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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.
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. 
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. 
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. 
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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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: |
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