Introduction

Jupiter is the largest and most massive planet in the solar system. It is known since pre-historic times as one of the brightest celestial bodies. In mythology , Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods and the patron of the Roman state.

Jupiter is the fifth planet from the Sun and the first of the gas-giant planets (the others are Saturn, Uranus and Neptune). It orbits the Sun at a mean distance of 5.203 AU (7.783e8 km), with an orbital period of 11.86 years and a mean orbital velocity of 13.1 km/s.

Jupiter is huge, as can be inferred from the following data:    

• mass:    1.90e27 kg (318 times Earth's mass and about 2.5 times the mass of all other planets, satellites, asteroids, meteoroids and comets in the solar system)
• equatorial diameter: 142,980 km (11.21 times Earth's diameter)
• polar diameter: 133,700 km (10.44 times Earth's diameter) 
• mean density: 1330 kg/m3 (less than 1/4 Earth's mean density)

Unlike the inner planets (Mercury, Venus, Earth and Mars), Jupiter consists mostly of hydrogen (82%), helium (17%) and about 1% of other heavier elements. This composition is similar to the one of the Sun. This is consistent with the theory that Jupiter was formed by condensation of light elements form primordial solar nebula. Jupiter's atmosphere Jupiter's atmosphere contains approximately 86% hydrogen, 14% helium and traces of methane, ammonia, water vapor and other gases. Because of the planet's rapid rotation, the weather patterns in the Jovian atmosphere are extremely complicated.

Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the core of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the core of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's core hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices".

Jupiter's magnetosphere is enormous and it's surrounded by a shock wave, where the supersonic solar wind is unexpectedly reduced to subsonic speeds. Most of the particles of the solar wind are detected around Jupiter in an unstable region. Particles trapped inside Jupiter's magnetosphere are spewed out into a vast current sheet by the planet rapid rotation.

Jupiter rotates very rapidly around its axis and so its shape is like a flattened sphere (ellipsoid of revolution). Even with small telescopes the difference between the equatorial and polar diameters can be discerned. The mean equatorial rotation period (calculated from the motion of the clouds) is 9h 50m 28s and internal rotation period (rotation period of the magnetic field) is approximately 9h 55m 30s. 

Jupiter has 39 known satellites , most notably the four large Galilean moons (Io, Europa, Ganymede and Callisto), discovered by Galileo in 1610. In fact the Jovian system is like a solar system in miniature 

Recently, it was discovered that Jupiter has rings ( Halo, Main and Gossamer). These are probably composed of very small particles of rocky material and unlike Saturn's rings they are dark and contain no ice.

Jupiter has been explored since pre-historic times and by many spacecrafts.