Turn left at Callisto
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The main purpose of today's flyby was to use the gravitational pull of Callisto to modify Galileo's orbit. After 14 months spent carefully studying Europa, Galileo is beginning a series of four encounters with Callisto spanning a six month period designed to bring the craft closer to Jupiter and Io.
Right: Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite Galileo image. On the left, over Io's limb, a new bluish plume rises about 86 miles above the surface of a volcanic caldera known as Pillan Patera. In the middle of the image, near the night/day shadow line, the ring shaped Prometheus plume is seen rising 45 miles above Io while casting a shadow to the right of the volcanic vent. Io is about the size of the Moon. More information.
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While mission planners are eager for a closer view of the giant planet, what they really have their eye on is Io, the innermost of Jupiter's large satellites. Io is one of the most exotic places in the solar system. It is literally bursting with volcanoes that spew sulfurous plumes over 300 km high. One called Prometheus may have been active for at least 18 years! In October or November, after a series of four orbit-changing encounters with Callisto, Galileo is scheduled to make two daring close approaches to Io, possibly flying through a volcanic plume.
Volcanic ejecta change the appearance of Io's surface on a daily basis and sulfurous material that escapes the moon form a gigantic torus of gas circling Jupiter. Nestled inside Jupiter's magnetosphere, the "Io torus" is enormous. With a diameter the size of Io's orbit it spans 844 thousand km and has an important impact on Jupiter's magnetic environment. As Io moves along its orbit and through this magnetized plasma torus, a huge electrical current flows between Io and Jupiter. Carrying about 2 trillion watts of power, it's the biggest DC electrical circuit in the solar system.
The dozens of active volcanoes on Io result from 100 meter high tides raised in its otherwise solid surface by nearby Jupiter and the other Galilean satellites. Although this process is fairly well understood, much of Io's forbidding environment remains a mystery. Galileo's final two orbits will feature close flyovers from 611 kilometers, then 300, kilometers away. Suspense will be high as Galileo flies right over Pillan Patera's active plume of frozen sulfur. If all goes well, Galileo's instruments will capture breathtaking images with 6 meters resolution.
In the vicinity of Io the radiation environment is severe, strong enough to kill a human. Nevertheless, Galileo's mission may not end with the Io flybys.
Whatever the fate of Galileo, the Io flybys will be a hard act to follow.
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Portions of this story were taken from the Water, Fire and Ice web site maintained by JPL, and from the "Today on Galileo" series by Ed Hirst, a Galileo mission planner. Web Links
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Galileo home page at JPL, with the latest on Europa, Callisto and Io
Io from the SEDS Nine Planets web site
Callisto from the SEDS Nine Planets web site
Jupiter from the SEDS Nine Planets web site
Hubble images of Io from the Space Telescope Science Institute
Io: The Prometheus Plume Aug. 18, 1997 Astronomy Picture of the Day
Close-up of an Io volcano Aug. 4, 1995 Astronomy Picture of the Day
Sizzling Io July 6, 1998 Astronomy Picture of the Day
Jet Propulsion Laboratory home page
Galileo buzzes Europa -- Feb. 2, 1999. Galileo executes a close flyby of Europa for the last time during the current mission.
The Frosty Plains of Europa -- Dec. 3, 1998. As Galileo returns new images of Europa, NASA scientists prepare to study samples from a potentially similar environment here on Earth.
Callisto makes a big splash -- Oct. 22, 1998. Scientists may have discovered a salty ocean and a possible ingredient for life on Jupiter's moon.
Galileo takes a close look at icy Europa -- Oct 2, 1998. The spacecraft flew within 2300 miles of the mysterious satellite last weekend.
Clues to possible life on Europa may lie buried in Antarctic ice -- Mar. 5, 1998. Exotic microbial forms turn up in ice above Antarctica's Lake Vostok.
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|For more information, please contact:
Dr. John M. Horack , Director of Science Communications
|Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack