The effective lifetime of the cryostat was determined by the rate at which the super-fluid helium in the main tank was depleted through venting of gaseous phase helium. This was proportional to the rate at which heat leaked into the cryostat from external sources or was generated by internal sources, such as internal calibration sources, cryomechanisms, etc. To optimise the lifetime of the cryostat it was necessary to minimise the heat dissipated within the cryostat and heat leaks entering the cryostat from external sources. A number of sources of heat input to the cryostat were identified and addressed as follows:
Service Module to Payload Module interface: a major source of heat input to the cryostat was the interface between the Service Module and the Payload Module. This was countered, and effectively eliminated, by allowing a portion of each connecting strut to act as a radiator, diverting Service Module heat into space. Although this considerably reduced the heat input to the cryostat, it came at a penalty of impaired platform stability from transient temperature gradients in the struts.
Instrument electronics: a source of heat within the cryostat were the electronic systems used by the instrument detector and readout systems. Considerable effort went into the development of detectors which were capable of operating without significant heat dissipation at the extremely cold temperatures within the cryostat. Those systems which could not be developed to these requirements were located in the warm electronics section of the Service Module.
External radiation: the radiation received by the cryostat from sources external to the spacecraft (i.e. Sunlight, Earthlight) would cause net heat flows into the cryostat without the presence of the exterior shielding. This consisted of a sun-shield, multi-layer insulation (MLI) and vapour-cooled radiation shields which, in combination, effectively isolated the interior of the cryostat from heat inputs from sources external to the spacecraft.
An important factor for the planning of ISO's scientific operations was the accurate knowledge of the amount of liquid helium remaining in the tank. The ability to make this `Direct Liquid Content Measurements' (DLCMs) under microgravity conditions was a novel development for ISO, which relied on the near-infinite thermal conductivity of the superfluid helium. A calibrated heat pulse was introduced into the tank, which increased the temperature of the helium by an amount directly proportional to the mass remaining. Three such measurements (see Section 5.7) were performed at various stages of the mission for estimating the mass flow rate of the boiled-off helium and determine with a better accuracy the lifetime expected for the satellite.
Due to excellent engineering and a fortunate combination of circumstances at launch, the liquid helium supply lasted over 10 months longer than the specified 18 months. Three months came from a prudent safety margin in the engineering calculations of the rate of loss of helium. Two months were the result of favourable circumstances in the launch campaign at Kourou in French Guiana, when, during a technical check of the Ariane 44P launcher, ISO's engineers seized the chance to recharge the helium, and the quick launch that followed meant that the outer parts of the cryogenic system of the spacecraft had little time to warm up in Kourou's tropical climate. Finally, the daily loss of helium turned out to be 17% less than expected, at the lower end of a range of possibilities considered by the engineers, giving another five months of additional life.
This extra lifetime not only led to many more observations but also made it possible to observe the Taurus/Orion region -- inaccessible in the nominal mission.