Goodbye Herschel

Arnold Benz, ETH Zürich


On April 29 this year the Herschel space observatory exhausted its supply of helium that cooled the sensors in the far-infrared and submillimeter range, wavelengths that cannot be observed from the ground. Herschel was put into the Lagrange position L2 in the Sun-Earth system, 1.5 million kilometres from us. Four years after its launch by the European Space Agency (ESA), the mission completed its novel observations about how stars and thus galaxies form and evolve throughout the universe. The Herschel observatory was sent to space with 2,300 litres of superfluid helium. The helium was pumped around the spacecraft in such a way as to cool the three observing instruments and gradually evaporated in the process. Herschel cannot observe without cooling below 1 K in the most critical components, but the amount was limited and chosen to outlast the expected lifetime of the cutting-edge, but new-to-space electronics.

Herschel exceeded expectations both in technology and science. The technical developments required by the unusual observing wavelengths delayed the start by more than two years. Swiss industry provided the large cryostat and the optical assemblies for the HIFI instrument; the low-noise low-power indium phosphide amplifiers for HIFI (Heterodyne Instrument for the Far Infrared) were developed at ETH Zürich, software for HIFI at the Fachhochschule FHNW in Windisch. After four years in space all three instruments on board were still fully operational.

Herschel's observations have revealed the cosmos in unprecedented detail at these wavelengths. This raised interest in the astronomical community and resulted in 160 "first results papers" within the first four months of scientific data taking. However, most results emerged and still do so after careful data analysis, modelling and interpretation. Some highlights of my personal selection are described below.

Herschel observations allowed studying galaxies in the early universe that form stars at prodigious rates and apparently also in the absence of mergers. Other processes like inflowing intergalactic streams of cold gas may thus be equally effective. In nearby active galaxies, on the other hand, Herschel found for the first time vast amounts of outflowing molecular gas that may deplete a galaxy's supply to form new stars.

In observations of our own galaxy, Herschel identified the presence of a filamentary network in the gas of molecular clouds, filaments which contain cloud cores that will collapse under their own weight to form stars. Herschel demonstrated that filaments are nearly everywhere in clouds and that they are a key to star formation. It is not clear how filaments originate; intersecting shock waves and magnetic fields are proposed.

A chief goal of Herschel is the study of the chemical composition of cosmic objects through high-resolution spectroscopy, and in particular the search for water in the gas state, which cannot be observed from the ground. Of special interest at ETH Zürich were the chemical network of water in star formation and the evolution of protostellar disks. Herschel found gaseous water already in a cold pre-stellar core before the start of star formation. It amounts to a few million times the amount of water in the Earth's oceans. The 10 million year old disc surrounding nearby star TW Hydrae still contains a water supply equivalent to several thousand times Earth's oceans. Disks older than some 60 million years were observed to contain not enough gas to form new planets. These findings suggest that water played an important role throughout the formation of the solar system.

Several molecules were discovered in interstellar clouds for the first time. We modelled the chemistry in regions of star formation in an attempt to predict which molecules – especially ionised ones – exist there and could be used to infer the physical conditions. In particular, ultraviolet and X-ray irradiation change the chemistry and heat the collapsing envelope and its walls to the outflows. The surprise: new molecules in interstellar space, like SH+, OH+, and H2O+ were discovered to be more abundant than expected. The region of ionized molecules in the inner parts of star and planet forming regions is certainly a field of future research. On the other hand, O2, also not observable from the ground, was much harder to detect than predicted. Free oxygen, not bound to carbon monoxide, doesn’t seem to be in molecular gas form, but might hide in silicates of the interstellar dust.

The water profile in the atmospheres of Mars is studied at the University of Bern. For the first time the water vapour was determined from a global perspective yielding a view on overall seasonal changes. The group was also part of the team that discovered O2 in the Mars atmosphere.

Two weeks after the end of the helium the Herschel observatory was moved away from the Lagrange point and the communication was terminated. The loss of pointing control ends the direct contact with Herschel. The observatory will slowly drift away from Earth and orbit indefinitely the Sun like a small planet.

Herschel has observed more than we expected, but the project is not over yet. More than 35,000 scientific observations were executed with Herschel, and more than 50.000 lines have been detected with the HIFI instrument alone. They will eventually be identified, analyzed in more detail, and modelled. Only limited parts of the data are fully exploited. It will probably be a long time before the next opportunity to gather this kind of data in space. Due to the breadth and completeness throughout the entire wavelength range, the Herschel data are bound to be unparalleled for many years to come. In a year, all the data will become a public legacy. Most important will be their combination with observations at other wavelengths, such as the millimetre/submillimetre telescope ALMA in Chile. ESA has just announced a Herschel data analysis course for beginners.



[Released: July 2013]