Galilean Moons of Jupiter

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The Galilean Moons of Jupiter

The Historic Galileo Mission

Donna L. Martin

A documentation of The Galilean Moons which includes the history of the Galileo Mission.


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In 1610 Galileo discovered four moons orbiting the planet Jupiter also known as the Failed

Star. In a proposal by Simon Marius they were officially named the Galilean moons. They are

the largest moons in our solar system. The discovery of Galileo made very radical changes in the

history of science and religion and has continued to do so in our time. 1977 almost four hundred

years after Galileos discovery official planning started to send a spacecraft in exploration of the

Jovian System. The first in depth look at Jupiter and its moons was the Galileo Mission.

The Galileo Mission was originally scheduled to be launched in 1986 but was unable due to the

tragedy of the Challenger. During this time changes because of safety concerns were made to the

propulsion design of the Galileo. The changes made to this resulted in the VEEGA Trajectory.

The VEEGA which means Venus-Earth-Earth-Gravity-Assist would allow the Galileo to run off

borrowed energy from Venus and Earth using their gravity to project it forward. The Galileo

would also run off radioisotope thermoelectric generators which were provided by the

Department of Energy.

The Galileo was launched in October 1989 from the Space shuttle Atlantis. It would provide

communications to NASA through the programs Deep Space Network. The transmissions

would take approximately 50 minutes to reach Galileo from Earth. It would arrive at Jupiter in

December 1995. The Galileo would orbit Jupiter for thirty-four times in eight years. Each orbit

would be named using the first letter of the moon it was closest to and the number of the orbit.


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On its way to Jupiter the Galileo made several new discoveries. In 1990 as it flew past Venus it

took pictures and tried out its instruments for the first time by testing the carbon dioxide and

sulfuric compounds of its clouds. In 1990 when the Galileo flew past Earth it was able to detect

small amounts of life in our atmosphere and also give us new pictures showing a different view

of the Earth and Moon together. The Galileo had to fly through the asteroid belt on its way to

Jupiter and it was able to capture over 150 remarkable images of several asteroids. In 1991 the

Galileo took very detailed pictures of the Gaspra asteroid and its many craters. In 1993 it also

discovered the Ida asteroid to have its own moonlet (Dactyl). While getting closer to Jupiter the

Galileo was able see the fragments from the Levy-Shoemaker 9 comet crash into Jupiter.

As the Galileo entered Jupiters atmosphere it sent out a probe to take what would be the first

measurements of the local weather including wind speeds and the amount of water in the

atmosphere. The probe would also document evidence of Jupiters evolution in relation to the

Sun. The results of the probe showed 450 mile per hour winds and no water in the clouds.

Scientists at NASA believe the probe accidently entered a Hot spot. The probe which was

covered by a heat shield descended further into the atmosphere where it was eventually crushed

by the pressure.

The Galileo was the first satellite to observe ammonia in another planets clouds. It also

documented huge lightning storms on Jupiter that is more powerful than any lightning we have

on Earth. From 1995 to 1997 Galileos new mission was to focus on Jupiters four largest

moons: Io, Ganymede, Callisto, and Europa, the Galilean Moons.


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Io is Jupiters innermost moon and approximately 2,236 miles in diameter. It is known to be the

densest and most volcanically active moon in our solar system. The data from Galileo shows that

Io does not eject lava but instead projects a sulfuric compound which gets caught into orbit and

creates a torus, or ring, around Jupiter. This also gives Io its orange color. It has no water but

does have liquid lakes of sulfur. The volcanic activity keeps it looking young because the craters

refill constantly. Io gets gravitational pull from Jupiter and its other moons which cause energy

to generate and produce volcanic activity. The Galileo also observed a magma ocean under Io

which helps to create the volcanic activity. In Journal Science Magazine a former investigator on

the Galileo team Krishan Khurana says, Scientists are excited we finally understand where Ios

magma is coming from and have an explanation for some of the mysterious signatures we saw in

some of the Galileos magnetic field data. Io has an iron core over 560 miles in diameter and it

is similar to our planet because we had a magma center which cooled over a billion years ago.

Ganymede is the largest moon in our solar system and is 3,268 miles in diameter. The Galileo

found it to be full of impact craters and three different types of terrains. One terrain is brightly

colored with numerous ridges and another is very dark thought to be over four billion years old.

It was found to have a core of iron and its own magnetosphere. Ganymede also has a thin

atmosphere of oxygen which is thought to be a product of the sunlight mixing with its surface.

Callisto was found to be the most cratered in our solar system. The Galileo found Callisto to

have no core but instead it is a mixture of rock, ice, and iron and a very thin atmosphere of

carbon dioxide. It has icy volcanoes and impact craters which are named Valhalla and Asgard.

Observations from Galileo did not show a magnetic field around Callisto as with Ganymede.


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Europa which was the reason for Galileos cataclysmic end was believed to have ocean water

beneath its mantle. It has a metallic core and its inner structure is thought to resemble Earths. Its

color is reddish brown and is thought to come from microorganisms. The data received from

Galileo proved the theory of an ocean below Europas surface to be correct. The interaction with

Jupiters magnetic field and produces a salty ocean. The atmosphere also has oxygen from the

water molecules that have seeped though geysers and interacted with the sunlight.

Even with all the data received the Galileo Mission had its share of problems. During the first

flyby attempt the large umbrella shaped antenna would not open and it caused a great deal of

reprogramming and upgrades to be done. NASA would not receive back as much information as

they originally planned. The trip to Io proved to be problematic because of all the radiation and it

caused the computers to go offline for awhile. The trip to Jupiter, still a success, was extended

until 2003 so that Galileo in a joint effort with Cassini could obtain more information about

Jupiter. It was renamed the Galileo Millennium Mission. It would look at the effects of radiation

on spacecrafts, as the Galileo had already received more radiation than it was designed to handle.

The Galileo was the first spacecraft to stay in such a close proximity of a planet to obtain

information about its structure. It measured particles in Jupiters rings to find out the

composition of them and the reason for their varied shapes. The Galileo had a computer

shutdown while doing its last flyby which was of the moon Almathea. NASA was able to

recover data after placing Galileo back online. The Galileo Mission ended in 2003 when NASA

directed it to burn up in Jupiters atmosphere so as not to upset possible life on Europa which

was one of Galileos major discoveries. The other major discovery was another theory of the

formation of our solar system and planet evolution. Plans to return to Jupiter are underway and

Juno should reach Jupiter in 2016.


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