But getting such a large mirror into space is neither easy nor cheap. To keep costs down, the mirror (twice the size of Hubble's) must be built of a lightweight, as-yet-undetermined material, and be able to fit into a small rocket. This poses what NASA project scientist John Mather calls the "telescope in a bottle" problem. Engineers plan to create a segmented mirror that unfolds in orbit. But its 36 segments will be flimsy, which means the materials can't be relied on to keep the mirror in perfect optical shape. The solution? Mechanical devices that minutely adjust each plate.