Launched in 1997, Cassini has been touring the Saturn system since arriving there in 2004 for an up-close study of the planet, its rings and moons.
During its journey, Cassini has made numerous dramatic discoveries, including a global ocean within Enceladus and liquid methane seas on Titan.
Engineers have been pumping up the spacecraft's orbit around Saturn this year to increase its tilt with respect to the planet's equator and rings.
Following a gravitational nudge from Saturn's moon Titan, Cassini will enter the first phase of the mission's dramatic endgame today.
"We're calling this phase of the mission Cassini's Ring-Grazing Orbits, because we'll be skimming past the outer edge of the rings," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in the US.
"In addition, we have two instruments that can sample particles and gases as we cross the ringplane, so in a sense Cassini is also 'grazing' on the rings," said Spilker.
Cassini will circle high over and under the poles of Saturn till Aprill 22 next year, diving every seven days - a total of 20 times - through the unexplored region at the outer edge of the main rings.
On many of these passes, Cassini's instruments will attempt to directly sample ring particles and molecules of faint gases that are found close to the rings.
"Even though we're flying closer to the F ring than we ever have, we'll still be more than 7,800 kilometers distant. There's very little concern over dust hazard at that range," said Earl Maize, Cassini project manager at JPL.
The F ring marks the outer boundary of the main ring system; Saturn has several other, much fainter rings that lie farther from the planet.
The F ring is complex and constantly changing: Cassini images have shown structures like bright streamers, wispy filaments and dark channels that appear and develop over mere hours. The ring is also quite narrow - only about 800 kilometers wide. At its core is a denser region about 50 kilometers wide.
Cassini's ring-grazing orbits offer unprecedented opportunities to observe the menagerie of small moons that orbit in or near the edges of the rings, including best-ever looks at the moons Pandora, Atlas, Pan and Daphnis.
Grazing the edges of the rings also will provide some of the closest-ever studies of the outer portions of Saturn's main rings (the A, B and F rings).
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