Cassini: the Beginning of the End

A Long-running Cassini Enters Its Final Orbits

Saturn and its rings
Saturn and its rings, as seen by Cassini on April 3, 2017. Courtesy Cassini Mission.

It has been nearly 20 years since the Cassini Mission to Saturn started its grand adventure of exploration. Folks, this has been a mission for the record books. Not only has it been extended several times (and for good reasons!), it has been giving us almost a “Webcam at Saturn” view for 13+ years. That’s a pretty good return for the money and time!

Saturn is a fascinating place. The more we study it, the more we learn about its rings, atmosphere, and collection of moons. Unfortunately, the spacecraft is running low on fuel, and when that happens, the mission ends. On September 15, 2017, the spacecraft will complete its final orbit and spiral into the massive atmosphere of Saturn. Between now and then, the spacecraft has a lot to do. For one thing, it will nudge into ever-closer orbits of the planet. It will pass close by Titan, the shrouded moon that the Huygens lander revealed to us early in the mission. Eventually, the spacecraft will be doing deep dives between the rings and the planet, and send back ever-more detailed images of both.

Mission planners are calling this “Cassini’s Grand Finale”, and for good reason. NO other spacecraft has gotten this close to Saturn, and given the vagaries of space exploration funding and planning these days, it’s not likely we’ll see another do the same thing for some years.

So, Why Does Cassini Study Saturn So Closely?

You might wonder why the multinational team and NASA decided to spend so much time on one planet? Actually, planetary scientists have spent a LOT of time at nearly all the planets except Uranus, Neptune, and, of course, distant Pluto — which was the latest one to be explored. In the cases of Uranus and Neptune, they also get to watch them through orbiting space telescopes, but the Voyager 2 mission did give close-up images and data.

The reason planetary scientists do this is to observe change over time, that is — the evolution of a planet’s surface, atmosphere, and other characteristics. It’s not like you go out, take a snapshot, and that tells you everything you want to know about a world. No, if you’re smart and have the capability, you look at it for a long time, under changing conditions. We do the same with Earth’s climate and surface features. And, as anybody interested in space exploration knows, we also do it with Mars, at Venus, and Jupiter.

In the case of the Cassini mission, the rewards for watching such a long time have been amazingly great:

1) as I mentioned above, the spacecraft’s Huygens lander showed close-up details of Titan, and the analysis of Titan data shows that while it’s a frozen desert, it’s quite Earth-like in many ways. So, studying Titan gave us another way of understanding how weather, climate, and geologic systems work there, and also on Earth. That’s a win.

2) Cassini has watched the Saturnian weather systems through a good part of the 29-Earth-year-long Saturnian year, giving insight into how it changes as the planet orbits the Sun; that’s a win for learning more about weather processes in the outer solar system.

Saturn propellor seen by cassini
A “propeller” of ring material swirling around a tiny moonlet near the Encke Gap in Saturn’s rings. This one is named “Earhart” after Amelia Earhart, a famous aviator in the early 20th century. Courtesy Cassini Mission.

3) The rings! OMG, the rings. They win just for being so gorgeous and intriguing. The Voyager spacecraft visits whetted our appetites for those rings; Cassini followed up with some incredible closeups. We may not yet know exactly what objects collided to form those rings, but planetary scientists can now tell us how those ring particles react and behave. Cassini images show how the shepherding satellites corral the rings. In the case of one ring, those images how the particles spewing from the moon Enceladus make their way to orbit around Saturn to form the ring.

4) What about those fascinating moons! I just mentioned Enceladus: it tops the list as a geologically active, water-rich world. That’s the one spewing ice particles to the E ring. Titan, of course, is a huge prize, despite those heavy clouds. It continues to fascinate us with its frigid surface. At least one other moon may have come from the outer solar system, and gotten captured into Saturn orbit millions of years ago. And, the list goes on — the moons themselves could be the subject of another mission, if we had the time, money, and equipment.

5) Let’s not forget the constant study of the magnetic field and radiation environment around Saturn over the years!  And, the aurorae that sizzled at its polar regions. I remember when discovering aurorae at any planet other than Earth was a new thing. Now, it’s completely obvious that planets with magnetic fields can experience them.

Science during Cassini’s Grand Finale

When September 15 rolls around, Cassini will be doing science right up to the end. The plan is to measure the atmosphere as the spacecraft plunges through Saturn’s cloud tops. It will transmit back information until the pressure is more than it can bear. This is the safest way to dispose of the spacecraft, since its rocket fuel will have run out. Scientists do not want to risk inadvertent biological contamination of the moons by letting Cassini drift in the system without guidance.  Better to let it plunge into Saturn and give us the first in situ look at the clouds as it roars through. The spacecraft probably won’t last very long after it enters the atmosphere, so it will be interesting to grab a look when we can.

You can follow the mission at the Cassini Web page, and monitor the latest images and data. I can confidently predict that the pictures are going to be spectacular and well worth the wait!

(Note: I received some emails from folks about  my recent absence from the pages here. I’m working on a book, which has engaged my writing chops in overdrive; however, I’ll endeavor to post here more often. Thanks for the notes!)

2 Comments

  1. C.C. Petersen

    Hi Mark,

    No sample return missions have happened from Saturn’s rings. I admit, it would be cool. But yes, the spectral analysis is good enough to give us a very good idea of the composition of the rings.

Leave a Reply

Your email address will not be published.