At this moment, a spacecraft is aimed straight at Pluto, heading out toward the edge of the Solar System at speeds faster than 50,000 miles an hour. The New Horizons probe was launched in 2006. But even at that blistering speed, it won't arrive until 2015.
Mission scientists don't want to waste a moment when it finally gets there, so they've been scouting ahead with the Hubble Space Telescope to see if there's anything unusual to photograph. They are also looking for any hazards to avoid—rings, for example, which could damage or even destroy a space probe that smashed through them at high speed.
The image that popped up in Hubble's gallery on June 28 revealed that Pluto has a moon nobody knew about. The moon will be known as P4 until it is given a real name. It joins Charon, discovered by a U.S. Naval Observatory telescope in 1978; and Nix and Hydra, spotted by Hubble, in 2005.
There's a good reason P4 escaped notice until now: its small size makes it all but impossible to see from Earth. "We always knew it was possible there were more moons out there," says Alan Stern of the Southwest Research Institute in Boulder. Stern is the principal investigator of the New Horizons mission and a co-discoverer of the new moon. "Lo and behold, there it was," Stern says.
A Close Encounter
Even though New Horizons will be flashing past Pluto at blinding speed, the total encounter will last for weeks. "Lots of people think we're going to go by on a Tuesday or something," says Stern. But New Horizons will start getting better images than the Hubble—and thus the best ever taken of Pluto and its moons—starting ten weeks before the flyby, and lasting ten weeks afterward. The probe's closest approach will happen on July 14, 2015.
Naming a New Moon
Tiny P4 needs a real name. "We're tossing around some ideas," says Mark Showalter of the SETI Institute in Mountain View, California. "But the name has to come out of Greek mythology associated with Hades and the underworld."
The International Astronomical Union has to formally approve the names of heavenly objects. There are strict guidelines for what's permitted. Underworld myths are the rule for moons of Pluto.
The IAU is also responsible for the decision in 2006 to demote tiny Pluto, just one-half the size of Earth's moon, to the status of "dwarf planet." Showalter says he doesn't think it matters what you call Pluto. "Think of bonsai trees," he says. "The fact that they're so small is what makes them interesting. So if you don't like the term 'dwarf planet,' just think of Pluto as a bonsai planet."