See also Herschel Space Observatory.
The Spectral and Photometric Imaging Receiver, SPIRE, is one of three scientific instruments that flew on the European Space Agency's Herschel Space Observatory.
Computer-generated picture of Herschel at L2 (Courtesy of ESA)
The Herschel Space Observatory (HSO, or simply Herschel) was launched on 14 May 2009, and operated until April 29 2013. During its operational lifetime it produced a wealth of observations which are still being analysed. Herschel carried a 3.5 metre diameter telescope, the largest astronomical telescope ever put into space, and three scientific instruments designed to carry out imaging and spectroscopy in the 60 – 700 µm wavelength range. Prior to Herschel, this far-infrared/submillimetre part of the spectrum was very poorly explored, as most of it is blocked by the Earth's atmosphere. But it is crucial to our understanding of star and galaxy formation and the nature and behaviour of interstellar matter. The key science goals of Herschel's observations were the detection and investigation of galaxies in the distant (and therefore early) universe, and the study of star formation and the interstellar medium in our own and nearby galaxies.
When carrying out observations in this part of the electromagnetic spectrum, it is important to cool the telescope and the instruments as much as possible, because warm objects are very bright in the far infrared and submillimetre. Herschel's telescope was cooled to about 85 K, but its three scientific instruments, PACS, HIFI, and SPIRE needed to operate at even lower temperatures. They were therefore installed in a large helium tank or “cryostat”. The lifetime of the cryostat in space was just under four years, after which the helium supply was exhausted and the instruments became too warm to operate. The Hershel spacecraft was finally switched off on June 17 2013, and is now in a safe orbit around the Sun with little chance of it ever encountering the Earth. During its operational lifetime, Herschel collected a huge amount of scientific data, and work will continue on the processing, analysis and scientific interpretation of the observations for many years to come. Although the operational part of the mission is over, the SPIRE team will continue to work during a three-year Post-Operations programme to put the SPIRE data in the best possible condition for the benefit of future research.
The computer-generated picture of Herschel above on the right shows the telescope mounted on top of the cryostat, both protected from direct sunlight by a large sunshade. The spacecraft control and telemetry systems and the instrument electronics are located at the bottom under the cryostat.
The Cardiff AIG is the lead institute for the Herschel-SPIRE instrument, which was built by an international consortium comprising more than 18 institutes from eight countries. SPIRE was assembled and tested at the Rutherford Appleton Laboratory in Oxfordshire. It was then installed in the Herschel spacecraft and underwent further testing as part of the complete system prior to launch.
The SPIRE instrument contained an imaging photometer (camera) and an imaging spectrometer. The camera operated in three wavelength bands centred on 250, 350 and 500 µm, and made images of the sky simultaneously in these three submillimetre “colours”. The spectrometer covered the range 200 – 670 µm, allowing the spectral features of atoms and molecules to be measured.
The camera and spectrometer occupied separate compartments in the instrument box, as shown in the computer generated images left. Each instrument compartment contained mirrors, submillimetre filters to define the wavelength bands observed, moveable mirrors to control the beam, internal calibration sources, and detector arrays. The detectors operated at a temperature of 0.3 K, provided by an internal cooler (located on the camera side).
The essential features of the SPIRE instrument are described in more detail in The Herschel -SPIRE instrument and its in-flight performance.
Much more information about the Herschel mission can be found at the website of the Herschel Science Centre.
The complete SPIRE instrument box is shown above, undergoing final check-out before delivery (top) and during installation in the Herschel cryostat with the other two instruments (below). Images courtesy of ESA.
SPIRE results offer the astronomical community a powerful tool for many astrophysical studies from our own solar system to the most distant galaxies. As of June 2013, more than 500 refereed scientific publications have used SPIRE data, and the number is increasing all the time. An up-to-date list of Herschel publications can be found here.
The design of SPIRE was dictated by two particular scientific programmes to which Herschel was particularly suited.
Stars form inside clouds of gas and dust in the interstellar medium (ISM), and the very first stages of this process are not well known. Dense regions inside such clouds may condense under their own gravity to form stars. The stars shine brightly in the visible and ultraviolet part of the spectrum, but that radiation doesn't escape from the cloud – it is absorbed by the dust, causing the cloud to heat up to a temperature of about 50 or 100 degrees Kelvin – not very hot but a lot warmer than it would be without the stars inside. Material at that temperature glows at wavelengths of about 100 µm, in the far infrared part of the spectrum – so that is where the energy comes out. So if we want to study star formation we need to look in the far infrared. SPIRE's camera allowed surveys to be made of nearby interstellar clouds, observing the warm dust directly, telling us about the amount of star formation and the processes that may hinder or promote the formation of stars.
SPIRE's high sensitivity to dust emission also made it the ideal instrument to study the material that is ejected in copious quantities from evolved stars, enriching the interstellar medium with heavy elements and providing the raw material for planetary systems like our own. Studies of star formation and of the interaction of forming and evolved stars with the ISM are also, of course, intimately related to the investigation of galaxy formation and evolution, which occur through just these processes.
Normal galaxies like the Milky Way emit about a third of their energy in the far infrared and subillimetre region due to the re-processing of stellar optical and UV radiation by interstellar dust grains. During the periods when the galaxies formed their stars, they were actually far brighter in the far infrared than in the optical/UV due to this effect. Galaxy formation can be viewed as star formation on a massive scale, and the study of the early stages of galaxy evolution thus requires us to detect emission from distant galaxies in the submillimetre, enabling their luminosities to be measured and their and star-forming activity to be estimated. SPIRE successfully carried out surveys to detect huge numbers of galaxies in the process of formation in the most distant (and therefore early) Universe. The results are giving astronomers for the first time the ability to study systematically how and when the galaxies grew and evolved over cosmic time, and how the chemical elements of which we are made were created in the process.
The SPIRE Consortium
The SPIRE project is a large international collaboration. Cardiff is the lead institute, with Prof. Matt Griffin as the Principal Investigator. The Consortium includes more than 300 scientists, engineers and managers from eight countries (Canada, China, France, Italy, Spain, Sweden, UK, USA). The following institutes have provided hardware and software elements to the instrument programme:
- Cardiff University: filters, internal calibrators, thermal system, instrument test support.
- CEA, Saclay, France: instrument control and detector readout electronics; Software contribution
- CEA, Grenoble, France: detector cooler
- Imperial College, London, UK: software contribution, instrument test support
- IAC, Tenerife, Spain: software contribution
- IPAC, Pasadena, USA: software contribution, instrument test support
- IFSI, Rome, Italy: digital electronics
- MSSL, Surrey, UK: instrument enclosure, thermal system
- JPL, Pasadena, USA; detector arrays, instrument test support
- NAOC, Beijing, China: software contribution
- OAMP, France: instrument optics; Spectrometer mechanism
- RAL, Oxfordshire, UK: instrument assembly and testing; project management and system engineering
- Stockholm Observatory, Sweden: software contribution
- UK ATC, Edinburgh, UK: beam steering mirror
- University of Colorado, USA: detector optics; instrument test support
- University of Lethbridge, Canada: instrument test components and support, software contribution
- University of Padua, Italy: software contribution
- University of Sussex, UK: software contribution
Funding for SPIRE has been provided by the national agencies of the participating countries and by internal institute funding: the Canadian Space Agency (CSA); NAOC in China; Centre National d'Etudes Spatiales (CNES), Centre National de la Recherche Scientifique (CNRS) and Commissariat à l'Énergie Atomique (CEA) in France; Agenzia Spaziale Italiana (ASI) in Italy; Ministerio de Educacion y Ciencia (MEC) in Spain, Stockholm Observatory in Sweden; the Science and Technology Facilities Council (STFC) and the UK Space Agency (UKSA) in the UK; and NASA in the USA. Additional funding support for some instrument activities has been provided by ESA.
The Herschel-SPIRE instrment and its in-flight performance
Griffin, M., Abergel, A., Ade, P., et al., Astronomy and Astrophysics 518, L3, 2010 (DOI: 10.1051/0004-6361/201014519)