Jupiter is the largest world in our solar system; four of its moons are the size of planets. It is different in structure from the solid inner planets. A part from a small rocky core, Jupiter is mainly hydrogen and helium. Below the cloudy atmosphere, the pressure is so great that these are liquid rather than gas. Deep down, the liquid hydrogen behaves like a metal. As a result, Jupiter has a strong magnetic field and fierce radiation belts. Jupiter emits more heat radiation than it receives from the Sun, because it continues shrinking at a rate of a fraction of an inch per year. Had Jupiter been only 13 times more massive, this contraction would have made the center hot enough for nuclear fusion reactions to begin, though not to be sustained for as long as in a star. It would have become a brown dwarf - a body between a planet and a star. The Galileo spacecraft, which orbited Jupiter from 1995-2003, photographed Jupiter and its moons.

The US Pioneer missions were sent past Jupiterin the early 1970s, Pioneer 10 sending back the first pictures. In 1977 the US sent two Voyager probes to explore Jupiter's cloud tops and five of its moons. Voyager 1 uncovered a faint ring circling the planet. The thin yellow ring meassures approximately 30 km thick

The cloud tops of Jupiter seem to be divided into a series of bands that are different colors. The light bands are called zones, and the dark bands belts. The north tropical zone (equivalent to our northern temperate zone) is the brightest, its whiteness indicating high-level ammonia clouds. The equatorial belt, surrounding Jupiter's equator, always seems in turmoil, with the atmosphere constantly whipped up by violent winds. Across the planet are a number of white or red ovals. These are huge cloud systems. The brown and orange bands indicate the presence of organic molecules including ethane.

JUPITERS MOONS: In 1610, Galileo made the first systematic study of the four largest moons of Jupiter. Since they seemed to change their positions relative to the planet every night, he concluded, correctly, that these objects must be revolving around Jupiter. This insight provided more ammunition for the dismantling of the geocentric theory, which placed Earth at the center of the universe. In 1892 another small moon _ was discovered circling close to the cloud tops of the planet. To date, a total of 63 moons have been discovered.