Jupiter, King of Worlds

Jupiter is the king of the planets of our solar system. Jupiter is almost 26,000 times more massive than the Moon and 318 times more massive than Earth! Even though Jupiter is still less than one tenth of one percent as massive as the Sun, the king of our worlds is over 2.5 times greater in mass than the rest of all of the planets in the solar system combined (unless there really is a Planet IX out there, which would only change that number a little bit; a quick back-of-the-envelope calculation tells me that if Planet IX is there and is 10 times the mass of the Earth, then Jupiter would be about 2.3 times more massive than all the other planets combined)!

Jupiter is a behemoth of planetary mass in our solar system. Here you can see the relative masses of the 8 planets. (image from Das steinerne Herz on Wikipedia)

Jupiter was one of the five planets beyond Earth known by many ancient peoples. Jupiter's apparent point of light in the night's sky was associated with the god Marduk by the ancient Babylonians, while ancient Chinese beliefs personified Jupiter as the Fu Star (Fuxing), association the bright light of Jupiter with prosperity. It was from the ancient Romans that Jupiter gained the name we use for it. Jupiter in Roman mythology is the king of the gods, just as Zeus was the king of the gods in Greek mythology. We even came to name one of the days of our weeks after Jupiter (though in English, our word for that day (Thursday) has derived from the Germanic mythologies and the god Thor).

It was Galileo's discovery of four of the moons of Jupiter in 1610 and his writing of them that nearly caused the Catholic Church to murder Galileo (the idea that there could be moons orbiting other planets was certainly considered blasphemy against the church at that time). Below you can see a page from the Sidereus Nuncius ("Starry Messenger"; Galileo's 1610 book on his astronomical observations), where Galileo drew Jupiter and the moons he had discovered:

Although Galileo's discoveries of Jupiter's moons propelled astronomy and our understanding of the universe forward, it was Giovanni Cassini who first drew pictures of the features on Jupiter that he could observe with his telescope. For instance, below are some images from Cassini where he first shows what we believe was the discovery of the Great Red Spot (or, at least, discovery of a spot which may have preceded the Great Red Spot by over a century).

Perhaps one of the coolest of the earliest drawings of Jupiter came in 1880, and can be found in the collections of astronomer and artist E.L. Trouvelot. His drawing of Jupiter was created during an observation on November 1st of 1880, and clearly shows the Great Red Spot:

These early observations and drawings must have sent people's minds racing with wonderment at what those features on the king of worlds could be. However, once we started incorporating photographic imaging with observational astronomy and, especially, started sending spacecraft to explore the other worlds we started constraining what it was we were seeing on Jupiter (while also raising lots of more questions!). 

The Pioneer 10 spacecraft was the first extension of humanity to fly by Jupiter back in 1973. Here's one of the images taken during that encounter with the giant world. Pioneer 10 was the first spacecraft to measure the radiation and magnetic fields surrounding Jupiter. It was also the first to take images of the moons of Jupiter up-close. Pioneer 10 passed within 81,000 miles of the cloudtops of Jupiter on it's flyby over 40 years ago.

Jupiter has since had several other spacecraft go zooming by, most of which at least took pictures if not full-on collecting data and targeting Jupiter and its moons for observation; for instance, check out this table I stole from Wikipedia on spacecraft that have flown by along with the dates of closest approach and the minimum distance from Jupiter at that time:

-----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 8, 1992	408,894 km
	February 4, 2004	120,000,000 km
Cassini	December 30, 2000	10,000,000 km
New Horizons	February 28, 2007	2,304,535 km

The only spacecraft so far to be sent into orbit of Jupiter was the Galileo Spacecraft, which operated in the Jovian system for over 8 years (from its arrival in December of 1995 until we crashed it into Jupiter (something we like to call "de-orbiting") in September of 2003). Galileo was used to study Jupiter's atmosphere and rings and to image and study the volcanoes on Io. Galileo discovered that Ganymede has its own, very strong magnetic field and really gave us most of the best data from which we have concluded that there is likely a deep subsurface ocean on Europa. The Galileo spacecraft really unlocked Jupiter and set the grounds for future spacecraft to visit that giant world.

An artist's concept of the Galileo spacecraft at Jupiter, with Io's volcanoes erupting nearby

Of the upcoming missions to Jupiter, one of them is actually going to get there very soon. The Juno spacecraft (see image to the left for a digital image of the spacecraft) is a NASA mission to Jupiter which was launched in August of 2011 and will arrive at Jupiter on July 4th of this year. Juno, named after the wife of Jupiter who was able to see through his clouds, will study Jupiter's composition, magnetic and gravitational fields, and will study the various processes within the Jovian atmosphere.

Following Juno, there are two missions in the works for studying the icy moons of Jupiter. JUICE (the Jupiter Icy Moon Explorer) is an ESA mission slated for launch in 2022. JUICE is designed to study primarily Ganymede and Callisto, though it should also fly by Europa a couple of times as well. A NASA mission set for launch on the early 2020s (likely 2022, as of now) is also in the works. The mission is currently called the Europa Multiple-Flyby Mission, though that name will surely change when the mission is more fully developed. This mission will focus on studying Europa. It will tell us about the surface processes and composition of Europa, will tell us a bit about the internal geology and composition of this moon, and, most importantly for astrobiology, will seek to determine the existence of Europa's ocean and determine what it can about the composition and possible surface-interactions of this ocean. 

Though JUICE and the Europa Multiple-Flyby Mission will target the icy moons of Jupiter, they will teach us a lot about the Jovian system in general (like how Jupiter's magnetic field and the chaotic whipping of particles through that field effect the moons of Jupiter). Taken along with Juno, the next couple of decades of research on Jupiter should be highly revealing, telling us a lot about the king of the planets but also helping us to uncover new mysteries and new questions about this behemoth in our solar system. 

In closing out this post, here is a quote about Jupiter, the god, from Ovid:

"Jupiter, from on high, smiles at the perjuries of lovers."

When it comes to the "perjuries of lovers" on Earth, Jupiter the planet, just as Jupiter the god, most certainly has no interest in our shortcomings. Though there is a beautiful history to consider in astrology, and Jupiter most certainly has played a prominent role there, the real test of time for Jupiter has revealed this king of our planets to be a massive and active monster of nature, though also beautiful in its presentation of itself to the universe.

Jupiter Drawing, from Kelvin Ma at Wikipedia