Sixty moons for Saturn

The scientists behind the Cassini orbiter have announced the discovery of Saturn's 60th moon, a little thing that showed up in time-lapse photography of the ringed planet. Jupiter still leads this moon race with 63 known satellites - but Saturn could soon pull ahead, at least temporarily.

Of course, astronomers don't focus on the mere numbers, which shift around every time a new batch of observations is made. They say they're more interested in what even the tiniest moons can tell us about the universe's biggest questions.

"The big questions are always about the formation," the University of Hawaii's David Jewitt, one of the leaders of the Hawaii Irregular Satellites Survey, told me today. "If you're not just butterfly-collecting, then you're answering questions about the way the solar system came to be."

This 60th Saturnian moon currently has the prosaic designation S/2007 S4, although the Cassini imaging team has nicknamed it "Frank" for the time being. It's up to the International Astronomical Union to approve an official name - traditionally, a figure from Greek mythology. (The Planetary Society's Emily Lakdawalla explains how names from Greek, Norse and Inuit lore are divvied up among Saturnian satellites. The mix also includes Gallic  mythological figures.)

Frank first showed up May 30 as a speck on a series of images from Cassini's wide-angle camera - in a region near the moons Methone and Pallene, which were discovered by the Cassini team back in 2004.

When the Cassini scientists looked back through their image database, they could track the tiny dot as it orbited the planet between the other two moonlets. This Web page includes an animated GIF image and a QuickTime video that flips through the most recent imagery.

"With these new data sets we were able to establish a good orbit for the new moon," Carl Murray, a member of the imaging team from Queen Mary, University of London, said in a NASA feature on the find. "Knowing where the moons are at all times is important to the Cassini mission for several reasons."

First of all, the scientists now know that there's another object they have to avoid as they plot Cassini's future trajectory. They estimate that Frank is about 1.2 miles (2 kilometers) wide and made of ice and rock.

But beyond that, studying Frank's course could shed additional light on the dynamics of the Saturnian system. "We've gone from two to three tiny little bodies all in a row," Carolyn Porco of the Space Science Institute, leader of the Cassini imaging team, told me today. "That's starting to suggest there may be more."

There could be a band of Frank-sized moonlets circling Saturn, and scientists might even be able to trace them back to a common origin - say, a protosatellite that broke into pieces long, long ago.

Those are the sorts of insights that raise the satellite search above the level of butterfly-collecting, Jewitt told me. Although Cassini is in a good location for spotting more Saturnian moons, Jewitt pointed out that ground-based programs such as HISS are even more prolific in the moon-spotting department.

In May, HISS reported three new irregular moons of Saturn (PDF file). Jewitt hinted that still more Saturnian satellites may be added to the list, if the HISS astronomers are able to verify what their observations seem to be telling them.

HISS is concentrating on irregular satellites, not only around Saturn but around the solar system's other giant planets as well. Unlike Frank and other regular satellites (including the solar system's marquee moons), irregular satellites tend to have wide, eccentric orbits. Most of them move in a retrograde fashion, opposite to their mother planet's direction of rotation. That's a tip-off that they weren't born in place when the planets coalesced, but were captured in orbit at a later time.

Thanks to advances in imaging technology, astronomers are getting much better at sizing up the solar system's irregular army. "The first irregular satellite was found about 100 years ago," Jewitt said. "In the whole of the 20th century, only eight were found, but now we have over a hundred."

So far, these multitudes of moons have raised more questions than they've answered. For example, how were the satellites captured in the first place? At one time, astronomers thought that an asteroid typically became a satellite when it passed through the upper layers of a planet's atmosphere and experienced drag, but Jewitt said that explanation is now falling out of favor.

"The one that looks most promising now is that capture might occur because two protosatellites either collide or scatter from each other in the vicinity of a planet," Jewitt said.

As more moons are added to the list, Jewitt and other planetary scientists could well close in on the answers to those big questions about the formation of planets and their satellites. "Each and every new discovery adds another piece to the puzzle and becomes another new world to explore," Murray said.

In coming years, Cassini could get an even closer look at Frank (or whatever it's eventually called): The spacecraft's current trajectory would put it within 7,300 miles (11,700 kilometers) of the moonlet in December 2009.

For now, Porco is tickled to think that Cassini is right in the middle of a bustling celestial neighborhood. "I like the idea that we're at 60 natural satellites and one artificial satellite," she said. "That is a great indication of where we are in the exploration of the solar system. We've come this far."