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Cassini steals a peek at Saturn's tenuous G ring

Morgan Rehnberg, Correspondent
Jun 5, 2015, 23:15 UTC

Sen—Like a photographer bringing out the best in her subject, sometimes finding the right angle is all that matters for NASA’s Cassini spacecraft. That was the case earlier this week as the robotic explorer’s cameras hunted down the perfect view of Saturn’s mysterious G ring.

This is not the first time that Cassini, in orbit about Saturn for 11 years, has studied this tenuous ring. Previous observations have helped answer fundamental questions about where the ring comes from and how it survives.

First discovered during NASA’s Voyager mission, the G ring has long puzzled astronomers. It is composed primarily of tiny dust grains that are small enough to be carried away in collisions with charged particles travelling along Saturn’s magnetic field.

The fact that the ring remains despite this erosion means that something must be replenishing the dust, yet, unlike Enceladus and the E ring, no moon orbits nearby. However, in 2009, similar observations by Cassini revealed a potential answer: the moonlet Aegaeon, just a half-kilometer across, is embedded within the ring. When tiny meteors strike Aegaeon, they may knock free enough dust to replenish the ring.

It is the size of these dust grains that complicates Cassini’s observations. Unlike larger objects, which reflect light back towards the source, small particles tend to scatter it forward instead. This effect, known as diffraction, results in transmission of light that is strongly dependent on the size of the dust being studied.

To make the G ring visible, Cassini found the correct angle between the Sun, the dust, and the camera, enabling astronomers to peer once more at Saturn's most isolated ring.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.