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NASA published a picture of a comet the other day. The image shows the comet being lit from above. See NASA's picture below.

enter image description here

However, since it is a radar image, I would have assumed to get a shading on all sides and grazing angles, like velvet or scanning electron microscopy. Or alternatively to have the sides facing the radar dish being shaded, and the edges being dim. For the velvety look, compare to this picture from Wikipedia:

enter image description here

So why is the comet's image shaded only from above?

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There is a very good blog article here that explains this effect but basically the Doppler shifting of the radar return means that certain parts of the asteroid light up more than others in the observing wavelengths from Earth. Its better explained in the article>>

Taken from the article "How radio telescopes get "images" of asteroids" Posted by Emily Lakdawalla:

As an asteroid rotates, some parts of it are moving toward us, while other parts are moving away. As the broadcast radio wavefronts hit the part of the asteroid that is moving toward us, the asteroid smacks into each wavefront faster than it would if it were not rotating. The speed of the wavefronts does not change, because the speed of light is constant, so the wavefronts end up being packed closer together. This is a Doppler shift. The asteroid has taken the broadcast wavelength and reflected it at a shorter wavelength from the parts of the asteroid that are rotating toward us. On the other side of the asteroid, which is rotating away, the opposite thing happens; each arriving wavefront smacks into the asteroid a little later than it would if the asteroid were not rotating, so the reflected waves are spread farther apart.

Source: http://www.planetary.org/blogs/emily-lakdawalla/2011/3248.html?referrer=https://en.wikipedia.org/

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  • $\begingroup$ You have to dig through that article quite a way before you get to the answer you should have summarised. These are delay-Doppler density plots, not "images" of the of the target. The vertical axis is the delay axis (decreasing upwards) and the horizontal the Doppler axis, the brightness to the signal strength corresponding to the return at the delay-Doppler corresponding to the pixel. $\endgroup$ – Conrad Turner Mar 29 '16 at 7:28

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