Hyperion

Moon · Moon of Saturn · orbits Saturn

Hyperion looks like nothing else in the Solar System: a potato-shaped moon whose surface is packed with deep, sharp-rimmed craters that make it resemble a giant sponge. Roughly 40% of the body is empty space, so impactors tend to punch in and compress the ice rather than blast debris outward - which is exactly why the sponge texture survives.

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Quick facts

About 40% of its volume is empty space - one of the most porous large bodies known.
Its density is roughly half that of water ice, so it is essentially a frozen rubble pile.
It may be a surviving fragment of a larger moon destroyed by an ancient impact.

Chaos, literally

Most moons rotate with clockwork regularity. Hyperion tumbles. Its lumpy shape, eccentric orbit and a 4:3 resonance with mighty Titan make its spin formally chaotic - measure its orientation today and no calculation can tell you which face will point where a few weeks from now. It was the first body in the Solar System confirmed to rotate chaotically.

Physical characteristics

Diameter	270 km
Dimensions	360 × 266 × 205 km
Mass	5.600e+18 kg
Surface gravity	0.02 m/s²
Density	0.54 g/cm³
Mean temperature	-180 °C
Geometric albedo	0.3

Atmosphere and composition

Atmosphere	None.
Composition	Highly porous water ice with dark organic dust settled in its crater floors.

Discovery

Discovered by	William Bond, George Bond and William Lassell
Year	1848
How	Found independently in the US and England within days of each other

Missions

Voyager 2 NASA · Flyby probe · 1977 · active

Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.

Cassini-Huygens NASA/ESA/ASI · Orbiter + lander · 1997–2017 · completed

Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.

What is still uncertain

Hyperion tumbles chaotically; it has no fixed day length.