiness, enthusiasm, good fortune, big-heartedness and success. In its positive form, Jupiter brings a zest for life and new experiences, good health, prominence, wealth, generosity, honesty and plain old good luck. In its negative form Jupiter's influence results in laziness, boastfulness, extravagance and blind, misdirected optimism.

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