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Herschel will be the largest space telescope of its kind when launched. Herschel's 3.5-metre diameter mirror will collect long-wavelength infrared radiation from some of the coolest and most distant objects in the Universe. Herschel will be the only space observatory to cover the spectral range from far-infrared to sub-millimetre wavelengths.
Infrared radiation is invisible for the human eye. It is actually 'heat', or thermal radiation. Even objects that we think of as being very cold, such as an ice cube, emit infrared radiation. For this reason, infrared telescopes can observe astronomical objects that remain hidden for optical telescopes, such as cool objects that are unable to emit in visible light.
Also, infrared instruments need to be cooled down to temperatures very close to absolute zero (-273.15°C), otherwise their own infrared emission would spoil the observations. Opaque objects, those surrounded by clouds of dust, are another speciality for infrared telescopes: the longer infrared wavelengths can penetrate the dust, allowing us to see deeper into such clouds.
However, Earth's atmosphere acts as an 'umbrella' for most infrared wavelengths, preventing them from reaching the ground. A space telescope is needed to detect this kind of radiation invisible to the human eye and to optical telescopes.
What's special?
If it was possible to look at the Universe from the outside it would probably appear as a foamy structure, with the galaxies distributed in curved walls surrounding huge areas of emptiness, like bubbles in a foam bath. Such is the overall picture of the present-day Universe.
However, it was not always like that. There was a time when galaxies were not there, simply because they did not even exist yet. Astronomers have several questions about this time. When did galaxies form? How did it happen? Did they all form at about the same time, or is there a non-stop galaxy-making machine at work? Were the first galaxies like those we see now? The galaxies are made of stars... Did the stars form first and then get together to form galaxies, or was it the other way round? How do stars form? When they form, do they normally form planets as well?
Astronomers dream of a telescope able to answer these kinds of questions. They want a telescope that fulfils at least two requirements. It has to be a giant space telescope, able to collect light from very distant galaxies. Secondly, it must be able to observe objects completely enshrouded by dust, as forming stars and galaxies are certainly dusty.
ESA's Herschel mission has been designed specifically to achieve these goals. With its ability to detect far-infrared light, it will let astronomers see, for the first time, dusty and cold regions that have been hidden so far. With its 3.5-metre mirror, Herschel will mark the beginning of a new generation of 'space giants'.
Spacecraft and telescope
The Herschel satellite is a tall 'tube' 7.5 metres high and 4 metres wide, with a launch mass of around 3.3 tonnes. It will carry the infrared telescope and three scientific instruments. The bulk of the spacecraft consists of a liquid helium thermos bottle inside which the instrument detectors sit and are cooled down to only a few degrees above absolute zero.
The telescope is a Cassegrain telescope, with a primary mirror diameter of 3.5 metres. This is the largest space telescope ever to be built and a great technological challenge that Europe will face alone. The contract signed between ESA and Astrium in Toulouse, France, to build Herschel's telescope makes it fully European.
Herschel's telescope has to meet demanding requirements. It has to be light enough to be placed into an orbit far more distant than, for example, that of Hubble. Also, the mirror's surface has to be extremely smooth; it had to be polished to make it so uniform that its 'bumps' are smaller than a few thousandths of a millimetre. It will have to withstand very hard environmental conditions. At launch, it will be 'shaken' with a force several times that of normal gravity on Earth.
In order to achieve its scientific objectives, Herschel's detectors have to operate at very low and stable temperatures. So the spacecraft is equipped with the means of cooling the detectors close to absolute zero (-273ºC), ranging from -265ºC to only a few tens of a degree above absolute zero.
Science payload
Herschel's science payload consists of three instruments:
* Photodetector Array Camera and Spectrometer (PACS), a camera and a low- to medium-resolution spectrometer for wavelengths up to about 205 micrometres. It uses two bolometer detector arrays in the camera and two photo-conductor detector arrays in the spectrometer.
* Spectral and Photometric Imaging Receiver (SPIRE), a camera and a low- to medium-resolution spectrometer for wavelengths longer than 200 micrometres. It uses five detector arrays: three to take images of infrared sources in three different infrared 'colours' and two to fully analyse the longer infrared light being released from the source.
* Heterodyne Instrument for the Far Infrared (HIFI), a highly accurate spectrometer that can be used to obtain information about the chemical composition, kinematics, and physical environment of infrared sources.
PACS and SPIRE are cameras and spectrometers that will allow Herschel to take pictures in six different 'colours' in the far-infrared. HIFI is a spectrometer with extremely high resolution. The scientific payload complement was approved by the ESA in February 1999.
Journey
Herschel will be launched in 2008 with another mission, Planck - a mission to study the cosmic microwave background radiation - on an Ariane rocket. The two spacecraft will separate about 2.5 hours after launch and will operate independently. In less than six months, Herschel will reach its operational orbit around a point in space known as the second Lagrangian point (L2), situated at 1.5 million kilometres away from the Earth.
Herschel has been designed to perform routine science operations for a minimum of 3 years at L2. The mission will end when the helium used to cool the focal plane of the scientific instruments is depleted. |
