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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.

“Once again, the Pluto system has surprised us,” New Horizons’ project scientist Hal Weaver, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory, said in Thursday’s image advisory.

The SETI Institute’s Mark Showalter, a co-investigator on the New Horizons team who was one of the discoverers of Kerberos, said “our predictions were nearly spot-on for the other small moons, but not for Kerberos.”

Showalter and his colleagues still don’t fully understand how they could have been so wrong.

To top it all off, the moon was apparently formed by the merger of two smaller objects, giving Kerberos a double-lobed, dog-bone shape. That’s apt, considering that the moon was named after the three-headed dog of the underworld in Greek and Roman mythology. The larger lobe is about 5 miles (8 kilometers) across, and the smaller lobe is about 3 miles (5 kilometers) in widh. Kerberos’ total length in the longest dimension is about 7.4 miles (12 kilometers).

Pluto's moons
This composite image shows a sliver of Pluto’s large moon, Charon, and all four of Pluto’s small moons, as resolved by the Long Range Reconnaissance Imager on the New Horizons spacecraft. All the moons are displayed with a common intensity stretch and spatial scale. Charon is by far the largest of Pluto’s moons, with a diameter of 751 miles. (Credit: NASA / JHUAPL / SwRI)

The newly released pictures were taken just before New Horizons’ closest approach to Pluto on July 14, and the spacecraft is now more than 74 million miles (120 million kilometers) beyond the dwarf planet. The probe is sending back gigabytes’ worth of images from the July encounter as it moves on to its next target, an object called 2014 MU69 in the broad ring of icy material known as the Kuiper Belt.

Today, the mission team reported that the spacecraft successfully executed the first of four maneuvers aimed at sending it past 2014 MU69 (a.k.a. Potential Target 1, or PT1) in 2019. The three other thruster firings are due to occur over the next two weeks.

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