From space, astronauts can see people making love as a tiny speck of light. Not light, exactly, but a glow that could be confused for light - a coital radiance that takes generations to pour like honey through the darkness to the astronaut's eyes. In about one and a half centuries - after the lovers who made the glow will have long since been laid permanently on their backs - the metropolitan cities will be seen from space. They will glow all year. Smaller cities will also be seen, but with great difficulty. Towns will be virtually impossible to spot. Individual couples invisible.