NASA's $3.4 billion Cassini spacecraft passed Jupiter at the closest point of its trip Saturday, using gravity from the solar system's largest planet to swing it toward Saturn.

   While passing Jupiter over the next few months, Cassini will team up with the Galileo spacecraft to make dual observations of the planet and its surroundings.

   Scientists at the Jet Propulsion Laboratory steered Cassini to within 6 million miles of Jupiter early Saturday to give the spacecraft a gravity boost as it heads for a rendezvous with Saturn in July 2004.

   Galileo, which was launched in 1989 and began orbiting Jupiter in 1995, will also test Cassini's instruments for Saturn.

   ``We have a chance to make observations with a well-instrumented spacecraft that has more capabilities than any spacecraft that has previously visited Jupiter,'' Cassini program manager Bob Mitchell said Saturday. ``Fortunately, Galileo is still operating there, so we can get a synergistic effect in studies of Jupiter by having spacecraft at two different locations at the same time.''

   The Cassini spacecraft was launched in 1997 on an 11-year mission. While near Jupiter, it will help with studies of the planet's atmosphere, moons, faint rings and magnetosphere, the massive magnetic field surrounding the planet.

   The Galileo spacecraft, which has been hampered by a high-gain antenna that never fully deployed, already sits deep within the Jovian magnetosphere, while Cassini just entered it early Thursday, said Dr. Stamatios Krimigis, principal investigator for Cassini's magnetospheric imaging instrument.

   The unique vantage points offered by the two spacecraft should help solve some riddles about the magnetosphere and its relationship to the solar wind - the protons, electrons and ionized hydrogen particles that erupt from the sun and spread throughout the solar system.

   ``When you see changes in the magnetosphere on Galileo we never know if that is unique to that location or forced by pressure from the solar winds,'' Krimigis said. ``Now with two spacecraft we'll be able to know an answer to that very clearly.''

   Scientists already have learned that the magnetosphere has doubled in size from when the Voyager spacecraft made observations in 1979, Krimigis said. It is believed that a drop in solar wind pressure has allowed the magnetosphere to expand, he said.

   The Cassini spacecraft in November recorded ion acoustic waves emanating from the magnetosphere, which sound ``sort of like whales in ecstasy,'' said Dr. Jay Bergstralh, NASA' acting director of solar system exploration and Cassini program scientist.

   Officials said the Cassini spacecraft has overcome maneuvering problems that forced NASA earlier this month to temporarily halt Jupiter observations.

   A communications problem with a companion probe remains unresolved, Bergstralh said. That problem involves the European Space Agency's Huygens probe, which will detach from Cassini and parachute to Saturn's moon Titan in 2004.

   The Cassini's probe's flyby of Jupiter will end in March. It will study Saturn for four years beginning in 2004.