After a nearly 30-year odyssey navigating political shoals and weathering solar storms, a 900-pound robotic explorer dubbed Ulysses is slowly beaming back its final dispatches.

Sometime in the next few weeks, the craft will fall silent, ending an era of solar exploration that has given scientists their first 3-D view of the vast magnetic cocoon the sun builds around the solar system.

Ulysses helped unveil processes that have given scientists a deeper understanding of how solar storms form and move through the solar system. It made the first direct measurements of the interstellar dust that the solar system collects as it plows through interstellar space. And Ulysses helped scientists study distant gamma ray bursts – a phenomenon one astrophysicist has dubbed "the birth-cry of a black hole."

The craft "has been a terrific old workhorse," says Richard Marsden, the Euro­­pean Space Agency's manager for the joint ESA/NASA mission. "It's produced great science and lasted much longer than we expected." Late last year the mission got its third extension.

The International Solar Polar Mis­sion, as it was formally called, also served as a crucible for developing and sustaining international partnerships in space exploration. Early on, the United States canceled the planned Ulysses twin, raising concerns that the US was an unreliable partner. The mission became a one-craft show.

But the relationship recovered. US scientists provided half the instruments on Ulysses. The craft was launched in 1990 from a space shuttle and has been controlled by a joint team based at the Jet Propulsion Laboratory in Pasadena, Calif. The National Aeronautics and Space Administration's Deep Space Network has served as the project's communications link.

Now, reinforced by the success of the Cassini-Huygens mission currently under

way at Saturn as well as Ulysses' success, ESA and NASA are holding talks that could lead to new joint space-science missions, says Gerhard Schwehm, who heads ESA's solar-system science division.

"After the problems we had in the beginning" of the Ulysses project, "we've gotten excellent support from NASA," Dr. Schwehm says. Word of Ulysses' imminent demise came toward the end of February. It was starting a third extended mission when it was hit with a peck of power problems.

The craft swings around the sun once every 6.2 years in an orbit that takes it as far out as Jupiter and as close to the sun as Earth. Its generators contain plutonium, whose radioactive decay yields the heat used to produce electricity. After 17 years in orbit, the generators no longer produce enough electricity to run the high-power transmitter, science instruments, and heaters at the same time. Now its high-power transmitter has quit. Designers were using the transmitter's warmth to keep fuel from freezing. So engineers are scrambling to download as much data as they can using the craft's slower, low-power transmitter before the fuel freezes and engineers lose the ability to keep the orbiter's big antenna aimed at Earth. Once the fuel freezes, it will be locked in its orbit around the sun.

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