ESA's Science Programme Committee, meeting at the European Space Research and Technology Centre, ESTEC, in Noordwijk (the Netherlands) on 3 and 4 June 1993, has accepted the recommendations of the Space Science Advisory Committee and confirmed INTEGRAL (International Gamma-Ray Astrophysics Laboratory) as the second "medium sized" mission (M2) within the framework of the "Horizon 2000" scientific programme (see also ESA Information Note to the Press nr. 07-93 of 22 April 1993).
INTEGRAL is a gamma-ray observatory to be developed in cooperation with NASA and the Russian Institute for Space Research, IKI. The instruments it will carry, 10 to 50 times more sensitive than those previously flown on other spacecraft of this kind, will allow it to greatly push the limits of gamma ray astronomy.
INTEGRAL's primary mission will be to observe the galactic plane and centre, but it will also be looking into other directions to search for extragalactic gamma-ray sources such as nuclei of active galaxies or clusters of galaxies. Another observing priority will be the study of compact objects (neutron stars, black holes) as well as supernovae and novae. Most of the observing time will be made available to the worldwide astronomy community through a "guest observer programme" for which calls for proposals will be issued. The rest of the mission will be reserved for the research institutes that will develop the payload, i.e. the instruments, and the data processing facilities.
The INTEGRAL satellite will use a platform -the so-called "bus"- identical to that currently being developed for the ESA X-ray Multi Mirror (XMM) cornerstone mission. ESA has the overall responsibility for spacecraft and mission design, instrument integration and testing, spacecraft operations and acquisition of data. Scientific instruments will be provided largely by the European science community. NASA will supply one or two ground stations and will be involved in the development of instruments, e.g. the spectrometer. Russia will provide a Proton launcher free of charge in exchange for observing time. A parallel option with an Ariane 5 launcher is also considered.
The launch of the 3.6 ton INTEGRAL spacecraft is planned early in the year 2001. The Proton launcher delivers the INTEGRAL into a 72 hour orbit with a high perigee of 48000 km and a 115000 km apogee at 51.6 degree inclination, while an Ariane 5 launch would inject the spacecraft into a 24 hour orbit with a 4000 km perigee and 68000 km apogee at 65 degree inclination. Both launch vehicles would thus make it possible to avoid the disturbance that is caused by high-energy particles trapped in the Earths's radiation belts and which actually permits observations to be carried out only at spacecraft altitudes above 40000 km. The INTEGRAL mission is expected to last 2 years, but could be extended to 5 years.
Note to Editors
In common with most of the components of the electromagnetic spectrum, gamma-rays, which are intercepted by the atmosphere, do not reach the surface of our planet. They can therefore only be observed from space. The first satellite observing gamma-rays that was launched by ESA, in 1975, was the COS-B spacecraft. It was initially planned to carry out a two year mission, but remained operational for 80 months. COS-B was the second satellite dedicated to observing this end of the spectrum following the American SAS-2 satellite (Small Astronomical Satellite) which had been launched in 1972. On the strength of these early successes, the Europeans and Americans began working on satellites for future missions. In 1989, the Soviet satellite Granat was launched by a PROTON carrying SIGMA (Syst.me d'Imagerie . Masque Al.atoire), a French coded mask (*) gamma-ray telescope. In the United States the Compton Gamma-Ray Observatory (CGRO) was launched by the Space Shuttle in April 1991.
The next stage in gamma-ray astronomy
Granat is now nearing the end of its mission, while the CGRO continues to explore the skies for gamma-ray sources. As the next logical step in gamma-ray astronomy, INTEGRAL, will fly a payload comprising two main instruments, a Caesium Iodide imager and Germanium spectrometer which are complementary to each other, together with two monitors: a CCD optical transient camera (OTC) and a coded mask X-ray monitor.
The two main instruments will bring major improvements in both spectral and angular resolution of observations. INTEGRAL's instruments will be 10 to 50 times more sensitive than those on earlier missions. This improvement in sensitivity will give access to a substantially increased portion of the universe that is observable in this part of the spectrum. With its Germanium detectors (which have a much better spectral resolution, and which are more sensitive than counters used so far), the spectrometer will be able to study typical radiation from violent processes in the 15 KeV to 10 MeV region about which little is as yet known: nuclear excitation, positron annihilation and cyclotron emission. The main task of the imager will be observation and mapping of sources with a much improved spatial accuracy and sensitivity. The monitor instruments will study the same part of the sky as the main instruments, but in different bands of the spectrum. The optical transient camera (OTC) could even make it possible for the first time to associate a gamma-ray burst source with a visible object.
(*) Coded masks to reconstitute images In gamma-ray astronomy one cannot use mirrors to concentrate photon fluxes, instead, spectrometer, imager and x-ray monitor will use coded masks to image photon sources. Images of the sources observed will be deduced from the shadow cast by the mask on the detectors. This technique, which provides "statistical images" has been used for some 15 years on gamma-ray telescopes flown by balloons. Granat is the first satellite on which it has been successfully applied.
On INTEGRAL, the spectrometer, imager and x-ray monitor will all be fitted with masks, each adapted to the type of observation to be carried out by the instruments concerned. The mask for the main INTEGRAL imager, which will consist of over 2000 elements, can be rotated so as to eliminate "background noise" likely to blur the images obtained