Before NASA’s New Horizons spacecraft completed its study of Pluto and continued on its way to the distant reaches of the Kuiper Belt, it took some parting images of Kerberos, Pluto’s smallest moon. This completed a series of "family portrait" images New Horizons has sent back of Pluto and its four other moons since last May.
Taken from a distance of nearly 255,000 miles (410,380 km) from Kerberos, the images show a double-lobed shaped mass less than 8 miles (13 km) across at its longest point and about 3 miles (4.8 km) at its shortest. In comparison, Pluto’s largest moon, Charon, has a diameter of 751 miles (1,209 km). Nix and Hydra, Pluto’s next largest moons, are approximately 25 miles (40 km) across, while Styx is similar in size to Kerberos at roughly 6-7 miles (10-11 km) across.
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According to NASA, scientists had previously thought that Kerberos was larger than has now been revealed due to its gravitational influence as measured by the Hubble Telescope. The reason for the weight vs. size anomaly remains a mystery, but scientists think that its lobed shape may be the result of two objects merging with one another at some point in the distant past.
"Our predictions were nearly spot-on for the other small moons, but not for Kerberos," said New Horizons co-investigator Mark Showalter, of the SETI Institute in Mountain View, California.
Data and images sent back from New Horizons also revealed that the surface of Kerberos appears to resemble that of some areas of the dwarf planet and its other small moons in that it is coated with fresh water ice.
New Horizons was launched in July 2006 specifically to understand the formation of the Pluto system, the Kuiper Belt and the early formation of our solar system. It first started sending images of Pluto in February 2015 on its way to the dwarf planet and, following its July flyby, is now over 75 million miles (121 million km) from Pluto. It is expected to reach the next defined object in the Kuiper Belt on January 1, 2019.
The video below provides a quick overview of the New Horizons mission.