Pluto's Family

Astronomers using the Keck telescope in Hawaii have taken the best photos yet of the Plutonian system. They hope to increase our basic kbowledge of Pluto and its three moons before the arrival next decade of the New Horizons probe.

For such a small body moving so slowly so far into the depths of the Solar System, Pluto has quite a bit going on around it. Charon, its largest moon, is roughly half the size of Pluto itself. We used to say Charon was the largest moon relative to its planet in the Solar System, but Pluto is no longer officially a planet. The largest moon relative to its planet now reverts back to Earth's Luna. Conspiracy? Anyway, Pluto also has two tiny moons, Nix and Hydra. Astronomers hope to get enough high quality images to pin down the smaller moons' orbits and masses.

The two small moons of Mars are thought to be captured asteroids. Mars orbits on the inner edge of the Main Belt of asteroids, so such captures seem plausible. It's likely fair to say Pluto orbits on the inner edge of the Kuiper Belt, but such a comparison is probably misleading. The outer Solar System is a vast realm, and Pluto is a featherweight; Pluto and Charon combined probably don't come to even half the mass of Mars. How Pluto could have captured two small bodies while bigger Mars captured the same number in a busier neighborhood seems to ask for an explanation. Part of that explanation likely is that bodies move more slowly in Pluto's neighborhood, and so would be more easily captured. Lack of speed coupled with the area involved, of course, also suggests close approaches that might allow captures would the extremeky rare.

The histories of Nix and Hydra might be quite interesting.