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ISO Mission Overview

1. Mission summary

The ISO mission resulted from a proposal made to ESA in 1979. After a number of studies ISO was selected in 1983 as the next new start in the ESA Scientific Programme. Following a Call for Experiment and Mission Scientist Proposals, the scientific instruments were selected in mid 1985. The two spectrometers (SWS, LWS), a camera (ISOCAM) and an imaging photo-polarimeter (ISOPHOT) jointly covered wavelengths from 2.5 to around 240 microns with spatial resolutions ranging from 1.5 arcseconds (at the shortest wavelengths) to 90 arcseconds (at the longer wavelengths). The satellite design and main development phases started in 1986 and 1988, respectively. ISO was launched perfectly in November 1995 by an Ariane 44P vehicle. Routine scientific operations commenced in February 1996 and continued until depletion of the liquid helium (LHe) in April 1998, with limited operations continuing through May. The first homogeneous ISO data archive opened to the community in December 1998 and, by August 1999, all data had entered the public domain. Work on supporting the community in their exploitation of ISO data and on bringing the legacy archive into its final shape continued until the end of 2006, in the Post Operations Phase (1998-2001) and Active Archive Phase (2002-2006), as outlined below. Maintenance of the historical archive is now carried out by the core archive team at ESAC.

2. Operations Phase (1995-1998)

In orbit, ISO was an outstanding technical, operational and scientific success. Due to excellent engineering and a somewhat fortunate combination of circumstances at launch, the liquid helium supply lasted over 10 months longer than the specified 18 months. The extra lifetime not only led to many more observations but also made it possible to observe the scientifically-important Taurus/Orion region - inaccessible in the nominal mission. By a combination of very good in-orbit performance, detailed analysis on the ground and a variety of tune-ups, the accuracy of the pointing system was improved to the arc second level. The absolute pointing error was reduced to around 1 arcsec - ten times better than specified - and the short-term jitter was about five times better than the specification of 2.7 arcsec (2 sigma, half cone, over a 30 second period of time). The optical performance of the telescope and baffle system was excellent, with stray light being too low to measure. All elements of the ground segment also performed excellently, leading to an overall availability of the system during routine phase of 98.3% of the time scheduled for science. Taking into account all possible reasons for failure, only 4% of observations were lost. Over 98% of the highest-priority observations were successfully executed. All the scientific instruments, including many delicate cryogenic mechanisms, performed extremely well. At a wavelength of 12 microns, ISO was one thousand times more sensitive and had one hundred times better angular resolution than IRAS.

3. Post Operations Phase (1998 - 2001)

The goal of the post operations phase was to maximise the scientific return of ISO by facilitating effective and widespread exploitation of data. This was accomplished by:

deepening the understanding of the performance of the instruments and satellite in orbit;

It was a collaborative effort between the ESA ISO Data Centre (IDC) in Villafranca Madrid, five specialist National Data Centres (NDCs) built around the teams that had provided the instruments, and the US ISO Support Center at IPAC.

At the start of the Post Operation Phase, all data were re-processed with the end-of-mission calibration to populate the first homogeneous ISO Data Archive, which opened to the community in December 1998. Some 10 versions of the pipeline were released, grouped under three major overall ISO pipeline versions, from Version 8 to Version 10. The improved products were made available to the community via on-the-fly reprocessing in the archive. Users then selected, from the archive, data sets of interest and used the provided (and evolving) interactive analysis tools for deeper study. ISO's Legacy Archive, containing the reference product set from the final pipeline processing, was released just after the end of this phase.

A major effort was made on calibration and cross-calibration, using topic-oriented Working Groups, each addressing one specific calibration topic but with many links between them and fluid exchange of useful information. These Working Groups were formed by members from the IDC and from the different NDCs and reported on: Beam Profiles/Pointing; Glitches; Line profiles; Photometric calibration; Polarisation; Spectral Matching; Transients; and Interactive Analysis. Cross-calibration specialists maintained an overview of the work of the Working Groups and cooperated with instrument-specific calibration manpower in efforts to understand inconsistencies of calibration relevant to more than one instrument. Every six months an overall cross-calibration report was compiled. The final Cross-Calibration Final Report was issued in January 2001.

A mid-term review was held in February 2000 to review progress and make recommendations re future directions. All data centres provided direct support to users and welcomed visitors. Eight general conferences and specific small "hands-on" data reduction workshops were organised with the proceedings being made widely available via the ESA SP publications series.

4. Active Archive Phase (2002 - 2006)

The main goal of the Active Archive Phase was "to bring the archive into its final shape, while further facilitating the direct and easy access of the world-wide community to the ISO data products, especially for the preparation and exploitation of upcoming missions". The main thrusts of the active archive phase were to continue the support of the community in their use of ISO data and to capture back into the archive as much as possible of the user knowledge so as to maximize the long-term value and usability of the archive. One specific direction set by the post-operations mid-term review was "scientifically focused interactive data processing by users with the aim of including the results in the archive".

The responsibilities of the ESA ISO Data Centre in the active archive phase included:

maintenance and improvement of software packages for interactive data reduction,

including making available new algorithms and calibration files

further work on instrument-specific calibration;

focused reduction of specific data sets to be ingested back into the archive.

This phase continued most of the collaborations between the centres involved in the previous phase. In general, the effort was concentrated at the start of the phase, allowing a smooth transfer of expertise to other missions.

5. Science Highlights and Publications

ISO has delivered important results in nearly all fields of astronomical research, almost literally from comets to cosmology. Some are already being followed up with existing telescopes; others have to await future facilities, such as the Herschel Space Observatory. Highlights include:

the discovery of crystalline silicates outside our own solar system, in the atmospheres of young and old stars as well as in comet Hale-Bopp giving a link between interplanetary and interstellar dust; showing that water is ubiquitous in the cosmos. A wealth of water vapour transitions were measured in objects such as Mars, Titan, the giant planets, comets including Hale-Bopp, in shocks, in the cold interstellar medium, in circumstellar envelopes and in the ultra-luminous galaxy Arp220;

the first detections of the lowest pure rotational lines of H2 (giving access to gas at temperatures less than a few hundred K) in many sources such as young massive stars, HH-objects, PDR's, the diffuse ISM, outer parts of edge-on galaxies; detailed investigations of interstellar solid state features, e.g. CO2 ices, leading to a very detailed interplay between observations and laboratory spectroscopy.

obtaining spectral energy distributions of extragalactic objects, which serve as templates for cosmological studies, helped to consolidate the unified scheme or to develop quasar evolutionary scenarios.

making deep cosmological surveys at near- and far-infrared wavelengths the results from which show: early star formation hidden by dust; the need for evolutionary models; resolution of part of the cosmic infrared background (CIB) into discrete sources; and detection of fluctuations in the CIB.

The results produced until late 2004 were reviewed in a special issue of Space Science Reviews (vol 119, 2005). This issue was reprinted into a 450-page book entitled "ISO Science Legacy - a compact review of ISO major achievements", consisting of 4 general articles reviewing four major themes investigated by ISO (crystalline silicates, molecular hydrogen, water in the universe, deep surveys), followed by 13 papers reviewing ISO science from the Solar System to the distant Universe.

Recent papers continue to develop these themes and address a number of important questions in all areas of astronomy, either based on the study of a particular object or, mostly, on samples of targets extracted from the ISO archive and often in combination with data from other facilities. ISO data are also resulting in feedback into modelling and laboratory research activities.

Nearly 1400 papers have been published in the refereed literature on ISO results. Note that the ISO Data Archive contains links from the individual observations to the papers that use them. On the other hand, ISO observations are linked from the associated papers in ADS.

6. ISO Data Archive (IDA)

ISO performed around 30,000 science observations. If account is taken of observations made in the parallel and serendipity modes of the instruments (observational modes in which an instrument could observe while another instrument was prime) almost 150000 data sets were recorded with ISO. Since ISO was operated as an observatory with four complex instruments, the resulting data is very heterogeneous. The data underwent sophisticated processing, including validation and accuracy analysis before products were made available via the archive.

The ISO Data Archive (IDA) opened to the external world on December 9, 1998, six months after the end of the operations. Around 400 GBytes of data are stored on magnetic disks. In addition to the observational data products, the archive also contains satellite housekeeping data, software tools, documentation, and externally derived products.

Users of the ISO Data Archive see a self-contained, fast and powerful interface to all ISO data products. Complex queries can be made against hundreds of database parameters using friendly and modular query panels. The user is helped in selection of observations by a clear and configurable results display that includes quick-look data browsing, access to auxiliary and ancillary files, access to related observations in the ISO catalogue, on-line help and access to other archives.

The IDA is based on an open and flexible 3-tier architecture comprising the data products and database, the business logic and the user interface. An important consideration was to separate the stored data from their final presentation to the user. The business logic and the user interface have been developed entirely in JAVA and XML. This has facilitated its re-use for other ESA science archives (such XMM-Newton, INTEGRAL and Planetary Missions).

In addition to the 'standard' data products generated for every ISO observation by an automatic data-analysis pipeline, the ISO archive contains so-called "Highly Processed Data Products (HPDP)". These are the result of further processing beyond the pipeline and/or use of new, refined algorithms mainly contributed by ISO instrument experts. They include processed images and spectra, object catalogs, and spectral and image atlases. A number of instrument-mode or object-type specific projects have been carried out, fulfilling the goal set at the start of the Phase to populate 30% of the scientific content of the archive with more readily usable products. Thirty sets of HPDPs are available. The total number of observations associated with one or more HPDP sets is 9607, i.e. 32% of its scientific content.

The ISO Data Centre has also fostered the ISO presence in the Virtual Observatory. The initial ad-hoc implementation of elements of interoperability between archives (e.g. with CDS/VizieR, SIMBAD, ADS and NASA's IRSA) evolved into a system complying with the new standards set by the Virtual Observatory international community. The IDA was one of the first archives to be part of the European Virtual Observatory prototypes, as demonstrated in January 2004 and 2005. ISO has played a key role in the definition of standards for interoperability within the International Virtual Observatory Alliance (IVOA). Access to ISO spectral and image data is provided, respectively, via the IVOA Simple Spectrum Access Protocol (SSAP) and the IVOA Simple Image Access Protocol (SIAP). ISO has also been a pioneer in Europe by implementing Basic SkyNode (another IVOA standard) access to the ISO products.

Extensive explanatory and technical support documentation is archived. This includes the five volumes of the ISO Handbook (released in their final form as ESA SP-1262 in 2003), gathering all the information needed to make efficient use of ISO data, and extensive technical documentation, tracing and explaining the experience of the instrument teams as they worked to understand the calibration of their instruments. Together with the ISO data products, the body of explanatory and support documentation is called the ISO Explanatory Library. About 200 major documents are archived in an organised and uniform fashion for the satellite and the four instruments.

ISO Data Centres

The Data Centres responsible for ISO User Support were as follows (Centres which continued to operate into the Active Archive Phase are marked with a (#) :