For six years, the Cassini spacecraft has been touring Saturn and its magnificent rings. Unlike previous spacecraft, which have snatched a passing glimpse of the planet on their way to the edges of the solar system, Cassini has beamed back stunningly detailed images year after year. Among the most striking are structures shaped like aircraft propellers. They are the wakes created by unseen moons as they plough through the icy rubble orbiting around Saturn. Dozens of propellers have been spotted in the A-ring, a band 14,000 kilometres wide and only 10 metres deep.
The moons themselves are too small even for Cassini to see directly because they are only a few kilometres to a few hundred metres across. In contrast, their propeller-shaped wakes can be thousands of kilometres long, and some have now been observed orbiting Saturn for several years. They form as a result of a moon's gravity tugging on the surrounding material. The debris inside its orbit, being closer to Saturn, is moving faster, and the perturbation therefore quickly overtakes the moon, creating a long, thin wake ahead of it. This is the leading blade of the propeller. Meanwhile, the material outside the moon's orbit is moving more slowly, creating the trailing blade.