From Bad Astronomy:
The ever-amazing Cassini spacecraft sent back this pretty nifty shot of Saturn’s icy moon Rhea playing peekaboo in the rings.
Beautiful, isn’t it? You can see that Rhea was on the other side of the rings from Cassini when this image was taken, and that the spacecraft was almost, but not quite, in the plane of the rings, too.
But there’s more to this shot… Take a closer look. What’s that, hiding in a gap in the rings, apparently hovering over Rhea’s terminator (the line dividing day and night)?
Surprise! It’s Prometheus, a tiny potato orbiting the planet much closer in. It’s far smaller than Rhea, only about 120 km (75 miles) long versus Rhea’s 1530 km (950 miles) diameter. Rhea is Saturn’s second largest moon — only Titan is bigger — and one of the ten biggest moons in the entire solar system. Prometheus, on the other hand, is so small it wasn’t even discovered until the Voyager 1 probe spotted it in 1980.