Basic Data

• Mean Distance from Sun: 778.5 million km.
• Equatorial Radius: 71,492 km. (11.21 Earth)
• Mass: 1.90 x 10²⁷ kg. (317.8 Earth)
• Surface Gravity: 3.7 m/s² (0.38 Earth)
• Length of Year: 11.86 years
• Length of Day: 9h 56m

Wikipedia - Jupiter

About Jupiter

Jupiter has been called the King of Planets. Some people suspect that there might have been some kind of "inspiration" that led the ancients to name this particular point of light after the king of the gods. An immense ball of mostly hydrogen—similar in composition to a star—Jupiter does not have a solid surface. Instead, as one plunges into its atmosphere one finds himself descending through layers of gas of ever-increasing density, until one's spacecraft or probe is crushed beneath the weight. And far from being a frozen wasteland, Jupiter is hot. The Galileo probe reported rapidly rising temperatures—well above the boiling point of water—after penetrating only a little more than a hundred and fifty kilometers.

The first thing you will notice as you look at Jupiter are the colorful bands of clouds. Actually, these clouds are merely a "thin scum" at the top of the immense Jovian atmosphere, formed of chemicals most likely created by both reactions deep within the planet and brought to the "surface" by convection, and reactions triggered by solar ultraviolet radiation striking the outermost layers of gas.

Jupiter has often been called "a star that failed". Actually, in order to begin nuclear reactions in its core, Jupiter would have to be many times more massive than it is. Nevertheless, gravitational compression does generate enough energy so that the planet radiates more than twice the amount of energy that it receives from the Sun.

Probably the deadliest aspect of Jupiter regarding the possibility of life among its moons is the planet's immense and powerful radiation belt. Vast quantities of charged particles, trapped in the biggest magnetic field in the solar system (except for the sun's own), produce a blizzard of particles that can kill very quickly. Even exploratory spacecraft have to be shielded in order for their electronics to survive.

The realm of Jupiter is immense and magnificent. The heavens truly do declare the glory of God. But it is also a cold and poisonous realm, bombarded with lethal radiation. It is not a place for man.

Jupiter's Moons

Jupiter has many moons. Most of them are small, asteroid-like rocks, many of them probably captured from the asteroid belt itself. Four of the moons are large: Io, Europa, Ganymede and Callisto. These are called the "Galilean satellites" because they were discovered by Galileo when he observed Jupiter through his telescope. (They were the first solid evidence that not everything in the universe revolves around the Earth.) All four possess very thin atmospheres (high vacuum by Earth standards), and are quite different in composition.

Io is a rocky body with large amounts of sulfur, giving it an orange color, and a number of active volcanoes. It is the most volcanically active body in the solar system. Io is about the same size as Earth's Moon.

Europa is slightly smaller and composed of both ice and rock. It appears as an almost billiard-ball-smooth object, and probes have essentially confirmed that it is largely covered with an ocean, with a layer of ice on top. There has been a lot of speculation about the possibilities of life on Europa, but recent discoveries about the chemical composition of the Europan ocean are casting doubt on that theory, even among wishful thinkers.

The outermost moons, Ganymede and Callisto, are also composed of a mixture of ice and rock, but instead of it being in distinct layers, on these moons it forms a sort of "dirty ice". Callisto is the most heavily cratered body in the solar system.

Observation from Earth

Jupiter was known to the ancients, being one of the brightest objects in the night sky after the Moon and Venus. The Babylonians called it Marduk and the Greeks Zeus. It was one of the first objects in the sky that Galileo aimed his telescope at—and discovered the four satellites that bear his name and noted that they revolved around Jupiter instead of the Earth.

There has also been speculation that Jupiter's motion through the sky is directly related to the divisions in the Zodiac; there are twelve signs, and Jupiter's year is twelve of our years. Each year it moves from one sign to the next.

Jupiter's magnetic field is, of course, invisible. But if it were visible, it would loom four times as large as the Moon in our sky.

Exploration of Jupiter

Jupiter has been visited by a number of probes. Most of them have been flybys, such as Pioneer and the two Voyagers. Only one, Galileo, was sent into orbit around the planet. It also released a probe that descended more than a hundred miles into the atmosphere before being destroyed by heat and pressure. In the end, mission controllers sent the Galileo orbiter crashing into Jupiter as well; they were afraid that it might someday crash on Europa and "contaminate" it.