Infrared astronomy is a young and exciting science. In recent decades infrared astronomers have unveiled tens of thousands of new galaxies, and have made surprising discoveries such as the huge amounts of water vapour that fill our Galaxy. Yet scientists know there is still much more to discover. Objects such as other planetary systems, or processes like the birth of galaxies in the early Universe, can best be studied with infrared telescopes situated in space and therefore freed from the restrictions imposed by the Earth's atmosphere. This is the reason ESA has constructed the Herschel observatory.
The Herschel spacecraft is approximately 7.5 metres high and 4 × 4 metres in overall cross section, with a launch mass of around 3.4 tonnes. The spacecraft comprises a service module and a payload module. The service module houses systems for power conditioning, attitude control, data handling and communications, together with the warm parts of the scientific instruments. The payload module consists of the telescope, the optical bench, with the parts of the instruments that need to be cooled, i.e. the sensitive detector units and cooling systems. The payload module is fitted with a sunshield, which protects the telescope and cryostat from solar visible and infrared radiation and also prevents Earth straylight from entering the telescope. The sunshield also carries solar cells for the electric power generation.
The Telescope and Instruments
The Herschel telescope is a Cassegrain design with a primary mirror diameter of 3.5 metres, the largest single mirror ever built for use in space. The three scientific instruments are:
- HIFI (Heterodyne Instrument for the Far Infrared), a very high resolution heterodyne spectrometer
- PACS (Photodetector Array Camera and Spectrometer) - an imaging photometer and medium resolution grating spectrometer
- SPIRE (Spectral and Photometric Imaging Receiver) - an imaging photometer and an imaging Fourier transform spectrometer
The instruments have been designed to take maximum advantage of the characteristics of the Herschel mission. In order to make measurements at infrared and sub-millimetre wavelengths, parts of the instruments have to be cooled to near absolute zero. The optical bench, the common mounting structure of all three instruments, is contained within the cryostat and over 2000 litres of liquid helium will be used during the mission for primary cooling. Individual instrument detectors are equipped with additional, specialised cooling systems to achieve the very lowest temperatures, down to 0.3 K for PACS and SPIRE.
Herschel is the only space facility ever developed to cover the far infrared to sub-millimetre parts of the spectrum (from 55 to 672 µm). It will open up an almost unexplored part of the spectrum, which cannot be observed well from the ground.
The Questions Herschel Will Answer
The questions that Herschel will seek answers to include:
- How galaxies formed and evolved in the early Universe
- How stars form and evolve and their interrelationship with the interstellar medium
Herschel will also investigate the chemistry of our Galaxy and the molecular chemistry of planetary, cometary and satellite atmospheres in the Solar System.
Herschel was carried into space by an Ariane 5 ECA launcher on 14 May 2009 and about sixty days after launch reached its orbit around L2. For reasons of cost effectiveness, ESA decided to launch Herschel together with Planck, a mission to study the cosmic microwave background radiation. The two spacecraft separated soon after launch and are being operated independently.
Herschel's operational orbit is located 1.5 million kilometres away from the Earth in a direction diametrically opposite the Sun, at the second Lagrange point of the Sun-Earth system (L2). There the spacecraft is in a Lissajous orbit around L2 with an average amplitude of about 700 000 km and an orbital period of about 178 days.
Herschel has a nominal routine operational lifetime of three years, with a possible extension of one year. About 7000 hours of science time will be available per year. Herschel is a multi-user observatory accessible to astronomers from all over the world.
In 1983 the US-Dutch-British IRAS satellite inaugurated infrared space astronomy by mapping 250 000 cosmic infrared sources and large areas of extended emission.
In November 1995 ESA launched its Infrared Space Observatory, ISO, which has allowed a much more detailed study of the infrared sky. ISO observed in the wavelength range from 2.5 to 240 µm and achieved an one thousand fold increase in sensitivity and a one hundred fold improvement in angular resolution (at 12 µm) compared to IRAS. ISO's operational lifetime was one year longer than planned, ending in May 1998.
The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched on 25 August 2003. During its nominal 2.5-year mission and subsequent extended operations, Spitzer obtained infrared images and spectra in the wavelength range 3 to 180 µm with its 0.85 metre telescope and three science instruments operating at cryogenic temperatures. Rather than operating at L2, as Herschel does, Spitzer is in an Earth trailing heliocentric orbit.
Why Observe in the Infrared?
Large parts of the Universe are too cold to radiate in the visible wavelength range or at shorter wavelengths. Study of these cooler objects is only possible by observing in the infrared spectrum or at even longer (sub-millimetre) wavelengths. Bodies with temperatures between five and fifty Kelvin have radiative emission peaks in the wavelength range observed by Herschel, and gases with temperatures between ten and a few hundred Kelvin exhibit their brightest molecular and atomic emission lines at these wavelengths.
Additionally, many objects of great interest to astronomers are concealed within or behind clouds of gas and dust. In the early stages of their formation, stars and planets are surrounded by the gas and dust clouds from which they are being created. Galactic cores and most of the remnants of the early Universe are also hidden from view by dust clouds. The dust particles in these clouds are comparable in size to the wavelength of visible light and are therefore efficient at scattering or absorbing radiation at these wavelengths. Infrared radiation is less affected by these clouds - the longer the wavelength, the thicker the dust cloud that it can penetrate.
Why Observe in Space?
Water vapour in the Earth's atmosphere absorbs radiation across large parts of the infrared and sub-millimetre wavebands, making ground based observations at these wavelengths impossible. Limited observations can be made using techniques such as high altitude balloons but a space-based observatory is the only truly satisfactory solution to this problem.
By orbiting at L2, some 1.5 million kilometres from Earth, Herschel is not troubled by any atmospheric absorption. In addition, the spacecraft avoids any problems caused by thermal infrared radiation from the Earth interfering with observations. The L2 orbit also prevents the occurrence of temperature changes due to the spacecraft moving in and out of eclipse in an Earth orbit, which are a particular problem for infrared instruments requiring extreme thermal stability.