Herschel – A Cool Mission Unveiling the Cold Universe with Göran Pilbratt (European Space Agency, Noordwijk, The Netherlands)


The ESA Herschel Space Observatory – generally known as Herschel – was the first space observatory dedicated to performing observations in the poorly explored far-infrared and submillimetre part of the spectrum. It operated as an observatory open to the entire community for almost four years in 2009-2013. This talk will introduce Herschel and provide a – by necessity – subjective overview of Herschel science achievements.

Herschel was until 2000 known as the Far Infra-Red and submillimetre Space Observatory (FIRST), it was one of the four Cornerstone missions incorporated into the ESA Horizon 2000 long term plan from its inception in 1984. Herschel covered the wavelength range 55-670 um, which corresponds to a blackbody temperature range of 5-55 K, hence it targeted the ‘cold dark’ universe, although to Herschel it was certainly anything but ‘dark’.

Herschel carried a 3.5 m diameter Cassegrain telescope, produced using new silicon carbide (SiC) technology. The telescope, which was passively cooled and had a very low emissivity, fed three focal plane instruments, called HIFI, PACS, and SPIRE. These instruments were very different in their designs and use of technology, but together were a complementary whole from a science perspective. The focal plane units were sitting on an optical bench inside a superfluid liquid helium cryostat, the cryogen supply constraining the mission lifetime.

Herschel was flawlessly launched on 14 May 2009 (together with Planck) and operated successfully for almost 4 years, its cryogen was exhausted on the 1447th operational day on 29 April 2013. In this period it successfully conducted a total of 23,400 hours of science observations approved by the Herschel Observing Time Allocation Committee, and in addition 2600 hours of science calibration, a total number of ~37,000 and ~6600 observations, respectively. All data are available at different levels of data processing through the Herschel Science Archive. By May 2020 close to 2700 papers had been published based on Herschel.

Herschel observations are very wide ranging, in terms of lookback time of its targets they cover 17 orders of magnitude, from a near Earth object to the current record of redshift z=6.95, well within the first billion years after the Big Bang.

The main science areas include star formation and cosmic star formation history. Herschel studied star formation in detail in nearby galactic molecular clouds, a main area was the study of ‘filaments’ and their role in connection with the physics of star formation. The cosmic history of star formation was studied through large (in time and sky coverage) extragalactic surveys of dust enshrouded submillimetre galaxies. A third ‘signature’ science area was water in the universe, also in terms of the ‘water trail’ and the origin of water on Earth. But there was much more.

Finally this talk will attempt to deal with the question of whether, why, and how Herschel can be considered a Game Changer. Spoiler alert – it can.

Göran Pilbratt obtained his PhD in 1986 from the Onsala Space Observatory at Chalmers University of Technology in Göteborg, Sweden. Moving to ESA/ESTEC, Noordwijk, The Netherlands, initially for a research fellowship, he was appointed Study Scientist for Herschel (then FIRST) in 1991, in 1995 becoming the Project Scientist, a post he held until the end of 2019.

Seminar was recorded on December 17, 2020