cassini Archives - The Spacewriter's Ramblings

The time is coming when we will all say goodbye to Cassini, the spacecraft exploring Saturn since it first turned its cameras to stare at the planet in 2002. It began its Grand Finale activities some months ago with final orbits, final images, and other “last” looks at the Saturn system. The last pass around the moon Titan is done. The spacecraft got a gravitational kick from that loop that sends it right into Saturn’s northern hemisphere clouds on Friday. The last command sequences are sent. All controllers can do now is wait, monitor transmissions coming back from Cassini, and make any last-minute changes as necessary.

Cassini Opened New Windows on Saturn

As I look over the many, many images and other data reports from the mission, it’s hard to select a “favorite” result. It continues to show us way more than simply a ringed planet with a bunch of moons and some gorgeous rings. Granted, we are used to seeing great images of the system from the Voyager spacecraft in the 1980s. But, Cassini did it in high-res for years.

One of the great truisms about planetary science tells us that while it’s good to take snapshots of a target, it’s even better to study how that target changes over time. It’s the difference between taking a baby picture of someone and having a series of pictures taken during their lifetimes. You get to see how they grow and change. Cassini opened windows of opportunity to watch the Saturn system change over time — from its storms and cloud belts and zones to activities on some of its moons and within its rings.

Encountering Enceladus

cassini — A 14-hour image sequence of Enceladus made by Cassini shows plumes spewing ice crystals out to space. Courtesy NASA/Cassini Mission.

Still, if I had to select just one result that’s really important, I’d go with the discovery and long-term study of the moon Enceladus and its jets. Those geysers spew out to space and create the E ring. Their ice particles told scientists about the ocean hidden beneath the icy crust of that little moon. The fact that there’s an ocean there at all is a major surprise. The finding that it’s salty and “warm” (i.e. not frozen) indicates conditions could be hospitable to life.

Now, to be clear, life hasn’t been discovered there. But, the conditions exist. For a world that far from the Sun’s heat, that’s a tantalizing finding. It rewrites what we know about frozen worlds in the outer solar system.

After Cassini, What Happens?

Saturn propellor — 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.

The Enceladus discoveries are enough to start people looking at future missions to Saturn. Those will follow up on Cassini’s accomplishments. While we’re thinking about sending more spacecraft to Saturn, it wouldn’t be a bad idea to check out Titan again. That world was another surprise, with its frigidly weird landscapes and lakes. The rings could use some more study, too. Those tiny little ‘propellers’ that interact with ring particles in an intricate dance are intriguing.

Both worlds merit closer looks and in-depth surveys, if not landers. It may be many years before those missions happen. NASA might not be leading those missions. Perhaps other countries will see what the international Cassini team has achieved and will launch probes to these worlds. However we get back to this system (and who ever does it next), Cassini made it possible.

Follow the final few days of Cassini’s historic mission as it sails into the clouds. NASA’s Cassini link is your best source of info.

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 pole — Aurorae 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!

The Spacewriter's Ramblings

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Cassini Enters the Final Plunge