Paris, 18 May 1998
Controllers at the ESA ground station at Villafranca (Madrid, Spain) witnessed the definitive end for the telescope but they didn't have to press any 'red button' or the like. The instructions for the switch off had already been introduced into ISO's computers earlier.
ISO's last month of life was used to gather as much technical data as possible. Various software and hardware systems that, due to the superb performance of the spacecraft, did not have to be used during the operational phase were subjected to detailed tests. Results from these tests will benefit future ESA missions, such as XMM and Integral, which use some of the same components, such as the Star Trackers guiding the spacecraft.
Also, ISO's farewell included a further last gift for the astronomers. A few of the detectors in the Short Wavelength Spectrometer (SWS), one of the four instruments on-board the satellite, could still be used after exhaustion of the liquid helium. In anticipation of this opportunity, a special scientific programme had been prepared and was interleaved with the technology tests. Some 150 extra hours were used to measure nearly 300 stars at wavelengths between 2.4 and 4 microns enabling astronomers to make a detailed spectral classification.
In fact, ISO continued to give scientific surprises to the very end. ISO's 'last light' observation --taken with the SWS instrument just before midnight on May 10-- was of emission lines from hydrogen in a hot supergiant star (Eta Canis Majoris). The preliminary results show that this star, supposed to be ordinary, is probably surrounded by a disk of matter.
Commenting on the satellite switch off, ESA's Director of Science, Roger Bonnet, said "ISO has allowed us to gain the first clear view of the universe at infrared wavelengths. A great amount of work still awaits us to interpret all ISO's exciting discoveries. We will miss ISO, of course -- new answers always bring new questions and the wish for yet more knowledge; that is why ESA is already working on one of ISO's successors, the Far Infrared and Submillimetre Space Telescope, FIRST."
ESA's ISO has revolutionised infrared astronomy; its discoveries have already unveiled a totally new face of the universe. Many more results are still to come in the months and years after ISO's switch off as astronomers continue mining its treasure trove of unique data.