Phoebe, a Planetesimal From 4.5 Billion Years Ago. En route to Saturn NASA-ESA Cassini-Huygens mission grazed the most important of Saturn's outer moons, Phoebe, on June, 11th 2004. Phoebe is dwelling in the outer reaches of the ringed planet and had been chosen as a science target as it was thought to be a possible remnant of solar system formation. First data were already leaning in that direction. Further data are confirming it. Phoebe really is one of these planetesimals which were the building blocks of the solar system 4.5 billion years ago. As gas giants cores were forming from such bodies, gravitational interactions pushed away much of the planetesimals to more distant orbits where they joined a native population, forming the Kuiper Belt. Phoebe staid behind and became a Saturn's moon. Looking at Phoebe we are seeing one these ancestral bodies which are the base of planets about the Sun or other stars. Phoebe was discovered in 1898 by William Pickering, an American astronomer. This nearly spherical moon is 143 x 137 x 131 mi (230 x 220 x 210 km); it is orbiting 8 million miles (13 million km) from Saturn in 550 days. Phoebe rotates in 0.4 days. Cassini-Huygens mission is reaching Saturn on July, 1st for a thoroughful mission at the planet's system and a dedicated study of Saturn's major moon, Titan. picture courtesy NASA/JPL/Space Science Institute