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Herschel - Looking at the Birth of Stars

* EADS Astrium builds largest infrared telescope
* Measurements near absolute zero
* Observation from a distance of about 1.5 mio. km

ILA/Berlin, Friedrichshafen, 10 May 2004

With the Herschel space telescope, the European Space Agency ESA will be able to look billions of light- years out into space to investigate the formation of stars. From 2007, Herschel will observe the evolution of stars and galaxies in the infrared spectrum with unprecedented resolution. As part of an international industrial consortium, EADS Astrium is developing the satellite's central element, i.e. the payload module with its "super cooling unit", and will assemble and test the spacecraft in Friedrichshafen. With its 3.5m diameter silicon carbide (SiC) mirror manufactured by EADS Astrium in Toulouse, Herschel will be the largest imaging space telescope ever built.

At a height of 7.5m and a diameter of 4.0m, Herschel is impressive in size. With a launch mass of approximately 3.25 tons, its three highly sensitive instruments, which include cameras, photometers and spectrometers will operate at various wavelengths in the focal plane.

These detectors operate in the far infrared spectrum over wavelengths of between 60 and 670 microns. Almost all objects emit heat in this range and so, in order to prevent the instruments' own infrared radiation from drowning out the received signal, they must be cooled inside a cryogenic unit – called a cryostat – down to minus 271 degrees Celsius (about two degrees above absolute zero).

The cryostat is the central unit of the payload module and is being built under the leadership of EADS Astrium. It will be three metres high and have a diameter of three metres. The low temperature will be achieved using superfluid helium. An aperture on the top of the cryostat will allow the entry of light from the telescope.

For the construction of cryostat, EADS Astrium was able to draw on the experience it gained from Herschel's precursor, the European Infrared Space Observatory (ISO) which was successfully operated from 1996 to 1998. An ISO test model is still available in Ottobrunn for to test the highly sensitive Herschel instruments in the "ultracold" environment.

While the ISO telescope could be housed completely within the cryostat, as its mirror was only 70cm diameter, the 3.5-metre mirror is positioned in front of, and outside, the cryostat. The payload module also includes a large sunshield which whilst protecting the spacecraft against scattered radiation, also accommodates the solar cells.

World's largest Silicon-Carbide telescope mirror

The size and surface quality of the primary mirror determine the performance of a telescope. The bigger the reflector, the more heat radiation it collects. This enables the observation of celestial bodies with very low luminosity. At the same time, the resolution increases with the mirror size, as well as the detail sharpness of the images. Thus, Herschel will be a super-class instrument.

The construction of such a large silicon carbide (SiC) mirror represents a great technological challenge. On the one hand, the mirror should be as lightweight as possible in order to reduce the launch mass. On the other, it must withstand extreme loads with high accelerations and shock loads during launch and considerable fluctuations in temperature. In space, the mirror will cool down to approximately minus 200 degrees Celsius but, at the same time, it must guarantee the utmost contour accuracy and ensure that the surface is perfectly smooth and highly polished.

The Herschel telescope will weigh 300kg overall with the 20-centimetre-thick primary mirror alone weighing 240kg. At the same time, the material is extremely stable and combines all the advantages of metal and glass: It hardly changes shape at all during temperature fluctuations and it is resistant to high stresses and fatigue.

With a mirror measuring 3.5 metres in diameter, Herschel will be the largest space telescope ever built. In comparison: Hubble has a 2.4-metre mirror. In addition to this, the large primary mirror, EADS Astrium is also building the secondary mirror. Located at the focal point directly in front of the primary mirror, it reflects the light from the primary mirror back through an aperture in the primary mirror to the measuring instruments behind. This secondary mirror and its support structure are also made of SiC.

Double launch into an unusual orbit

To ensure an unimpeded view out into space, Herschel is located 1.5 million kilometres from the Earth on the opposite side from the sun, i.e. the so-called second Lagrangian point. This is about four times the distance between the Earth and the Moon. From here Herschel can easily avoid the infrared radiation of the Earth and the Moon and the gravitational pull of the Sun and the Earth are cancelled out by the centrifugal force acting on it. Thus, Herschel moves around the Sun in parallel with the Earth.

Herschel will be the first spacecraft to arrive at the second Lagrangian point. A second observatory, named Planck will join it there. Both spacecraft will be launched together on on Ariane 5, thus considerably reducing the launch costs. After launch Herschel and Planck will separate from the Ariane upper stage and then detach from each other. Using their own propulsion systems they will go into different orbits to carry out their own completely different tasks. Planck is going to measure the so-called cosmic background radiation for at least 15 months. This is a type of "Big Bang echo" which encompasses a wealth of information about the formation of the Universe.

A look into Deep space

In the infrared spectral range, a completely different Universe is disclosed to astronomers than that seen in visible light. Accordingly, Herschel will be a universal device with almost inexhaustible fields of application. One core field of research will be the study of evolving stars.

Huge dust clouds are scattered everywhere in the Milky Way. Under specific conditions and due to gravitational forces, individual areas inside such clouds will concentrate more and more matter and thus form new stars. The Sun was also created in this way. Only when the stars begin to shine brightly, do they clear the environment of the remaining dust and become visible over great distances.

In visible light it is not possible to witness these early stages in the birth of a star because they take place deep inside the cloud. Infrared radiation, by contrast, makes this possible. It penetrates the dust clouds and reveals a momentary glimpse into the formation of new stars.

ISO, Herschel's precursor, already enabled astronomers to make relevant findings. By virtue of its much larger primary mirror and its more sensitive detectors, Herschel can find objects which are considerably fainter. In addition, it will provide more detailed images than its precursor. This is primarily due to the rapid development of infrared detectors within the past ten years.

Furthermore, the infrared detectors on Herschel make it possible to reach the longer wavelengths. ISO operated between 2 and 240 microns. Herschel, however, will study celestial bodies over wavelengths of 80 to 670 microns. Thus, this telescope closes the gap between the infrared and radio spectra, allowing the investigation of extremely cold objects, such as those existing mainly within dense dust clouds.

In addition, the researchers want to use Herschel to study individual substances, such as ice particles consisting of water, carbon dioxide and hydrogen cyanide, or minerals within clouds. All these substances play an important role in the early stages of star formation. The new findings on the chemical composition of star matter will substantially expand understanding of the formation of stars and thus on the solar system itself.

A second core field of research relates to the formation and evolution of galaxies. Astronomers assume that in the young galaxies existing for a few billion years after the Big Bang, up to one hundred times more stars per year were formed than today. The newborn hot stars have heated the surrounding dust to such an extent that it emits intensive heat radiation in the infrared range.

These young galaxies are many billions of light-years away. By virtue of their escape speed, their light is displaced towards longer wavelengths. Astronomers call this phenomenon spectral redshift. Thus, the infrared radiation emitted by the dust at 100 microns, for example, reaches Earth at a wavelength of 120 microns. In this range, Herschel will be the first space telescope operating at this frequency range. This frequency range, however, is inaccessible from Earth because of the atmospheric absorbtion.

The same applies to distant galaxies which are thought to conceal black holes. They absorb the surrounding gas, which was previously heated to millions of degrees. Thus, dust clouds in the local vicinity are also heated, which then radiate in the infrared range. This infrared radiation also reaches the Earth in the submillimetre range.

FIRST becomes Herschel

Herschel's former scientific name was the Far Infra-Red and Sub-millimetre Telescope (FIRST). Later, ESA renamed it in honour of Friedrich Wilhelm Herschel, the most famous astronomer of his time, who was the first to discover infrared radiation in the solar spectrum in 1800. He became known for the detection of the planet Uranus in 1781.

EADS Astrium is Europe’s leading satellite system specialist. Its activities cover complete civil and military telecommunications and Earth observation systems, science and navigation programmes, and all spacecraft avionics and equipment. EADS Astrium is a wholly owned subsidiary of EADS SPACE, which is dedicated to providing civil and defense space systems. In 2003 EADS SPACE had a turnover of € 2.4 billion and 12,000 employees in France, Germany, the United Kingdom and Spain.

EADS is a global leader in aerospace, defense and related services. In 2003, EADS generated revenues of € 30.1 billion and employed a workforce of more than 100,000.

Friedrichshafen, May 2004/04009

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