As humankind’s first robotic visitor to Pluto approaches its destination, astronomers working to understand what it will find there have uncovered a tiny moon orbiting the dwarf planet.
The moon is the fifth known natural satellite of Pluto and has been informally labeled P5. It was discovered Saturday, July 7, in images taken by the Hubble Space Telescope as part of a campaign to identify possible hazards to the New Horizons spacecraft, now en route to Pluto for a 2015 flyby. Dust rings encircling Pluto, or small moons shedding unseen debris, could endanger the $700-million mission. So far, the search has not identified any dangerous dust bands around Pluto, but it has turned up two newfound moons—a small object called P4 last year, and now P5.
P5 is incredibly faint—half as bright as P4, and roughly one one-hundred-thousandth as bright as Pluto—and orbits relatively close to the dwarf planet. The newfound moon’s faintness implies that it has a diameter of just 10 to 25 kilometers. “They’re very close, and this is a very small object.
P4 and P5 join Charon, a large moon of Pluto whose discovery was announced exactly 34 years prior to the day Showalter spotted P5 in new Hubble imagery as well as the small moons Nix and Hydra, discovered by astronomers using Hubble in 2005. All of those satellites could be remnants of one giant collision early in solar system history.