This true-color simulated view of Jupiter is composed of four images taken by Cassini on December 7. Photo: NASA/JPL/University of Arizona. SEE THE FULL PAGE IMAGE

Joint observations of Jupiter by NASA's Cassini and Galileo spacecraft are providing an unprecedented look at the giant planet's atmosphere and magnetosphere, scientists said Saturday.

Cassini made its closest approach to the solar system's largest planet at 5:12 a.m. EST (1012 GMT) Saturday, passing 9.7 million kilometers (6 million miles) from Jupiter's cloud tops. The spacecraft has been observing the planet for several weeks in conjunction with Galileo, a spacecraft in orbit around Jupiter for more than five years.

While Cassini's presence near Jupiter is primarily designed to give the spacecraft a gravity assist to enable it to continue to Saturn, the flyby also provides "a unique scientific opportunity afforded by the fact that we have two operating planetary spacecraft in the vicinity of Jupiter," said Jay Bergstralh, acting director of solar system exploration at NASA headquarters and Cassini program scientist.

Cassini's flyby, known officially as the Jupiter Millennium Flyby, marks the first time that two independent missions have simultaneously observed any planet other than the Earth.

Some of those taking advantage of the flyby are scientists examining the planet's powerful magnetosphere. While Galileo is located within the magnetosphere, Cassini was, until a few days ago, outside of it, giving scientists a different perspective on the magnetosphere and its interaction with the solar wind.

An artist's concept of Cassini during the flyby of Jupiter. Photo: NASA/JPL

The passage of Cassini through the magnetosphere's bow shock early Thursday -- one day earlier than predicted -- indicates to scientists that the magnetosphere is considerably larger than expected.

"We're seeing a magnetosphere twice the size of what it was during Voyager 1 encounter," said William Kurth of the University of Iowa.

Kurth speculated that a decrease in the solar wind measured by the ACE spacecraft near the Earth two weeks earlier might have decreased pressure on the magnetosphere, allowing it to expand. Such an observation was only possible with the existence of a second "weather station," as Kurth described Cassini, located outside of the solar wind.

Cassini has also taken the first images of Jupiter's magnetosphere, using a special instrument on the spacecraft sensitive to ions and electrons. "This is an image that has never been made before, since this is an instrument that did not previously exist," said Stamatios "Tom" Krimigis, principal investigator for the Magnetospheric Imaging Instrument team. Analysis of those images is ongoing, he said.

The atmosphere and volcanic hotspots of Jupiter's moon Io are apparent in this view of the moon in eclipse, taken by Galileo. Photo: NASA/JPL

In addition, a spectrometer on Cassini detected oxygen, sulfur, and sulfur dioxide ions in the magnetosphere up to 21 million kilometers from the Galileo moon Io, whose volcanoes are thought to be the source of those ions. "This represents a new dimension that we did not know before," said Krimigis, "namely that there is a big nebula that is created by the volcanoes of Io that actually accompanies Jupiter as it moves around the Sun."

Scientists are also pleased with the images being returned by Cassini's camera. "The imaging experiment on Cassini has been very successful so far, and has in fact performed beyond our wildest imaginings, which is saying something since we've been imagining this for a decade now," said the University of Arizona's Carolyn Porco, Cassini camera team leader. The camera's "remarkably stable" platform, she explained, creates images that are "startlingly sharp and clear."

Porco and colleagues have used Cassini's powerful camera to look at Jupiter, its rings, and moons. Because of the distant nature of the flyby, Cassini is not able to take the same high-resolution images Galileo is capable of. Instead, they are focusing on temporal variability, looking for changes in features over time.

"It turned out to be a very complimentary set of observations," Porco said. "Galileo goes in for the very high-resolution close-up view, and we stand back and watch things change with time."

NASA's Galileo spacecraft captured this false-color image of a storm on Jupiter. The inset shows areas of lightning at the same location as the storm, as viewed from the planet's dark side. Photo: NASA/JPL

Those observations include images of Jupiter's faint rings: while the Cassini images may not show temporal variations in the rings, Porco said observations of the small moons associated with the rings will allow scientists to refine their orbits and see if the moons themselves are the source of the material in the rings.

Joint observations by Galileo and Cassini are also planned to study Io and look for any glows and emissions as the moon's tenuous atmosphere interacts with Jupiter's magnetic field.

Cassini is also performing time-lapse imaging of Jupiter to probe the circulation of the planet's atmosphere as various altitudes. Such images may provide insights into how storms are powered, said Andrew Ingersoll, a Caltech planetary scientist who is on both the Galileo and Cassini imaging teams.

Two competing explanations exist: a "parasite" model where small storms feed off the energy of larger ones, and a "fish" model where large storms are powered by assimilating smaller storms.

Galileo captured this false-color image of a storm on Jupiter. The inset shows areas of lightning at the same location as the storm, as viewed from the planet's dark side. Photo: NASA/JPL. SEE THE FULL PAGE IMAGE

Data from the two Voyager spacecraft and Galileo have tended to support the "fish" hypothesis, but until Cassini scientists have been unable to put all the pieces together. "Cassini has a Galileo -- or better -- camera and has the coverage in time Voyager has," explained Ingersoll. While the images are still being analyzed the preliminary data also supports the fish model.

Such imaging "has never been done before, despite the fact that there have been several spacecraft that have visited Jupiter so far," explained Porco. "Because of the distant nature of this flyby, the imaging team took as its highest objective to image the atmosphere of Jupiter in a way Galileo could not."

Galileo had intended to perform similar time-lapse imaging, but those plans were set aside after problems with Galileo's antenna early in its mission lowered the rate at which the spacecraft could transmit data, sharply reducing the number of images it could return. "Cassini is ideal," said Ingersoll. "It totally recoups the problems we had with the Galileo antenna. I'm as happy as a clam."

Cassini will continue to observe Jupiter through March as it moves away from the planet, including observations of the planet's night side as well as continued time-lapse imaging of the planet's atmosphere. Scientists also plan to spend months analyzing the data returned by Cassini and Galileo.

"Stay tuned, folks," advised Ingersoll. "There's a lot more to come."