At the Shape Limit

You hover above Proteus, studying its lumpy, irregular shape. It's not quite round - more like a badly shaped ball of clay.

"This is the largest a moon can be without becoming spherical," your science officer explains. "Any larger, and gravity would overcome the structural strength of the rock and ice. It would collapse into a sphere."

You look at the massive Pharos crater - 255 km across on a 420 km moon. The impact that created that crater nearly destroyed Proteus entirely. A little more energy, and Proteus would have shattered into a ring around Neptune.

"Proteus is living on the edge," you say.

"In every way," the officer replies. "Edge of shape stability. Edge of destruction from that impact. Edge of Neptune's ring system. And with a 27-hour orbit, time moves fast here. Two sunrises per Earth day. Neptune rotating overhead constantly."

You watch Neptune below - a massive blue-green sphere filling 23 degrees of sky. Proteus is so close to Neptune that the planet dominates everything. And because Proteus orbits every 27 hours, you see Neptune's features rotate past twice during each Proteus day.

"Time and gravity," you muse. "Both trying to reshape Proteus. Time trying to darken it with radiation. Gravity trying to round it. So far, Proteus is winning. But just barely."

Below, the potato-shaped moon continues its rapid orbit, defying both time and gravity, existing at the edge of what's possible for a moon.