NASA's Cassini spacecraft is set to begin its final five ultra-close orbits around Saturn, before the probe plunges into the atmosphere of the ringed planet and ends its epic 20-year-long journey. The spacecraft will enter new territory in its final mission phase, the Grand Finale, making the first of the five passes over Saturn on August 13. 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 the US. The pass will be considered nominal if the thrusters operate between 10 and 60 per cent of their capability. If the thrusters are forced to work harder - meaning the atmosphere is denser than models predict - engineers will increase the altitude of subsequent orbits. Referred to as a "pop-up manoeuvre," thrusters will be used to raise the altitude of closest approach on the next passes, likely by about 200 kilometres. If the pop-up manoeuvre is not needed, and the atmosphere is less dense than expected during the first three passes, engineers may alternately use the "pop-down" option to lower the closest approach altitude of the last two orbits, also likely by 200 kilometres. Doing so would enable Cassini's science instruments, especially the ion and neutral mass spectrometer (INMS), to obtain data on the atmosphere even closer to the planet's cloud tops. "As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe," said Linda Spilker, Cassini project scientist at JPL. Other Cassini instruments will 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 the Grand Finale. On September 11, a distant encounter with Titan will serve as a gravitational version of a large pop-down manoeuvre, slowing Cassini's orbit around Saturn and bending its path slightly to send the spacecraft toward its plunge into the planet. During the half-orbit plunge, the plan is to have seven Cassini science instruments turned on and reporting measurements in near real time. The spacecraft is expected to reach an altitude where atmospheric density is about twice what it encountered during its final five passes. Once Cassini reaches that point, its thrusters will no longer be able to work against the push of Saturn's atmosphere to keep the spacecraft's antenna pointed toward Earth, and contact will permanently be lost. The spacecraft will break up like a meteor moments later, ending its long and rewarding journey.
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