The Cassini spacecraft has discovered a pair of 2-mile-wide moons, mission scientists announced today. The satellites, provisionally named S/2004 S1 and S/2004 S2, are the smallest bodies so far seen around the ringed planet. The finds raise Saturn’s known number of moons to 33.
As Cassini fell toward Saturn in early June, scientists began using the spacecraft’s cameras to scan for new satellites. “One of our major objectives in returning to Saturn was to survey the entire system for new bodies,” said Carolyn Porco, leader of Cassini’s imaging team at the Space Science Institute, in Boulder, Colorado. “So it’s really gratifying to know that among all the other fantastic discoveries we will make over the next 4 years, we can now add the confirmation of two new moons, skipping unnoticed around Saturn for billions of years until just now.”
Sebastien Charnoz, a planetary dynamicist working with Andre Brahic, a Cassini imaging team member at the University of Paris, found the new moons. “Discovering these faint satellites was an exciting experience, especially the feeling of being the first person to see a new body of our solar system,” Charnoz said. For weeks from his Paris office, Charnoz had searched Cassini images for saturnian moons using a special computer program, but he didn’t score a hit until he went on vacation with his laptop. “This tells me I should take more holidays,” he joked.
Both of the new moons orbit Saturn in a little more than one Earth day. S/2004 S1 and S/2004 S2 are, respectively, about 2 miles (3 kilometers) and 2.5 miles (4 km) across, orbiting 120,000 miles (194,000 km) and 131,000 miles (211,000 km) from Saturn’s center. This places them between the two large saturnian moons Mimas and Enceladus. From Earth, the new moons appear fainter than magnitude 22.
Scientists are investigating a possibility that S/2004 S1 is actually a recovery of an object first detected by the Voyager spacecraft 23 years ago. That object, dubbed S/1981 S14, appeared in a single image taken August 23, 1981.
Before today’s announcement, the smallest moons known to orbit Saturn were about 12 miles (20 km) across. Scientists expect to find small moons within the gaps in Saturn’s rings, where their gravitational pull helps clear out particles and maintain the gaps. Finding a pair of moonlets beyond the rings, between two major moons, is something of a surprise.
Cassini scientists are trying to improve their views of S/2004 S1 and S/2004 S2 even as they continue the search for additional moons. “We are at this very moment looking to see what the best times are for retargeting,” Porco said. “Hopefully, we haven’t seen the last of them.”