Some of the choices—polystyrene foam or CFRP—may at first seem impractical. But the potential cost-saving (the factor of 16) is so vast that they are worth examining. There are ways of casting a thin film of silicone rubber or of epoxy onto the surface of the mirror-backing (the polystyrene or the CFRP) to give an optically smooth surface that could be silvered. The most obvious obstacle is the lack of stability of polymers—they change dimensions with age, humidity, temperature, and so on. But glass itself can be reinforced with carbon fibers; and it can also be foamed to give a material that is denser than polystyrene foam but much lighter than solid glass. Both foamed and carbonreinforced glass have the same chemical and environmental stability as solid glass. They could provide a route to large cheap mirrors.