Are the individual instruments cooled to that temperature though? Because if they are not, then the difference doesn't matter. Had a look at the SPIRE manual [

Are the individual instruments cooled to that temperature though? Because if they are not, then the difference doesn't matter. Had a look at the SPIRE manual [0][1] and it seems that Herschel does include an Helium cooler to cool SPIRE and PACS to 300mK. On the other hand a closed-cycle system can also extend the mission, since you won't run out of cooling at a fixed point in time. [0] http://herschel.esac.esa.int/Docs/SPIRE/html/spire_handbook.html http://herschel.esac.esa.int/Docs/SPIRE/html/spire_handbook.... [1] http://herschel.esac.esa.int/Docs/Herschel/html/ch02.html http://herschel.esac.esa.int/Docs/Herschel/html/ch02.html