Herschel to finish observing soon
ESA’s Herschel space observatory is expected to exhaust its supply of liquid helium coolant in the coming weeks after spending more than three exciting years studying the cool Universe. Herschel was launched on 14 May 2009 and, with a main mirror 3.5 metres across, it is the largest, most powerful infrared telescope ever flown in space. Herschel carries three science instruments: a high-resolution spectrometer, HIFI (Heterodyne Instrument for the Far Infrared), and the two cameras and imaging spectrometers, SPIRE (Spectral and Photometric Imaging REceiver) and PACS (Photodetector Array Camera and Spectrometer). MPE has led the development and operation of PACS.
As a pioneering mission, Herschel is the first 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. To be able to make such sensitive far-infrared observations, the detectors of the three science instruments must be cooled to a frigid ‑271°C, close to absolute zero. They sit on top of a tank filled with superfluid liquid helium, inside a giant thermos flask known as a cryostat. The superfluid helium evaporates over time, gradually emptying the tank and 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.
Indeed, 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. However, all good things must come to an end, and engineers believe that almost all of the liquid helium has now gone, although it is not possible to predict the exact day the helium will finally run out. A hint comes from an alternative method to monitor the helium content continuously over the mission developed at MPE: this measurement indicates that the helium will be exhausted in the second half of March.
“Herschel and PACS have already surpassed our most optimistic expectations,” says Albrecht Poglitsch, who led the development of the PACS instrument from MPE. “Our instrument was actively observing during more time than the other two instruments combined, with a nearly flawless, fully autonomous operation in its distant orbit. And, of course, we are determined to continue observing to the last drop of liquid helium!”
Herschel will continue communicating with its ground stations for some time after the helium is exhausted, allowing a range of technical tests. Finally, in early May, it will be propelled into its long-term parking orbit around the Sun.
Notes for Editors
An announcement will follow to confirm when the liquid helium coolant has been exhausted.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.