Herschel: the infrared Universe

At this moment in time, 2 European space observatories are flying to reach their orbit. Their common aim: to get a better understanding of our Universe.

 

A short while after their launch via Ariane 5, Planck and Herschel (right) are making their way to Lagrange point L2, 1.5 million km from the Earth. Both European Space Agency space observatories will go into orbit around this point in July 2009. Credit: ESA/AOES Medialab

On 14 May 2009, the Ariane 5 rocket achieved a double feat: after a successful lift-off from the Guiana Space Centre (CSG), it launched satellites Herschel and Planck on their trajectories, enabling them to travel to Lagrange point L2, 1.5 million km from our planet. This is a point of equilibrium between the gravitational field of the Sun and that of the Earth. There are, in fact, 5 similar points which are not — eccentricity of physics which goes against common sense — necessarily positioned between our planet and the celestial body of the day! For Herschel and Planck, L2 is above all a “haven of peace” around which they will orbit.

Herschel and Planck will orbit around Lagrange L2. Not to scale. Credit: Enjoy Space

Once there, they will not be subjected to any cold passage through the Earth’s shadow or the influence of the heat rejected by the Earth, both of which would cause thermal instability. Such instability is incompatible with the precision of their onboard instruments.
Lastly, the final advantage of L2 is that it obviously follows the Earth on its annual path around the Sun, and will take the duo with it. Hence, the pair of satellites will permanently remain within listening and control range of the engineers and scientists. The first section of this feature will be given over to Herschel.

The biggest space mirror
Paying tribute to astronomer William Herschel (1738-1822) who notably discovered the planet Uranus and infrared radiation, the space telescope of the same name is, above all, characterised by its primary, 3.5 m diameter mirror. This is the biggest mirror ever sent into space, bigger even than that of the famous Hubble space telescope by a good 110 centimetres. Let’s be honest about things, Herschel is going to be making observations in the infrared light range whereas Hubble also operates in the visible range; infrared light is “satisfied” with a mirror of lesser “precision”.

With a diameter of 3.5 m, the Herschel mirror is the biggest ever sent into space. Credit: ESA

Nevertheless, infrared wavelengths are of enormous interest to astronomers. Firstly, objects that are too cold do not give out light in the visible range and only infrared sensitive detectors can see them. Telescopes such as Herschel are, therefore, going to show us a whole hidden section of the Universe. Secondly, infrared light, contrary to visible light, goes through gas and dust clouds much more easily: we are, then, lifting a curtain that ordinarily makes objects and phenomena invisible. In concrete terms, astronomers are hoping not only to be able to better determine what elements are present in interstellar clouds, but also to get a better understanding of star and galaxy formation. Herschel’s capacities can also be put to good use as regards the study of giant planets (such as Jupiter and Saturn), their satellites and comets.

The Universe is not the same in the infrared light range! In the right-hand photograph, we can see the hands hidden by the bag. In astronomy, infrared light can be used to see through the gas clouds and, for example, study the stars forming within the obscure nebulae. A simulation photograph created by NASA’s infrared Spitzer telescope team. Credit : NASA

3 to 4 years in which to succeed
And thirdly, light from the most distant galaxies tends towards the red due to the expansion of the Universe — a phenomenon similar to the change in tonality of a fire engine’s siren as it speeds away from you — to the point that it “leaves” the visible spectrum for the infrared and the far infrared, the area to which Herschel is partial. Scientists will, therefore, be able to spy on the first galaxies which were formed more than 12 billion years ago.

Position of the cryostat in Herschel Placed under the mirror and containing 2,300 litres of liquid helium, it maintains the instruments at a temperature of only several degrees above absolute zero. In space, Herschel also benefits from the lack of atmosphere which blocks a large part of the infrared spectrum on Earth. Credit: ESA/AOES Medialab

But a 3.5 m diameter mirror is not enough to achieve this, extremely precise, infrared sensitive equipment is also required. The three instruments in the European Space Agency’s satellite, namely PACS, SPIRE and HIFI, were developed in collaboration with scientific institutes from different countries with France playing a key role co-ordinated by the CNES, the French Space Agency. So that they operate correctly, they are cooled by 2,300 litres of liquid helium and must not exceed a temperate of -269 °C, which is only 4°C above absolute zero. However, this extreme performance will not last; inside 3 to 4 years, the liquid helium will have evaporated, heat will then take over and Herschel will become “blind”. The other challenge of this mission is, therefore, to make the best of the observation time available and maximise the scientific return. So despite all the technology developed, it will be the men and women on the ground that will make the difference.

This feature continues with Planck.

Published on 16 June 2009