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Galileo on Galileo, Part II
Color painting of spacecraft flying near a small moon and Jupiter.
An artist's concept of the Galileo orbiter flying past Jupiter's moon Amalthea in 2002.

Editor's Note:Twenty years ago, on 18 Oct. 1989, the Space Shuttle Atlantis carried the Galileo orbiter into space to embark on an historic mission to explore Jupiter and its moons. This is also the year the world is celebrating 400 years of science since Galileo Galilei, the man, discovered four moons orbiting Jupiter and helped revolutionize our understanding of our place in the unvierse. To mark the anniversaries, former Galileo science team members imagined how Galileo the man might sum up the spectacular results of the machine that carried his name to Jupiter. The note is a follow up to the team's original imagining of Galileo's thoughts as the spacecraft approached Jupiter in 1995.

A Second Communique From Somewhere in Outer Space -- Overheard on the World Wide Web

17th century Painting of Galileo Galilei
Galileo Galilei
Galileo Project Science Group

Project Scientist Torrence Johnson
Probe Scientist Rich Young

To My Esteemed Fellow Scientists,

When I last wrote to you, the Galileo spacecraft, my namesake, was approaching that magnificent celestial body, Jupiter. The World Wide Web has provided me with the opportunity to observe its travels as well as your discoveries. What wonders your extraordinary machine has revealed! While it was almost inconceivable to my colleagues that the moons I observed nearly 400 years ago could attend Jupiter in its orbit, the nature of those moons revealed by your machine would have left them dumbfounded. Even my countryman, Dante Alighieri, in his most fevered imagination, could not have foreseen the complexity encompassing those worlds!

In my time, the Englishman William Gilbert demonstrated that the Earth itself was like a giant lodestone. While unknown in its mechanism, mariners had for years used the lodestone's magnetic effect on compasses to navigate the seas. Jupiter is also now known to possess an extreme amount of such magnetic force. Your investigations have shown that even Ganymede has such magnetic force, the first and only moon known to have such a characteristic.

Color photo of cracks and freckles on the surface of Europa.
A Galileo spacecraft image of Europa's surface.

The ingenuity of your magnetic sensing continued to the exploration of Europa, leading to the inference of a subsurface ocean therein, a body of water even larger than the one that covers the surface of our own world! Combined with views of the almost craterless surface, so very unlike my own observations of our profusely cratered Moon, you were able to support the earlier Voyager-inferred youthfulness of Europa's surface. And those huge ice rafts, analogous to icebergs in our own oceans, lent further credence to the idea that a vast ocean exists but a short distance under the currently frozen crust.

Of course, it is necessary to discuss the atmospheric Probe, which in December 1995, bravely made the first direct measurement of Jupiter's outer layers, in effect touching and tasting that planet. I, too, watched as the Probe experienced a very turbulent ride during its nearly one-hour descent, when it measured the water abundance to be much lower than that of the Sun, a perplexing result. Apparently, the Probe had entered a desert-like area of Jupiter where dry air was rising. I understand it is now thought that this region is not representative of Jupiter as a whole, and lightning models using other probe data suggest that the overall amount of water is at least half that of the Sun. Interestingly, carbon, sulfur, and nitrogen abundances were three times that of the Sun. Could it be that comets may have delivered those chemicals to Jupiter after its formation?

With each increment to our knowledge it becomes obvious that many phenomena that we see on Earth have analogs in the cosmos. With the patience of a hunter, your Galileo has spied volcanic lakes and lava fountains on Io that are hotter than a blacksmith's furnace. I was amazed to learn the craft has also proven that the rings of dust around Jupiter's midsection are supplied as a consequence of the endless meteoritic impacts on the small inner moons. Even tenuous atmospheres around Ganymede and Callisto have been detected by your sensors.

As if by magic, your craft was able to "see" inside the moons using the new idea of gravity sensing, determining that Europa, Io, and Ganymede have metallic cores; a similar assessment for Callisto will have to wait for the next emissary, the Europa Jupiter System Mission, a joint mission between the space-faring nations of the Old and New Worlds planned for 10 years hence. That mission promises to again revolutionize our knowledge of Jupiter and his retinue, both building on past glories and opening doors unimagined today.

For almost eight years, the Galileo craft explored the Jovian system diligently. While I was sad when the spacecraft perished in September 2003 in a fiery crash into Jupiter himself, I am comforted that the knowledge it provided will last into eternity.
What a joy it has been for me to observe the travels of your spacecraft and to follow the resulting discoveries and conclusions via the World Wide Web!

My deepest regards and with great anticipation of the future missions, I remain your humble servant,

Galileo Galilei

(Translated by Jan Ludwinski, Jean Aichele, and Bruce McLaughlin, 14 October 2009)

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Last Updated: 24 January 2011

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Last Updated: 24 Jan 2011