Space Telescope Herschel Ran Out of Liquid Helium Today as Planned

The crucial liquid helium coolant of ESA’s Herschel space observatory ran out today, which means the end of the device that served astronomers for more than three exciting years studying the cool Universe.

A pioneering mission, it was the first telescope to cover the entire wavelength range from the far-infrared to submillimetre, making it possible to study previously invisible cool regions of gas and dust in the cosmos, and providing new insights into the origin and evolution of stars and galaxies. Herschel was launched on 14 May 2009 and, with a main mirror 3.5 m across, it is the largest, most powerful infrared telescope ever flown in space.

In order to make such sensitive far-infrared observations, the detectors of the three science instruments – two cameras/imaging spectrometers and a very high-resolution spectrometer – are housed inside a giant thermos flask known as a cryostat so they can be cooled down to –271°C, close to absolute zero. This is achieved by a finite amount of superfluid liquid helium that evaporates over time, gradually emptying the helium tank and thus determining Herschel’s scientific life. At launch, the cryostat was filled to the brim with over 2300 litres of liquid helium, weighing 335 kg, for 3.5 years of operations in space.

But in this limited time, Herschel has made extraordinary discoveries across a wide range of topics, from starburst galaxies in the distant Universe to newly forming planetary systems orbiting nearby young stars. The science observing programme was carefully planned to take full advantage of the lifetime of the mission, with all of the highest-priority observations now completed.

It is planned to propel Herschel into its long-term stable parking orbit around the Sun in early May.

The confirmation that the helium is finally exhausted came this 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 Prof. Alvaro Gimenez, 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 2000 hours of calibration observations also contribute to the rich dataset, which is based at ESA's European Space Astronomy Centre, near Madrid in Spain.

The archive will become the legacy of the mission. It 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. The mission saw 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 Goeran Pilbratt, ESA's Herschel Project Scientist.

Herschel's stunning images of intricate networks of dust and gas filaments within our Milky Way Galaxy provide an illustrated history of star formation. These unique far-infrared observations have given astronomers a new insight into how turbulence stirs up gas in the interstellar medium, giving rise to a filamentary, web-like structure within cold molecular clouds.

If conditions are right, gravity then takes over and fragments the filaments into compact cores. Deeply embedded inside these cores are protostars, the seeds of new stars that have gently heated their surrounding dust to just a few degrees above absolute zero, revealing their locations to Herschel's heat-sensitive eyes.

Over the first few million years in the life of newborn stars, the formation of planets can be followed in the dense discs of gas and dust swirling around them. In particular, Herschel has been following the trail of water, a molecule crucial to life as we know it, from star-formation clouds to stars to planet-forming discs.

Herschel has detected thousands of Earth ocean's worth of water vapour in these discs, with even greater quantities of ice locked up on the surface of dust grains and in comets.

Closer to home, Herschel has also studied the composition of the water-ice in Comet Hartley-2, finding it to have almost exactly the same isotopic ratios as the water in our oceans.

These findings fuel the debate about how much of Earth's water was delivered via impacting comets. Combined with the observations of massive comet belts around other stars, astronomers hope to understand whether a similar mechanism could be at play in other planetary systems, too.