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When NASA's New Horizons mission launched in January 2006 the known moons of Pluto consisted of Charon (arguably more of a binary companion than a moon), and Nix and Hydra - spanning some 42 and 55 kilometers across respectively.
Remarkably, during the spacecraft's ten year, 5 billion kilometer cruise to Pluto, astronomers were able to use the Hubble Space Telescope to spot two more natural satellites - Kerberos (announced in 2011) and Styx (announced in 2012) as tiny points of reflected light.
Hubble discovery image of the smallest moons of Pluto (Credit: NASA, ESA, L. Frattare STScI)
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Now the data returning from the spacecraft, carefully processed and analyzed, has provided our first ever - and possibly our last ever - family portrait of all of these diminutive bodies.
A portrait of moons (Credit: NASA/JHUAPL/SwRI)
Little Kerberos turns out to be smaller than expected, and seems to consist of two 'lobes', one about 8 kilometers across, the other about 5 kilometers. Its reflectivity suggests a water ice surface. This shape also suggests that it formed by the merger of two bodies, slapping together non-destructively.
Intriguingly, a smaller Kerberos contradicts predictions from measurements of its gravitational influence on the other satellites - which indicated a more massive body. What's up with Kerberos? We don't yet know.
The best image of Kerberos we may ever get - made by combining images and software deconvolution and smoothing (Credit: NASA/JHUAPL/SwRI)
