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New Horizons completes the 'family portrait' of Pluto's moons

Kerberos is the last one to be photographed

New Horizons has sent back an image of Kerberos, the smallest moon in the Pluto system. Now, just a little more than three months after the Pluto flyby, we finally have a full "family portrait" of Pluto and its five moons.

The low-resolution picture of Kerberos (seen below) is actually made of four separate images taken by New Horizons' Long Range Reconnaissance Imager, or LORRI. It was taken from about 245,600 miles (396,100 km) away, just seven hours before the spacecraft's closest approach to Pluto back in July.

Pluto's biggest moon, Charon, is relatively massive compared to the dwarf planet. But the rest of its moons are tiny, and Kerberos is the smallest. It measures just 7.4 miles (12 kilometers) across and 2.8 miles (4.5 kilometers) on its shortest side. While Pluto's other small moons are more potato-shaped, Kerberos is "double-lobed," similar to the now-famous Comet 67P (but more than twice its size).

NASA thought Kerberos was a lot bigger

The small moons of Pluto were a bit of a mystery before the flyby, as they were all but impossible to photograph in any detail without getting relatively close. (NASA was't even completely sure of the number of moons, either, and was only able to confirm after the flyby that Pluto only has five moons.) But Kerberos was the most mysterious of all. NASA says that New Horizons scientists believed Kerberos was rather large based on gravitational measurements made with the Hubble Space Telescope, and that it hadn't appeared in any low-resolution images of Pluto because it was likely covered in dark material. The new image shows a different story: Kerberos has a bright, reflective surface, which hints at the presence of water ice — a common trait shared by Pluto's other small moons.

The view of Kerberos will likely get better as more data is downlinked from the New Horizons spacecraft over the next year, just like we've seen increasingly detailed views of the rest of the Pluto system. Meanwhile, the spacecraft itself is already almost 100 million miles past Pluto, and the New Horizons team is preparing to adjust its course as it sets its sight on a new target: Kuiper Belt object 2014 MU69.

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