May 31 saw NASA's Cassini spacecraft make its last close up pass with Saturn's moon Hyperion. The moon has a fascinating, almost spongy appearance due to the fact that its surface is actually less dense than water.

Because of Hyperion's low surface density, any collisions simply compress the surface instead of excavating it. Therefore the moon is given a porous appearance rather than being pockmarked by the impact craters that so often characterize other moons throughout the solar system.

Hyperion is the largest of Saturn's irregularly shaped moons, with a diameter of roughly 270 km (168 miles). The moon had been visited by Cassini once before in September 2005 when it made a much closer pass – a mere 505 km (314 miles) away from the unusual celestial body.

Then, and now, imaging Hyperion is essentially a game of chance. Unlike Saturn's larger moons, Hyperion tumbles unpredictably as it orbits the ringed giant, making it very difficult to predict what parts of the moon are likely to be photographed during a pass.

The most recent, and final, pass saw Cassini observe Hyperion from a distance of 34,000 km (21,000 miles). During the encounter, the probe's camera snapped numerous images of the moon, that were transmitted and received back on Earth the following day.

Source: NASA

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