Unique Solar Probe Freezing to Death

Seth Borenstein in Washington
Associated Press
February 25, 2008

The Ulysses solar probe is about to die by freezing to death after 17 years of studying the sun and solar system, NASA and the European Space Agency said.

The satellite had long outlasted the five-year mission it began in 1990, but it continued to transmit useful data on space dust and solar winds.

More recently its plutonium power source had slowly weakened and its fuel was freezing as the probe made a wide circle of the sun, traveling as far as Jupiter.

"Rewritten Textbooks"

In January engineers tried a long shot maneuver to heat up the fuel. Instead, their effort backfired and hastened Ulysses' death by several months.

The 250 million U.S. dollar probe was a joint European-NASA project. After being released from orbit by astronauts on the space shuttle Discovery in October 1990, Ulysses made nearly three full wide circles of the sun from above and below its poles. It also circled over Jupiter's poles, logging about 6 billion miles overall.

When the satellite recently started to fail, the probe had just finished examining the sun's north pole for a third time.

"This mission has rewritten textbooks," said Arik Posner, NASA's Ulysses program scientist.

What made Ulysses unique and crucial to scientists was its orbit and perspective.

It provided astronomers with a three-dimensional look at the sun and the rest of the solar system. Most of the planets line up along the same geometric plane generally around the middle of the sun and that's where most of the space probes orbit, too.

But Ulysses made long wide circles of the sun's poles, essentially gazing down at the sun and solar system from above and below instead of around the middle.

"We Understand It Now"

That three-dimensional data from Ulysses—which was not devised to take pictures—was important for scientists trying to understand the solar winds.

These winds blast away from the sun at 1 million miles per hour in all directions, said David McComas, a Ulysses scientist with the Southwest Research Institute in San Antonio.

The solar wind is crucial because it protects Earth from deadly cosmic radiation, causes geomagnetic storms on Earth, and it's responsible for the aurora borealis (see photos).

"We understand it now; we didn't understand it before," McComas said.

As the fuel began to freeze in recent months, engineers shut off its radio transmitter to divert what little power was left to its heaters. The effort failed and the radio transmitter could not be turned back on.

"It was rather uncertain it would work; it's so harsh and cold out there," Posner said. "It was our only option."

Had it worked, engineers figured they would have gotten an extra two years of life from Ulysses. The final transmitter will probably quit in the next few weeks, according to the European Space Agency.

Copyright 2008 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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