Jupiter_(planet)

Editing of this article by unregistered or newly registered users is currently disabled due to vandalism. If you cannot edit this article and you wish to make a change, you can discuss changes on the talk page, request unprotection, log in, or create an account.

For other uses, see Jupiter (disambiguation).

Jupiter  

This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
Orbital characteristicsYeomans, Donald K. (2006-07-13). HORIZONS System. NASA JPL. Retrieved on 2007-08-08. — At the site, go to the "web interface" then select "Ephemeris Type: ELEMENTS", "Target Body: Jupiter Barycenter" and "Center: Sun".Orbital elements refer to the barycenter of the Jupiter system, and are the instantaneous osculating values at the precise J2000 epoch. Barycenter quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from to the motion of the moons.
Epoch J2000
Aphelion	816,520,800 km 5.458104 AU
Perihelion	740,573,600 km 4.950429 AU
Semi-major axis	778,547,200 km 5.204267 AU
Eccentricity	0.048775
Orbital period	4331.572 days 11.85920 yr
Synodic period	398.88 daysWilliams, Dr. David R. (November 16, 2004). Jupiter Fact Sheet. NASA. Retrieved on 2007-08-08.
Average orbital speed	13.07 km/s
Mean anomaly	18.818°
Inclination	1.305° 6.09° to Sun\'s equator
Longitude of ascending node	100.492°
Argument of perihelion	275.066°
Satellites	63
Physical characteristics
Equatorial radius	71,492 ± 4 kmSeidelmann, P. Kenneth; Archinal, B. A.; A’hearn, M. F.; et.al. (2007). "Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006". Celestial Mechanics and Dynamical Astronomy 90: 155–180. doi:10.1007/s10569-007-9072-y. Retrieved on 2007-08-28. Refers to the level of 1 bar atmospheric pressure 11.209 Earths
Polar radius	66,854 ± 10 km 10.517 Earths
Flattening	0.06487 ± 0.00015
Surface area	6.21796×1010 km²NASA: Solar System Exploration: Planets: Jupiter: Facts & Figures 121.9 Earths
Volume	1.43128×1015 km³ 1321.3 Earths
Mass	1.8986×1027 kg 317.8 Earths
Mean density	1.326 g/cm³
Equatorial surface gravity	24.79 m/s² 2.528 g
Escape velocity	59.5 km/s
Sidereal rotation period	9.925 hSeidelmann, P. K.; Abalakin, V. K.; Bursa, M.; Davies, M. E.; de Burgh, C.; Lieske, J. H.; Oberst, J.; Simon, J. L.; Standish, E. M.; Stooke, P.; Thomas, P. C. (2001). Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000. HNSKY Planetarium Program. Retrieved on 2007-02-02.
Equatorial rotation velocity	12.6 km/s 45,300 km/h
Axial tilt	3.13°
North pole right ascension	268.057° 17 h 52 min 14 s
North pole declination	64.496°
Albedo	0.343 (bond) 0.52 (geom.)
Surface temp.    1 bar level    0.1 bar	min mean max 165 K 112 K
Apparent magnitude	-1.6 to -2.94
Angular diameter	29.8" — 50.1"
Adjectives	Jovian
Atmosphere
Surface pressure	20–200 kPaAnonymous (March 1983). "Probe Nephelometer". Galileo Messenger (6). NASA/JPL. Retrieved on 2007-02-12.  (cloud layer)
Scale height	27 km
Composition	89.8±2.0% Hydrogen (H2) 10.2±2.0% Helium ~0.3% Methane ~0.026% Ammonia ~0.003% Hydrogen deuteride (HD) 0.0006% Ethane 0.0004% water Ices: Ammonia water ammonium hydrosulfide(NH4SH)

Jupiter (pronounced /ˈdʒuːpɨtɚ/Jupiter, entry in the Oxford English Dictionary, prepared by J. A. Simpson and E. S. C. Weiner, vol. 8, second edition, Oxford: Clarendon Press, 1989. ISBN 0-19-861220-6 (vol. 8), ISBN 0-19-861186-2 (set.)) is the fifth planet from the Sun and the largest planet within the Solar System. It is two and a half times as massive as all of the other planets in our solar system combined. Jupiter, along with Saturn, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.

The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.8, making it the third brightest object in the night sky after the Moon and Venus. (However, at certain points in its orbit, Mars can briefly exceed Jupiter\'s brightness.)

The planet Jupiter is primarily composed of hydrogen with a small proportion of helium; it may also have a rocky core of heavier elements under high pressure. Because of its rapid rotation, Jupiter\'s shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the seventeenth century. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.

Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The latest probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed and adjust its trajectory toward Pluto, thereby saving years of travel. Future targets for exploration include the possible ice-covered liquid ocean on the Jovian moon Europa.

Contents 1 Structure 1.1 Composition 1.2 Mass 1.3 Internal structure 1.4 Cloud layers 1.5 Great Red Spot and other storms 1.6 Planetary rings 1.7 Magnetosphere 2 Orbit and rotation 3 Observation 4 Research and exploration 4.1 Ground-based telescope research 4.2 Exploration with space probes 4.2.1 Flyby missions 4.2.2 Galileo mission 4.2.3 Future probes 5 Moons 5.1 Galilean moons 5.2 Classification of moons 6 Interaction with the Solar System 7 Possibility of life 8 Human culture 9 See also 10 References 11 Additional reading 12 External links

Structure

Jupiter is one of the four gas giants; that is, it is not primarily composed of solid matter. It is the largest planet in the Solar System, having a diameter of 142,984 km at its equator. Jupiter\'s density, 1.326 g/cm³, is the second highest of the gas giant planets, but lower than any of the four terrestrial planets. (Of the gas giants, Neptune has the highest density.)

Composition

Jupiter\'s upper atmosphere is composed of about 90% hydrogen and 10% helium by number of atoms, or 86% H₂ and 13% He by fraction of gas molecules—see table to the right. Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described in terms of the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining 1% of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulphide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.Gautier, D.; Conrath, B.; Flasar, M.; Hanel, R.; Kunde, V.; Chedin, A.; Scott N. (1981). "The helium abundance of Jupiter from Voyager". Journal of Geophysical Research 86: 8713–8720. Retrieved on 2007-08-28. Kunde, V. G. et al (September 10, 2004). "Jupiter\'s Atmospheric Composition from the Cassini Thermal Infrared Spectroscopy Experiment". Science 305 (5690): 1582–86. Retrieved on 2007-04-04.  Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.Kim, S. J.; Caldwell, J.; Rivolo, A. R.; Wagner, R. (1985). "Infrared Polar Brightening on Jupiter III. Spectrometry from the Voyager 1 IRIS Experiment". Icarus 64: 233–48. doi:10.1016/0019-1035(85)90201-5. Retrieved on 2007-08-28. 

The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. However, neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.Niemann, H. B.; Atreya, S. K.; Carignan, G. R.; Donahue, T. M.; Haberman, J. A.; Harpold, D. N.; Hartle, R. E.; Hunten, D. M.; Kasprzak, W. T.; Mahaffy, P. R.; Owen, T. C.; Spencer, N. W.; Way, S. H. (1996). "The Galileo Probe Mass Spectrometer: Composition of Jupiter\'s Atmosphere". Science 272 (5263): 846–849. Retrieved on 2007-02-19.  Helium is also depleted, although to a lesser degree. This depletion may be a result of precipitation of these elements into the interior of the planet.Mahaffy, Paul. Highlights of the Galileo Probe Mass Spectrometer Investigation. NASA Goddard Space Flight Center, Atmospheric Experiments Laboratory. Retrieved on 2007-06-06. Abundances of heavier inert gases in Jupiter\'s atmosphere are about two to three times that of the sun.

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.Ingersoll, A. P.; Hammel, H. B.; Spilker, T. R.; Young, R. E. (June 1, 2005). Outer Planets: The Ice Giants (PDF). Lunar & Planetary Institute. Retrieved on 2007-02-01. However, because of the lack of atmospheric entry probes, high quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter.

Mass

Approximate size comparison of Earth and Jupiter, including the Great Red Spot

Jupiter is 2.5 times more massive than all the other planets in our solar system combined--this is so massive that its barycenter with the Sun actually lies above the Sun\'s surface (1.068 solar radii from the Sun\'s center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. Jupiter\'s volume is equal to 1,317 Earths, yet is only 318 times as massive.Gierasch, Peter J.; Nicholson, Philip D. (2004). Jupiter. World Book @ NASA. Retrieved on 2006-08-10. Burgess, Eric (1982). By Jupiter: Odysseys to a Giant. New York: Columbia University Press. ISBN 0-231-005176-X. 

Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink. For small changes in mass, the radius would not change appreciably, and above about four Jupiter masses the interior would become so much more compressed under the increased gravitation force that the planet\'s volume would actually decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved as in high-mass brown dwarfs around 50 Jupiter masses.Guillot, Tristan (1999). "Interiors of Giant Planets Inside and Outside the Solar System". Science 286 (5437): 72–77. Retrieved on 2007-08-28.  This has led some astronomers to term it a "failed star", although it is unclear whether or not the processes involved in the formation of planets like Jupiter are similar to the processes involved in the formation of multiple star systems.

Although Jupiter would need to be about seventy-five times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30% larger in radius than Jupiter.Burrows, A.; Hubbard, W. B.; Saumon, D.; Lunine, J. I. (1993). "An expanded set of brown dwarf and very low mass star models". Astrophysical Journal 406 (1): 158–71. Retrieved on 2007-08-28.  Queloz, Didier. "VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars", European Southern Observatory, November 19, 2002. Retrieved on 2007-01-12.  In spite of this, Jupiter still radiates more heat than it receives from the Sun. The amount of heat produced inside the planet is nearly equal to the total solar radiation it receives.Elkins-Tanton, Linda T. (2006). Jupiter and Saturn. New York: Chelsea House. ISBN 0-8160-5196-8.  This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 2 cm each year.Guillot, T.; Stevenson, D. J.; Hubbard, W. B.; Saumon, D. (2004). "Chapter 3: The Interior of Jupiter", in Bagenal, F.; Dowling, T. E.; McKinnon, W. B.: Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 0521818087.  When it was first formed, Jupiter was much hotter and was about twice its current diameter.Bodenheimer, P. (1974). "Calculations of the early evolution of Jupiter". Icarus 23: 319–25. Retrieved on 2007-02-01. 

Internal structure

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen. NASA background image

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). The existence of the core is suggested by gravitational measurements indicating a mass of from 12 to 45 times the Earth\'s mass or roughly 3%-15% of the total mass of Jupiter.Guillot, T.; Gautier, D.; Hubbard, W. B. (1997). "New Constraints on the Composition of Jupiter from Galileo Measurements and Interior Models". Icarus 130: 534–539. Retrieved on 2007-08-28.  The presence of the core is also suggested by models of planetary formation involving initial formation of a rocky or icy core that is massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. The core may in fact be absent, as gravitational measurements aren\'t precise enough to rule that possibility out entirely. Assuming it does exist, it may also be shrinking, as convection currents of hot liquid metallic hydrogen mix with the molten core and carry its contents to higher levels in the planetary interior.

The core region is surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet. Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.Lodders, Katharina (2004). "The new astrometry". Jupiter Formed with More Tar than Ice 611: 587–597. Retrieved on 2007-07-03. 

Above the layer of metallic hydrogen lies a transparent interior atmosphere of liquid hydrogen and gaseous hydrogen, with the gaseous portion extending downward from the cloud layer to a depth of about 1,000 km. Instead of a clear boundary or surface between these different phases of hydrogen, there is probably a smooth gradation from gas to liquid as one descends.Guillot, T. (1999). "A comparison of the interiors of Jupiter and Saturn". Planetary and Space Science 47 (10–11): 1183–200. Retrieved on 2007-08-28.  Lang, Kenneth R. (2003). Jupiter: a giant primitive planet. NASA. Retrieved on 2007-01-10. This smooth transition happens whenever the temperature is above the critical temperature, which for hydrogen is only 33 K (see hydrogen).

The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa.

Cloud layers

See also: Cloud pattern on Jupiter

This looping animation shows the movement of Jupiter\'s counter-rotating cloud bands. In this image, the planet\'s exterior is mapped onto a cylindrical projection

Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. The clouds are located in the tropopause and are arranged into bands of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.Ingersoll, A. P.; Dowling, T. E.; Gierasch, P. J.; Orton, G. S.; Read, P. L.; Sanchez-Lavega, A.; Showman, A. P.; Simon-Miller, A. A.; Vasavada A. R.. Dynamics of Jupiter’s Atmosphere (PDF). Lunar & Planetary Institute. Retrieved on 2007-02-01. The zones have been observed to vary in width, color and intensity from year to year, but they have remained sufficiently stable for astronomers to give them identifying designations.

The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.) These electrical discharges can be up to a thousand times as powerful as lightning on the Earth. Watanabe, Susan:Surprising Jupiter: Busy Galileo spacecraft showed jovian system is full of surprises. NASA (February 25, 2006). Retrieved on 2007-02-20. The water clouds can form thunderstorms driven by the heat rising from the interior.Kerr, Richard A. (2000). "Deep, Moist Heat Drives Jovian Weather". Science 287 (5455): 946–947. Retrieved on 2007-02-24. 

The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.Strycker, P. D.; Chanover, N.; Sussman, M.; Simon-Miller, A. (2006). "A Spectroscopic Search for Jupiter\'s Chromophores". DPS meeting #38, #11.15, American Astronomical Society. Retrieved on 2007-02-20.  These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.

Jupiter\'s low axial tilt means that the poles constantly receive less solar radiation than at the planet\'s equatorial region. Convection within the interior of the planet transports more energy to the poles, however, balancing out the temperatures at the cloud layer.

Great Red Spot and other storms

Main article: Great Red Spot

This dramatic view of Jupiter\'s Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million mi) from Jupiter. Cloud details as small as 160 km (100 mi) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter\'s scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.

The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator that is larger than Earth. It is known to have been in existence since at least 1831,Denning, W. F. (1899). "Jupiter, early history of the great red spot on". Monthly Notices of the Royal Astronomical Society 59: 574–584. Retrieved on 2007-02-09.  and possibly since 1665.Kyrala, A. (1982). "An explanation of the persistence of the Great Red Spot of Jupiter". Moon and the Planets 26: 105–7. Retrieved on 2007-08-28.  Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.Sommeria, Jöel; Steven D. Meyers & Harry L. Swinney (February 25, 1988). "Laboratory simulation of Jupiter\'s Great Red Spot". Nature 331: 689–693. Retrieved on 2007-08-28.  The storm is large enough to be visible through Earth-based telescopes.

The oval object rotates counterclockwise, with a period of about 6 days.Cardall, C. Y.; Daunt, S. J.. The Great Red Spot. University of Tennessee. Retrieved on 2007-02-02. The Great Red Spot\'s dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth\'s diameter.Jupiter Data Sheet. Space.com. Retrieved on 2007-02-02. The maximum altitude of this storm is about 8 km above the surrounding cloudtops.Phillips, Tony (March 3, 2006). Jupiter\'s New Red Spot. NASA. Retrieved on 2007-02-02.

Storms such as this are common within the turbulent atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last for hours or centuries.

Time-lapse sequence from the approach of Voyager I to Jupiter, showing the motion of atmospheric bands, and circulation of the great red spot. NASA image.

Even before Voyager proved that the feature was a storm, there was strong evidence that the spot could not be associated with any deeper feature on the planet\'s surface, as the Spot rotates differentially with respect to the rest of the atmosphere, sometimes faster and sometimes more slowly. During its recorded history it has traveled several times around the planet relative to any possible fixed rotational marker below it.

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller in size. This was created when several smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were first observed in 1938. The merged feature was named Oval BA, and has been nicknamed Red Spot Junior. It has since increased in intensity and changed color from white to red.Jupiter\'s New Red Spot (2006). Retrieved on 2006-03-09. Steigerwald, Bill (October 14, 2006). Jupiter\'s Little Red Spot Growing Stronger. NASA. Retrieved on 2007-02-02. Goudarzi, Sara (May 4, 2006). New storm on Jupiter hints at climate changes. USA Today. Retrieved on 2007-02-02.

Planetary rings

Main article: Rings of Jupiter

The rings of Jupiter.

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer "gossamer" ring.Showalter, M.A.; Burns, J.A.; Cuzzi, J. N.; Pollack, J. B. (1987). "Jupiter\'s ring system: New results on structure and particle properties". Icarus 69 (3): 458–98. doi:10.1016/0019-1035(87)90018-2. Retrieved on 2007-08-28.  These rings appear to be made of dust, rather than ice as is the case for Saturn\'s rings. The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational pull. The orbit of the material veers towards Jupiter and new material is added by additional impacts.Burns, J. A.; Showalter, M.R.; Hamilton, D.P.; et.al. (1999). "The Formation of Jupiter\'s Faint Rings". Science 284: 1146–50. doi:10.1126/science.284.5417.1146. Retrieved on 2007-08-28.  In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the gossamer ring.

Magnetosphere

Main article: Jupiter\'s magnetosphere

Jupiter\'s broad magnetic field is 14 times as strong as the Earth\'s, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest in the solar system (with the exception of sunspots). This field is believed to be generated by eddy currents—swirling movements of conducting materials—within the metallic hydrogen core. The field traps a sheet of ionized particles from the solar wind, generating a highly-energetic magnetic field outside the planet—the magnetosphere. Electrons from this plasma sheet ionize the torus-shaped cloud of sulfur dioxide generated by the tectonic activity on the moon Io. Hydrogen particles from Jupiter\'s atmosphere are also trapped in the magnetosphere. Electrons within the magnetosphere generate a strong radio signature that produces bursts in the range of 0.6–30 MHz."Jupiter\'s Magnetosphere", The Astrophysics Spectator, November 24, 2004. Retrieved on 2006-05-24. 

At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter\'s magnetosphere is a magnetopause, located at the inner edge of a magnetosheath, where the planet\'s magnetic field becomes weak and disorganized. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter\'s lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.

Aurora borealis on Jupiter. The three brightest regions are created by tubes of magnetic flux that connect to the Jovian moons Io, Ganymede and Europa.

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet\'s polar regions. Volcanic activity on the Jovian moon Io (see below) injects gas into Jupiter\'s magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfven waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When the Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.Radio Storms on Jupiter. NASA (February 20, 2004). Retrieved on 2007-02-01.

Orbit and rotation

The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. The elliptical orbit of Jupiter is inclined 1.31° compared to the Earth. Because of an eccentricity of 0.048, the distance from Jupiter and the Sun varies by 75 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.

The axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.Interplanetary Seasons. Science@NASA. Retrieved on 2007-02-20.

Jupiter\'s rotation is the solar system\'s fastest, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. This rotation requires a centripetal acceleration at the equator of about 1.67 m/s², compared to the equatorial surface gravity of 24.79 m/s²; thus the net acceleration felt at the equatorial surface is only about 23.12 m/s². The planet is shaped as an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles.

Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter\'s polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three "systems" are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10° N to 10° S; its period is the planet\'s shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet\'s magnetosphere; its period is Jupiter\'s "official" rotation.Ridpath, Ian (1998). Norton\'s Star Atlas, 19th ed., Prentice Hall. ISBN 0582356555. 

Observation

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus); however at times Mars appears brighter than Jupiter. Depending on Jupiter\'s position with respect to the Earth, it can vary in visual magnitude from as bright as −2.8 at opposition down to −1.6 during conjunction with the Sun. The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds. Favorable oppositions occur when Jupiter is passing through perihelion, an event that occurs once per orbit. As Jupiter approaches perihelion in March 2011, there will be a favorable opposition in September of 2010.Anonymous. Favorable Appearances by Jupiter. Retrieved on 2008-01-02. (Horizons) The next opposition of Jupiter occurs on July 9,2008; it is unusual because, as seen from Jupiter, Earth will transit (pass in front of) the Sun.^{[citation needed]}

The retrograde motion of an outer planet is caused by its relative location with respect to the Earth.

Earth overtakes Jupiter every 398.9 days as it orbits the Sun, a duration called the synodic period. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars. That is, for a period of time Jupiter seems to move backward in the night sky, performing a looping motion.

Jupiter\'s 12-year orbital period corresponds to the dozen constellations in the zodiac. As a result, each time Jupiter reaches opposition it has advanced eastward by about the width of a zodiac constellation. The orbital period of Jupiter is also about two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.

Because the orbit of Jupiter is outside the Earth\'s, the phase angle of Jupiter as viewed from the Earth never exceeds 11.5°, and is almost always close to zero. That is, the planet always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained.Encounter with the Giant. NASA (1974). Retrieved on 2007-02-17.

Research and exploration

Ground-based telescope research

In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa, Ganymede and Callisto (now known as the Galilean moons) using a telescope; thought to be the first observation of moons other than Earth\'s. Note, however, that Chinese historian of astronomy, Xi Zezong, has claimed that Gan De, a Chinese astronomer, made this discovery of one of Jupiter\'s moons in 362 BC with the unaided eye, nearly 2 millennia earlier.Xi, Z. Z. (1981). "The Discovery of Jupiter\'s Satellite Made by Gan-De 2000 Years Before Galileo". Acta Astrophysica Sinica 1 (2): 87. Retrieved on 2007-10-27.  Dong, Paul (2002). China\'s Major Mysteries: Paranormal Phenomena and the Unexplained in the People\'s Republic. China Books. ISBN 0835126765.  Galileo\'s was also the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus\' heliocentric theory of the motions of the planets; Galileo\'s outspoken support of the Copernican theory placed him under the threat of the Inquisition.Westfall, Richard S. Galilei, Galileo. The Galileo Project. Retrieved on 2007-01-10.

During 1660s, Cassini used a new telescope to discover spots and colorful bands on Jupiter and observed that the planet appeared oblate; that is, flattened at the poles. He was also able to estimate the rotation period of the planet.O\'Connor, J. J.; Robertson, E. F. (April, 2003). Giovanni Domenico Cassini. University of St. Andrews. Retrieved on 2007-02-14. In 1690 Cassini noticed that the atmosphere undergoes differential rotation.

False-color detail of Jupiter\'s atmosphere, imaged by Voyager 1, showing the Great Red Spot and a passing white oval.

The Great Red Spot, a prominent oval-shaped feature in the southern hemisphere of Jupiter, may have been observed as early as 1664 by Robert Hooke and in 1665 by Giovanni Cassini, although this is disputed. The pharmacist Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot in 1831.Murdin, Paul (2000). Encyclopedia of Astronomy and Astrophysics. Bristol: Institute of Physics Publishing. ISBN 0122266900. 

The Red Spot was reportedly lost from sight on several occasions between 1665 and 1708 before becoming quite conspicuous in 1878. It was recorded as fading again in 1883 and at the start of the twentieth century.SP-349/396 Pioneer Odyssey—Jupiter, Giant of the Solar System. NASA (August 1974). Retrieved on 2006-08-10.

Both Giovanni Borelli and Cassini made careful tables of the motions of the Jovian moons, allowing predictions of the times when the moons would pass before or behind the planet. By the 1670s, however, it was observed that when Jupiter was on the opposite side of the Sun from the Earth, these events would occur about 17 minutes later than expected. Ole Rømer deduced that sight is not instantaneous (a finding that Cassini had earlier rejected), and this timing discrepancy was used to estimate the speed of light.Roemer\'s Hypothesis. MathPages. Retrieved on 2007-01-12.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch refractor at Lick Observatory in California. The discovery of this relatively small object, a testament to his keen eyesight, quickly made him famous. The moon was later named Amalthea.Tenn, Joe (March 10, 2006). Edward Emerson Barnard. Sonoma State University. Retrieved on 2007-01-10. It was the last planetary moon to be discovered directly by visual observation.Amalthea Fact Sheet. NASA JPL (October 1, 2001). Retrieved on 2007-02-21. An additional eight satellites were subsequently discovered prior to the flyby of the Voyager 1 probe in 1979.

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter.Dunham Jr., Theodore (1933). "Note on the Spectra of Jupiter and Saturn". Publications of the Astronomical Society of the Pacific 45: 42–44. Retrieved on 2007-02-18. 

Three long-lived anticyclonic features termed white ovals were observed in 1938. For several decades they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.Youssef, A.; Marcus, P. S. (2003). "The dynamics of jovian white ovals from formation to merger". Icarus 162 (1): 74–93. Retrieved on 2007-04-17. 

In 1955, Bernard Burke and Kenneth Franklin detected bursts of radio signals coming from Jupiter at 22.2 MHz. The period of these bursts matched the rotation of the planet, and they were also able to use this information to refine the rotation rate. Radio bursts from Jupiter were found to come in two forms: long bursts (or L-bursts) lasting up to several seconds, and short bursts (or S-bursts) that had a duration of less than a hundredth of a second.Weintraub, Rachel A. (September 26, 2005). How One Night in a Field Changed Astronomy. NASA. Retrieved on 2007-02-18.

Scientists discovered that there were three forms of radio signals being transmitted from Jupiter.

• Decametric radio bursts (with a wavelength of tens of meters) vary with the rotation of Jupiter, and are influenced by interaction of Io with Jupiter\'s magnetic field.Garcia, Leonard N.. The Jovian Decametric Radio Emission. NASA. Retrieved on 2007-02-18.
• Decimetric radio emission (with wavelengths measured in centimeters) was first observed by Frank Drake and Hein Hvatum in 1959. The origin of this signal was from a torus-shaped belt around Jupiter\'s equator. This signal is caused by cyclotron radiation from electrons that are accelerated in Jupiter\'s magnetic field.Klein, M. J.; Gulkis, S.; Bolton, S. J. (1996). Jupiter\'s Synchrotron Radiation: Observed Variations Before, During and After the Impacts of Comet SL9. NASA. Retrieved on 2007-02-18.
• Thermal radiation is produced by heat in the atmosphere of Jupiter.

During the period July 16, 1994 to July 22, 1994, over twenty fragments from the comet Shoemaker-Levy 9 hit Jupiter\'s southern hemisphere, providing the first direct observation of a collision between two solar system objects. This impact provided useful data on the composition of Jupiter\'s atmosphere.Baalke, Ron. Comet Shoemaker-Levy Collision with Jupiter. NASA. Retrieved on 2007-01-02. Britt, Robert R.. "Remnants of 1994 Comet Impact Leave Puzzle at Jupiter", space.com, August 23, 2004. Retrieved on 2007-02-20. 

Exploration with space probes

Main article: Exploration of Jupiter

Since 1973 a number of automated spacecraft have visited Jupiter. Flights to other planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Reaching Jupiter from Earth requires a delta-v of 9.2 km/s,Wong, Al (May 28, 1998). Galileo FAQ - Navigation. NASA. Retrieved on 2006-11-28. which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.Hirata, Chris. Delta-V in the Solar System. California Institute of Technology. Retrieved on 2006-11-28. Fortunately, gravity assists through planetary flybys can be used to reduce the energy required to reach Jupiter, albeit at the cost of a significantly longer flight duration.

Flyby missions

Flyby missions

Spacecraft	Closest approach	Distance
Pioneer 10	December 3, 1973	130,000 km
Pioneer 11	December 4, 1974	34,000 km
Voyager 1	March 5, 1979	349,000 km
Voyager 2	July 9, 1979	570,000 km
Ulysses	February 1992	409,000 km
	February 2004	240,000,000 km
Cassini	December 30, 2000	10,000,000 km
New Horizons	February 28, 2007	2,304,535 km

Voyager 1 took this photo of the planet Jupiter on January 24, 1979 while still more than 25 million mi (40 million km) away.

Beginning in 1973, several spacecraft have performed planetary flyby maneuvers that brought them within observation range of Jupiter. The Pioneer missions obtained the first close-up images of Jupiter\'s atmosphere and several of its moons. They discovered that the radiation fields in the vicinity of the planet were much stronger than expected, but both spacecraft managed to survive in that environment. The trajectories of these spacecraft were used to refine the mass estimates of the Jovian system. Occultations of the radio signals by the planet resulted in better measurements of Jupiter\'s diameter and the amount of polar flattening.Lasher, Lawrence (August 1, 2006). Pioneer Project Home Page. NASA Space Projects Division. Retrieved on 2006-11-28.

Six years later, the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter\'s rings. They also confirmed that the Great Red Spot was anticyclonic. Comparison of images showed that the Red Spot had changed hue since the Pioneer missions, turning from orange to dark brown. A torus of ionized atoms was discovered along Io\'s orbital path, and volcanoes were found on the moon\'s surface, some in the process of erupting. As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere.Jupiter. NASA Jet Propulsion Laboratory (January 14, 2003). Retrieved on 2006-11-28.

The next mission to encounter Jupiter, the Ulysses solar probe, performed a flyby maneuver in order to attain a polar orbit around the Sun. During this pass the spacecraft conducted studies on Jupiter\'s magnetosphere. However, since Ulysses has no cameras, no images were taken. A second flyby six years later was at a much greater distance.Chan, K.; Paredes, E. S.; Ryne, M. S. (2004).