That's no moon... that's a fully opera - No, wait, it is a moon. Ganymede, in fact, as captured disappearing behind Jupiter by the Hubble Telescope. Click through for video.
Although the images and movie - made from 540 stills taken over a two hour period - were only released just before Christmas, they were actually shot in April 2007. The reason for the delay in their release? We're saying that it's got something to do with the amount of time the NASA animators were distracted watching Wall-E.