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By Carolyn Collins Petersen, TheSpacewriter
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Cassini’s Grand Finale Begins

Saturday, August 12, 2017 18:26
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I don’t want to sound like one of those crazies standing in the public square holding a sign that says, “The End is Nigh”. However, in the case of the Cassini spacecraft, the end really is near. In fact, it’s five orbits away.

What Cassini mission scientists plan is a grand finale as the spacecraft makes its last orbits and plunges into the planet’s atmosphere.

The last of its trips around Saturn begins on Sunday (August 13), and these orbits will see it dip very close to the planet. Cassini’s final orbit will take it into the planet on September 15, 2017. Just before that, a quick gravity maneuver with Titan will slow the spacecraft down. That prepares it for the final dive into Saturn. On the way in, all its science instruments will be turned on. They’ll transmit data until it breaks up under the dense atmospheric pressure of the lower atmosphere. The idea is to get information about a region of Saturn hitherto unexplored and Cassini will be the first official probe of the planet’s atmosphere.Leading Up to the Finale

Cassini grand finale

An artist’s concept of the Cassini mission as it begins its final five orbits of Saturn. Courtesy Cassini Mission/NASA.

In case you haven’t kept score on this long-running mission, the Cassini spacecraft left Earth on its one-way trip to Saturn twenty years ago, on October 14, 1997. I knew some of the folks on the team but was immersed in my own research into comets at the time. So, I followed it from afar for a while before getting hooked on its fantastic images a few years ago.

What a trip it’s been on! Cassini spent seven years getting to its target. It looped past Venus twice, and back by Earth before heading to Jupiter. Then, it slipped into orbit in the Saturnian system on July 1, 2004. By that time, it had already been imaging the planet and its moons and was ready to begin serious data-gathering.

Cassini’s Accomplishments In Review pre grand finale aurorae sighted over southern poleAurorae over Saturn’s south pole. Courtesy NASA.

Over the years since then, the spacecraft has given us amazing looks at the moon Titan (both from orbit and via the Cassini-Huygens lander).  It also revealed incredible details about the moon Enceladus, including in-depth looks (and fly-throughs) of the jets of ice particles spewing from that moon. Of course, the rings and the planet’s atmosphere were major targets. Who would have thought that the rings had propellers? And kinks? And waves? And aurorae in the atmosphere? And that the atmosphere had such fascinating structures? The Hexagon alone is worth another spacecraft visit, in my opinion! For 13 years, the Cassini spacecraft has delivered the best solar system gift a planetary scientist could ask for: an in-depth look at a distant gas giant world. It will be a long time before humans can get out there and study it in person.

The Grand Finale Orbits

Now that it’s down to the last five orbits, what will Cassini be doing? As it gets closer to the planet, the spacecraft’s instruments will continue to measure Saturn’s immense gravitational field and its magnetic field. Such observations give valuable clues to the interior structure of the planet. It may actually reveal just how fast Saturn’s interior is rotating (which is where its magnetic field is generated).

Not surprisingly, with the spacecraft drawing ever-closer to the planet, the cameras are going to get a good look at the structure of the upper atmosphere as well as the rings. While it passes through ring-plane crossing, specialized instruments called “particle detectors” will study the tiny bits of ice that exist in the rings. In addition, Cassini scientists should get a very good feel for just how dense those rings really are. Finally, as the spacecraft plunges into the atmosphere, the mass spectrometer on board will sample the “air” and reveal more about the mix of gases that float high above the planet.

Why Send it Into Saturn?

It seems rather odd to be sending a working spacecraft into the planet at the end of its mission. In truth, the propellants are running low. In addition, Cassini’s instruments will eventually stop working. But, why not leave the spacecraft in a final orbit as a testament to the fantastic mission? The answer lies on the Saturn moons: pristine ice surfaces. Enceladus and Titan, in particular, could be targets if the spacecraft’s orbit should decay and cause it to stray through their orbits. That would bring Earth-based contaminants to these worlds. It’s not something anybody wants, particularly if there’s a chance of life existing on either of those moons. Or, what’s worse, if they’re home to compounds that could form life in the future. In either case, sending an Earth-based spacecraft their way would mess up the delicate balance that leads to or supports life. So, the safest thing is to send Cassini into the Saturnian atmosphere. There, it will burn up and any bio-contaminants would presumably be destroyed.

There’s a bit over a month left to enjoy the fruits of Cassini’s final orbits. Follow the news on the mission Web page and watch for announcements of new findings and images. It’s going to be a tremendous grand finale!

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