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ISO

The next major astrophysical mission of ESA is the Infrared Space Observatory (ISO), scheduled for launch in late 1995. Selected in 1983, ISO will expand upon the work of the pioneering US-UK-Netherlands Infrared Astronomical Satellite (IRAS), launched in 1983. However, "compared with IRAS, ISO will have a longer operational lifetime, wider wavelength coverage, better angular resolution, more sophisticated instruments, and, through a combination of detector improvements and longer integration times, a sensitivity gain of several orders of magnitude" (Reference 228). Whereas IRAS was designed to map the IR celestial sphere, ISO will make more detailed observations of selected objects.

ISO will have an initial mass of 2.5 metric tons in a compact structure 5.3 m in length and 23 m in width. The precision attitude control system will provide a pointing accuracy of 2.7 arcseconds. Electrical power will be furnished by solar cells mounted on the exterior of the Sun shield. ISO's operational orbit will be 1,000 km for perigee and 70,000 km for apogee at a low inclination, and the spacecraft will be controlled from ESA's Villafranca ground station in Spain (References 228-235).

The heart of ISO is a cryogenically-cooled Ritchey-Chretien telescope with an effective aperture of 60 cm. Approximately 2,300 liters of superfluid helium will be carried to cool the infrared detectors and scientific equipment to 23 degrees K for a period of at least 18 months. The principal instruments are (1) ISOCAM camera and polarimeter operating at 2.5-17 µm, (2) ISOPHOT imaging photopolarimeter operating at 2.5-200 µm, (3) SWS short-wavelength spectrometer operating at 2.4-45 µm, (4) LWS long wavelength spectrometer operating at 45-180 µm.

ISO's prime contractor is Aerospatiale with a team of about 35 subcontractors, including Fokker, DASA, and Dornier. Difficulties with the cryogenic cooling system have been the principal reason for the more than two year delay encountered thus far in the project. In addition, the flight model telescope was rejected due to excessive contamination and blemishes on the primary mirror. The mirror was replaced, and the telescope rebuilt.