A Big Moon Close Up

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A Big Moon Close Up

NASA's Galileo spacecraft zoomed by the biggest moon in the Solar System last Saturday.

May 23, 2000 -- On Saturday, May 20, 2000, NASA's Galileo spacecraft successfully flew past the largest moon in our solar system -- Ganymede, which orbits around Jupiter. Galileo dipped to 809 kilometers (503 miles) above the surface in the spacecraft's first flyby of Ganymede since May 7, 1997.

"It's great that things went so smoothly," said Galileo Project Manager Jim Erickson of NASA's Jet Propulsion Laboratory (JPL). "The team was ready for any problems, but they got to relax on this one. We're really looking forward to the new pictures and learning more about this largest of all moons."

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Above: Ganymede, which orbits Jupiter, is slightly larger than the planet Mercury and more than three-quarters the size of the planet Mars. If it orbited the Sun, Ganymede would surely be considered a planet. Shown here in their correct relative sizes are a Hubble Space Telescope image of Mars, a Galileo image of Ganymede, a Mariner 10 mosaic of Mercury (the smooth stripe represents an area of missing data), and a Galileo spacecraft picture of the Moon.

At 4:00 a.m. PDT, mission controllers at JPL received a signal indicating that the flyby had taken place. The spacecraft's camera and other instruments were set to capture the flyby with images and other observations. If all goes as planned, the data will be transmitted to Earth over the next several months for processing and analysis.

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To fly by Ganymede, Galileo had to approach Jupiter's powerful radiation belts. Not surprisingly the radiation -- which can affect spacecraft instruments, components and systems -- did cause two main re-sets of Galileo's main computer. Onboard software correctly diagnosed this as a false indication, and proceeded with the Ganymede encounter as programmed.

"It appears that this workhorse spacecraft has done it again," Erickson said. Galileo has already survived three times the radiation it was designed to withstand.

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Jupiter's largest moon Ganymede is even larger than Mercury and Pluto. Its surface is a mixture of clean white ice and dirty dark ice, with varied geological formations including craters, basins, grooves and rough mountain areas.

Galileo was launched from the Space Shuttle Atlantis on October 18, 1989. After a long six-year journey to Jupiter, Galileo began orbiting the huge planet and its moons on December 7, 1995. It successfully completed its two-year primary mission on December 16, 1997. That was followed by a two-year extended mission which concluded in December 1999. Galileo is now continuing its studies under yet another extension, called the "Galileo Millennium Mission."

JPL, a division of the California Institute of Technology in Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, DC.

Web Links

Great Ganymede! -Describes the science objectives of Galileo's May, 2000, flyby.

Ganymede -from the SEDS Nine Planets

The Facts about Ganymede -from JPL's Galileo Home Page

Galileo Plasma Wave Investigation: Observations at Ganymede -This web site hosted by the University of Iowa summarized the evidence that Ganymede has its own magnetic field.

Galileo Finds a Magnetic Field on Ganymede -- 1996 JPL press release

Jupiter's Largest Moon Has a Oxygen Thin Atmosphere -- 1996 Hubble press release