The closest-ever image of Jupiter's moon Io, taken during a daring flyby of the volcanic moon by NASA's Galileo spacecraft on October 10, 1999, shows a lava field near the center of an erupting volcano.
The image, available at http://www.jpl.nasa.gov/pictures/io , was taken from an altitude of 671 kilometers (417 miles) and is 50 times better than the previous best, taken by the Voyager spacecraft in 1979.
Visible in the image are new lava flows from the volcanic center named Pillan, an area with erupting lava hotter than any known eruption that occurred on Earth within billions of years. Scientists will be studying this image to determine the characteristics of the eruption, along with other data due to be sent back by the spacecraft in coming weeks.
Not surprisingly, fierce radiation took its toll on the spacecraft. Io's orbit lies in a region of intense radiation from Jupiter's radiation belts, which can affect the performance of or even knock out various spacecraft instruments. A mere fraction of the dose that Galileo received would be fatal to a human. Because of the radiation risk, the Io encounters were scheduled for the end of the two-year extended mission, after the spacecraft had already fulfilled its other mission objectives.
Most of the Io images were taken using a "fast camera" mode, where the camera itself pre-processes the image to average the brightness in adjacent parts of the picture. Galileo engineers say it appears that Jupiter's radiation caused the process to get out of sync, which degraded the quality of the images. Fortunately, images that were taken in other camera modes, including the newly released image, apparently did not suffer ill effects from the radiation.
"When we're flying the spacecraft through this high- radiation zone near Io's orbit, we have to plan for the likely radiation and figure out how to deal with it," said Galileo Project Manager Jim Erickson. "We used several different modes to see how each would work. Now that we know this particular camera mode didn't work well amidst the radiation, we'll use other modes from our six different types for the next Io flyby."
That second Io flyby is scheduled for November 25 at an altitude of only 186 miles (300 kilometers).
Galileo's original mission was to spend two years studying Jupiter, its moons and magnetic environment. That mission ended in December 1997, then was followed by a two-year extended mission scheduled to end in January 2000. Galileo, the first spacecraft to orbit Jupiter, has revolutionized our knowledge of the giant planet and its moons and has provided thousands of colorful images.
During the October 10 Io flyby, the radiation also apparently triggered a problem with Galileo's near-infrared mapping spectrometer. The instrument has a grating that allows it to measure different wavelengths of light as they are reflected onto a sensor. This enables the instrument to produce a spectrum of the light from objects it observes. During the flyby, the grating did not move as it should have, which means that only one set of wavelengths was measured instead of the complete spectrum. The resulting data provides maps at each of several wavelengths in very high spatial resolution. These maps can be used to show the distribution of materials on the surface and measure the temperature of the lava in Io's volcanoes, but detailed spectral information for identifying materials on the surface will be limited to the early part of the encounter where full spectral data were acquired.
The Galileo flight team is still evaluating the status of another instrument, the ultraviolet spectrometer, which has been acting up for two months. Since this instrument was not scheduled to be used during the Io encounter, it was switched off while engineers diagnose its grating problem.
Additional information and pictures taken by the Galileo spacecraft are available at the mission's web site: http://galileo.jpl.nasa.gov.
Galileo was launched from the Space Shuttle Atlantis on October 18, 1989. It entered orbit around Jupiter on December 7, 1995. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is operated for NASA by the California Institute of Technology, Pasadena, Calif.
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