Jupiter - giant failed star

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Vital Stats

Jupiter is the fifth planet from the Sun, and is easily the largest planet in the solar system. It is orbited by 61 (sixty-one) moons, only four of which have significant size: Ganymede, Callisto, Io, and Europa (shown below right). These are the Galilean Satellites, discovered by Galileo Galilei (1564-1642) in 1610!

Amazing Collision in Space!

In 1994, comet Shoemaker-Levy 9 crashed into Jupiter! This was the first time such an event had been witnessed in the solar system, and struck home how chillingly vulnerable we are here on earth.

Read about all this, and more, in Jupiter Odyssey - the Story of NASA's Galileo Mission (UK | US).

Once in a New Moon!

In 2001, 10 further moons were found. In 2002, astronomers discovered 11 more. In 2003, the number of known moons rose yet again to 61! Many are remarkably tiny, some as small as one or two kilometres. The new moons tend to have retrograde orbits which may indicate that they were captured by Jupiter's gravity, rather than forming at the same time as their parent.

Jupiter's J-ring
Jupiter's Faint Ring System

Jupiter has a faint ring system, not visible from Earth. It was first discovered when the Voyager probes flew by, photographing it in the shadow behind the planet.

Giant Among Giants

Jupiter's moons, and Great Red SpotJupiter is referred to as a "gas giant", 11 times wider than Earth. It is nearly all atmosphere, with only a relatively small solid core. The atmosphere is composed mainly of hydrogen and helium, with the upper clouds formed from droplets of ammonia. The vivid visible surface shows distinct weather systems. The Great Red Spot (left), is a gigantic whirling storm, bigger than the Earth, rotating every 12 days. Also visible are bands, representing different currents occurring in the cool Jovian atmosphere. These rotate with Jupiter, the polar regions moving more slowly than the equatorial regions.

Under Pressure

The planet's size creates enormous pressure at its core, producing large amounts of radiation and heat. The core may be as hot as 30,000°C! Radiation belts extend far beyond the planet into space, providing a lethal environment for spacefaring craft venturing too close. The planet also produces a powerful magnetic field. Had the planet been somewhat more massive than it is, it would be a star. There would have been enough pressure, due to gravity, to start nuclear fusion in its core. It would shine with its own light, giving the solar system two stars (a commonly observed occurrence in other parts of the Galaxy). Had this happened, life could not have existed on Earth.

Top Right ^^

Comet Collision

On the 18th July, 1994, Jupiter was hit by 19 fragments of comet Shoemaker-Levy that it had pulled apart the year before. Despite striking the far side of the planet, as seen from Earth, the impacts caused explosion after explosion - and it was to be recorded by every telescope on Earth, the H.S.T. in orbit, and the Galileo probe, still 18 months away from Jupiter. Comet Shoemaker-Levy collides with Jupiter

Bruised JupiterThe collisions were more powerful than atomic bombs, and left impact bruises twice the size of the Earth in Jupiter's atmosphere! The photo shows a number of impacts strewn across the southern hemisphere. It was taken by the H.S.T. on 22nd July, 1994.

What a Sight!

Close up details of Jupiter and its moons have been received by various probes launched since 1972. The first to arrive was Pioneer 10 in 1973, followed by Pioneer 11 and Voyagers 1 and 2 in 1979.

Voyager Schematics

Voyager draw






In December 1995, NASA's Galileo probe arrived, for a 3 year tour of the Jovian system, launching its own probe into the turbulent atmosphere. Much like Star Trek, the mission far outlasted its original design, surviving radiation exposure more than twice the level it was designed to withstand. In December 2000, the Cassini probe (on its way to Saturn) flew past uniquely allowing joint observations. Galileo's mission finally ended on the 21st September 2003 when it plunged into Jupiter's atmosphere. NASA had planned this to prevent the probe contaminating Europa, which may just have life beneath its icy surface.


Map of Jupiter | Mythical Icons

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