The Cassini spacecraft has found two new moons around Saturn, each about two miles across. They rank as some of the smallest of Saturn's known satellites, and their existence may give astronomers some clues to the number of comets in the outer solar system.
The new astronomical bodies, provisionally called S/2004 S1 and S/2004 S2, are located about 194,000 kilometers and 211,000 kilometers from the planet center, between the orbits of Mimas and Enceladus--the two closest of Saturn's eight major satellites. Sebastien Charnoz of the University of Paris discovered the moons using a computer code that looks for spots of light moving with respect to the background stars. "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," Charnoz says. "This tells me I should take more holidays."
The number of Saturnian moons now stands at 33, but currently Jupiter has the most companions with 60. The satellite count for these planets has dramatically risen over the past few years. "It wouldn't surprise anyone if there are hundreds of these rock satellites around the gas giants," says Kevin Grazier of the Cassini imaging team. And the more that are found, the fewer comets that are expected in the outer solar system. This is because a collision with a comet would destroy a small moon. With better satellite estimates, astronomers hope to refine their understanding of the Kuiper Belt, a repository of comets beyond the orbit of Neptune, as well as the history of craters on moons around the outer planets.
With the close-up view that Cassini provides, Grazier and his colleagues anticipate seeing even tinier satellites around the ringed planet. But this begs the question: when does an orbiting rock become a moon? "It's something we haven't defined yet," admits Grazier. But S/2004 S1 and S/2004 S2 "are absolutely moons." --Michael Schirber