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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 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!

Classifying Exoplanets

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

In the wake of the long-teased and barely embargoed story about the discovery of Earth-like planet Proxima Centauri b last week, the press really ran with the idea of it being “just like Earth”. That’s really a misleading way to report on an astonishing finding. Judging by some reports (and comments) I’ve seen in social media and in the mainstream media, you’d think that little green Proxima Centaureans were already panicking at the thought of Earthlings finding them and hogging up all the spaces in the drive-up at the local Starbucks.

What Makes a Planet Earth-like?

For the record, “Earth-like” is a term that says a planet has some characteristic like Earth. It means that world is similar in some way to Earth. It might be a rocky world, it might be about the same size as Earth, it might be in the habitable zone of its star. But, it does NOT mean that it’s exactly like Earth in every way. That’s just incorrect to assume. Why?

Earth formed and evolved in an environment that supplied what it needed to become the Earth we know. There were planetesimals rich in water and minerals that slammed together to make early Earth (and supply some or most of its water). The life that exists on our planet reflects the chemical mix that existed in the system.

The planet around any other star is also a creature of its birth environment. The mix of chemical elements and the dynamics of its formation all affect what it turns out to be. They also affect the ultimate forms of life that may evolve on other worlds. Of course, in any case of a planet around a star, the energy output of the star (and its activity) also affect the chances for the origin and evolution of life. Each situation has a complex set of factors that govern its ultimate form.

“Earth-like” and Planetary Science

Planetary scientists use the term “Earth-like” to describe a world that is more like Earth than it is any other type of planet. It may not be Earth 2.0 hosting people with three eyes and giant brains. But, it can still be a rocky world with oceans or other surface water features. It might be a little smaller than Earth or a little larger, or it might be exactly the same size. The point is, it’s more Earth-like” than, say, a super-Jupiter. In the case of Proxima Cen b, it means a rocky planet in the habitable zone of the star it orbits. It’s too soon yet to know if it has water vapor or oxygen in its atmosphere, or water on its surface. That understanding will come with more and better observations.

Proxima Centaur b is “Earth-like”

So, for the record, “Earth-like” does not mean Earth 2.0. You have actually read and/or listen to the scientists when they describe the planet they’ve found. The answer is NOT in soundbites such as “Earthlike”, but in the fuller, more nuanced descriptions they give about where the planet orbits, its composition, and so on. It pays big dividends to actually pay attention to what the scientists say, to look beyond the clickbait headlines by Websites that are harvesting your “likes”, and to totally ignore the conspiracy theorists who are clamoring for your attention by using misleading characterizations of what the scientists DID say in their ignorant social media postings.

Maybe “Earthlike” isn’t the best term. I’ve also seen “Earth analog” and “rocky world”, but neither of those has the same punch as “Earth-like”. It conveys a meaning that gives you a general idea about a planet. It’s not talking about a gas giant or a tiny frozen world. It’s the scientists’ job to decipher the meaning for us, and their press offices need to learn to write better headlines. However, it’s up to us as consumers of science knowledge to actually read what the scientists are telling us.

The Spacewriter's Ramblings

Category Archives: planets

Cassini’s Grand Finale Begins