The Ulysses solar probe, after 17 years of studying the sun and solar system, is about to die by freezing to death, NASA and the European Space Agency said Friday.
The satellite had long outlasted the five-year mission it began in 1990, but it continued to transmit useful data on 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.
In January, engineers tried a longshot maneuver to heat up the fuel. Instead, their effort backfired and hastened Ulysses' death by several months.
The $250 million 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.
That three-dimensional data from Ulysses — not devised to take pictures — was important for scientists trying to figure out the solar wind. 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 is responsible for the aurora borealis.
"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.