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Cassini snaps a tiny moon shaping Saturn's ring

A little satellite can make big waves.

NASA/JPL-Caltech/Space Science Institute

Saturn has moons like a dog has fleas, and they're all fascinating in their own ways. This new picture from NASA's Cassini probe shows off the peculiarity of Daphnis, a moon so tiny it wasn't discovered until 2005. It's just 8 kilometres (5 miles) across, and irregularly shaped, which gives it a wobbly orbit.

If it was elsewhere, maybe this would be unremarkable, but Daphnis coasts around inside Saturn's A ring, in a space called the Keeler Gap (here's a diagram). It's also what is called a shepherd moon (named after a shepherd from Greek mythology), which is a moon that clears a gap in a planetary ring.

The Keeler gap is just 42 kilometres (26 miles) wide, and Daphnis' distance varies from Saturn by about 9 kilometres (5.6 miles), while it moves up and down by about 17 kilometres (10.5 miles). This eccentricity, and Daphnis' gravity, causes peaked waves to form on the edges of the gap, in both vertical and horizontal directions. In this new image, taken from a distance of 28,000 kilometres (17,000 miles), you can see a faint tendril of material to Daphnis' left, following in its wake.

Cassini's mission is due to come to an end in September of this year.

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