Close observations of Jupiter first occurred in the early 1970’s. In 1972, the United States launched Pioneer 10, which became the first space probe to fly past Jupiter. This unmanned probe flew within 81,000 miles (130,357 km) of the planet on December 3, 1973. The probe measured Jupiter’s radiation belt and reported the amount of hydrogen and helium in the planet’s atmosphere. Several probes followed Pioneer 10 including Pioneer-Saturn and U.S. Voyager 1. These probes provided valuable information and photographs of Jupiter’s polar regions, Great Red Spot, magnetic field, and temperature.

The United States’ most recent spacecraft, Galileo, began a six-year journey to Jupiter in October 1989. The Galileo Orbiter and Probe were launched on the Space Shuttle Atlantis. The probe remained attached to the base of the Orbiter until about 5 months before arrival at Jupiter. In December, 1995, Galileo dropped the Probe to study the planet’s atmosphere beneath the clouds.

The Galileo Probe spacecraft design reflects the very demanding conditions of a first entry into the atmosphere of a gas giant planet. The Probe must traverse the intense inner radiation belts of Jupiter, which are far stronger than the Van Allen radiation belts about Earth, as it approaches the top of the atmosphere. No spacecraft has previously journeyed into such a high radiation environment. The Probe must also be rugged enough to withstand the extreme heat generated by the high-speed plunge into Jupiter’s atmosphere. Finally, the Probe’s scientific instruments and supporting systems must be able to function at an atmospheric pressure 20 times that of Earth, which the Probe will encounter in Jupiter’s atmosphere.

The Galileo Probe successfully descended into Jupiter’s atmosphere on December 7, 1995, and directly measured the atmosphere of the giant planet for the first time. Results from this most difficult atmospheric entry in the solar system have permitted us to better understand many of the scientific mysteries of the largest planet in our solar system. The Galileo Probe no longer exists and is now part of Jupiter’s atmosphere as expected. The Galileo Orbiter successfully entered orbit well above the cloud tops of Jupiter on December 7, 1995, and is currently observing the Jupiter system.

In June, 1996, the Galileo Orbiter began 18 months of very detailed imaging and other observations of Jupiter’s four large intriguing moons and atmosphere along with further measurements of the magnetosphere.

Various scientific discoveries by the Galileo Orbiter have been reported recently. In one discovery, a cloud of microscopic dust grains surrounding Jupiter’s large moon, Ganymede, has been found by Galileo’s Dust Detector System. Scientists believe this dust cloud is created when meteoroids slam into its surface.

Scientist are also poring over interesting findings about surface temperatures on Jupiter’s moon, Europa, gathered by Galileo’s Photopolorimeter Radiameter, which measures temperature and other traits of Jupiter’s atmosphere, clouds, and moons. Information reveals that while Europa’s daytime temperatures are as expected, its nighttime temperatures are puzzling. At night, it appears the temperatures vary considerably from place to place.

In a recent flyby of Jupiter’s pockmarked moon, Callistro, pictures reveal a cratered terrain that will help scientists calculate the age of its surface, and other observations of the young Bran crater, which offers a good view of Callistro’s crust.

Recently, NASA released photogrophs of Europa and Io that were taken in late 1999.  In March, 2000, NASA announced an extension of the Galileo mission through the end of 2000.  NASA hopes to accomplish two main objectives during this extension:

	An observation of the Jupiter system's magnetic environment.   This will be done in conjunction with the Cassini spacecraft in December, 2000.
	Two additional flybys of Ganymede, Jupiter's largest moon.

Results from this extended mission are expected by April, 2001.   Galileo mission planners are investigating the possibility of scheduling additional visits to Io and Callistro.  The Galileo mission has provided a wealth of information about Jupiter and its moons..

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