The event was not unexpected: The 2,300 liters of liquid helium used to cool the spacecraft's sensitive instruments have been slowly evaporating since the final top-off the day before Herschel's launch on May 14, 2009, the European Space Agency reported Monday.
Herschel's instruments, operating close to absolute zero, have made three years' worth of highly sensitive observations of the universe.
"Herschel has exceeded all expectations, providing us with an incredible treasure trove of data that that will keep astronomers busy for many years to come," said Alvaro Gimenez Canete, ESA's director of science and robotic exploration.
At the beginning of the spacecraft's daily communication session with its ground station in Western Australia, a clear rise in temperatures was seen in all of Herschel's instruments.
Since its launch, Herschel has probed the universe's "coolest" secrets by observing the frigid side of planet, star and galaxy formation, officials said.
"Herschel gave us the opportunity to peer into the dark and cold regions of the universe that are invisible to other telescopes," John Grunsfeld of NASA's Science Mission Directorate at NASA headquarters in Washington said.
NASA helped build instruments for Herschel and process data returned by the spacecraft.
"This successful mission demonstrates how NASA and ESA can work together to tackle unsolved mysteries in astronomy," Grunsfeld said.
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