Galileo Spacecraft Visits Jupiter Moon

Early Thursday, the spacecraft Galileo made its first scheduled visit to Jupiter’s largest moon, Ganymede--embarking on a looping, 18-month journey around the giant planet and its satellites.

About 250 scientists and family members gathered at the Jet Propulsion Laboratory in Pasadena to celebrate the news, which arrived via radio signal at 12:04 a.m. For two hours, up to the point of closest approach, cameras on the spacecraft snapped pictures of the enormous moon, which is bigger than the planet Mercury.

Mission Director Neal Ausman called the encounter “up close and personal.” The pictures will be released to the press and public after they are beamed down to Earth next week.

“It was an exciting moment for everyone. Some of these people have dedicated most of their careers to this project,” JPL’s Franklin O’Donnell said.

Among other things, scientists will be looking for earthquakes on Ganymede’s rocky surface, which appears to have continental plates and fault lines much like Earth’s.

The only glitch of the day occurred when one of the spacecraft’s 10 detectors sent a message to the central computer to “Please turn us off,” Ausman said. “We wanted to take no risks, so we left it off,” he said.

The malfunction means that JPL scientists lost a portion of the picture they were building of Jupiter’s magnetic field, but it should not affect data from the moons.

The 2.5-ton Galileo has been beset with problems since its inception about two decades ago. Because of a change in propulsion systems, it was forced to take a six-year detour; then, in 1991, its giant antenna failed to unfurl. Without its main antenna, data and pictures trickle back at a rate far below that of a standard computer modem.

Galileo’s luck turned last Dec. 7, however, when the cone-shaped probe it dropped into Jupiter’s atmosphere landed right on target and rockets fired to steer the satellite into its present looping orbit.

For the next year and a half, Galileo will make a series of giant loops around the planet, repeatedly returning to four of Jupiter’s 16 moons. It will be propelled mostly by gravity assists from the moons themselves. “With the gravity assist, we’ll be back in 75 days,” said Ausman. “Plus, we save enormous amounts of propellant.”

The last close look at Jupiter’s planetary system came in 1979, when Voyager satellites flew by on their grand tour of the solar system. Galileo is getting 100 times closer.

Jupiter is of special interest to planetary scientists because the giant planet is a somewhat smaller version of our sun. (If it were three times bigger, it would ignite and become a star). Its large and varied moons are similar in many ways to the sun’s planets. By studying Jupiter’s miniature solar system, scientists hope to gain insights into how our solar system evolved from its primordial cloud of gas and dust.

The probe that entered Jupiter’s atmosphere last December beamed back news of extremely high winds and an unexpectedly dry climate. But while the probe had a scientific life of less than an hour before it was destroyed by Jupiter’s enormous pressures, the mother ship Galileo will make nine more close approaches--stopping by Ganymede three more times, and visiting Io, Europa and Callisto.