Galileo Millenium Mission Status 16 Aug 2001

NASA’s Galileo spacecraft is transmitting to Earth scientific information from its dash past
Jupiter’s moon Io last week, including top-priority measurements of magnetic forces above Io’s north
pole.

A first look at the data already suggests that Io’s internally generated magnetic field is either
absent or quite weak, said the principal investigator for Galileo’s magnetometer instrument, Dr.
Margaret Kivelson of the University of California, Los Angeles. Stronger conclusions will have to
await more analysis. Determining whether Io has an intrinsic magnetic field is the primary science
goal for the flyby on Aug. 6, Universal Time, and for Galileo’s next encounter, an Oct. 16 pass near
Io’s south pole.

Sampling of other early data indicates that Galileo’s camera appears to have resumed
functioning in time to capture some of the final images planned during last week’s flyby, said Dr.
Eilene Theilig, Galileo project manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.
Some exposures planned while Galileo was closest to Io were lost because of an intermittent
electronic problem that has affected the camera for more than a year. Project engineers believe the
problem probably results from cumulative exposure to intense natural radiation near Jupiter.

“We’re now expecting to get images from five of the 16 planned observations, including
global images of Io,” Theilig said. Stored data on Galileo’s tape recorder from the camera and other
instruments will continue to arrive at Earth antennas of the JPL-managed Deep Space Network
throughout the next eight weeks.

Galileo has been orbiting Jupiter since 1995. After its original two-year tour of the Jovian
system, NASA extended the intrepid robot’s mission three times to take advantage of Galileo’s
continuing ability to return valuable scientific information. The spacecraft has handled more than
three times as much radiation from Jupiter’s radiation belts as it was engineered to withstand. Its
nuclear electrical power source continues to provide reliable electricity for its on-board
instruments, computers, radio and other systems.

Data from the Aug. 6 flyby are still being analyzed to determine whether Galileo flew
through an active plume as it skimmed over Io. The Tvashtar volcanic area near Io’s north pole was
spouting a tall plume when last observed seven months earlier. Io is the most volcanic world known.
Observations by Galileo’s infrared and radiometer instruments, as well as the camera’s images,
should provide new details about volcanic activity near the pole, Theilig said.

Determining whether Io has an intrinsic magnetic field will narrow the possible models for
the moon’s interior and give better understanding of planetary interiors in general, said Galileo
Project Scientist Dr. Torrence Johnson of JPL. Magnetic fields such as those that exist at Earth,
Jupiter and Jupiter’s moon Ganymede are generated by movement of fluid material deep inside.

Io’s density and heat output tell scientists that it, like Earth, has a molten iron core. If Io
indeed lacks an intrinsic magnetic field, that would suggest its molten core lacks the vigorous
convective motion that is believed to result in Earth’s field, Johnson said. “That might fit a model
where Io’s core is swaddled by a hot, warm blanket that heats the core from outside,” he said.

Fluctuations in the pull of Jupiter’s gravity as the moon’s distance from the planet varies are
believed to generate much of Io’s internal heat by flexing a stiffer mantle layer surrounding Io’s core.
New magnetic data will help scientists evaluate and refine that model of Io’s heating, Johnson said.

Data from instruments that monitor particles and energy fields around the spacecraft will also
help in understanding currents that couple Io to the atmosphere and ionosphere of Jupiter, he said.

Additional information about Galileo, Io and Jupiter is available online at
http://galileo.jpl.nasa.gov. JPL, a division of the California Institute of Technology in Pasadena,
manages the Galileo mission for NASA’s Office of Space Science, Washington, D.C.