Press Release


By SpaceRef Editor
October 22, 1999
Filed under

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Contact: Jane Platt (818) 354-0880



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

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:

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.


SpaceRef staff editor.