The main focus of the experiment will be Jupiters enormous magnetosphere, the invisible region surrounding the planet where its magnetic field holds sway. If somehow illuminated from within, said Galileo Project Scientist Torrence Johnson, Jupiters magnetosphere would appear as large as the full moon to someone standing on Earth.

"Initially, it was not thought Galileo would be alive, well and functioning at this point because of radiation damage."

Like a stream flowing against a stalwart rock, the solar wind constantly buffets Jupiters magnetosphere, molding it and allowing it to expand and contract with the whims of the suns own weather.

"The question is, how much is the change in the solar wind affecting what is going on at Jupiter?" Johnson said. "By actually seeing the changes in the solar wind and then seeing what happens within the magnetosphere, well be able to find out."

For most of the three- to four-month joint mission, Galileo will lurk inside the magnetosphere, while Cassini will remain outside, making its closest approach of 6.7 million miles (10.6 million kilometers) to Jupiter around December 30

That will allow Cassini to remain upstream, as it were, of the solar wind, measuring it before it comes into contact with the balloon-like expanse of Jupiters magnetosphere.

Days or hours later, Galileo will then measure any changes within the magnetosphere itself, which scientists can use to gauge the cause and effect of the solar wind.

NASA intended Cassini to fly by Jupiter all along in a bid to gain a slingshot-like gravity boost to carry it out to Saturn, where it will arrive July 1, 2004. The spacecraft, launched in 1997, has already made similar swings by Venus and Earth.

That Galileo will be there to greet it at Jupiter should prove an unexpected bonus to the scientific community.

"Initially, it was not thought Galileo would be alive, well and functioning at this point because of radiation damage," said Bob Mitchell, the Cassini project manager at NASAs JPL. "Then the realization came we would still have Galileo sitting in close and Cassini flying by farther out."

Although fraught with glitches, Galileo has persevered since its launch in 1989, twice winning life-extending reprises after its original mission drew to a close in 1997.

Blasted with nearly three times as much radiation as planned for, Galileo should now continue to make observations throughout 2000, including flybys of the jovian moon Ganymede on May 20 and December 28. The icy moon is the largest natural satellite in the solar system.

It will likely be on its last legs when Cassini appears on the scene.

Galileo or no Galileo, Cassinis flyby will also be a dry run for the intense scientific work it will carry out at Saturn farther down the road.

Mitchell said they would turn on 11 of the robotic probe's 12 instruments to collect data during its slow cruise around Jupiter.

Scientists hope to undertake a broad range of experiments, including efforts to: pinpoint the source of the radio noise the planet generates; measure the composition of dust kicked off the volcanic moon Io and accelerated by Jupiter; create spectacular movies of the swirling complexity that is Jupiters colorful atmosphere.

Johnson said the movies, captured over the course of months, should easily rival in resolution those made using the Hubble Space Telescope.

At one point during the joint encounter, Cassini and Galileo will likely swap places, with the former plunging into the magnetosphere and the latter zipping out of it, Mitchell said.

Each of the spacecraft cost NASA about $1.4 billion to build.