Cassini is in the seventeenth grand finale orbits, where the spacecraft dives between Saturn and the innermost rings of the ring system. There are five complete orbits left before the spacecraft burns up in the atmosphere of the gas giant, in a science rich dive after a final partial orbit. In the last five remaining complete orbits, Cassini will dive closer than ever to the upper atmosphere of Saturn, encountering dense matter in the process.
To maintain its orbit during the dives grazing the atmosphere, Cassini will have to fire its thrusters. The previous close flybys of Saturn's moon Titan with its hazy atmosphere have prepared the Cassini team for executing the close flybys on Saturn.
Cassini bid farewell to Titan one last time before initiating the set of grand finale orbits.
If the thrusters have to be fired too hard to maintain the orbit, the scientists will know that the upper atmosphere of Saturn is denser than models suggest. In this case, the altitude of the spacecraft will be slightly raised in the subsequent orbits, an approach known as the pop-up maneuver. If the atmosphere is less dense, then the altitude of the spacecraft will be lowered further, known as the pop-down maneuver.
Linda Spilker, a Cassini project scientist says, "As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe. 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."
During the final orbits, Cassini will study the auroras on the poles of Saturn, the temperatures, and the hexagonal vortexes at the poles. The radar on board will also capture information on the atmospheric features at a never before seen resolution. Finally, Cassini will dive right into the atmosphere, beaming back observations from seven sensors on board, almost till the very moment it burns up in the atmosphere.
In the last but fifth dive, Cassini will collect direct samples of Saturn's atmosphere for the first time, study the concentration of ammonia, observe mysterious features on the C ring known as streaks, and measure the temperature of different layers of Saturn's atmosphere. In the subsequent orbit, Cassini will observe the auroras on the north pole of Saturn, the temperatures of the southern polar vortex, and produce a mosaic of the southern polar auroras.
In the third of the final five orbits, Cassini will study the temperatures in the upper troposphere of Saturn, create a map of the equatorial region, and sample the atmosphere again. This will be deepest dive into Saturn's atmosphere before the final orbit. In the penultimate full orbit, Cassini will actually observe Titan's atmosphere and observe the northern auroral region while it is sunlit. Cassini will also collect samples of Saturn's atmosphere again.
Finally, in the last full orbit, Cassini will measure the amount of helium, hydrogen and ions in Saturn's atmosphere. Cassini is expected to capture a breathtaking image of Saturn's rings, looking outwards. Then Cassini will slingshot around Titan one last time, in a distant flyby to ensure that the final half orbit will let it crash into Saturn. This maneuver has been dubbed as the "goodbye kiss." On 15 September, Cassini will begin its final dive into Saturn.
The Cassini mission is a collaborative effort by NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI). Cassini has been a constant companion of Saturn for the past thirteen years. There is not enough fuel on board to maintain control of the spacecraft, which means that Cassini could drift into one of the Moons of Saturn. The spacecraft is being crashed into Saturn to prevent potential contamination of the moons, which could harbour life.