The largest space telescope ever launched is due to stop operating later this month, bringing to a close more than 22,000 hours of observation in the far infrared and submillimetre wavelengths.

The Herschel Space Observatory, launched in May, 2009, will run out of its stores of liquid helium onboard, used to cool its three instruments to temperatures just above absolute zero – the lowest temperature possible.

The frigid conditions are necessary to allow the instruments to track traces of cold gases and dust in the many star-forming pockets of the universe as well as probe the innards of distant supernovae. “When an object is warm, the atoms and electrons inside it are oscillating and moving randomly with an intensity that depends on temperature,” said Matt Griffin, lead scientist of the SPIRE instrument onboard Herschel. So, supercooled instruments are quieter internally, and stable enough to measure very faint signals that the telescope picks up.

However, as the helium continuously boils off, the instruments will heat up rapidly within a few hours once the liquefied gas is exhausted, rendering the eye in the sky blind. “Herschel has told us so much, how molecular clouds make stars and why some don't; that half of the total star formation in the universe is obscured by dust,” Goran Pilbratt, the mission’s project scientist at the European Space Research and Technology Centre in Noordwijk, the Netherlands, told The Hindu.

An Ariane 5 rocket launched the telescope into the Lagrangian point L2 1.5 million km away, where gravitational pulls due to Earth and the Sun match to lock Herschel into a fixed point above Earth. It has three on-board instruments called PACS, SPIRE and HIFI. These instruments they have amassed a wealth of data, not to mention stunning images of budding stars and galaxies.

Once the telescope turns blind in the second half of March, the mission scientists will propel Herschel out of L2 and into a stable orbit around the Sun. Pilbratt clarified that the European Space Agency (ESA) has no concrete plans now for a replacement mission, although ground-based observatories will follow up on Herschel’s observations.

The ALMA observatory in Chile, which came online on March 13 and also observes in the submillimetre range, will also be of great interest. Commenting on its specific importance, Leo Matcalfe, the Mission Manager at ESA’s European Space Astronomy Centre in Madrid, Spain, said, “ALMA provides extremely high spatial resolutions, far finer than Herschel could achieve. So, ALMA is well suited for the detailed follow-up.”

Now, scientists will focus on creating a legacy archive. This includes the best images, spectra and other data-sets.

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