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Kirk Johnson's Iowa Angel

Larger than it Appears

An Amazing Picture of the Launch of STS-98

This explanation comes from David Bohlin, a senior scientist in Code SR at NASA.

The reason for the gradations of light and color in the Shuttles exhaust plume is a combination of the facts that the sun had set at the Cape and the effect of the Rayleigh scattering of sunlight as it passes through the Earth's atmosphere. Rayleigh scattering of electromagnetic waves by gas molecules is proportional to the inverse of the 4th power of the wavelength, which explains why the sky is blue to our eyes and the setting sun is red. That also explains the colors of the plume. The lower part is in the shadow of the earth's local western horizon (which can also be seen as the dark horizontal band immediately above the horizon in the background eastern sky). As you move up to a higher altitude on the plume the setting sun comes into view, but the only light getting through the long air-path length is the red end of the spectrum so the plume looks red (as does the next higher band of the background sky). As you go higher the sun is effectively rising above the horizon to the plume, the air-path length for the sunlight is diminishing so less blue light is being scattered away, and the residual light moves to the yellow part of the spectrum. Finally the upper part of the plume is in full sunlight and appears white.

As for the apparent "convergence" of the shadow to the full moon, my training in architectural drawing suggests to me that this is a simple effect of perspective of how the more or less rectangular shadow of the plume looks as it projects through space in a direct line from the sun to the moon. The sun is far enough away that the shadow retains structural identity, just as does the earth's shadow on the moon during a lunar eclipse.

Physics aside, though, this is a stunning picture that ought to take its place in the pantheon of the most interesting ones ever made of a Shuttle launch. It was possible only because of the fact that the sky was absolutely clear from east to west and the timing of the launch with the rising of the full moon. I congratulate you getting it!!!

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