Infrared Space Telescope Herschel Runs Out of Coolant

The crucial liquid helium coolant of ESA’s Herschel space observatory is set to run out in just a few weeks, which will mean 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 not possible to predict the exact day the helium will finally run out, but engineers believe that almost all of the liquid helium has now gone.

“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,” says Micha Schmidt, the Herschel Mission Operations Manager at ESA’s European Space Operations Centre in Darmstadt, Germany.

In addition, Herschel is performing numerous other interesting observations specifically chosen to exploit every last drop of helium.

“When observing comes to an end, we expect to have performed over 22 000 hours of science observations, 10% more than we had originally planned, so the mission has already exceeded expectations,” says Leo Metcalfe, the Herschel Science Operations and Mission Manager at ESA’s European Space Astronomy Centre in Madrid, Spain.

“We will finish observing soon, but Herschel data will enable a vast amount of exciting science to be done for many years to come."

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