NerdModo: Can Spy Satellites read your Car Number Plate?

Mathematically, the highest possible resolution for Hubble comes out to be around 16cm which is around 6.3 inches or half a foot. Now that is good enough to spot a car and a building in detail but not good enough to read a newspaper headline.

This is an analysis using Rayleigh’s criterion, but no other tricks or effects (i.e. single image). Hubble wasn’t designed for this kind of imagery, but is a reasonable order-of-magnitude example, because it’s in a fairly low orbit. You can squeeze out a factor of two, perhaps, in altitude and aperture size, for spy satellites. But planes are another story, since they are at a much lower altitude. Examples of some different resolutions in the link.