Rosetta took the photo from 200 miles away. The orbiter was on the outward leg of a 600 mile-or-so journey away from the comet and back again. Comets are mostly made of ice, and they give off varying amounts of gas and dust depending on how warm they are. Rosetta was taking a quick jaunt away from its orbit to see what those gases might look like at the edge of their reach.
Scientists are interested in studying comets like 67P because they were probably formed at the very start of our solar system's history. Their icy hearts contain the molecules that were around at that time, so studying the composition of that ice — either by touching down on the surface, as Philae did, or by analyzing it as it sublimates, as Rosetta does — can tell us what materials were available when the planets were just starting to form.
The image looks so striking because the sun, the comet and the orbiter were almost perfectly aligned. The resulting backlit effect shows off that aforementioned environment of gas and dust. And boy, is it gorgeous.
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