Saturn watch: Cassini finds two new moons and lightning.

The Cassini spacecraft has just begun its 4-year tour around Saturn, but the mission is already proving a tour de force.

An analysis of images taken by Cassini in early June, several weeks before the craft settled into orbit about Saturn, has revealed two tiny moons orbiting the ringed planet. Size estimates suggest that the orbs are the smallest ever found around Saturn, researchers reported this week. Earlier this month, other scientists described short-lived storms in Saturn's atmosphere.

The newly detected moons, each of which takes about a day to orbit Saturn, lie between the paths of two larger satellites, Mimas and Enceladus. One of the new moons, dubbed S/2004 S1, was measured at 194,000 kilometers from Saturn's center and has an estimated diameter of 3 km. It may be the same object spotted in a single image taken by one of the Voyager spacecraft in 1981.

The second body, referred to as S/2004 S2, lies 211,000 km from Saturn's center and has a diameter of about 4 km.

"It feels wonderful to have gone all the way to Saturn and discovered new real estate," says Cassini imaging-team leader Carolyn Porco of the Space Science Institute in Boulder, Colo. She presented her team's findings Aug. 17 at the Western Pacific Geophysics meeting in Honolulu.

The moons provide new evidence that the Kuiper belt, a reservoir of icy bodies at the solar system's outer edge, might not have quite as many small, house-size comets as planetary scientists have assumed, Porco says. Small Kuiper belt comets whiz past Saturn at speeds several times as fast as that of a bullet. If such comets were common, some probably would have collided with Saturn's small moons, breaking the satellites into bits too small for Cassini to detect, says Cassini imaging-team member Luke Dones of the Southwest Research Institute in Boulder.

The suggestion that the Kuiper belt may have a lower-than-expected number of small comets dovetails with a finding about Jupiter's moon Europa made by the Galileo spacecraft several years ago, Dones notes. Galileo observed that Europa has few craters that are small, an indication that few small comets have pummeled its surface over the past 50 million years or so.

During a teleconference with reporters on Aug. 5, researchers announced that Cassini had detected bursts of radio waves generated by lightning. The intensity of the bursts varied greatly from day to day, indicating that they had come from several different short-lived storms at mid to high latitudes, says Cassini researcher Bill Kurth of the University of Iowa in Iowa City.

That's in contrast to findings from Voyager, which observed long-lived storm systems at low latitudes in the early 1980s. The difference may be traced to changes in the shadow cast by Saturn's rings. Two decades ago, the rings cast a shadow near Saturn's equator, so the cold, shaded part of Saturn's atmosphere resided next to the hottest part. Turbulence between the hot and cold parts could have led to long-lived storms.

Cassini finds a different set of conditions. Summer reigns in Saturn's southern hemisphere and the rings have cast a broad shadow over Saturn's northern hemisphere. In this configuration, hot and cold regions in the planet's atmosphere are much farther apart, and long-lived storms may be less likely.