Technicians and scientists check out one of the Webb telescope's first two flight mirrors on Sept. 19, 2012 in the clean room at NASA's Goddard Space Flight Center in Greenbelt, Md.
The mirrors are going through receiving and inspection and will then be stored in the Goddard clean room until engineers are ready to assemble them onto the telescope's backplane structure that will support them.
One of the Webb’s science goals is to look back through time to when galaxies were young. To see such far-off and faint objects, Webb needs a large mirror. A telescope’s sensitivity, or how much detail it can see, is directly related to the size of the mirror area that collects light from the objects being observed. A larger area collects more light, just like a larger bucket collects more water in a rain shower than a small one.