Semi-official...Pluto's new moon will be called "P4"

An image of the Pluto system taken by the Hubble Space Telescope's Wide Field Camera 3 ultraviolet visible instrument with newly discovered fourth moon P4 circled, taken on on July 3rd, 2011.

"Pass Out the Cigars! Pluto Is a Papa"

by

Michael D. Lemonick

July 25th, 2011

Time

With all the attention places like Jupiter and Mars have been getting as NASA prepares to send two new probes their way, it's easy to forget that a spacecraft is even now heading out toward the edge of the Solar System at speeds exceeding 50,000 mph (80,000 kph), aimed straight at Pluto. Even at that blistering speed, the New Horizons probe, launched back in 2006 before Pluto was downgraded from a fully certified planet to a dwarf planet, won't arrive until 2015.

But 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 or 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 didn't show any rings — but it did show that Pluto has a moon nobody knew about. Temporarily known as P4 until it's granted 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 diameter, somewhere between 8 and 21 mi. (13 and 33 km), 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, the principal investigator of the New Horizons mission and a co-discoverer of the new moon, "and lo and behold, there it was."

It almost wasn't, as far as the astronomers were concerned. Stern, along with planetary scientist Mark Showalter of the SETI Institute in Mountain View, California, put in a proposal about a year ago asking for some of Hubble's precious time to look for rings around Pluto. "It must have rings, at least from time to time," says Stern. The reason: Nix and Hydra, like pretty much every other object in the Solar System, get bombarded with meteorites or bits of comet every so often. "I guarantee that when we get there," says Stern, "we'll see craters." Those impacts will throw particles of ice into space and those paricles will form themselves into rings. "The only question," says Stern, "is how long they last." The Hubble folks, though, turned the scientists down. So they appealed, and the second time around their project was approved.

Showalter and Stern are not done yet. Along with several colleagues, they have submitted a second proposal before the Hubble time-allocation committee, which fields hundreds of such pitches per year. Scientists whose requests are granted don't always have as much time as they'd like with the Hubble, but Showalter and Stern will take what they can get, since they wouldn't be at all surprised if there are still more Plutonian satellites to be found. That's because the four known moons were likely born when something huge smacked into Pluto ages ago. If that collision produced big debris chunks, it surely produced smaller, still-undetected ones too.

There's not a lot of time to lose, though. Even though New Horizons won't get within shouting distance of Pluto for nearly four more years, the mission scientists have to lock in their sequences of observations well in advance. "We have to write that script, test it, make sure it's all worked out," says Stern, and that takes time. "If we start discovering things too late, we're not going to be able to adjust."

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 while the probe's closest approach will indeed be a one-day event, occurring on July 14, 2015, to be precise, 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.

By that time, tiny P4 should have a real name. "We're tossing around some ideas," says Showalter, "but the name has to come out of Greek mythology associated with Hades and the underworld." That's according to the International Astronomical Union, which has to formally approve the names of heavenly objects — and which has strict and sometimes arcane guidelines for what's permitted. Underworld myths are the rule for moons of Pluto; for moons of Uranus, it's got to be characters from the works of Shakespeare and Alexander Pope — specifically Pope's poem "The Rape of the Lock." That required Showalter to learn the verses well. "I'm the discoverer of two moons of Uranus," he says. "We named them Cupid and Mab."

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." That ruling caused anguish to schoolchildren around the world while making some scientists rejoice. Stern, an unabashed Pluto lover, is philosophical. He has no doubt that Pluto is indeed a planet no matter what the IAU says, but he's not considering trying to get the decision reversed. "We've move past that," he says. "I believe that most planetary scientists know it's a planet, and we don't need the IAU to tell us it is."

Showalter, on the other hand, doesn't think it matters what you call Pluto. "I don't see 'dwarf planet' as a demotion," he says. "Think of bonsai trees. 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."