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JOVIAN DREAMS
Galileo Comes Alive On Callisto Final Approach: Camera Snaps Last Flyby

Callisto [kah-LISS-toe] is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It orbits just beyond Jupiter's main radiation belt. Callisto is the most heavily cratered satellite in the solar system. Its crust is very ancient and dates back 4 billion years, just shortly after the solar system was formed. Callisto lacks any large mountains. This is probably due to the icy nature of its surface. Impact craters and associated concentric rings are about the only features to be found on Callisto. More on Callisto at Planetscapes.com
Pasadena - May 25, 2001
NASA's Galileo spacecraft has successfully completed a flyby of Jupiter's moon Callisto, closer than any of the spacecraft's 30 previous flybys of Jovian moons.

Galileo's camera appeared to be working well from the time it was given a command Thursday afternoon to turn off then back on, right through and after the spacecraft's closest approach to Callisto at 4:24 a.m. (PDT) today, said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Earlier, the camera appeared to be malfunctioning and probably did not capture some intended images taken of the moon Io from greater distance.

Other instruments appear to have worked well throughout the encounter, Theilig said. "This incredible spacecraft has come through for us again." she said.

Galileo passed about 138 kilometers (86 miles) above the surface of Callisto. The spacecraft has been orbiting Jupiter since 1995. Its closest previous encounter came within about 198 kilometers (123 miles) of the volcanic moon Io in February 2000.

Today's pass was designed to use Callisto's gravity to alter the shape of Galileo's orbit so that the spacecraft will fly near Io in early August. As a bonus, the flyby gave scientists an opportunity to point their instruments for a close look at Callisto, a heavily cratered moon about the size of the planet Mercury.

"It appears Galileo is on track for a polar pass by Io in August," Theilig said. "Because this spacecraft has already outlived expectations, the flight team prepared for contingency situations, but is always relieved to get through without encountering significant problems."

If all goes well, images and other data will be transmitted to Earth by Galileo over the next two months, with an interruption of three weeks in June when Jupiter and Galileo will be behind the Sun from Earth's point of view.

Intense radiation near Jupiter poses a risk to the spacecraft's electronics. Galileo's closest approach to Jupiter on this orbit was at a distance of about 460,000 kilometers (about 285,000 miles) from the giant planet's cloud tops on May 23. It will pass about 20 percent closer than that to Jupiter the same hour it flies by Io in early August.

Galileo, built at JPL, has already received more than three times the cumulative radiation exposure it was designed to withstand and has continued making valuable scientific observations more than three years after its original two-year mission in orbit around Jupiter. The spacecraft's nuclear electrical power source -- two radioisotope thermoelectric generators continues to provide power to Galileo's instruments, computers, radio and other systems.

The radio signals indicating today's Callisto flyby had taken place traveled for about 50 minutes at the speed of light and reached a large dish antenna at the Madrid station of NASA's Deep Space Network at about 5:15 a.m. PDT. The network relayed the signals to mission controllers at JPL.

As of 11 a.m. today, the spacecraft had recorded about 90 percent of the scientific data that its instruments had been programmed to collect during this swing through the inner portion of the Jovian system. During the weekend, Galileo is scheduled to make additional observations of Callisto and of Jupiter's clouds.

Magnetometer readings by Galileo during earlier flybys of Callisto indicated that this moon may have a layer of melted, salty water deep beneath its surface. However, unlike its sister moon Europa, which is likely to have liquid water much nearer its surface, Callisto shows a heavily cratered surface bearing the record of impacts by comets and other objects over billions of years. High-resolution images can help scientists understand the bombardment history of the Jovian system.

Related Links
Galileo at JPL
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JOVIAN DREAMS
Keep Galileo's Eyes Open, Say Petitioning Scientists
Leavenworth - April 24, 2001
NASA recently extended the successful Galileo spacecraft's mission until January 2003 to continue study of Jupiter's fascinating moons, particularly the extremely volcanic moon Io. But scientists now say that a planned powerdown of Galileo's imaging suite at the end of this year will hamstring efforts to solve Io's many mysteries.



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