May 5, 1999

Turn left at Callisto

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Galileo heads for a daring encounter with Io's volcanoes
Galileo image of volcanoes on Jupiter's moon Io
May 5, 1999: This morning at 6:56 am Pacific Daylight Time NASA's Galileo spacecraft zoomed past Jupiter's moon Callisto at a distance of only 1315 km, 94 times closer than Voyager's closest approach in 1979. Early reports indicate that the encounter was a success and that Galileo is operating normally.

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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Scientists hope that a close-up view of Jupiter's atmosphere will reveal important new details of wind and storm patterns, including the billowing thunderstorms that grow to heights several times those on Earth. Water circulates vertically in Jupiter's top layers, leaving large areas drier than the Sahara desert, and others drenched like the tropics. Mapping the distribution of water and its role in Jupiter's weather might help scientists understand Earth's more fast-paced weather changes.

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.
Ra Patera, a large shield volcano on Io
Left: This Voyager image of Ra Patera, a large shield volcano on Io, shows colorful flows up to about 200 miles long emanating from the dark central volcanic vent. More information.

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.
Hubble image of volcano Pillan
Right: Io's volcanic plumes are so large that they can be seen from Earth orbit. In this Hubble image the mound rising from Io's surface is an eruption from Pillan, a volcano that had previously been dormant. Pillan's plume is very hot and its ejecta is moving at speeds of 1,800 mph. Based on information from the Galileo spacecraft, Pillan's outburst is at least 2,240 degrees Fahrenheit (1,500 degrees Kelvin). The hot sulfur dioxide gas expelled from the volcano cools rapidly as it expands into space, freezing into snow.

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.
JPL Callisto 21 flyby animations
Left: Ron Baalke and David Seal of NASA's Jet Propulsion Laboratory have prepared several computer-generated animations of today's Callisto flyby. Click here for viewing options.

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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Ice, Water and Fire background information about the Galileo Europa Mission from JPL

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

Related Stories:

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