One of these things is not like the others:
The Cassini spacecraft took this lovely image in December 2011, during a close pass of Saturn's moon Dione. Ignoring Saturn's rings slashing through the picture, we see, from left to right, the moons Dione, Prometheus, and Epimetheus. Which is the odd moon out? Here's a hint: Dione is 1100 km (700 miles) across, Prometheus 86 km (53 miles) along its longest axis, and Epimetheus 113 km (70 miles). Got it now? Yeah, sure, Dione is far larger than the other two! But that's not my point: Dione is round, while the other, smaller moons are lumpy and rather potato-shaped. Why? Size matters. In this case, a bigger moon means more mass, and that means more gravity. In general, the force of gravity points toward the center of an object. As you add more mass to an object, gravity gets stronger. On ...
