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Largest space telescope to lose infrared vision

Posted: March 14, 2013

The infrared eyes of Europe's Herschel observatory are about to go dark, but the space-based telescope will leave a legacy of data to keep astronomers occupied for years to come.

Artist's concept of the Herschel observatory against a background image of baby stars forming in the Rosette Nebula. Credit: ESA/C. Carreau
Some time in the next few weeks, coolant inside Herschel's instrument bay will run out, leaving the telescope's detectors feckless in their mission to glimpse the coolest reaches of the universe.

The best estimate puts the mission-ending event in the second half of March, according to Göran Pilbratt, Herschel's project scientist at the European Space Agency.

Herschel's observing life is limited by a finite supply of super-cold superfluid helium. The observatory's three science instruments are immersed in the cryogenic fluid, keeping their detectors just above absolute zero, colder than anything observed in the solar system.

The observatory's sensors are kept at cryogenic temperatures to allow the detectors to see cold pockets in star-forming regions, interstellar dust, and galaxies formed just after the birth of the universe.

Herschel's coldest detector has an operating temperature of 0.3 Kelvin, or minus 459 degrees Fahrenheit, just above absolute zero.

There is no gauge aboard Herschel indicating how much helium is left in the observatory's cryogenic reservoir, so it is impossible to tell when the coolant will be gone.

An analysis of temperatures inside Herschel's instrument compartment led to a prediction the helium would run out later this month. But the estimate is just an approximation with a margin of error, according to Pilbratt.

"It is no surprise that this will happen, and when it does we will see the temperatures of all the instruments rise by several degrees within just a few hours," said Micha Schmidt, Herschel mission operations manager at the European Space Operations Center in Darmstadt, Germany.

Herschel's three instruments will rapidly lose their sensitivity, and the world's largest space telescope will cease observations.

Photo of the Herschel spacecraft before launch. Credit: ESA
Named for astronomer William Herschel, who discovered Uranus and infrared light, the observatory launched in May 2009 in tandem with the Planck telescope, which looks at the cosmic microwave background, the light left over from the Big Bang.

The Herschel telescope's primary mirror spans 11.5 feet, the largest astronomical telescope ever launched into space - some 50 percent wider than the primary mirror on Hubble.

"When observing comes to an end, we expect to have performed over 22,000 hours of science observations, 10 percent more than we had originally planned, so the mission has already exceeded expectations," said Leo Metcalfe, manager of the Herschel mission at the European Space Astronomy Center in Madrid.

The $1.4 billion Herschel telescope peered inside star-forming regions, discovered distant galaxies, studied interstellar dust, and observed objects within the solar system, balancing a bounty of scientific data with stunningly artistic views of the cosmos.

The craft is stationed a million miles from Earth at the L2 Lagrange point, where the pull of gravity from the sun and Earth keep the telescope roughly the same location in space.

Herschel launched with 2,300 liters, or more than 600 gallons, of liquid helium, but the fluid gradually boils off. Engineers topped off the observatory's helium supply the day before launch to maximize Herschel's operating time.

The mission's three science instruments, embedded inside Herschel's cryogenic tank, are capable of capturing and analyzing images in a wide swath of the electromagnetic spectrum, ranging from infrared to submillimeter wavelengths.

"When it comes to the performance of the instruments, for most observing modes, we're actually doing better than we dared to believe," Pilbratt said in an interview with Spaceflight Now.

Astronomers have published more than 500 papers using data acquired with the Herschel observatory.

Herschel's PACS and SPIRE instruments captured this infrared image of the Andromeda galaxy. Credit: ESA/Herschel/PACS & SPIRE Consortium, O. Krause, HSC, H. Linz
"We have so many stunning images of molecular clouds all over the place, and those just look beautiful," Pilbratt said. "We have observed water in all kinds of places. We have determined the isotopic composition of water in a comet, and for the first time, detected a comet with the same type of water we have on Earth."

Herschel has finished its highest-priority science program, but managers have enough observations requested from the science community to utilize the telescope until its last day.

"When the coolant is gone, then Herschel is useless as an astronomical facility," Pilbratt said.

ESA officials considered driving the spacecraft into the moon after its mission is complete, but managers elected to guide the observatory into a stable orbit around the sun.

But astronomers have years of work ahead to cull and analyze Herschel's extensive data catalog.

"On the day that Herschel runs out of helium, some astronomer is probably going to get his first observation," Pilbratt said.

After a few weeks of engineering tests, controllers will fire Herschel's rocket thrusters to depart the L2 point and enter a storage orbit some time in early May, according to ESA.

Once engineers dispose of the spacecraft, Pilbratt's team still has much work to do.

"I see 2017 as the end of the Herschel mission," Pilbratt said.

The science team will spend the next few years building a legacy for the Herschel mission, archiving data and documentation, and making the data accessible, according to Pilbratt, a Swedish astronomer who started working on the Herschel mission in 1991.

"It has been an adventure," Pilbratt said. "It's not over. Herschel data will be valuable for astronomers for decades."