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	Jupiter At a Glance
	Discovery Known since antiquity Name King of the Roman gods Average Distance from Sun 483,682,810 miles 778,412,020 km 5.2 Astronomical Units Mass 318 times Earth's mass Equatorial Diameter 88,846 miles 142,984 km Length of Day 9 hours, 56 minutes Length of Year 11.9 Earth years Surface Gravity 2.14 that of Earth (If you weigh 100 pounds, you would weigh about 214 pounds on Jupiter.) Known Moons 63 (January 2006) Io, Europa, Ganymede and Callisto Compare Planets
	Exploration
	Pioneer 10 and 11 blazed the trail to the outer solar system during the 1970s. Pioneer 10 became the first spacecraft to visit any planet beyond the asteroid belt when it flew past Jupiter on December 3, 1973. Pioneer 10 found that Jupiter's interior is hotter than previously thought, discovered that the planet's radiation belts are strong enough to kill a human being, and discovered that Jupiter's moon Io is embedded in a giant cloud of hydrogen that encircles Jupiter. Pioneer 11 flew past Jupiter a year later, then in 1979 became the first craft to encounter Saturn. More»

Weather systems on Earth blow themselves out in a few days or weeks. On the solar system's giant planets, though, weather systems can last for decades. The record holder is Jupiter's Great Red Spot, which was first observed no later than 1830 -- and perhaps a good bit earlier. This oval storm system is wide enough to swallow two Earths, and the winds at its perimeter blow several times faster than the strongest hurricane ever recorded on Earth.

View in 2009

The Great Red Spot (upper right) is wide enough to swallow two Earths.

Despite its great size and its brightness in Earth's night sky, however, much about Jupiter remains poorly understood. Scientists must infer quite a bit about it from observations by the spacecraft that have visited the planet.

Much of the mystery surrounds Jupiter's interior. From the planet's mass, density, and magnetic field, scientists surmise that it probably has a dense core of rock and metal at least 10 times as massive as Earth. A thick layer of hydrogen surrounds the core. Jupiter's gravity squeezes this layer so tightly that it acts as a metal. The metallic hydrogen probably rotates at a different speed from the core, so it produces a "dynamo" effect, generating electric currents that create Jupiter's magnetic field.

Layers of hydrogen and helium gas surround the metallic hydrogen, and a relatively thin layer of clouds envelops the entire planet. (This layer is actually dozens of miles thick, but compared to Jupiter's great size, that's like the skin on an onion.) Jupiter's rapid rotation stretches the clouds into narrow bands that circle the entire planet.

The bands show different colors, which means their clouds are made of different materials and hover at different altitudes. The highest layers of clouds, which are white, are made of frozen ammonia. Clouds in the next layer contain ammonia mixed with other chemicals, so they look brown or orange. The lowest layers contain water vapor and water ice, so they look blue.

These bands form alternating light and dark stripes. The light-colored stripes are decks of clouds that are pushed high into the atmosphere by rising bubbles of warm gas. The darker stripes consist of cooler material that is dropping back into the planet's atmosphere.

Much of the weather in these zones draws its energy not from the Sun, as is the case with storms on Earth, but from deep within Jupiter itself. As Jupiter's gravity squeezes the planet, it produces heat that wells up through the atmosphere then escapes into space as infrared energy. Jupiter radiates more energy into space than it receives from the Sun.

Rings encircle Jupiter, although they are far darker and skimpier than those of the flashier planet Saturn. The rings may consist of material that was "sandblasted" off the surfaces of Jupiter's moons by collisions with meteorites.

Since Jupiter has no solid surface, no human will ever walk on the planet. In fact, any attempt to visit the Jovian system will require heavy shielding. Jupiter's magnetic field captures electrically charged particles from the Sun and from the planet's volcanic moon Io. These particles create strong radiation belts. Around Io's orbit, the radiation belts are powerful enough to kill an unprotected human in a few minutes. That adds one more item to the list of Jupiter's superlatives: deadliest radiation belts.

View in 2009
The largest planet in our solar system is a commanding presence for much of the year. It looks like an intensely bright cream-colored star, shining brighter than anything else in the night sky except the Moon and Venus. It climbs into view in the pre-dawn sky in February. It rises earlier as the months go by, and moves into the evening sky in late spring. Jupiter is at opposition in mid-August, when it appears brightest for the year and remains visible all night. Late in the year it is visible only in the evening sky, dropping lower each night.

Jupiter's Moons

1. Metis 2. Adrastea 3. Amalthea 4. Thebe 5. Io 6. Europa 7. Ganymede 8. Callisto 9. Themisto 10. Leda 11. Himalia 12. Lysithea 13. Elara 14. S/2000 J11 15. Iocaste 16. Praxidike

17. Harpalyke 18. Ananke 19. Isonoe 20. Erinome 21. Taygete 22. Chaldene 23. Carme 24. Pasiphae 25. S/2002 J1 26. Kalyke 27. Magaclite 28. Sinope 29. Callirrhoe 30. Euporie 31. Kale 32. Orthosie

33. Thyone 34. Euanthe 35. Hermippe 36. Pasithee 37. Eurydome 38. Aitne 39. Sponde 40. Autonoe 41. S/2003 J1 42. S/2003 J2 43. S/2003 J3 44. S/2003 J4 45. S/2003 J5 46. S/2003 J6 47. S/2003 J7 48. S/2003 J8

49. S/2003 J9 50. S/2003 J10 51. S/2003 J11 52. S/2003 J12 53. S/2003 J13 54. S/2003 J14 55. S/2003 J15 56. S/2003 J16 57. S/2003 J17 58. S/2003 J18 59. S/2003 J19 60. S/2003 J20 61. S/2003 J21 62. S/2003 J22 63. S/2003 J23

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This document was last modified: May 04, 2009