The Galileo probe to Jupiter and its moons was launched aboard the space shuttle Atlantis in 1989. After flybys of Earth, Venus and the asteroid belt, Galileo approached its final destination in 1994 and returned images of the comet Shoemaker-Levy 9 crashing into Jupiter.
A parachute probe, which separated from from the orbiter in 1995, gathered data on the planet's turbulent atmosphere as it descended. Galileo continued to orbit Jupiter and make close flybys of its main moons until it was intentionally destroyed in 2003.
Photo: Artist's impression of the Galileo probe arriving at Jupiter (NASA)
The careful planning that went into the Galileo mission to Jupiter couldn't predict that the atmospheric probe would fall between Jupiter's clouds during its descent in 1995 and miss out on key data.
Images from the Galileo and Voyager missions (launched in 1989 and 1977 respectively) showed scientists that Europa is a relatively smooth icy moon with a network of fractures that may erupt liquid water from an ocean beneath the surface.
BBC News's Dr David Whitehouse reports on the Galileo entry probe's plunge into Jupiter and its early findings. NASA scientists said that the atmosphere contains ammonia, hydrogen sulphide and methane, a combination of smelly gases.
Galileo was an unmanned NASA spacecraft which studied the planet Jupiter and its moons. Named after the Renaissance astronomer Galileo Galilei, it was launched on October 18, 1989, by the Space Shuttle Atlantis on the STS-34 mission. Galileo arrived at Jupiter on December 7, 1995, via gravitational assist flybys of Venus and Earth, becoming the first spacecraft to orbit Jupiter. Despite suffering from antenna problems, Galileo conducted the first asteroid flyby near 951 Gaspra and discovered the first asteroid moon, Dactyl, around the asteroid 243 Ida. It furthermore launched the first probe into Jupiter's atmosphere. The mission's total cost was estimated at approximately US$1.4 billion.
The spacecraft measured the atmospheric composition of Jupiter and directly observed ammonia clouds, which seem to be created by an outflow from the lower depths of Jupiter's atmosphere. Galileo also registered Io's volcanism and the plasma interactions between its and Jupiter's atmospheres. Other studies gave support for the popular theory of liquid oceans under the icy surface of Europa. There were also indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto, while Ganymede was shown to possess a magnetic field. New evidence was also found for the existence of exospheres around Europa, Ganymede, and Callisto.
Galileo furthermore discovered that Jupiter's faint ring system consists of dust from impacts on the four small inner moons. The extent and structure of Jupiter's magnetosphere was also mapped. In 1994, Galileo provided the only direct observation of Comet Shoemaker-Levy 9's impact into the atmosphere of Jupiter.
On September 21, 2003, after 14 years in space and 8 years in the Jovian system, Galileo's mission was terminated by sending the orbiter into Jupiter's atmosphere at a speed of over 48 kilometres (30 mi) per second, to avoid any chance of it contaminating local moons with terrestrial bacteria. Of particular concern was the ice-crusted moon Europa, which, due to Galileo's findings, scientists now suspect harbors a potentially life-supporting subsurface saltwater ocean.
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