NASA's Cassini to make final, closest observations of Saturn

After studying Saturn, its rings and moons for more than 12 years, NASA's Cassini spacecraft has entered the final year of its epic voyage during which it will make the closest-ever observations of the planet.

The conclusion of the historic scientific odyssey is planned for September next year, but not before the spacecraft completes a daring two-part endgame.

Beginning on November 30, Cassini's orbit will send the spacecraft just past the outer edge of the main rings. These orbits, a series of 20, are called the F-ring orbits.

During these weekly orbits, Cassini will approach to within 7,800 kilometres of the centre of the narrow F ring, with its peculiar kinked and braided structure, NASA said.

"During the F-ring orbits we expect to see the rings, along with the small moons and other structures embedded in them, as never before," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in California.

"The last time we got this close to the rings was during arrival at Saturn in 2004," she added.

Cassini's final phase, called the Grand Finale, begins in earnest in April next year. A close flyby of Saturn's giant moon Titan will reshape the spacecraft's orbit so that it passes through the gap between Saturn and the rings, an unexplored space only about 2,400 kilometers wide.

The spacecraft is expected to make 22 plunges through this gap, beginning with its first dive on April 27.

During the Grand Finale, Cassini will make the closest-ever observations of Saturn, mapping the planet's magnetic and gravity fields with exquisite precision and returning ultra-close views of the atmosphere.

Scientists also hope to gain new insights into Saturn's interior structure, the precise length of a Saturn day, and the total mass of the rings, which may finally help settle the question of their age.

The spacecraft will also directly analyse dust-sized particles in the main rings and sample the outer reaches of Saturn's atmosphere, both first-time measurements for the mission.

"It's like getting a whole new mission. The scientific value of the F ring and Grand Finale orbits is so compelling that you could imagine a whole mission to Saturn designed around what we're about to do," said Spilker.

Since the beginning of 2016, mission engineers have been tweaking Cassini's orbital path around Saturn to position the spacecraft for the mission's final phase.

They have sent the spacecraft on a series of flybys past Titan that are progressively raising the tilt of Cassini's orbit with respect to Saturn's equator and rings.

This particular orientation enables the spacecraft to leap over the rings with a single (and final) Titan flyby in April, to begin the Grand Finale.

The Grand Finale will come to a dramatic end in September next year, as Cassini dives into Saturn's atmosphere, returning data about the planet's chemical composition until its signal is lost.

Friction with the atmosphere will cause the spacecraft to burn up like a meteor soon afterwards.