Despite decades of space exploration, we've only visited a handful of comets up close. That's because most comets move on orbits that give them a high velocity when they pass the Earth, making a rendezvous difficult. But in 2010, the Deep Impact spacecraft (renamed to EPOXI) flew past the nucleus of the comet Hartley 2 and returned amazingly detailed images.
This shot shows two views from the flyby. On the right you can see one part of the comet's peanut-shaped nucleus, and it's emitting a storm of snow - literally, ice made of water - ranging in size from snowflakes up to snowballs a few centimeters across. On the left is the longer (and in my opinion, more beautiful) view, showing the ice being jetted off the nucleus, as well as the shadow of the nucleus itself on the material it had previously
As much as I love this image, I couldn't find a high-enough resolution shot of it to warrant putting it in the Top 14 (so I chose a closeup of the asteroid Lutetia instead), but I think you'll agree this is an astonishing view that we very rarely see.Get the hi-res version here.Image credit: NASA/JPL-Caltech/UMD