By Dr David
BBC News Online science
The Cassini spacecraft, on its final approach to Saturn before it enters orbit, is to make a close pass of its mysterious moon Phoebe.
Mysterious Phoebe is soon to be revealed
Phoebe is only 220km across and has never been seen in detail. Cassini's images will be twice as good as those obtained by Voyager 2 in 1981.
Already tall sunlit peaks and deep shadowy craters have been seen in the approach pictures.
On Friday, Cassini will sweep past Phoebe at a distance of about 2,000km.
Varied and interesting surface
The approach images also show a great deal of contrast which scientists say is indicative of a varied and interesting surface.
Phoebe was discovered in 1898. It is small and very dark.
Long-distance images were obtained by the Voyager 2 flyby in 1981, but Cassini's images - with a resolution of a few tens of metres - will be far superior.
The probe will enter orbit around Saturn in 1 July
Phoebe orbits Saturn in a direction opposite to that of the larger, and closer, Saturn moons.
It is thought to be a captured body that originated in the outer Solar System, possibly one of the building blocks of the outer planets.
If this is true then observations of Phoebe will provide valuable information about how the various worlds that inhabit the cold, outer reaches of our Solar System were formed.
Cassini is a joint Nasa-European Space Agency probe which will enter orbit around Saturn on 1 July. Next year, it will deliver the Huygens probe into the atmosphere of Saturn's major moon, Titan.