NASA's Cassini spacecraft is set to embark on a set of ultra-close passes through Saturn's upper atmosphere with its final five orbits around the planet.

Cassini will make the first of these five passes over Saturn at 12:22 a.m. EDT on August 14, NASA said.

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The spacecraft's point of closest approach to Saturn during these passes will be between about 1,630 and 1,710 kilometres above Saturn's cloud tops.

The spacecraft is expected to encounter atmosphere dense enough to require the use of its small rocket thrusters to maintain stability – conditions similar to those encountered during many of Cassini's close flybys of Saturn's moon Titan, which has its own dense atmosphere.

"Cassini's Titan flybys prepared us for these rapid passes through Saturn's upper atmosphere," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in California. 

"Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict," Maize said.

Cassini instruments will obtain data on the atmosphere close to the planet's cloud tops, make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortexes at the planet's poles. 

Its radar will peer deep into the atmosphere to reveal small-scale features as fine as 25 kilometres wide – nearly 100 times smaller than the spacecraft could observe prior to its final mission phase, the Grand Finale.

"As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe," Linda Spilker, Cassini project scientist at JPL, said. 

"It's long been a goal in planetary exploration to send a dedicated probe into the atmosphere of Saturn, and we're laying the groundwork for future exploration with this first foray," Spilker added.

On September 11, a distant encounter with Titan will serve as a gravitational version of a large pop-down maneuvre, slowing Cassini's orbit around Saturn and bending its path slightly to send the spacecraft toward its September 15 plunge into the planet, ending its long and rewarding journey. 

Launched in 1997, Cassini arrived at Saturn in 2004. 

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency.