Our upcoming infrared space observatory, @NASAWebb, will launch in 2021. It will spy the first luminous objects that formed in the universe and shed light on how galaxies evolve and many other mysteries. Dive deeper with this advanced Q&A: https://bit.ly/2LJYSR5 pic.twitter.com/Q66J8v7myM
-
Testing the whole observatory at once at thermal vacuum is extraordinarily difficult if not infeasible. Even with significant upgrades Plum Brook probably wouldn't achieve the boundary conditions necessary to get good information. There are other better ways to test. 1/
. Where? 
large enough to test entire system. 
All this plus how heat moved around in our Chamber A tests tells us about all observatory heat flow and that thermally the design will work. Other key flight pieces (flight midboom assembly/stowed spacecraft element, etc) have been/will be tested at thermal vacuum. 3/