The majority of ISO's observing time is being made available via Calls for Observing proposals, which are open, in principle, only to the astronomical community in ESA member states, the USA and Japan. In addition to this ``Open Time'', there is ``Guaranteed Time'', for those involved in preparing and operating the mission and ``Discretionary Time'' for observations which could not have been foreseen at the time of the proposal process.
ISO was launched in the night of 16-17 November 1995. The first 21 days after launch were devoted to the commissioning the overall satellite. The following 56 days (from 8 December to 3 February) were devoted to the so-called ``Performance Verification Phase'', during which a detailed assessment of the in-flight performance of the scientific instruments was made, their core calibrations established and planned operating modes validated. Routine operations, i.e. executing the planned open and guaranteed time observations started on 4 February 1996.
ISO's lifetime is limited by its supply of liquid helium. Current indirect
estimates of the remaining liquid helium coolant indicate that ISO's
lifetime will be 24 
2 months--compared to the specification of 18
months. A direct measurement of the remaining helium will be made in
September 1996. Thus, current expectations are that ISO will be
operational until about December 1997.
In response to proposals from the pre-launch Call, the Observing Time
Allocation Committee recommended observations totalling 3000 hours in
priorities 1 and 2 (see the glossary and section 4.3.2). Based on the above
lifetime, it is estimated that there will be some 2000 hours of open time
available for Supplemental Call proposers. Included in this estimate are the
instrument exposure and overhead times as explained in the four Instrument
Observer's Manuals and the spacecraft slew times as explained in the ISO
Observer's Manual. Excluded from this time are the times needed for
spacecraft and instrument switch-on and stabilisation and the routine
observations necessary to maintain the calibration of the overall facility.
In addition to this 
2000 hours, there will be up to 300 hours
available via discretionary time (see section 3.3).
This Supplemental Call solicits proposals to be carried out in the period from December 1996 to the end of the mission. Observations resulting from the Supplemental Call will be merged with those from the pre-launch Call and carried out in parallel.