The Herschel Space Telescope is to astronomers what the Large Hadron Collider particle accelerator is to physicists. Now the wait is finally over: after the first plans were drafted twenty-seven years ago and construction work eventually began eleven years ago, the space telescope is ready for take-off. The countdown has begun and the astronomers have eagerly been counting the days until the launch.

Hot on the trail of far infrared radiation

A successful launch will also give ETH Zurich scientists reason to celebrate. After all, parts for the Heterodyne Instrument for the Far Infrared (HIFI) – one of three instruments that Herschel is equipped with – were developed at ETH Zurich’s Institute for Astronomy and the Institute of Field Theory and High Frequency Engineering. The HIFI will be used to measure far infrared radiation.

A mirror with a diameter of 3.5 meters gives Herschel ten times more optics than previous mirrors for this wavelength and enables seven times more spatial and 100 times more spectral resolution than conventional telescopes. Consequently, the European Space Agency (ESA) has constructed a telescope that is to examine wavelengths between the infrared and sub-millimeter range never before investigated from an observatory. “With Herschel we’re opening the last unexplored window in the measurable frequency ranges of cosmic radiation”, Arnold Benz, Professor at the Institute for Astronomy at ETH Zurich, proudly explains. The space observatory is therefore also named after the Anglo-German astronomer Friedrich Wilhelm Herschel, who discovered infrared radiation in 1800. “The telescope enables us to observe water molecules at very low temperature ranges – 10 to 20 degrees Kelvin – for the first time”, says Benz. At such temperatures, optical photons are no longer visible. These conditions prevail where stars and planets emerge, in cold cloud cores where oxygen and hydrogen presumably accumulate around dust particles and combine to water through a catalytic reaction.

Three-month path to optimum conditions

We need a space observatory like Herschel to be able to observe this kind of process and verify theories. After all, it is not possible to observe the far infrared radiation in the sub-millimeter range from Earth as the water vapor in the atmosphere absorbs the cosmic radiation. There is also the fact that every body in the infrared range shines and this “background noise” has to be kept to a minimum to ensure that the measured data is not distorted. Consequently, the telescope has to be orbited far away from the Earth’s thermal radiation. After a three-month journey – during which it is to cover about 1.5 million kilometers – it will finally reach its destination on the so-called Lagrange point, L2. This is where the conditions are ideal for its use: on the one hand, the interference from infrared radiation from the Earth and moon is minimal; on the other hand, the gravitational forces of the sun and the Earth and the centrifugal force of the orbit cancel each other out, meaning that the satellite does not need any fuel.

The telescope has to be permanently shaded with a solar sail and the measuring electronics cooled. To shield them from their own thermal radiation, the instruments are housed in what looks like a huge thermos flask where – at about one degree Kelvin – superfluid helium maintains the temperatures just above absolute zero.

Hot on the trail of the universe

Arnold Benz and his team will focus on the spectral lines of water molecules and their relatives. It initially works like an optical telescope to be able to record HIFI spectral lines from the frequency range as yet unexplored. In this, HIFI uses mirrors to focus the waves in seven different wavebands on mixers which transform the radiation on a much smaller intermediate frequency like a radio receiver, where it is then reinforced electronically. The physical state of the water vapor can be determined if at least three lines characteristic of the water molecule are identified with intensity whilst observing an object. After molecular hydrogen (H₂) and carbon monoxide (CO), water is the third most important molecule in the universe, Benz explains enthusiastically: “It’s an exciting and interesting molecule that cannot be observed from Earth. Herschel offers us the first opportunity to monitor the origins of its formation and its impact on how planets are formed.”

More accurate knowledge of the significance of the water could indicate how high- and low-mass planets are formed, for instance. After all, in the formation of planets, water is credited as making an important contribution during accretion: it forms an ice mantle around the dust particles, thus changing their ability to coagulate into larger bodies. In the formation of stars, water is important for the stars’ energy balance as it regulates the temperatures and enables the stars to cool down. Herschel could also help answer many open questions regarding the formation and development of galaxies from the early stages of the universe until today.

Decades of work

While the telescope is in use, about 75,000 spectral lines from different molecules are to be monitored by HIFI. “We expect to find spectral lines that will take us years to identify. We’re also bound to find many surprises and discover new molecules.” Consequently, it does not really matter that the flood of data will dry up in a maximum of four years, which is when the helium supply used to cool the system will run out; the data collected is bound to keep scientists busy for decades to come – of that Benz is convinced.

For the time being, all that remains is the satellite launch. As the precise launch time cannot be predicted, you can keep abreast of the current status of the launch preparations on the Institute for Astronomy web page.