http://news.bbc.co.uk/2/hi/science/nature/3798485.stm The Cassini spacecraft, which is en route to Saturn, has made a close pass of the planet's mysterious moon Phoebe.
The US-European spacecraft made its closest approach to the moon on Friday at 2156 BST at a distance of 2,000km.
As the probe flies past, it will gather data that will tell scientists about Phoebe's internal structure, its composition and its history.
Phoebe is 220km across and Cassini's images of it will be far superior to those taken by Voyager 2 in 1981.
By determining the mass and volume of the moon, the scientists can also determine its density. This will tell them whether the moon is solid all the way through or is essentially a mass of rubble.
A radar instrument aboard Cassini will be able to answer whether Phoebe is composed primarily of ice or of rock.
The moon is intriguingly dark - it reflects only 6% of the sunlight it receives - and it orbits Saturn in a direction opposite to that of the larger, and closer, Saturn moons.
Its darkness and retrograde orbit have led some scientists to wonder if Phoebe is a Centaur: an object that migrated from the outer Solar System.
Objects of this type - from the region known as the Kuiper Belt - are thought to have served as 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.
Images taken by Cassini so far show a body heavily pitted with craters, including a very large one, roughly 50km (31 miles) across.
The photos reveal a substantial amount of variation in surface brightness on Phoebe. Features that appear to be cliffs may be the boundaries between large craters.
The pattern of cratering will give clues to Phoebe's history. By looking at the surface properties of the youngest crater, Researchers can begin to understand how Phoebe started out.
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.
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.
The small moon was discovered in 1898 by the US astronomer William Henry Pickering.
Topic: Cassini flies past Saturn moon (Read 1009 times)