Prof. Marcia Grail, supervillain and superscientist, wishes to spy on her foes (they'll all pay!) from orbit. She wishes to be able to resolve detail on order the size of the letters on a license plate (approx. 5cm), in a wavelength of approximately 550 nm, from an orbit approximately 4.2 x 10^7 m above the surface of the Earth. If her resolution is limited only by diffraction, then what is the smallest aperture telescope that she can get away with