Jupiter
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Jupiter is the fifth planet from the Sun and the largest planet within the Solar System.As of 2008, the largest known planet outside the Solar System is TrES-4. It is a gas giant with a mass slightly less than one-thousandth of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the planets.
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 −2.94, making it on average the third-brightest object in the night sky after the Moon and Venus. ( Mars can briefly match Jupiter's brightness at certain points in its orbit.)
Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. 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 17th century when it was first seen by telescope. 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 most recent 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. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.
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.Composition
Jupiter's upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume or 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 as the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent 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 sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.{{cite journal |author=Gautier, D.; Conrath, B.; Flasar, M.; Hanel, R.; Kunde, V.; Chedin, A.; Scott N. |title = The helium abundance of Jupiter from Voyager |journal = Journal of Geophysical Research |volume = 86|pages = 8713–8720|year = 1981 |url = http://adsabs.harvard.edu/abs/1981JGR....86.8713G |accessdate=2007-08-28 |doi = 10.1029/JA086iA10p08713 }}{{cite journal |author=Kunde, V. G. et al. |title=Jupiter's Atmospheric Composition from the Cassini Thermal Infrared Spectroscopy Experiment |journal=Science|date=September 10, 2004 |volume=305|issue=5690|pages=1582–86 |url=http://www.sciencemag.org/cgi/content/full/305/5690/1582 |accessdate = 2007-04-04 |doi = 10.1126/science.1100240 |pmid=15319491 }} Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.{{cite journal |journal = Icarus| volume = 64| pages = 233–48|year = 1985 |title = Infrared Polar Brightening on Jupiter III. Spectrometry from the Voyager 1 IRIS Experiment |url=http://adsabs.harvard.edu/abs/1985Icar...64..233K |author= Kim, S. J.; Caldwell, J.; Rivolo, A. R.; Wagner, R. |doi = 10.1016/0019-1035(85)90201-5|accessdate=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.{{cite journal |author=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. |title=The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere |journal=Science|year=1996|volume=272 |issue=5263|pages=846–849 |url=http://adsabs.harvard.edu/abs/1996Sci...272..846N |accessdate = 2007-02-19 |doi = 10.1126/science.272.5263.846 |pmid=8629016 }} Helium is also depleted, although only to about 80% of the Sun's helium composition. This depletion may be a result of precipitation of these elements into the interior of the planet.{{cite web |first=Paul|last=Mahaffy |url = http://ael.gsfc.nasa.gov/jupiterHighlights.shtml |title = Highlights of the Galileo Probe Mass Spectrometer Investigation |publisher = NASA Goddard Space Flight Center, Atmospheric Experiments Laboratory |accessdate = 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.{{cite web |author=Ingersoll, A. P.; Hammel, H. B.; Spilker, T. R.; Young, R. E. |date=June 1, 2005 |url = http://www.lpi.usra.edu/opag/outer_planets.pdf |format=PDF|title = Outer Planets: The Ice Giants |publisher = Lunar & Planetary Institute |accessdate = 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
Jupiter is 2.5 times the mass of all the other planets in our Solar System combined — this is so massive that its barycenter with the Sun 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 1,321 Earths, yet is only 318 times as massive.{{cite book |first=Eric|last=Burgess|year=1982 |title=By Jupiter: Odysseys to a Giant |publisher=Columbia University Press|location=New York |isbn=0-231-05176-X}} A "Jupiter mass" (MJ or MJup) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. So, for example, the extrasolar planet HD 209458 b has a mass of 0.69 MJ, while CoRoT-7 b has a mass of 0.015 MJ. 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 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.{{cite journal |last = Guillot|first = Tristan |title=Interiors of Giant Planets Inside and Outside the Solar System |journal=Science|year=1999|volume=286|issue=5437 |pages=72–77|accessdate=2007-08-28 |url=http://www.sciencemag.org/cgi/content/full/286/5437/72 |doi=10.1126/science.286.5437.72 |pmid=10506563}} This has led some astronomers to term it a "failed star", although it is unclear whether 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 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.{{cite journal |author = Burrows, A.; Hubbard, W. B.; Saumon, D.; Lunine, J. I. |title=An expanded set of brown dwarf and very low mass star models |journal=Astrophysical Journal|year=1993|volume=406 |issue=1|pages=158–71 |url=http://adsabs.harvard.edu/abs/1993ApJ...406..158B |accessdate=2007-08-28 |doi = 10.1086/172427 }} The presence of a core during at least part of Jupiter's history is 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. Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.{{cite book |author=Various |editor=McFadden, Lucy-Ann; Weissman, Paul; Johnson, Torrence |year=2006|title=Encyclopedia of the Solar System |edition=2nd|publisher=Academic Press |isbn=0120885891|page=412}} The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J6) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure. The JUNO mission, scheduled for launch in 2011, is expected to narrow down the value of these parameters, and thereby make progress on the problem of the core.{{cite journal |author=Horia, Yasunori; Sanoa, Takayoshi; Ikomaa, Masahiro; Idaa, Shigeru |title=On uncertainty of Jupiter's core mass due to observational errors |journal=Proceedings of the International Astronomical Union |year=2007|volume=3 |publisher= Cambridge University Press |doi=10.1017/S1743921308016554|pages=163–166}} The core region is surrounded by dense metallic hydrogen, which extends outward to about 78 percent 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.{{cite journal |last = Lodders|first = Katharina |title=Jupiter Formed with More Tar than Ice |journal=The Astrophysical Journal |year=2004|volume=611|issue=1|pages=587–597 |url=http://www.journals.uchicago.edu/doi/full/10.1086/421970 | doi = 10.1086/421970 |accessdate=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.{{cite journal |last=Guillot|first=T. |title=A comparison of the interiors of Jupiter and Saturn |journal=Planetary and Space Science|year=1999|volume=47 |issue=10–11|pages=1183–200 |url=http://adsabs.harvard.edu/abs/1999astro.ph..7402G |accessdate=2007-08-28 |doi=10.1016/S0032-0633(99)00043-4}} 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
.]] 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.{{cite web |author=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 |url = http://www.lpl.arizona.edu/~showman/publications/ingersolletal-2004.pdf |format=PDF|title = Dynamics of Jupiter’s Atmosphere |publisher = Lunar & Planetary Institute |accessdate = 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.{{cite web |editor=Watanabe, Susan|date = February 25, 2006 |url = http://www.nasa.gov/vision/universe/solarsystem/galileo_end.html |title = Surprising Jupiter: Busy Galileo spacecraft showed jovian system is full of surprises |publisher = NASA|accessdate = 2007-02-20}} The water clouds can form thunderstorms driven by the heat rising from the interior.{{cite journal |last = Kerr|first = Richard A. |title=Deep, Moist Heat Drives Jovian Weather |journal=Science|year=2000|volume=287|issue=5455 |pages=946–947 |url=http://www.sciencemag.org/cgi/content/full/287/5455/946b |doi=10.1126/science.287.5455.946b |accessdate = 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.{{cite conference |author=Strycker, P. D.; Chanover, N.; Sussman, M.; Simon-Miller, A. |title = A Spectroscopic Search for Jupiter's Chromophores |booktitle = DPS meeting #38, #11.15 |publisher = American Astronomical Society|year = 2006 |url = http://adsabs.harvard.edu/abs/2006DPS....38.1115S |accessdate = 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.{{cite web |author=Gierasch, Peter J.; Nicholson, Philip D. |year = 2004 |url = http://www.nasa.gov/worldbook/jupiter_worldbook.html |title = Jupiter|publisher = World Book @ NASA |accessdate = 2006-08-10}} 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
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,{{cite journal |last=Denning|first=W. F. |title=Jupiter, early history of the great red spot on |journal=Monthly Notices of the Royal Astronomical Society |year=1899|volume=59|pages=574–584 |url=http://adsabs.harvard.edu/abs/1899MNRAS..59..574D |accessdate = 2007-02-09}} and possibly since 1665.{{cite journal |last = Kyrala|first = A. |title=An explanation of the persistence of the Great Red Spot of Jupiter |journal=Moon and the Planets|year=1982|volume=26 |pages=105–7 |url=http://adsabs.harvard.edu/abs/1982M&P....26..105K |accessdate=2007-08-28 |doi=10.1007/BF00941374}} Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.{{cite journal |doi=10.1038/331689a0 |title=Laboratory simulation of Jupiter's Great Red Spot |first=Jöel|last=Sommeria |coauthors=Steven D. Meyers & Harry L. Swinney |journal=Nature|volume=331|pages=689–693 |url=http://adsabs.harvard.edu/abs/1988Natur.331..689S |date=February 25, 1988 |accessdate=2007-08-28}} The storm is large enough to be visible through Earth-based telescopes with an aperture of or larger.{{cite book |first=Michael A.|last=Covington|year=2002 |title=Celestial Objects for Modern Telescopes|page=53 |publisher=Cambridge University Press|isbn=0521524199}} The oval object rotates counterclockwise, with a period of about six days.{{cite web |author=Cardall, C. Y.; Daunt, S. J |url = http://csep10.phys.utk.edu/astr161/lect/jupiter/redspot.html |title = The Great Red Spot |publisher = University of Tennessee |accessdate = 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.{{cite web |url = http://www.space.com/scienceastronomy/solarsystem/jupiter-ez.html |title = Jupiter Data Sheet|publisher = Space.com |accessdate = 2007-02-02}} The maximum altitude of this storm is about 8 km above the surrounding cloudtops.{{cite web |first=Tony|last=Phillips|date = March 3, 2006 |url=http://science.nasa.gov/headlines/y2006/02mar_redjr.htm |title=Jupiter's New Red Spot|publisher=NASA |accessdate = 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 as little as a few hours or stretch on for centuries. to Jupiter, showing the motion of atmospheric bands, and circulation of the Great Red Spot.]] 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. 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.{{cite web |url=http://science.nasa.gov/headlines/y2006/02mar_redjr.htm |title=Jupiter's New Red Spot|year=2006 |accessdate = 2006-03-09}} 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.{{cite web |last = Weintraub|first = Rachel A. |date = September 26, 2005 |url = http://www.nasa.gov/vision/universe/solarsystem/radio_jupiter.html |title = How One Night in a Field Changed Astronomy |publisher = NASA|accessdate = 2007-02-18}} Scientists discovered that there were three forms of radio signals 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.{{cite web
- 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.{{cite web
- Thermal radiation is produced by heat in the atmosphere of Jupiter.
Exploration with space probes
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,{{cite web |last = Wong|first = Al |date= May 28, 1998 |url = http://www2.jpl.nasa.gov/galileo/faqnav.html |title = Galileo FAQ - Navigation|publisher = NASA |accessdate = 2006-11-28}} which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit. 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
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 near 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.{{cite web |last = Lasher|first = Lawrence |date= August 1, 2006 |url = http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhome.html |title = Pioneer Project Home Page |publisher = NASA Space Projects Division |accessdate = 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. The next mission to encounter Jupiter, the Ulysses solar probe, performed a flyby maneuver 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. In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet. On December 19, 2000, the spacecraft captured an image of the moon Himalia, but the resolution was too low to show surface details. The New Horizons probe, en route to Pluto, flew by Jupiter for gravity assist. Its closest approach was on February 28, 2007. The probe's cameras measured plasma output from volcanoes on Io and studied all four Galilean moons in detail, as well as making long-distance observations of the outer moons Himalia and Elara. Imaging of the Jovian system began September 4, 2006.{{cite web|last = Alexander|first = Amir |date= September 27, 2006 | url = http://www.planetary.org/news/2006/0927_New_Horizons_Snaps_First_Picture_of.html|title = New Horizons Snaps First Picture of Jupiter|publisher = The Planetary Society|accessdate = 2006-12-19}}Galileo mission
.]] So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter on December 7, 1995. It orbited the planet for over seven years, conducting multiple flybys of all the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 as it approached Jupiter in 1994, giving a unique vantage point for the event. However, while the information gained about the Jovian system from Galileo was extensive, its originally designed capacity was limited by the failed deployment of its high-gain radio transmitting antenna. An atmospheric probe was released from the spacecraft in July 1995, entering the planet's atmosphere on December 7. It parachuted through 150 km of the atmosphere, collected data for 57.6 minutes, and was crushed by the pressure to which it was subjected by that time (about 22 times Earth normal, at a temperature of 153 °C). It would have melted thereafter, and possibly vaporized. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, to avoid any possibility of it crashing into and possibly contaminating Europa—a moon which has been hypothesized to have the possibility of harboring life.Future probes and canceled missions
NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2011.{{cite web |first=Anthony|last=Goodeill|date=2008-03-31 |url=http://newfrontiers.nasa.gov/missions_juno.html |title=New Frontiers – Missions - Juno|publisher=NASA |accessdate=2007-01-02}} The Europa Jupiter System Mission (EJSM) is a joint NASA/ ESA proposal for exploration of Jupiter and its moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission.{{cite web |author=Talevi, Monica; Brown, Dwayne|date=2009-02-18 |title=NASA and ESA Prioritize Outer Planet Missions|url=http://www.nasa.gov/topics/solarsystem/features/20090218.html |accessdate=2009-02-18 }}{{cite news |first=Paul|last=Rincon|date=2009-02-18 |title= Jupiter in space agencies' sights |url=http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |publisher=BBC News|accessdate=2009-02-28}} ESA's contribution will still face funding competition from other ESA projects.{{cite news |first=Sergio|last=Volonte|date=2007-07-10 |title=Cosmic Vision 2015-2025 Proposals|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=41177 |publisher=ESA|accessdate=2009-02-18}} Launch date will be around 2020. EJSM consists of the NASA-led Jupiter Europa Orbiter, and the ESA-led Jupiter Ganymede Orbiter.{{cite web |url=http://sci.esa.int/science-e/www/area/index.cfm?fareaid=107 |title=Laplace: A mission to Europa & Jupiter system |publisher=ESA|accessdate=2009-01-23}} Because of the possibility of subsurface liquid oceans on Jupiter's moons Europa, Ganymede and Callisto, there has been great interest in studying the icy moons in detail. Funding difficulties have delayed progress. NASA's JIMO (Jupiter Icy Moons Orbiter) was cancelled in 2005.{{cite news|first=Brian|last=Berger |title=White House scales back space plans|publisher=MSNBC |date=2005-02-07|url=http://www.msnbc.msn.com/id/6928404/ |accessdate = 2007-01-02}} A European Jovian Europa Orbiter mission was also studied.{{cite web |last=Atzei|first=Alessandro|date=2007-04-27|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35982 |title=Jovian Minisat Explorer|publisher=ESA |accessdate=2008-05-08}} These missions were superseded by the Europa Jupiter System Mission (EJSM).Moons
Jupiter has 63 named natural satellites. Of these, 47 are less than 10 kilometres in diameter and have only been discovered since 1975. The four largest moons, known as the " Galilean moons", are Io, Europa, Ganymede and Callisto.Galilean moons
, Ganymede, Europa and Io.]] The orbits of Io, Europa, and Ganymede, some of the largest satellites in the Solar System, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits. The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction. This is seen most dramatically in the extraordinary volcanic activity of innermost Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface (indicating recent resurfacing of the moon's exterior).Classification of moons
Europa.]] Before the discoveries of the Voyager missions, Jupiter's moons were arranged neatly into four groups of four, based on commonality of their orbital elements. Since then, the large number of new small outer moons has complicated this picture. There are now thought to be six main groups, although some are more distinct than others. A basic sub-division is a grouping of the eight inner regular moons, which have nearly circular orbits near the plane of Jupiter's equator and are believed to have formed with Jupiter. The remainder of the moons consist of an unknown number of small irregular moons with elliptical and inclined orbits, which are believed to be captured asteroids or fragments of captured asteroids. Irregular moons that belong to a group share similar orbital elements and thus may have a common origin, perhaps as a larger moon or captured body that broke up.Interaction with the Solar System
Along with the Sun, the gravitational influence of Jupiter has helped shape the Solar System. The orbits of most of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane ( Mercury is the only planet that is closer to the Sun's equator in orbital tilt), the Kirkwood gaps in the asteroid belt are mostly caused by Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of the inner Solar System's history.{{cite journal |last = Kerr|first = Richard A. |title=Did Jupiter and Saturn Team Up to Pummel the Inner Solar System? |journal=Science|year=2004|volume=306|issue=5702 |pages=1676 |url=http://www.sciencemag.org/cgi/content/full/306/5702/1676a?etoc |accessdate=2007-08-28 |doi=10.1126/science.306.5702.1676a |pmid=15576586}} s in Jupiter's orbit, as well as the main asteroid belt.]] Along with its moons, Jupiter's gravitational field controls numerous asteroids that have settled into the regions of the Lagrangian points preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to commemorate the Iliad. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then more than two thousand have been discovered.{{cite web |url=http://www.cfa.harvard.edu/iau/lists/JupiterTrojans.html |title=List Of Jupiter Trojans|accessdate=2009-07-10 |publisher=IAU Minor Planet Center}} The largest is 624 Hektor. Most short-period comets belong to the Jupiter family—defined as comets with semi-major axes smaller than Jupiter's. Jupiter family comets are believed to form in the Kuiper belt outside the orbit of Neptune. During close encounters with Jupiter their orbits are perturbed into a smaller period and then circularized by regular gravitational interaction with the Sun and Jupiter.{{cite journal |author=Quinn, T.; Tremaine, S.; Duncan, M. |title=Planetary perturbations and the origins of short-period comets|journal=Astrophysical Journal, Part 1|year=1990 |volume=355|pages=667–679 |url=http://adsabs.harvard.edu/abs/1990ApJ...355..667Q |accessdate = 2007-02-17 |doi=10.1086/168800}}Impacts
image taken on July 23 showing a blemish of about 5,000 miles long left by the 2009 Jupiter impact.]] Jupiter has been called the Solar System's vacuum cleaner,{{cite news |first=Richard A.|last=Lovett |title=Stardust's Comet Clues Reveal Early Solar System |publisher=National Geographic News |date=December 15, 2006|url=http://news.nationalgeographic.com/news/2006/12/061215-comet-stardust.html |accessdate = 2007-01-08}} because of its immense gravity well and location near the inner Solar System. It receives the most frequent comet impacts of the Solar System's planets.{{cite journal |author=Nakamura, T.; Kurahashi, H. |title=Collisional Probability of Periodic Comets with the Terrestrial Planets: An Invalid Case of Analytic Formulation |journal=Astronomical Journal|year=1998|volume=115 |issue=2|pages=848–854|url=http://www.iop.org/EJ/article/1538-3881/115/2/848/970144.html |accessdate=2007-08-28 |doi = 10.1086/300206}} It was thought that the planet served to partially shield the inner system from cometary bombardment. However, recent computer simulations suggest that Jupiter doesn't cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward in roughly the same numbers that it accretes or ejects them.{{cite journal |author=Horner, J.; Jones, B. W. |year=2008 |title=Jupiter - friend or foe? I: the asteroids |journal=International Journal of Astrobiology |volume=7|issue=3–4|pages=251–261 |doi=10.1017/S1473550408004187|accessdate=2009-07-27 |url=http://arxiv.org/abs/0806.2795}} This topic remains controversial among current astronomers, as some believe it draws comets towards Earth from the Kuiper Belt while others believe that Jupiter protects Earth from the alleged Oort Cloud.{{cite news |first=Dennis|last=Overbyte|date=2009-07-25 |title=Jupiter: Our Comic Protector? |work=Thew New York Times| accessdate=2009-07-27|url=http://www.nytimes.com/2009/07/26/weekinreview/26overbye.html?hpw }} A 1997 survey of historical astronomical drawings suggested that the astronomer Cassini may have recorded an impact scar in 1690. The survey determined eight other candidate observations had low or no possibilities of an impact.{{Cite journal |author=Tabe, Isshi; Watanabe, Jun-ichi; Jimbo, Michiwo |year=1997|month=February|title=Discovery of a Possible Impact SPOT on Jupiter Recorded in 1690 |journal=Publications of the Astronomical Society of Japan |volume=49|pages=L1–L5|url=http://articles.adsabs.harvard.edu//full/1997PASJ...49L...1T/L000001.000.html |bibcode=1997PASJ...49L...1T}} During the period July 16, 1994 to July 22, 1994, over 20 fragments from the comet Shoemaker-Levy 9 (SL9, formally designated D/1993 F2) collided with 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.{{cite web |last = Baalke|first = Ron |url = http://www2.jpl.nasa.gov/sl9/ |title = Comet Shoemaker-Levy Collision with Jupiter |publisher = NASA|accessdate = 2007-01-02}}- {{cite web
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