When the satellite became visible to ESA's Villafranca Tracking station at about 20.00 CET on 30 May, it was in its "autonomy" mode with some temperatures in the cryogenic Payload Module being higher than nominal. The optical baffle had increased to about 42K and the optical support structure was around 10K. The satellite was re-configured to its normal operating mode and executed the planned sequence of pointings. The instruments were not switched on due to the temperatures being out of limits.
For the following perigee passage, on the afternoon of 31 May, the autonomous functions on board the spacecraft were disabled. A safe attitude was maintained in the usual way by following a set of specific pointings that had been uplinked from the ground. Perigee passage was achieved successfully.
By the start of revolution 197, the temperatures were again within operational values. Each of the four scientific instruments was activated manually with specialists monitoring the telemetry. These activities started at around 22.00 CET on 31 May and lasted for much of the revolution.
All instruments appeared to operate nominally and the go-ahead was given to re- start science observations. About 1/3 of the planned observations for revolution 197 were carried out.
Perigee passages to revolutions 198 and 199 (1 and 2 June) were uneventful.
Normal science observations were carried out on revolutions 198 - 200. Revolution 201 was re-planned to include a set of specific measurements to check for any longer-term impacts on the instruments' calibration. These observations showed that there was no change in calibration for ISOPHOT, SWS and LWS. (Note that the analysis for the short wavelength Fabry Perot of the LWS is still to be completed but no impact is expected). For ISOCAM - which was on at the time of the anomaly - a depression of approximately 15% in the response of its long-wavelength detector was seen for the following 1 - 2 revolutions.
The net loss of science time is about 1.7 revolutions directly and about 1.5 days in lifetime.
The timing of this anomaly - the most serious of ISO's lifetime - is a good example of Murphy's law. It occurred in the middle of the "First ISO Science Workshop", held in ESTEC Noordwijk, where 260 astronomers had gathered for three days to assess the initial scientific results from ISO. This meeting generated much excitement and enthusiasm about the quantity and quality of the data from ISO. A summary of the meeting and a Press Release have been released. The main scientific results of the meeting will be submitted to Astronomy and Astrophysics as letters.
Martin F. Kessler,
ISO Project Scientist.