The Cassini spacecraft has discovered two moons of Saturn that rank as the smallest bodies seen so far around the ringed planet, NASA and the mission's imaging team reported Monday.
The moons, provisionally named S/2004 S1 and S/2004 S2, are thought to be about 2 miles and 2.5 miles wide (3 and 4 kilometers across). Their orbits put them 120,000 miles and 131,000 miles (194,000 and 211,000 kilometers) from the planet's center, between the orbits of the moons Mimas and Enceladus, NASA said.
S/2004 S1 may be the same object that was spotted in a single frame taken by NASA's Voyager spacecraft 23 years ago, the space agency said. The object was then called S/1981 S14.
The newfound moons will not be named until their orbits are confirmed by the International Astronomical Union. Before the latest discoveries, Saturn had 30 named moons and yet another provisionally designated moon, S/2003 S1. Jupiter still leads the moon parade with 63 satellites, but Cassini could well reduce the gap further.
Auspicious beginning
The Cassini spacecraft arrived at Saturn just last month to begin a four-year scientific mission, and the probe already has provided unprecedented views of the planet's rings and major moons. The $3.3 billion mission is a joint project of NASA and the European and Italian space agencies.
"It's really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons, skipping unnoticed around Saturn for billions of years until just now," imaging team leader Carolyn Porco of the Colorado-based Space Science Institute said in Monday's announcement of the discovery.
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Sebastien Charnoz, a planetary dynamicist working with imaging team member Andre Brahic at the University of Paris, detected the moons with the aid of image-comparison software, NASA said.
"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 was quoted as saying. "I had looked for such objects for weeks while at my office in Paris, but it was only once on holiday, using my laptop, that my code eventually detected them. This tells me I should take more holidays."
Torrence Johnson, another member of the imaging team who is based at NASA's Jet Propulsion Laboratory, told MSNBC.com that the discovery required a little more work than Charnoz jokingly made it sound. Cassini's camera snapped pictures of a region where scientists suspected there may be satellites, then "several people on the team developed software to go through these pictures," Johnson said.
"We're certain that these things are natural chunks of rock and ice going around Saturn — probably mostly ice, based on where they are," Johnson said. Scientists are still working to define the objects' orbits fully so that they can be put on the IAU's list of named satellites.
Surprising find
Johnson, who is a veteran of the Voyager imaging team, noted that Voyager flagged other objects as possible moons more than two decades ago. Cassini may well confirm their existence, he said, and find yet other moons that earlier probes completely missed.
"We expect to find them — but where we find them, and what types of things they are, is going to be very interesting," he said.
Johnson and his Cassini colleagues had expected to find moons within gaps in the rings and perhaps near Saturn's thinnest ring, the F ring — so they were surprised to detect these small bodies between two major moons. Small comets careening around the outer solar system would be expected to collide with small moons and break them to bits.
"It's a little bit unusual to have found these right off the bat here," Johnson said. "I don't know of any other examples of small moons existing between orbits of big moons. ... You've got to assume that there are some other bodies that can be fed into this system to resupply them."
Another imaging team member, Luke Dones of the Southwest Research Institute in Boulder, Colo., said the discovery may indicate that small comets are rarer than scientists think, and that small moons could survive for billions of years in unexpected places within the Saturnian system.
"It's a very junky system," Johnson observed. "There's more debris in every dynamically protected spot in this system than we've seen with any other giant planet."
That means moon-hunters should have plenty of images to look through over the next four years and beyond.
"A lot of these things take time," Johnson said. "And in these searches, you have to have a little luck, also."
The smallest previously known moons of Saturn are thought to be 4.4 miles (7 kilometers) wide. The initial version of this report implied a larger diameter for those moons.
