“I compiled the pictures I took into a time lapse movie. The blue cloud appears to grow where the shuttle main engines passed through a particular layer of atmosphere. The cloud grew in coverage and changed from bright white to electric blue as it expanded.” — Adam Bojanowski
Note: The expanding vivid white cloud appears after tiny droplets of water condense and then freeze around particles in Discovery’s exhaust after passing through a saturated layer of atmosphere. The ice in this new cloud preferentially scattered the blue light from the setting Sun (this is called Rayleigh scattering, the same mechanism for giving the sky its blue hue).