Overview

• Origin of name: From the king of the gods, Zeus or Jupiter.
• Orbit: Radius 5.20 AU [period 11.9 yr, eccentricity 0.05].
• Viewing: From Earth can see a few moons and cloud bands; from space, its moons look like a complex miniature solar system.
• Size: 11.2 Earth radii, containing 318 Earth masses, or 0.1% of the Sun's mass [its surface gravity is 2.64 times the Earth's]; Jupiter is the largest planet, and has more mass than all the others together, but it is not very dense.
• Rotation: Differential, fast (one full rotation takes about 10 hours) and bulge.

  Exploration

• Early fly-bys: Pioneer 10 (with plaque) and 11 in 1973-1974, now leaving the Solar System [possibly detecting extra forces?]; Voyager 1 and 2 in 1979, also on their way out, and still functioning; Ulysses in 1992, while on a gravity assist toward the Sun.
• Recent: The Galileo spacecraft was in orbit 1995-2003 (and then plunged into the planet); The Cassini spacecraft flew by in 2000 on its way to Saturn; The Juno spacecraft is currently on its way.
• Future plans: A Jupiter Icy Moons Orbiter was planned, then cancelled; Possible future landers and exploration of the moons, especially under Europa's icy crust.

Atmosphere and Interior

• Composition: Mostly H₂ (70%), some He; little H₂O; The average density is only 1.3 times that of water.
• General appearance: Circulating, global cloud bands, with darker, rising "belts" and lighter, sinking "zones".
• Specific features: Great Red Spot (a thunderstorm larger than Earth due to a changing high pressure region, at least 300 years old, but may be shrinking over many years), Great Dark Spot (at least as large, near the North Pole), and many smaller spots; Powered by internal heat and the Sun (the Earth might have them too, if the Sun's energy was not dissipated by turbulence and friction); [the polar regions have auroras like Earth, but with additional satellite footprints!]
• Colors: Turbulent and changing, complex cloud chemistry of white zones and dark belts (methane, ammonia, sulfur and phosphorus compounds).
• Core: Jupiter may have a "small" rocky core or none; Most of its heavy elements are mixed with the rest.
• Energy: Jupiter emits more than it receives, and was even brighter in the past! Why?

  Magnetosphere and Ring

• Magnetic field: 20,000 times stronger than Earth's; Filled with energetic charged particles emitting radio waves; Deformed by the solar wind, has a long tail extending beyond the orbit of Saturn!
• Rings: Thin, made of rock fragments and dust, unstable and constantly resupplied; They were not expected, but were seen by Voyager I; Located inside the moons' orbits; Dark; Origin?

Galilean Moons: Jupiter's attendants

• Overview: The ones that Galileo saw; From Earth, they appear to be aligned; Why?
• Io: R & mass like our Moon; Smooth, colored; Active volcanoes, more than 300 [including Loki, with a lava flow the size of Connecticut] and tall mountains; In constant geological activity from tidal stress heating, leaves a trail of plasma along its orbit, shoots ash and dust like bullets into space.
• Europa: Surface of cracked ice, with few craters (also very thin oxygen atmosphere); There may be water underneath! Could life have developed there? A probe to look under the crust is being planned.
• Ganymede: Largest moon in the solar system, larger than Mercury! Cratered and ridged surface, shaped by tectonic forces, criss-crossed with surprisingly smooth tracts, maybe deep trenches that were later filled with ice; 1:2:4 resonance with Io and Europa.
• Callisto: Similar to Ganymede (will be resonant in a few hundred million years...).

Other Moons

• Overview: Total = 63 so far known (as of 2004), far more than any other planet; and there may be more.
• Inner: Metis, Adrastea, Amalthea (one of the most unusual moons in the Solar System, because it gives off more heat than it receives from the Sun), Thebe; rotating synchronously with their orbit.
• Outer: Leda, Himalia, Lysithea, Elara; Ananke, Carme, Pasiphae, Sinope; ... many unnamed.
• Origin: Some are probably captured asteroids, or groups of asteroid fragments from collisions with other moons; many are on tilted, eccentric and retrograde orbits; some are as small as 1 km across.

Other Aspects

• Big events: 1994, The plunging of (pieces from) comet Shoemaker-Levy 9 into the planet; 2006, The appearance of Red Jr and its close encounter with the Great red Spot.
• Puzzle: Pulsating X-rays coming every 45 minutes from the polar regions.