Just like barely visible jet aircraft can create vivid contrails against the blue sky, so too do tiny moonlets embedded in Saturn's outermost A ring whip up easily spotted disturbances in the ring material. The disturbances, which scientists have dubbed "propellers" because of their shape, were discovered in 2006 by the Cassini spacecraft. Since then, scientists have been tracking  their movement around the gas-giant planet. Today in The Astrophysical Journal Letters, members of the Cassini team describe how the features, which can stretch hundreds of kilometers, are being caused by the movement of moonlets only about 1 kilometer wide plowing through the A ring. The research marks the first time anyone has tracked the motion of solid bodies within a disk or ring of material. By continuing to study the propellers, scientists think they can learn more about how flat, spinning disks of gas and dust eventually accreted into stars and planets.
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