The satellite is essentially a large cryostat containing over 2000
litres of superfluid helium at launch to maintain the telescope and
scientific instruments at temperatures between 2 and 8 K originally
for a lifetime of 18 months (see Fig. 1). Results of
monitoring of the spacecraft functions in orbit, however, show that now a
lifetime of 24 
2 months
may be expected.
A direct measurement to confirm that
estimate will be performed in September 1996.
ISO's overall dimensions are: height 5.3m, width 3.6m and depth 2.8m. Its mass at launch is 2200kg. Its payload module consists of this helium tank, the telescope and the four scientific instruments. Some of the detectors are directly coupled to the helium tank and are at 2 K. The other units are cooled using the cool boil-off gas from the liquid helium. The gas is first routed through the optical support structure where it cools the telescope and the scientific instruments to 3-4 K. It then passes the baffles and radiation shields before it is vented into space. A sunshield which prevents direct sunshine on the cryostat is mounted on the outside of the vacuum vessel.
The service module provides the basic spacecraft functions. This includes the solar array mounted on the sunshield and sub-systems for thermal control, data handling, power conditioning, telemetry and telecommand, attitude and orbit control. The last provides the three-axis stabilized accuracy of a few arcseconds and also the raster pointing facilities needed for the mission. The spacecraft axes are shown in Fig. 2. The attitude and orbit control system consists of Sun and Earth sensors, star trackers, a quadrant star sensor on the telescope axis, gyros and reaction wheels. It uses a hydrazine reaction control system. The nominal data transmission rate from the satellite to the ground station, the down-link bit rate, is 32kbps of which about 24kbps are dedicated to scientific instruments.
Figure 2: Definition of the spacecraft axes