After making more than 35,000 observations and logging more than 25,000 hours of studying previously invisible celestial objects, the Herschel Space Observatory has exhausted its supply of liquid helium and will close its eyes on the universe. The European Space Agency mission — launched aboard an Ariane 5 rocket from French Guiana on May 14, 2009 — has revealed some of the universe’s “coolest” secrets by observing the frigid side of planet, star and galaxy formation. The school-bus-sized telescope is the largest, most powerful infrared telescope ever flown in space. Artist’s concept of the Herschel Space Observatory front view. Courtesy of ESA/AOES Medialab. “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, associate administrator for NASA's Science Mission Directorate at NASA headquarters in Washington. “This successful mission demonstrates how NASA and ESA can work together to tackle unsolved mysteries in astronomy.” NASA’s Jet Propulsion Laboratory built components for two of Herschel’s three science instruments. NASA also supports the Herschel Science Center, located at the California Institute of Technology’s Infrared Processing and Analysis Center. Two Canadian Space Agency teams of astronomers also were an integral part of the development and operations of two of the three science instruments: the Heterodyne Instrument for the Far Infrared (HIFI) and the Spectral and Photometric Imaging Receiver (SPIRE). Confirmation of the exhausted helium supply, which cools the telescope’s instruments, was made during the spacecraft's daily communication session with its ground station in Western Australia. A clear rise in temperature was measured in all of Herschel's instruments. Herschel's detectors were designed to pick up the glow from celestial objects with infrared wavelengths as long as 625 µm. Because heat interferes with these devices, they were chilled to temperatures as low as 2 K using liquid helium. The detectors also were kept cold by the spacecraft's orbit, which is around a stable point called the second Lagrange point, about 930,000 miles from Earth. This location gave Herschel a better view of the universe. Herschel has served as a particularly useful tool for following the trail of water throughout the universe, from star-formation clouds to stars to planet-forming discs. The space observatory detected thousands of Earth oceans’ worth of water vapor in these disks, with even greater quantities of ice locked up on the surface of dust grains and in comets like Hartley-2, which was found to have almost exactly the same isotopic ratios as the water in our oceans, fueling the debate about how much of Earth's water originated from impacting comets. View of the Orion nebula, taken by the Herschel and Spitzer space telescopes, highlights fledgling stars hidden in the gas and dust clouds. Couretsy of NASA/ESA/JPL-Caltech/IRAM. Other findings from the mission include the discovery of some of the youngest stars ever seen in the nearby Orion "cradle" and a peculiar planet-forming disk of material surrounding the star TW Hydra, indicating that planet formation may happen over longer periods of time than expected. Herschel also has shown stars interacting with their environment in many surprising ways, including leaving trails as they move through clouds of gas and dust. “Herschel is challenging us by raising more questions than answers so far,” said professor Michel Fich of the University of Waterloo. "For instance, one of our projects for the HIFI instrument began with a deceptively simple question: Why do high-mass stars form differently from low-mass stars? What we have found is that every object in our sample is different from the others in some significant way. This is really exciting because it was completely unexpected, and it is going to take years to work out why.” This month, Herschel will be propelled into its long-term stable parking orbit around the sun. Although it will no longer be making observations, discoveries will continue. Astronomers are still looking over data, much of which already is public and available through NASA’s Herschel Science Center. The final batch of information will be made publicly available in about six months. “Herschel has improved our understanding of how new stars and planets form but has also raised many new questions,” said Paul Goldsmith, NASA Herschel project scientist at JPL. “Astronomers will be following up on Herschel's discoveries with ground-based and future space-based observatories for years to come.” For more information, visit: www.nasa.gov/herschel or www.asc-csa.gc.ca