HYPERION

The spongy satellite

Mass: 5.6 x 1018kg (1.2 x 1019lbs) Diameter: 270km (168 miles) Parent planet: Saturn Discovered: 1848, William Bond, George Bond and William Lassell

Hyperion is the strangestlooking satellite in the Solar System, its surface resembling a sponge or coral with deep, dark pits rimmed by razorsharp ridges of brighter rock and ice. But that’s not the only thing that’s strange about Hyperion: it was the first non-spherical moon to be discovered and has a distinctly eccentric orbit.

Rather than matching its rotation to its orbital period, it spins in a chaotic pattern, with its axis of rotation wobbling unpredictably. Like all moons in the outer Solar System, it’s mostly made of water ice, but its surface is unusually dark. When Cassini flew past it measured its density to be 55 per cent that of water – its interior is mostly empty space.

One popular theory to explain these weird features is that Hyperion is the surviving remnant of a larger satellite that once orbited between Titan and Iapetus, and which was largely destroyed by a collision with a large comet. Material that survived in a stable orbit then came together again to create Hyperion as we know it.