NASA's Cassini spacecraft has discovered a new object in Saturn's rings.

By capitalizing on the angle of sunlight cast on the rings as the planet nears its August 11 equinox, Cassini captured the 25-mile long shadow cast on Saturn's B ring by a tiny moonlet that is probably around 1,300 feet in diameter.

It's no coincidence that the spacecraft's rendezvous with the planet coincides with its equinox, which occurs twice a Saturn year, or every 15 Earth years. The mission was planned to take advantage of the 27 months during which the sun is directly over the planet's equator at noon. This positioning causes any three-dimensional object in the plane of the rings to cast a long shadow and stand out.

Because Saturn's main rings, A, B, C and D, are a scant 30 feet thick, Cassini's cameras can't even see them edge on, which makes it tough to spot anything sticking out from them. The equinox is the only time these objects will be plainly visible.

Image: NASA/JPL/Space Science Institute

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