Starship Asterisk*

The shadow will always appear straight, just as crepuscular and anticrepuscular rays appear straight. No matter the shape of the object casting a shadow like that, it's basically going to appear to an observer on the ground as a flattened 'projection' upon the sky. The shadow is, of course, a three-dimensional shape, but you can't really -see- it as one.

It's hard (for me at least) to describe this in words. I should try to draw a picture or something. XD A quick example I suppose, if you're familiar with something like Adobe Photoshop, is to draw a straight line using a wiggly-shaped brush. No matter what shape of brush you use (assuming it's a solid shape), the line you draw will be straight.

And yes, the shadow is being cast upon haze and dust particles in the air, which are being illuminated by the sunlight.

Good questions. I was just puzzling over those myself. The shadow is presumably cast on haze in the atmosphere...water droplets and dust particles.

As for why it's arrow straight, I think it's the magnitude of the irregularity compared to the distance to the horizon. The various turns of the plume are stretched by the angles so far that they become basically indistinguishable.

First of all a big thank you to everyone involved in APOD in any way, it's a wonderful window into the amazing world of modern astronomy. I have a really simple question about the Shuttle Plume Shadow image, which I don't feel was covered in the accompanying text: How is it that we can see the shadow at all? One normally expects a shadow to be 'cast' onto some kind of pale reflective surface, returning the light from the illuminated surface surrounding the darker shadow area to the observer. In this case the shadow is being cast onto what, exactly? Scattering particles in the atmosphere returning the light to ground level?

A subsidiary question is: why is the shadow arrow-straight when the plume appears to be seriously bent? Does the bent appearance result from foreshortening of the plume which is actually as straight as the shadow suggests?

Is there any chance that the 'shadow' is actually some other kind of optical effect?