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After successful completion of its in-orbit mission, the ISO satellite was switched off at 14.00 CET on 16 May. The two key final commands had been sent up to the spacecraft earlier and executed by 'time tag'. At 13.00 CET, the batteries were isolated and at 14.00 CET the transmitter was turned off. Prior to the switch-off, two 'de-orbiting' manoeuvres had been carried out on 11 and 14 May to lower the perigee from 1380km to 715km. It is expected that ISO will re-enter the Earth's atmosphere in some 20 years time.
Since depletion of the liquid helium supply on 8 April, an extensive 'technology test programme' has been carried out with the spacecraft. Interleaved with these technical tests -filling any gaps- were observations using the shortest wavelength detectors of the SWS instrument. 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, e.g. XMM and Integral, which use some of the same components, such as the Star Trackers guiding the spacecraft.
The SWS band 1 detectors (covering wavelengths from 2.38 - 4.08 micron) were successfully used for some 150 hours, mainly to take spectra of nearly 300 stars in a programme of detailed spectral classification. ISO's last 'last light' observation -taken with the SWS just before midnight on May 10- was of Brackett alpha emission in a hot supergiant star and resulted in a surprise. Preliminary results show that this star, supposed to be ordinary, is probably surrounded by a disk of matter.
There remains much work to be done on the detailed analysis and interpretation of the ISO results. All ISO data will be re-processed with the latest software (off-line processing version 7.0) over the summer to provide a homogeneous archive. This archive, located at Villafranca Spain, will be opened to the astronomical community in the autumn, and all data will enter the public domain by spring 1999.
A collaborative effort, co-ordinated by the ESA ISO Data Centre at Villafranca in Spain, is already underway to maximise the exploitation of the ISO data and to prepare the best possible final archive to leave as ISO's legacy. The centres involved in this effort are:
Further details on the post operations activities will continue to be posted on this WWW server.
Martin F. Kessler,
ISO Project Scientist.