Herschel (ESA, 2009-2013) carried the largest, most powerful infrared telescope ever flown in space. The mission studied the origin and evolution of stars and galaxies to help understand how the Universe came to be the way it is today.
Herschel was the first observatory to cover the entire range from far-infrared to submillimetre wavelengths and bridge the two. The space telescope peered deeper into the far-infrared than any previous mission, studying otherwise invisible dusty and cold regions of the cosmos, both near and far.
Herschel made observations from a location 1.5 million km from Earth and carried three scientific instruments. HIFI (Heterodyne Instrument for the Far-Infrared), one of the three instruments, was developed under supervision of SRON.
Not only the technology but also the organisation of the HIFI project was complex. No less than 25 partners from more than 10 different countries were involved. All partners have contributed to HIFI with their own specific and often unique knowledge. SRON was the Principal Investigator responsible for coordinating the cooperation.
Thanks to HIFI, scientists now much better understand the cosmic cycle of gas which leads to for instance the birth of stars and planets, and the role (water)molecules play.
Our Milky Way is far from empty. The space between the stars we see - individually or in the glow of the Milky Way - is filled with clouds of gas and dust. It is in these remnants of previous stellar generations that new stars and planets form - stars like our Sun and planets like in our Solar System. Through all the information that is enclosed in the spectral lines that HIFI measured, we now much better understand the details of the cosmic cycle of gas and dust.
HIFI has extracted a wealth of unique information from very different environments – from shells of dying stars to galaxy cores and comets. The Dutch Molecule Hunter was able to do this because of an unbeatable combination of uninterrupted spectral coverage, high spectral resolution and calibration accuracy. This combination may not be available for another 40 years.
On the water trail
Water has been one of the most important targets for HIFI because of its rich spectrum and high abundance, and because it plays such an important role in star formation. Because of the high water vapor content of our own atmosphere, the data from space obtained with HIFI are a true legacy for decades to come.
Water was found in the tenuous torus of gas released from Saturn's moon Enceladus, in gas streaming off comets, in planetary atmospheres, external galaxies and around evolved stars. Water has been used to trace new types of shock waves present in the outflows that are part of the formation processes of stars. It has also been used to detect material flowing onto a star in the very first stages of its formation and to probe the cold water reservoir in the outer regions of planet-forming disks, with the water released from ice by a weak UV radiation field triggered by cosmic rays. Together, these data will allow to put together the water trail from collapsing clouds to planetary systems.
SRON researcher Floris van der Tak: "HIFI also found OH+ and H2O+ closely related to water. The discovery of these ionized molecules was a complete surprise, but their ubiquitous presence in our own Galaxy and beyond gives us a new handle on the number of high energy particles travelling through galaxies with speeds close to that of light and on the molecule-poor phase of the interstellar medium. OH+ en H2O+ are formed in a phase with an abundance of atomic hydrogen waterstof H instead of molecular hydrogen H2. Other molecules provide insight in the way molecular material reacts to ultraviolet radiation of hot stars."
Cosmic rays were not only detected through the ionizing influence they have on interstellar gas. Several times HIFI was interrupted by a hit of such a high energy particle in the electronics of HIFI (local cosmic rays). Every time HIFI was successfully restarted.
The clouds in Herschel pictures look quite like clouds in our own atmosphere. They are dynamic and turbulent. Based on HIFI measurements of molecules like CH+ scientists have been able to see and quantify this turbulence. There are also areas where the gas is so tenuous that previous telescopes could not observe it. This 'CO-dark' gas has been successfully probed by ionized carbon observations of HIFI and provides the basis for future missions which will take the study of star formation and galactic evolution one step further.
Besides observations of single molecules, HIFI also excelled in making complete spectral scans. Such scans contain tens of thousands of lines from, in some cases, over 50 molecular species arising from within water and organic rich interstellar gas clouds. These observations directly characterize the chemical composition of star-forming gas with unprecedented accuracy while offering the unprecedented ability to probe gas physics with hundreds to thousands of lines of a single molecule.
Herschels enormous main mirror is the largest satellite mirror to date with a diameter of 3.5 metres and it captures the cosmic radiation. Via a second mirror the radiation ends up in the focal point, where the scientific instruments are located. The receiving mirror of HIFI sends the radiation into the instrument.
The largest part of HIFI is located in the central cryostat of Herschel. The cryostat is just like an enormous 'thermos flask' but filled with a very cold fluid: 2200 liters of superfluid helium. This ensures that parts of HIFI are kept at a constant temperature of 2 to 10 degrees Kelvin (0 degrees Kelvin, absolute zero, is -273°C). Besides HIFI there are two other instruments in the cryostat, namely PACS and SPIRE.
The cosmic radiation observed by HIFI has a very high frequency and is called terahertz radiation. One problem in processing this high frequency radiation was that there were no electronics available that could work so fast. Therefore the radiation was mixed in HIFI with an artificially generated signal within the satellite. Then, just as with sound, a tremolo tone considerably lower in frequency was produced that could be properly processed while still retaining all of the scientific information. The superconducting detectors of HIFI converted the signal into an electrical current that will eventually be transmitted to Earth.
HIFI worked with superconducting receivers that are cooled using super-cold liquid hydrogen. For the spectrum ranging from 240 to 625 micrometers, HIFI had sensitive heterodyne SIS (superconducting-isolator-superconducting) receivers, and for the 157 to 210 micrometer area there were the Hot Electron Bolometers. Both types of receiver convert the (high-frequency) signal that is received into a lower frequency without losing any information. Just as for FM radio, this process makes the reception clearer and the signal can be amplified better. The functioning of the mixers was based on advanced physics, which was the result of more than 15 years of research and development. Four mixers have been constructed by SRON and the others came from scientific institutes in France, Germany, Sweden and the US.