Herschel is the largest astronomical telescope ever launched. Its 3.5 m-diameter primary mirror is giving astronomers their best view yet of the Universe at far-infrared and submillimetre wavelengths.

The mission builds upon the legacy of ESA’s Infrared Space Observatory and subsequent infrared missions such as NASA’s Spitzer and JAXA’s Akari.

The telescope is named after the German–British astronomer William Herschel. In 1800 he discovered infrared radiation while studying the Sun. He also discovered Uranus, the seventh planet in the Solar System, and surveyed the sky.

The Herschel mission has an unprecedented view of the cold Universe, bridging the gap in the spectrum between what can be observed from the ground and earlier space missions of this kind. Infrared radiation can penetrate the gas and dust clouds that hide objects from optical telescopes, allowing astronomers to see deep into star-forming regions, galactic centres and planetary systems.

Cooler objects, such as tiny stars and molecular clouds, and even galaxies enshrouded in dust (which barely emit optical light) are visible in the infrared. It can even detect emissions from dust itself. Observing in the infrared provides us with a complementary view of the Universe.

But why go into space to do this? The simple reason is that Earth’s atmosphere blocks most infrared wavelengths. In addition, the atmosphere produces its own infrared radiation. So, observing in the infrared from the ground is like trying to view stars on a cloudy day.

Herschel was launched together with ESA’s Planck satellite. Since operations began, the two missions have been studying different aspects of the cold cosmos.
 
Objectives
 
The key science objectives of the Herschel observatory are to study the formation of stars and galaxies, and to investigate the relationship between the two. The observing programmes are proposed by the wide astronomical community. Examples of Herschel’s specific investigations are:

• A survey of the formation and evolution of elliptical galaxies and the central bulges in other galaxies during the first third of the Universe’s history.
• Detailed follow-up observations of particularly interesting objects found in the survey. These will concentrate on understanding the physical processes and energy-generating mechanisms in galaxies.
• Detailed studies of the physical and chemical processes in the gas and dust that are not yet bound into stars and planets. These investigations are being conducted in our Galaxy and others. They are helping to investigate how and why stars form from interstellar clouds, and planets form from circumstellar discs. They will also provide fundamental clues about the complex organic molecules found, for instance, in the atmospheres of comets.
• Targeted observations of star formation and both young and old stars, to reveal the physical and chemical processes in the early and later phases of a star’s life.
• Detailed observations of the atmospheres of the cool outer worlds in our Solar System and of comets.

Launch

Herschel was launched on an Ariane 5 ECA from Europe’s Spaceport in Kourou, French Guiana, on 14 May 2009. It was launched together with ESA’s Planck spacecraft.
 
Mission lifetime
 
Herschel is designed to perform routine science operations for a minimum of 3 years. The mission will end when the helium used to cool the focal plane of the scientific instruments is depleted, which is predicted to happen sometime between late 2012 and spring 2013.
 
Spacecraft
 
Design

Herschel is a tall ‘tube’. The most obvious feature is the 3.5 m-diameter primary mirror that sits on top of the cryostat and is protected by a sunshade from direct sunlight. The mirror focuses infrared light, via a smaller secondary mirror, down into the focal plane inside the cryostat. This sits under the mirror and provides cooling of the scientific instrument package to within a fraction of absolute zero. The sunshade and is also partially covered with solar cells to provide the power required to operate the space telescope

Below the cryostat is the service module. It contains the essential computers and other systems that allow the spacecraft and its instruments to function, and also allow handling and compression of the raw scientific data.

In order to achieve its scientific objectives, Herschel’s detectors have to operate at very low and stable temperatures. So the spacecraft cools the detectors close to absolute zero (–273ºC), ranging from –265ºC to only a few tenths of a degree above absolute zero.

Mass

Approximately 3400 kg at launch. This included 2300 litres of liquid helium that is used to cool the instruments to their operating temperatures.

Dimensions

Herschel is 7.4 m high and 4.0 m in width.

Industrial involvement

The prime contractor is Thales Alenia Space (Cannes, France). It leads a consortium of industrial partners with Astrium (Friedrichshafen, Germany) responsible for the Extended Payload Module (EPLM) and Thales Alenia Space (Turin, Italy) responsible for the Service Module (SVM). There is also a host of subcontractors spread throughout Europe.
 
What’s on board?
 
Herschel carries the largest telescope mirror ever to be launched into space. It measures 3.5 m across and collects infrared light for three instruments.

Photodetector Array Camera and Spectrometer (PACS)
PACS is a camera and a low- to medium-resolution spectrometer for wavelengths up to about 205 microns. It uses two bolometric detector arrays in the camera and two photo-conductor detector arrays in the spectrometer.

Principal Investigator (PI):: Albrecht Poglitsch, Max Planck Institute for Extraterrestrial Physics (Garching, Germany).

PACS was designed and built by a consortium of scientists and institutes – under their own funding – from Germany (country of the PI), Belgium (country of the co-PI), Austria, France, Italy and Spain. They are all led by the PI.

Spectral and Photometric Imaging Receiver (SPIRE)
SPIRE is a camera and a low- to medium-resolution spectrometer for wavelengths longer than 200 microns. 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.

Principal Investigator: Matthew Griffin, Cardiff University (Wales, UK).

SPIRE was designed and built by a consortium of scientists and institutes – under their own funding – from the United Kingdom (country of the PI), France (country of the co-PI), Canada, China, Italy, Spain, Sweden and USA. They are all led by the PI.

Heterodyne Instrument for the Far Infrared (HIFI)
HIFI is a very-high-resolution spectrometer that can obtain information about the chemical composition, kinematics and physical environment of infrared sources.

Principal Investigator: Frank Helmich, SRON Netherlands Institute for Space Research (Groningen, The Netherlands).

HIFI was designed and built by a consortium of scientists and institutes – under their own funding – from the Netherlands (country of the PI), France, Germany and USA (countries of the co-PIs), and from Canada, Ireland, Italy, Poland, Russia, Spain, Sweden, Switzerland and Taiwan. They are all led by the PI.
 
Operations
 
Primary Ground Station: ESA’s deep space antenna in New Norcia (Australia).
Mission Operations Centre (MOC): provided by ESA at the European Space Operations Centre (ESOC), Darmstadt, Germany.
Herschel Science Centre (HSC): provided by ESA at the European Space Astronomy Centre (ESAC), in Villafranca, Spain.
NASA Herschel Science Centre (NHSC): provided by NASA at the California Institute of Technology (Caltech) Infrared Processing and Analysis Center (IPAC), Pasadena, California, USA.
Instrument Control Centres (ICCs):
- PACS ICC: Max Planck Institute for Extraterrestrial Physics, Garching, Germany;
- SPIRE ICC: Rutherford Appleton Laboratory, Didcot, UK;
- HIFI ICC: SRON Netherlands Institute for Space Research, Groningen, the Netherlands.

Herschel Project Scientist: Göran Pilbratt
Herschel Mission Manager: Johannes Riedinger
 
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