The Herschel telescope ended a nearly four-year campaign watching the cosmos Monday, when the telescope lost its sharp infrared vision as the last of a reservoir of liquid helium coolant evaporated into space - an expected ending to Herschel's stellar mission.

ESA's Herschel space observatory set against a background image of the Vela C star-forming region. Credits: ESA/PACS & SPIRE Consortia, T. Hill, F. Motte, Laboratoire AIM Paris-Saclay, CEA/IRFU - CNRS/INSU - Uni. Paris Diderot, HOBYS Key Programme Consortium
 
Launched in May 2009, the European-led mission was the largest telescope ever flown into space, collecting faint light from some of the coolest parts of the universe.

Check out a sampling of Herschel's most spectacular images.

Herschel has helped astronomers study star birth, discover the signature of water all around, and peer billions of years back in time to track the evolution of thousands of ancient galaxies, revealing many more stars than expected formed in the first few billion years of the universe's existence.

Although Herschel's observing campaign is over, the mission leaves a rich legacy of data to be made available to scientists in the coming decades.

"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, the European Space Agency's director of science and robotic exploration.

The final batch of Herschel's data will be made public in about six months, officials said.

The observatory is stationed at the L2 libration point about a million miles away - well away from interference from reflected light off the Earth and the moon.

Herschel's three science instruments, covering a swath of the electromagnetic spectrum in the far-infrared and submillimeter wavelengths, were immersed in a thermos-like container of liquid helium to keep the mission's detectors chilled close to absolute zero.

Cold detectors were required for Herschel to resolve the mission's targets, which included distant galaxies, cool pockets of gas and dust, and stellar nurseries where matter coalesces into stars and planets.

"To observe very cold regions with high sensitivity, Herschel's instruments needed to be at even lower temperatures - only a few degrees above absolute zero," said Thomas Passvogel, former Herschel program manager and current head of the projects department in ESA's science and robotic exploration directorate. "This was achieved through the use of 2,300 liters of superfluid helium, which granted us over three and a half years of observations of the cool and distant universe."

The emptying of Herschel's helium tank proved difficult to predict. There is no gauge aboard the telescope to measure how much of the cryogenic helium was left inside, so engineers had to use thermal models and knowledge of the reservoir's capacity to forecast when the helium would run out.

The consensus was the helium would run out some time after mid-March. It turned out Herschel squeaked out an extra six weeks of observations.

"Herschel has been productive up to the last drop," said Göran Pilbratt, Herschel's project scientist.

Officials collected enough research proposals for an extra few months of Herschel operations, just in case the mission ran longer than expected.

The $1.4 billion mission is named for William Herschel, who discovered Uranus and infrared light, and launched in tandem with the Planck telescope on an Ariane 5 rocket in May 2009.

The Carina Nebula, by ESA's Herschel space observatory. The image shows the effects of massive star formation - powerful stellar winds and radiation have carved pillars and bubbles in dense clouds of gas and dust. Credits: ESA/PACS/SPIRE/Thomas Preibisch, Universitats-Sternwarte Munchen, Ludwig-Maximilians-Universitat Munchen, Germany.
 
Herschel'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.

"Herschel gave us the opportunity to peer into the dark and cold regions of the universe that are invisible to other telescopes," said John Grunsfeld, head of NASA's science mission directorate. "This successful mission demonstrates how NASA and ESA can work together to tackle unsolved mysteries in astronomy."

NASA supplied components for two of Herschel's three instruments.

Since first light in July 2009, Herschel made more than 35,000 scientific observations, collecting more than 25,000 hours of data for about 600 programs. Astronomers have published more than 500 papers using data acquired by Herschel, according to Pilbratt.

"Herschel has allowed us to see for the first time a huge population of intensively star-forming galaxies that make up about half of the entire star formation budget of the universe," said Matt Griffin, principal investigator of Herschel's Spectral and Photometric Imaging Receiver instrument, in a ESA press release. "This highlights the importance of looking at the distant universe in the spectral range covered by Herschel. Stars form in clouds of gas and dust, and most of the energy released by forming stars is absorbed by the dust grains, causing them to warm up. We can detect this warm dust with Herschel and so measure the energy produced by the young stars."

Herschel's highest-resolution spectrometer, known as the HIFI instrument, picked out the presence of water around a nearby star and identified water in a comet in our own solar system with the same isotopic composition as the water in Earth's oceans.

"The comparison between the composition of water in comets and on Earth's oceans may have far-reaching implications: if comets did contribute to enriching our planet with water, they might also be responsible for the delivery of other species, such as the carbon- and nitrogen-based compounds that are of great importance to the emergence of life," said Frank Helmich from SRON Netherlands Institute for Space Research, principal investigator for HIFI.

Controllers will put the Herschel spacecraft through engineering tests before commanding the telescope on a trajectory away from the L2 point and into a stable solar parking orbit in May.