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Galileo Passes Close by Jupiter’s Moon Callisto

November 5, 1996

LOS ANGELES (AP) _ The unmanned Galileo spacecraft on Monday passed within 686 miles of Jupiter’s Callisto, taking a close look at ancient craters left by two huge meteors that slammed into the frozen moon.

Callisto is believed to be one of the most heavily cratered objects in the solar system.

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena received the signal 46 minutes after the flyby Monday morning.

``Everything is going very smoothly,″ said Galileo project manager Bill O’Neil.

Scientists hope the close approach will hint about how Jupiter formed, but they will have to wait until next week to begin playing back Galileo’s taped data.

They also are counting on some help from down under to capture the data, since the spacecraft’s high-power antenna jammed in 1991.

On Friday, to maximize information from Galileo’s low-gain antenna, scientists began linking antennas at several powerful tracking stations near Barstow, Calif., and at Canberra, Australia.

The idea is ``to make the biggest ear we can possibly make″ for Galileo’s relatively weak radio signal, O’Neil said.

With a diameter of 2,986 miles, Callisto is nearly as big as Jupiter’s moon Ganymede, the largest moon in the solar system at 3,269 miles across. Earth’s moon has a diameter of 2,155 miles.

Callisto also is believed to have a rocky core and a deep, icy crust like that of Ganymede, which the Galileo spacecraft closely examined in June and September.

Heavily pockmarked with craters from space debris hits, Callisto appears to be the oldest and least active of the four major Jovian moons discovered by Italian astronomer Galileo Galilei in 1610. It’s also the farthest from Jupiter.

The Voyager spacecraft passed by Callisto in 1979 and found fewer signs of earthquakes and volcanoes than on the surfaces of the other moons, said Ken Klaasen, a member of the Galileo imaging science team.

Galileo looked closely at two ancient basins, named Asgard and Valhalla, formed when meteors slammed into Callisto.

``We want to try to understand how the surface has changed since the time of those impacts; what kinds of geologic forces caused those changes to happen,″ Klaasen said.

During this near-pass, which lasts a week, Galileo also will get its closest peek yet at the frozen ocean of Europa. But the best closeup of that Jovian moon will come on Dec. 19 when the spacecraft zooms within 435 miles of the surface.

Galileo, launched from a shuttle in 1989, began a two-year orbital tour of Jupiter and its major moons last December.

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