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Jupiter (a.k.a. Jove; Greek Zeus) was the king of the gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus (Saturn).

Jupiter is the fifth planet from the Sun and by far the largest in the solar system, more than twice as massive as all the other planets combined (318 times Earth). It is a gas giant.

Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter). It has been known since prehistoric 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's heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition.

Jupiter is composed of about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. Jupiter has a faint planetary ring system composed of dust particles.

A number of probes have visited Jupiter, all of them American in origin. Pioneer 10 flew past Jupiter in december of 1973, followed by Pioneer 11 exactly one year later. Voyager 1 flew by in 1977 and Voyager 2 in 1979. The Galileo probe went into orbit around Jupiter in 1998(?), dropping a smaller subprobe into Jupiter's atmosphere and conducting multiple flybys of all of the Galilean moons. The Galileo probe also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet, giving a unique vantage point for this spectacular event.

One of Jupiter's most distinctive features is the Great Red Spot, a large hurricane coloured by reddish methane-rich gasses welling up from lower in the Jovian atmosphere. The Great Red Spot is remarkably stable, having first been spotted by Galileo over 300 years ago.

Jupiter's moons fall into four major groups:

  1. The inner group were all discovered during the Voyager project (except for Amalthea), all have diameters of less than 200 km and orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.
  2. The Galilean moons were all discovered by Galileo Galilei, orbit between 400,000 and 2,000,000 km, and include the largest moons in the solar system.
  3. The third group were all discovered in the 20th century but before Voyager, have diameters less than 200 km, and orbit between 11,000,000 and 12,000,000 km with an orbital inclination between 26° and 29°.
  4. The outer moons were also discovered in the 20th century before Voyager, but have diameters under 50 km and orbit between 21,000,000 and 24,000,000 km. They are particularly notable for having retrograde orbits with inclinations between 147° and 163°.

It is thought that the three 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.

The Jovian moons in order of increasing orbital radius:

GroupNameDiameter (km)Orbital radius (km)Orbital period
1Metis 40 127,600 7.08 hours
Adrastea 20 (23 x 20 x 15) 134,000 7.11 hours
Amalthea 189 (270 x 166 x 150)181,30011.92 hours
Thebe 100 (100 x 90) 222,000 16.23 hours
2Io 3632 421,600 1.76 days
Europa 3126 670,900 3.55 days
Ganymede 5262 1,070,000 7.16 days
Callisto 4820 1,883,000 16.69 days
3Leda 16 11,100,000238.7 days
Himalia 18611,470,000250.6 days
Lysithea 36 11,710,000259.2 days
Elara 76 11,743,000259.7 days
4Ananke 30 20,700,000617 days
Carme 4022,350,000692 days
Pasiphae 50 23,300,000735 days
Sinope 36 23,700,000758 days

All Jovian moons are tidally locked with Jupiter, and therefore have the same rotational period as their orbital period.

(Some information pasted from http://nineplanets.org/ using google for search. Additional information added since then.)

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets