In-orbit operations of ISO continue to run extremely smoothly. The spacecraft, the instruments and the ground segment all continue to perform excellently. Very efficient schedules containing an average of 45 highly-graded observations per day are routinely carried out. On the 14th May, the second direct measurement of the remaining liquid helium was made during a station-keeping manoeuvre.
Like the first measurement in September 1996, the second was made by using heaters to inject a precisely-known quantity of heat into the liquid helium and by measuring the temperature increase. The May measurement gave a remaining mass of 116 Kg. Combining the results of the May 1997 and September 1996 measurements yields a best estimate of 10 April 1998 (± 2.5 weeks) for the exhaustion of ISO's liquid helium. Thus, the in-orbit lifetime is now estimated to be some 10 months more than the required 18 months. This is due to meticulous engineering and a fortuitous combination of circumstances at the time of launch.
Extension of operations into 1998 will permit a second visit to the Taurus/Orion region. This is of particular scientific importance because during the first window (August - October 1997) ISO will be undergoing prolonged eclipses, during which operations will have to be reduced.
For the second station keeping (or orbit correction) manoeuvre, the satellite's hydrazine reaction control system was used for a `burn' of 3 minutes and 12 seconds duration. As a result, the satellite's velocity was altered by 0.7 m/s and the apogee height increased by some 20 km. The eastward drift of the satellite has been stopped and a small westward drift induced. The third (and probably last) manoeuvre is being planned for 11 December 1997.
Since the last newsletter, the ISOCAM polarisation (CAM05), the ISOPHOT absolute photometry (PHT05/25) and the LWS FP range (LWS03) modes have all been released. As foreseen, full operations of the LWS were successfully resumed on 31 January, following changes to the operating parameters for the FP interchange wheel.
The capability of ISO to respond swiftly to unexpected astronomical events was graphically demonstrated in April when the Italian-Dutch X-ray satellite BeppoSAX reported detection of a gamma ray burst (GRB970402). Within 40 hours of notification, ISO was making detailed observations with its camera and photometer of the error box.
A workshop entitled `` ISO to the Peaks'' on analytical spectroscopy with ISO will be held at VILSPA on 6-8 October.
Even with the extended lifetime, the end of ISO's in-orbit operations are sadly approaching. In order to maximise the return from all the observations, detailed plans - and associated software developments - are now being put into place both at ESA and at various National Data Centres. The next issue of ISO INFO will contain more information on these plans.