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Known Features and Moons of Jupiter
If you were visiting our solar system from the vast depths of space, Jupiter would definitely get your attention: fifth from the Sun, it's as massive as two and a half times the rest of the planets combined. It is a gas giant, a planet whose mass consists primarily of materials in gaseous form, in Jupiter's case, predominantly hydrogen and helium. Apart from being the biggest and most massive planet, it has the distinct advantage of also having the most satellites (a total of 62), though most of them were undiscovered until the mid-1970s. Its principal moons are Io, Callisto, Europa, and Ganymedes, and they were first observed by Galileo Galilei in the year 1610. One might think, since Jupiter is visible by naked eye, and since its moons are known for hundreds of years, there's not much to discover, right ?
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What We Learned Just By Looking at the Sky
Thankfully, science by its nature seeks to answer everything. So astronomers always kept a wary eye on Jupiter, and with the continued development of telescopes, many more details were added to Jupiter's profile. First of all, Giovanni Cassini observed that the planet's atmosphere has a differential rotation, meaning that the atmosphere in the northern hemisphere rotates with different speed (angular velocity) than the atmosphere in the southern hemisphere, creating massive storms in the planet's equator. The equator belt of Jupiter is also infamous for its aptly named Great Red Spot, a huge feature of Jupiter, which some say was possibly observed as early as the middle of the 17th century by Robert Hooke and Cassini respectively. The spot is actually a storm even larger in size than earth, and seems to be a stable metereological feature of the planet. Recently a new smaller spot was observed in the southern hemisphere, and it was given the extraordinary name, Red Spot Junior. Astronomers maybe don't have such a vivid imagination when it comes to names, but when it comes to designs and ideas, they go ballistic, almost literally.
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NASA has visited Jupiter seven times with unmanned probes: Pioneer 10 and 11, Voyager 1 and 2, Ulysses, Galileo, and just in 2007 by New Horizons. We learned more about its cloud layers, what we would call its "terrain", about its very strong magnetosphere that shields its atmosphere from being withered away by the solar wind (and adds extreme radiation belts to the hazards that space vehicles have to face - don't even mention astronauts). The probes provided us with detailed information about its atmosphere's consistency, uncovered a system of faint rings (a multitude of small objects, akin to the rings of Saturn, but barely visible). We also learned that water clouds do not exist on Jupiter, but at a certain "depth" (a better word than altitude since there's no solid surface to relate to) the temperature is very comfortable for life as we know it, pretty much room temperature. The Galileo probe was fortunate enough to observe the collision of the comet Shoemaker-Levy 9, a grandiose spectacle never seen before. Any volunteers for colonizing?
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NASA's Upcoming Mission
Do we know everything there is to know about Jupiter? Not just yet. The pressure is crushing for most human constructs to survive there, the journey to Jupiter is a long and dangerous one, let's take it a step at a time (Mars first). But there's more we need to know before even contemplating such an endeavour: The Juno mission, another probe in NASA's New Horizons program, (anyone remember the early Juno rockets? No relation apart from the name) set for launch in 2011 will provide us with more data to answer our main questions about Jupiter: it will investigate the existence of a rocky core (that would help to explain its magnetosphere's inner workings better), the quantity of water in its atmosphere (not enough for clouds, but maybe enough for some lifeforms), and finally tell us more about Jupiter majestic winds and storms, in order to better understand its complex meteorological system. Learn more about Jupiter's next visitor - the JUNO mission.
Gierasch, Peter J., and Philip D. Nicholson. "Jupiter." World Book Online Reference Center. 2004. World Book, Inc. (http://www.worldbookonline.com/wb/Article?id=ar293080.)
Pictures from http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-jupiter.html