Pluto’s dog-bone moon poses a puzzle

This image of Kerberos was created by combining four individual images from New Horizons’ Long Range Reconnaissance Imager that were captured on July 14, approximately seven hours before the spacecraft’s closest approach to Pluto, at a range of 245,600 miles (396,100 kilometers) from Kerberos. The image has bee processed to recover the highest possible resolution. (Credit: NASA / JHUAPL / SwRI)

NASA’s New Horizons probe has finally filled out its family portrait of Pluto and its moons – and Kerberos, the last moon to get its closeup, turns out to be nothing like what scientists expected.

Before the piano-sized spacecraft’s July flyby, an analysis of Kerberos’ gravitational influence on Pluto’s four other moons suggested that it had some heft. But the fact that it was so dim led the mission team to conclude it must have a dark surface. Otherwise, why would an object so large reflect so little light?

It turns out that Kerberos is almost as tiny as Pluto’s smallest moon, Styx. Like Styx, Kerberos’ surface appears to consist of relatively clean water ice, making it bright enough to reflect about half of the sunlight it receives.

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By Alan Boyle

Mastermind of Cosmic Log, contributor to GeekWire and Universe Today, author of "The Case for Pluto: How a Little Planet Made a Big Difference," past president of the Council for the Advancement of Science Writing.

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