Alan Stern of the Southwest Research Institute is understandably anxious to find out if Pluto has more than one moon. Not just because he's curious. Not just because it would be a notable discovery. More important, Stern is responsible for the scientific success of NASA's roughly $500 million New Horizons mission to Pluto and the Kuiper Belt of frozen objects in which the planet resides.

When New Horizons nears the planet in 2015, it would be a shame, Stern said in a recent telephone interview, not to have learned in advance whether there are other science targets orbiting Pluto that might help astronomers learn more about the development of planetary satellites and the solar system in general.

Timing will be crucial. Unlike probes that go into orbit around other planets, New Horizon's must perform all its science during flyby opportunities.

Pluto has one known moon, Charon which, at 745 miles (1,200 kilometers) wide is more than half the size of Pluto and the largest satellite in our solar system relative to its planet. What makes Stern suspect the planet might harbor others?

"I don't suspect," he says. "I simply want to know."

There are reasons, however, why a search makes sense. In fact, Pluto may have some distinct advantages over Earth in the satellite-garnering department.

A popular notion for small rocky satellites -- like Charon and any other that might exist around Pluto -- is that they are captured. But that's a rare event for any planet. "The probability of a two-body gravitational capture is phenomenally low," Stern said, even for objects that pass reasonably close. "Less than one-in-a-million."

Yet Pluto has had plenty of opportunities in its roughly 4.5 billion-year existence.

The Kuiper Belt is loaded with small and large objects, frigid rocks that are the leftovers of the solar system's formation. Nearly 400 have already been discovered -- including a few about half as big as Pluto -- and researchers estimate there could be millions or even billions the size of small cities.

"There are a lot of objects whizzing through the Pluto system all the time, and at relatively low speeds," Stern said. "Very rare but not impossible multi-body encounters could put Kuiper Belt Objects in orbit around Pluto."

It's more likely, however, that one or more other satellites could have formed directly around Pluto during its birth, Stern said. Or the planet could harbor a cloud of small objects that were broken up from a collision that shattered a satellite of Pluto long ago, like the scenario that seems to have played out around Jupiter. There could also be a moon that formed prior to Charon and was later pushed out to a more distant orbit.

Once Pluto has an object in orbit, the planet is surprisingly capable of keeping it there.

Earth has to fight gravitationally with the Sun to retain a satellite. Because our planet is much closer to the Sun than Pluto, it has a relatively small "stability zone" for companions, Stern explains. In fact, Earth has managed to retain only one Moon that we know of, a big, bright and obvious one that orbits at an average distance of 238,900 miles (384,402 kilometers).

Pluto, less massive but more distant from the Sun, could retain a satellite orbiting as far as 620,000 miles (1 million kilometers) away.

"That's a huge volume of space that could have satellites," Stern points out.

The technology is in place to find moons of Pluto down to 6 miles (10 kilometers) in diameter, if they exist. It's unlikely any moon would be larger than 62 miles (100 kilometers), because a satellite that large would probably already have been spotted.

Stern and his colleagues, John Spencer at the Lowel Observatory and Harold Weaver of the Johns Hopkins University Applied Physics Laboratory, recently applied for special, immediate Hubble Space Telescope time to scout the system out. But Hubble officials refused the request for mission support. The researchers will now reapply for regular Hubble time.

Meanwhile, they plan also to conduct surveys from ground-based observatories. Mid-size professional telescopes, with apertures of 10-13 feet (3-4 meters) have already proved capable of detecting Kuiper Belt Objects in the size range that one might expect of a second Plutonian moon.

Spencer said it looks as if they will get some time on the Japanese Subaru telescope, which sits atop Mauna Kea in Hawaii, later this month. Subaru has a much wider field of view than Hubble, so a single snapshot will gather light from the entire region in which moons of Pluto might exist, Spencer said.

The trick, then, is to sort any potential Moon from the background of stars and other possible Kuiper Belt Objects.

"It's a very crowded part of the sky with lots of stars," Spencer said. At least one more snapshot will be taken, likely on the same night, and the astronomers will then compare the two images to see how all the pinpoints of light moved. "In the course of a night you won't see [moons] moving around Pluto much, but you'll see them moving at same speed as Pluto across the sky" compared to stars and other objects, he explained.

If anything is discovered, Spencer said, it should then be pretty easy to convince a telescope operator to provide time to go back and examine the object in more detail, with the hope of pinning down its orbit and learning something about its composition.

One way or another, if Pluto has other moons, we'll know before long.

"Before New Horizons flies, we are going to find out if Charon orbits Pluto in isolation or not," Stern promised.