Editing Infrared Space Observatory (section)

== The satellite ==
[[File:Animation of Infrared Space Observatory's orbit.gif|thumb|Animation of Infrared Space Observatory's orbit<br>{{legend2| Magenta | Infrared Space Observatory}}{{·}}{{legend2| RoyalBlue | Earth }}]]
The basic design of ISO was strongly influenced by that of its immediate predecessor. Like IRAS, ISO was composed of two major components:
* '''Payload module''', composed of a large [[cryostat]] holding the telescope and the four scientific instruments.
* '''Service module''', supports the activities of the payload module by providing [[Electricity|electrical power]], thermal control, [[Attitude dynamics and control|attitude and orbit control]] and [[telecommunication]]s.

The payload module also held a [[cone (geometry)|conical]] sun shade, to prevent [[stray light]] from reaching the telescope, and two large [[Attitude dynamics and control|star trackers]]. The latter were part of the Attitude and Orbit Control Subsystem (AOCS) which provided three-axis [[Directional stability|stabilisation]] of ISO with a pointing [[Accuracy and precision|accuracy]] of one [[Minute of arc#Astronomy|arc second]]. It consisted of Sun and Earth sensors, the before-mentioned star trackers, a quadrant star sensor on the telescope axis, [[gyroscope]]s and [[reaction wheel]]s. A complementary [[reaction control system]] (RCS), using [[hydrazine]] [[propellant]], was responsible for orbital direction and finetuning shortly after [[Rocket launch|launch]]. The complete satellite weighed just under 2500&nbsp;kg, was 5.3 m high, 3.6 m wide and measured 2.3 m in depth.

The service module held all the warm [[electronics]], the hydrazine propellant tank and provided up to 600 [[watt]]s of electrical power by means of [[solar cell]]s mounted on the sunpointing side of the service module-mounted sunshield. The underside of the service module sported a load-bearing, ring shaped, physical interface for the launch vehicle.

The [[cryostat]] of the payload module surrounded the telescope and science instrument with a large [[Vacuum flask|dewar]] containing a [[Toroid (geometry)|toroidal]] tank loaded with 2268 [[litre]]s of superfluid helium. [[Conduction (heat)|Cooling]] by slow [[evaporation]] of the helium kept the [[temperature]] of the telescope below 3.4 K and the science instruments below 1.9 K. These very low temperatures were required for the scientific instruments to be sensitive enough to detect the small amount of infrared radiation from cosmic sources. Without this extreme cooling, the telescope and instruments would see only their own intense infrared [[Emission (electromagnetic radiation)|emissions]] rather than the faint ones from afar.