NASA - JWST launch in 2018

The James Webb Space Telescope (JWST), scheduled for launch in 2018, is expected to show us the most distant galaxies that formed in the early life of the universe. To do this, it has light gathering capability unprecedented in a space telescope—its 6.5-meter-diameter mirror has more than six times the light-collecting area of Hubble’s mirror. Also, it is designed to “see” mainly infrared radiation so it can detect the red-shifted radiation of early stars and make out stars and planets that are hidden from visible-light telescopes by clouds of gas and dust.

The prior experience of team leaders Keith Parrish and Shaun of this very large observatory at a very low temperature despite Thomson from Goddard Space Flight Center and Perry being in an environment where it will constantly be bombarded Knollenberg from Northrop Grumman was essential to ensuring with 200,000 watts of solar energy—and to provide enough a robust thermal system. Through their past experience, they design margin to be certain the thermal design will work. understood the risks and challenges of designing a passively With JWST a million miles away, a Hubble-style repair cooled cryogenic system. People who have done cryogenic work mission is out of the question. If the thermal design fails, so before—even if less demanding work than JWST requires— will the mission. have a feel for how much margin is needed and how much This task can be accomplished without radically new difference a seemingly tiny variation in temperature can make. technical innovation. 

The basic principles of the sun shields Newcomers to the field are likely to underestimate both. and radiators have been used before. But the extremes of the These team leads understood the critical cryogenic demands temperature requirements and the constraints imposed by mass and reminded the team to “concentrate on detailed heat-flow and budget limitations have made this a daunting task. diagrams and follow the milliwatts.” Previous cryogenic space hardware had masses of not more Members of the JWST team also had prior large-telescope than about 500 kg; JWST needs to cool over 3,700 kg. Not experience from working on the Hubble, Chandra, or only did we have to make sure that no more than 1 watt of Spitzer telescopes. Part of the Hubble experience—being a the 200,000 watts of solar energy would get through JWST’s member of a “badgeless” system team where contractors and heat shield, we had to fully understand the paths that 1-watt civil servants worked side by side, not concerned about which seepage would take to “sneak” around from the hot side to the organization signed their paycheck—proved essential to the cold side of the observatory, tracking down to tens of milliwatts JWST work.