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.
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.
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 views expressed are those of the author(s) and are not necessarily those of Scientific American.
Caleb A. Scharf is director of astrobiology at Columbia University. He is author and co-author of more than 100 scientific research articles in astronomy and astrophysics. His work has been featured in publications such as New Scientist, Scientific American, Science News, Cosmos Magazine, Physics Today and National Geographic. For many years he wrote the Life, Unbounded blog for Scientific American. Follow Caleb A. Scharf on Twitter