By Geoff Koch

The Dallas Morning News

DALLAS – If you’re keeping an updated census of solar system moons, it’s time to add seven to the list.

Last week, the Cassini-Huygens spacecraft spotted two new moons orbiting Saturn. The two satellites, named S/2004 S1 and S/2004 S2, are both about 2 miles across – the smallest ever observed orbiting the ringed giant. Saturn’s smallest previously known moon was about 12 miles across.

The discovery, made by University of Paris planetary scientist Sebastien Charnoz, increases Saturn’s moon-count to 33.

“The two new satellites are a kind of mystery,” wrote Charnoz in an e-mail interview. “How were they formed? How did they get there? We have no good answers.”

Charnoz, part of Cassini’s imaging team, observed the two new satellites relatively close to Saturn. This location surprised scientists, who suspected that comets flying about the early solar system would probably have pulverized the small moons orbiting near the giant planets – Jupiter, Saturn, Uranus and Neptune.

Saturn’s two newfound moons “are either recently captured asteroids, and thus relatively rare,” said J.J. Kavelaars, an astrophysicist with the National Research Council of Canada, “or they’re evidence that densities of comets in the early solar system weren’t as high as astronomers think.”

New knowledge of ancient comet density may help explain a cloud of icy, rocky debris known as the Kuiper Belt that today orbits the sun beyond Neptune and Pluto.

Saturn is not the only giant to add to its orbiting brood. Kavelaars is among several authors of an Aug. 19 Nature article describing a multiyear effort to observe a spate of new moons around Neptune.

The international team, which used several ground-based telescopes, announced some of its results last year online. But the group’s current paper confirms the addition of five moons for Neptune, bringing its official total to 13.

Three of the five newly discovered satellites have retrograde orbits, circling Neptune in the opposite direction than the planet orbits the sun. All five moons are probably “asteroids captured when passing by the planet,” Kavelaars said.