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Cassini makes final visit to peculiar Hyperion

Morgan Rehnberg, Correspondent
Jun 2, 2015, 16:55 UTC

Sen—The Cassini spacecraft made a final visit on May 31 to one of the Solar System’s strangest objects. Hyperion is one of the largest non-spherical bodies, but that is only the start of its incredible strangeness.

It is not unusual to find moons in the Solar System whose shapes bear a remarkable similarity to a potato, but to find one as large as Hyperion certainly is. At more than 200 km across, only Neptune’s Proteus is larger without also being round.

There is one distinction, however, that Hyperion does not have to share. Unlike every other known object in the Solar System, it rotates chaotically. That means that no matter how precisely Hyperion’s current orientation and direction of rotation is known, you can never predict how it will spin in the future. In the near-ideal world of planetary dynamics, Hyperion has defied more than a century and a half of observation.

These final observations highlight one other puzzling aspect of this satellite: From certain angles its surface can better resemble a sponge than a pockmarked asteroid. What causes this? Scientists are not completely sure, but it likely stems from Hyperion’s incredibly low density—just over half that of water. In fact, some astronomers believe that the moon has more in common with a rubble pile than a solid object.

These sorts of farewell visits are beginning to become routine for Cassini as it winds down its 13-year mission studying Saturn.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.


This raw image was captured on May 31, 2015 by NASA's Cassini spacecraft on its final visit to Saturn's moon Hyperion. Image credit: NASA/JPL-Caltech/Space Science Institute