The Herschel space observatory has exhausted its supply of coolant concluding more than three years of observations.
The European Space Agency (ESA) mission began with over 2300 litres of liquid helium, which has been slowly evaporating since the final top-up the day before Herschel's launch on May 14, 2009.
The evaporation of the liquid helium was essential to cool the observatory's instruments to close to absolute zero, allowing Herschel to make highly sensitive scientific observations of the cold Universe until yesterday.
The confirmation that the helium is finally exhausted came yesterday afternoon at the beginning of the spacecraft's daily communication session with its ground station in Western Australia, with a clear rise in temperatures measured in all of Herschel's instruments.
"Herschel has exceeded all expectations, providing us with an incredible treasure trove of data that will keep astronomers busy for many years to come," says Proffessor Alvaro Giménez, ESA's director of Science and Robotic Exploration.
Herschel has made over 35,000 scientific observations, amassing more than 25,000 hours of science data from about 600 observing programmes. A further 2,000 hours of calibration observations also contribute to the dataset, which is based at ESA's European Space Astronomy Centre, near Madrid in Spain.
The archive is expected to provide even more discoveries than have been made during the lifetime of the Herschel mission.
"Herschel's ground-breaking scientific haul is in no little part down to the excellent work done by European industry, institutions and academia in developing, building and operating the observatory and its instruments," adds Thomas Passvogel, ESA's Herschel and Planck Project Manager.
The mission resulted in a number of technological advancements applicable to future space missions and potential spin-off technologies, including the development of advanced cryogenic systems, the construction of the largest telescope mirror ever flown in space, and the utilisation of the most sensitive direct detectors for light in the far-infrared to millimetre range.
Manufacturing techniques enabling the Herschel mission have already been applied to the next generation of ESA's space missions, including Gaia.
"Herschel has offered us a new view of the hitherto hidden Universe, pointing us to previously unseen processes of star birth and galaxy formation, and allowing us to trace water through the Universe from molecular clouds to newborn stars and their planet-forming discs and belts of comets," says Göran Pilbratt, ESA's Herschel Project Scientist.
The telescopes 3.5m diameter mirror and extremely sensitive scientific instruments made it the most powerful infrared observatory ever launched.
The SPIRE (Spectral and Photometric Imaging Receiver) instrument, responsible for keeping all the spacecraft’s instrumentation cool, making images of the sky simultaneously in three submillimetre “colours” and measuring the spectral features of atoms and molecules, was led by the UK, with support from the UK Space Agency.
Herschel's instruments did not just produce pictures, but also carried spectrometers meaning astronomers could study the full range of wavelengths in great details allowing the identification of specific atoms and molecules in interstellar space and in distant galaxies.
“Herschel's spectrometers cover a huge range of wavelengths, most of which has never been studied in such detail,” says Bruce Swinyard of University College London and STFC's Rutherford Appleton Laboratory.
“Not only can we study the detailed composition of gas and dust, but we can study the importance of specific types of molecules, such as water, in the formation of stars and planets.”