The Cassini spacecraft is closing in fast on its first target of observation in the Saturn system: the small, mysterious moon Phoebe, only 220 kilometers (137 miles) across.
The three images shown here, the latest of which is twice as good as any image returned by the Voyager 2 spacecraft in 1981, were captured in the past week on approach to this outer moon of Saturn. Phoebe's surface is already showing a great deal of contrast, most likely indicative of topography, such as tall sunlit peaks and deep shadowy craters, as well as genuine variation in the reflectivity of its surface materials. Left to right, the three views were captured at a phase (Sun-Saturn-spacecraft) angle of 87 degrees between June 4 and June 7, from distances ranging from 4.1 million kilometers (2.6 million miles) to 2.5 million kilometers (1.5 million miles). The image scale ranges from 25 to 15 kilometers per pixel.
Phoebe rotates once every nine hours and 16 minutes; each of these images shows a different region on Phoebe. Phoebe was the discovered in 1898. It has a very dark surface.
Cassini's powerful cameras will provide the best-ever look at this moon on Friday, June 11, when the spacecraft will streak past Phoebe at a distance of only about 2,000 kilometers (1,240 miles) from the moon's surface. The current images, and the presence of large craters, promise a heavily cratered surface which will come into sharp view over the next few days when image scales should get as small as a few tens of meters.
Phoebe orbits Saturn in a direction opposite to that of the larger interior Saturnian moons. Because of its small size and retrograde orbit Phoebe is believed to be a body from the distant outer solar system, perhaps one of the building blocks of the outer planets that were captured into orbit around Saturn. If true, the little moon will provide information about these primitive pieces of material.