Since its launch in 1989, the sturdy spacecraft traveled more than 4.6 billion kilometers (almost 2.8 billion miles), about the equivalent of seven times the distance between Earth and Jupiter. Despite communication problems and a temperamental tape recorder, Galileo returned 30 gigabytes of data, including 14,000 pictures.

This wealth of information drastically expanded our understanding of the solar system's biggest planet and its moons. The mission was possible because it drew its power from two long-lasting radioisotope thermoelectric generators provided by the Department of Energy.

Passing Asteroids
The exciting list of discoveries started even before Galileo was able to get a close glimpse of Jupiter. As it crossed the asteroid belt in October 1991, Galileo snapped images of Gaspra, returning the first ever close-up image of an asteroid. Less then a year later, the spacecraft got up close and personal with yet another asteroid, Ida. Images from Ida revealed the asteroid has its own little "moon," Dactyl, the first known moon of an asteroid.

Location, Location
In 1994 the spacecraft was in the right place at the right time and made the only direct observation of a comet impacting a planet. It took images of fragments of comet Shoemaker-Levy 9 crashing into Jupiter. Images of the impact, which was not visible from Earth, helped scientists better understand this type of event.

At Jupiter
Galileo began its tour of the jovian system in December 1995. Carefully designed orbits allowed the spacecraft to observe Jupiter's atmosphere, revealing numerous large thunderstorms many times larger than those on Earth, with lightning strikes up to 1,000 times more powerful than terrestrial lightning. Data collected by the descent probe made the first in-place studies of the planet's clouds and winds, and it furthered scientists' understanding of how Jupiter evolved. The probe also made measurements designed to assess the degree of evolution of Jupiter compared to the Sun.

As the first spacecraft in long-term residence in jovian orbit, Galileo also successfully studied the global structure and dynamics of Jupiter's magnetic field. Galileo also determined that Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons. Data also showed that Jupiter's outermost ring is actually made up of two rings, one embedded within another.

Moons' Wonders
Galileo extensively investigated the geologic diversity of Jupiter's four largest moons: Ganymede, Callisto, Io and Europa. Stunning images revealed the contrasting and changing surfaces of these moons.

Io has extensive volcanic activity, which is continually modifying the surface. The heat and the frequency of eruption can be 100 times more than that of Earth, something reminiscent of Earth's early days. The similarities make Io an ideal laboratory for the study of what Earth was like more than 3 billion years ago.

The moon Europa, Galileo unveiled, could be hiding a salty ocean up to 100 kilometers (62 miles) deep underneath its frozen surface. Images also reveal ice "rafts" the size of cities that have broken and drifted apart to create a scalloped and broken surface. There are also indications of volcanic ice flows, with liquid water flowing across the surface. These discoveries are particularly intriguing since liquid water is a key ingredient in the process that may lead to the formation of life.

The biggest discovery surrounding Ganymede was the presence of a magnetic field, the first moon of any planet known to have one. Images of this moon featured a faulted and fractured surface that demonstrated high tectonic activity. Like Europa and Io, Ganymede has a metallic core. Galileo magnetic data also provided evidence that Ganymede might have a liquid-saltwater layer as well.

Galileo determined that, while Callisto doesn't have a metallic core, its surface shows evidence of extensive erosion. Data collected raise the question of whether Callisto's surface may also hide an ocean.

Last Dance
Galileo's own discovery of a likely ocean hidden under Europa's surface raises the possibility of life there and concern about protecting it. For that reason, in its final victory lap the Galileo spacecraft will dive into the atmosphere of the gaseous planet and disintegrate. Predictably, some of the spacecraft findings raised intriguing questions that will have to be answered by future mission. But Galileo Galilei, the first modern astronomer, would be immensely proud of the discoveries made by the spacecraft that carries his name.

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