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," says Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, Calif., who led the campaign. "So that's what Hubble can do."