Artificial comet. After a mission of nearly 2 decades, the Ulysses spacecraft will orbit the sun silently.

End of a Space Odyssey

After almost 17 years in orbit, the scientific mission of the Ulysses spacecraft will end tomorrow. The satellite, originally designed for a 5-year study of the solar wind and interstellar dust from a unique orbit over the sun's poles, has suffered a fatal defect in its attitude-control system. So technicians at NASA's Jet Propulsion Laboratory in Pasadena, California, along with their mission counterparts from the European Space Agency (ESA), are pulling the plug on the 300-kilogram probe, whose bountiful data transmissions helped transform what is known about our nearest star.

"Ulysses extended our knowledge of the sun's influence on its surroundings through its outflowing solar wind from two dimensions into three," says space physicist Robert Forsyth of Imperial College London in the U.K. Forsyth has worked on the mission since its inception in the late 1980s. "I did the data processing of the first magnetic field vector that came from the spacecraft," he says, "and by a quirk of fate and personnel changes, it will also fall back to me to process the last one later this week."

Launched from the space shuttle Discovery on 6 October 1990, the probe's rocket sent it first toward Jupiter, where the gas giant's gravity kicked the spacecraft into an orbit perpendicular to the plane of the solar system. From that vantage point, Ulysses determined that the sun's magnetic field can capture charged particles ejected from the sun's poles and redirect them toward Earth and the other planets--thereby creating an unexpected source of harmful radiation.

"I think it's fair to say Ulysses created a massive change in the way we think about the global structure of the solar wind and the sun's magnetic field," says space physicist Mathew Owens, also at Imperial College London. "Before Ulysses, measurements were confined to the solar equator, and we assumed they were representative of the global sun." But after the spacecraft began transmitting data from the solar poles, he explains, a different picture of the sun's activity emerged. "Like the blind men in a room with an elephant, it's only after complete coverage that the bigger picture is revealed."

Ulysses delivered another unexpected discovery: Instruments aboard the spacecraft designed to study interstellar dust found a surprising amount of neutral helium atoms arriving from deep space--a find from which scientists have deduced that the universe contains too little matter to halt its expansion. And the spacecraft found that the dust itself was 30 times more abundant than expected. Such information has made Ulysses "one of the most productive science missions ever flown," says space physicist David McComas of the Southwest Research Institute in San Antonio, Texas. "We certainly hope to see more examples of such long-term collaborative activities."

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