Cassini-Huygens Mission

The Cassini-Huygens mission ends this week, after 13 years in orbit around Saturn. Scientists found answers to some questions they had, and uncovered new questions.

I think they’ll be studying Cassini’s and Huygens’ data for years. Decades.

I’ll take a quick look at what we’ve learned, and why scientists want follow-up missions to the Saturn system.

The Enceladan subsurface ocean wasn’t a complete surprise.

Theoretical work based on images and data from the Voyager flybys and Galileo Jupiter mission suggested that liquid water lay under Europa’s crust.

Cassini ‘saw’ geysers at the moon’s south pole in 2005. Their water is salty, with some organic matter.

“Organic” doesn’t mean “living.” But scientists think there could be life in the Enceladan ocean. That’s a major reason for having Cassini dive into Saturn’s atmosphere.

The spacecraft is running out of fuel. Left in orbit around Saturn, it might drift into an orbit that ended with an impact on Enceladus. That would complicate any study of possible Enceladan life.


Science and Imagination


(From NASA/Jet Propulsion Laboratory-Cal Tech, used w/o permission.)
(“Inspiration for this retro poster came from the famous 1963 diptych by Roy Lichtenstein titled Whaam
(NASA/JPL))

Until the last decades of the 20th century, we didn’t know much about the moons of Jupiter, Saturn, and the other outer planets.

Astronomers could tell how far from their planet the moons were, had a pretty good idea of how big they were, and could make educated guesses about what they were made of: but that was about it.

Scientists were limited to facts, and what they could extrapolate from those facts.

Writers and artists could stay ‘in the box,’ or let their imaginations off the leash: which they frequently did.

Then we started sending robot spaceships to the outer Solar System.

Some moons looked a bit like ours, crater-covered desolate spheres. Others are — different.

Nodding Mimas


(FromNASA/JPL-Caltech/Space Science Institute, used w/o permission.)
(Mimas, with Saturn’s rings in the background, image from Cassini. October 23, 2016.)
(Tony Greicius, NASA))

Most moons in the Solar System, including Earth’s, are in a 1:1 spin-orbit resonance. One side always faces the planet. Pretty much.

The moons, including ours, nod and wobble. “Libration” is what astronomers call this rocking motion.

Depending on who you read, the word comes from a Latin word meaning balance, or swinging, or something else.

You can call it non-uniform rotational motion or oscillations about an equilibrium. But “nodding” is shorter, and they all describe the same thing. More or less.

Mimas moves more than our moon. A point on its surface moves back and forth by as much as six kilometers. That’s a sizable fraction of the moon’s 396 kilometer diameter.

By measuring Mimas’s motions, scientists can learn what sort of material is under its surface. More accurately, they will: after they figure out why it nods so much.

Scientists figured Mimas would nod. Data from Cassini’s Image Science Subsystem, ISS for short, shows it has twice the expected wobble.

It’s a near-certainty that the extra wobble comes from something odd inside Mimas, but scientists haven’t worked out the details. Not yet.

One model that fits the data is a subsurface ocean, a deep one.

A problem there is that tidal stresses would make Mimas at least as tectonically active as Europa. That would give Mimas surface features like Europa’s cracks, which it doesn’t have. Or maybe the Mimantean crust is really thick and strong.

Another model that also fits is a lopsided core. Some scientists agree that the ocean is unlikely, but that a lopsided core isn’t the best explanation.

They say it’s more likely that something denser than its surroundings is under the crater Herschel.

That’s the big crater that gives this moon its “Death Star” moniker.

That discussion was still going on the last time I checked.

I figure we’ll find an answer, probably after collecting more data. Probes with seismometers on Mimas would help.

I don’t know when that will happen.

A joint NASA-ESA Titan Saturn System Mission would have launched in 2020, arriving at Saturn in 2029. It’s been postponed, but not canceled.

NASA and ESA are working on the Laplace Mission now. That mission’s focus will be the Jupiter system, particularly Europa. Japan’s JAXA, Russia’s Roscosmos, and probably other outfits will probably contribute, too.1

I’m quite sure someone will send explorers, robotic and otherwise, to the Saturn system. The question is when.

We’ve been wondering what’s over the horizon for a very long time. All that’s been changing is how far away our horizons are.


1. Cassini: Final Hours


(From NASA/JPL-CalTech/SSI, via BBC News, used w/o permission.)
(“Just returned from Cassini: An image of Titan and its northern seas of liquid methane”
(BBC News))

Cassini conducts last picture show
Jonathan Amos, BBC News (September 14, 2017)

Engineers now have a precise expectation of when they will lose contact with the Cassini probe.

“The spacecraft is being ditched in the atmosphere of Saturn on Friday, bringing to an end 13 amazing years of discovery at the ringed planet.

“The team hopes to receive a signal for as long as possible while the satellite plummets into the giant world.

“But the radio will likely go dead at about 6 seconds after 04:55 local time here at mission control in California.

“That is 11:55:06 GMT (12:55:06 BST). This is the time that antennas on Earth lose contact.

“Because of the finite speed of light and the 1.4 billion km distance to Saturn, the event in space will actually have occurred 83 minutes earlier….”

The Tidbinbilla Deep Space Tracking Station’s big antenna near Canberra, Australia, will be picking up Cassini’s final signals: along with several backup receivers.

Cassini’s systems can’t work fast enough to let scientists get pictures during the probe’s last moments.

Before Cassini stops taking and processing images, scientists are having it ‘look’ at Titan and Enceladus, and Saturn’s dark side. The Saturn night side images will be in infrared and ultraviolet.

Cassini’s magnetic and chemical sensors will be sending data directly to Earth during the last probe’s three hours. Cassini usually stored data in its onboard solid state memory before transmitting it back to Earth. There won’t be time for that as the probe enters Saturn’s atmosphere.

I expect that we’ll learn more about Saturn from Cassini’s last transmissions. We’ve already learned a great deal.


2. Saturn: 13 Years of Science


(From NASA/JPL-CalTech/SSI, via BBC News, used w/o permission.)
(“Cassini began its study of the ringed planet in 2004”
(BBC News))

Cassini: Saturn probe turns towards its death plunge
Jonathan Amos, BBC News (September 12, 2017)

The international Cassini spacecraft at Saturn has executed the course correction that will send it to destruction at the end of the week.

“The probe flew within 120,000km of the giant moon Titan on Monday – an encounter that bent its trajectory just enough to put it on a collision path with the ringed planet.

“Nothing can now stop the death plunge in Saturn’s atmosphere on Friday….”

Folks from 17 countries designed and built the tech, and have been working on the Cassini-Huygens mission.

The project’s named after Giovanni Domenico Cassini and Christiaan Huygens. Huygens discovered Titan in 1655.

Cassini discovered some of Saturn’s moons — Iapetus and Rhea in 1671 and 1672, Tethys and Dione in 1684.

He also spotted divisions in Saturn’s rings. The Cassini divisions are named after him. So is an asteroid, 24101 Cassini, and craters on Earth’s Moon and Mars.

Huygens has namesakes, too: asteroid 2801 Huygens, a Lunar mountain and a Martian crater.

The Cassini-Huygens mission started in 1982, when the European Science Foundation and American National Academy of Sciences formally looked at cooperative efforts.

Folks at the Jet Propulsion Laboratory built the Cassini orbiter. Alcatel Alenia Space built the Huygens lander. The combined spacecraft left Cape Canaveral’s Launch Complex 40 on October 15, 1997.

Two fly-by passes of Venus, one of Earth, and finally one by Jupiter, sent the Cassini-Huygens probes toward Saturn.

Statistics and Silliness

The Earth fly-by was on August 18, 1999; accompanied by the usual ‘something causes cancer’ articles and protests.

There was a little truth to them. Folks running the mission had crunched numbers and done a risk assessment.2

If everything that could possibly wrong had gone wrong — in the worst possible way — Cassini’s plutonium power source would have burned up, along with the rest of the spacecraft, in Earth’s atmosphere.

And then, we were told by ardent defenders of humanity, 500,000 innocent victims would die from cancer.

Their angst may have been real. Their statistics weren’t. That doesn’t mean that activists were lying.

The numbers they used may have started as a transcription error. Hypertrophied fear of radiation — and new tech in general — could have taken over at that point.

There really was a risk. But not much of a one.

The risk assessment’s worst-case scenario might have been detectable. But not, I think, particularly significant.

Cancer would kill 0.0005 per cent more folks — if Cassini dove into Earth’s atmosphere at exactly the wrong angle. Eventually. In a worst-case scenario.

That would have been around 5,000 additional deaths from cancer, over a typical human lifespan. I value human life, including folks I don’t know. (June 4, 2017; August 14, 2016)

But I’d seen the assessment results, and have some understanding of statistics.

Personally, I’m more frightened of walking down a flight of stairs. Some real-life Mr. Squibbs analogs got their 15 minutes of fame, and that’s another topic. (July 28, 2017; May 21, 2017; October 16, 2016)

Back to Saturn and science.

Cassini-Huygens started orbiting Saturn on July 1, 2004. Cassini has been collecting and sending data ever since.

Huygens reached Titan’s surface on January 14, 2005.

Huygens sent back data for about 90 minutes after landing. The data included 700 images. Or would have.

A design glitch kept one of the lander’s communications channels from working. We lost 350 images that way, along with Doppler radio measurements between Cassini and Huygens.

That’s the bad news. The good news is that we got data from the Huygens accelerometers and other instruments: plus that 90 minutes of data from the surface.

The probe’s designers had focused on keeping the lander working during descent, gathering data about Titan’s atmosphere.

When they designed Huygens, they were pretty sure it would touch down in a mountain range. Or maybe a flat plain. On the other hand, maybe it would splash into a Titanian ocean, or onto some other sort of surface.

Since they didn’t know what Huygens would land on, I think they did a good job.


3. Titan, Mostly


(From NASA/JPL-Caltech, used w/o permission.)
(“This illustration shows Cassini diving through the Enceladus plume in 2015. New ocean world discoveries from Cassini and Hubble will help inform future exploration and the broader search for life beyond Earth.”
(NASA/JPL))

Nine Ways Cassini Matters: No. 1
NASA/JPL (September 11, 2017)

NASA’s Cassini spacecraft and ESA’s Huygens probe expanded our understanding of the kinds of worlds where life might exist.

“With discoveries at Saturn’s moons Enceladus and Titan, Cassini and Huygens made exploring ‘ocean worlds’ a major focus of planetary science. Insights from the mission also help us look for potentially habitable planets—and moons—beyond our solar system.

“Life as we know it is thought to be possible in stable environments that offer liquid water, essential chemical elements, and a source of energy (from sunlight or chemical reactions). Before Cassini launched in 1997, it wasn’t clear that any place in the icy outer solar system (that is, beyond Mars) might have this mix of ingredients. By the next year, NASA’s Galileo mission revealed that Jupiter’s moon Europa likely has a global ocean that could be habitable. Since its 2004 arrival at Saturn, Cassini has shown that Europa isn’t an oddball: Potentially habitable ocean worlds exist even in the Saturn system—10 times farther from the sun than Earth….”

Christiaan Huygens spotted Titan in 1655.

Josep Comas i Solà noticed that Titan’s edge was unusually dark in 1903. He said the moon might have an atmosphere.

That was enough to let science fiction writers imagine life on Titan.

Frank R. Paul’s illustration for “Golden City of Titan” was far more fiction than science, which is appropriate for the November 1941 Amazing Stories back cover.

I suspect tales like “Sojarr of Titan” and “The Puppet Masters” fueled interest in space exploration, and discouraged some folks from taking science seriously.

I’d like to think many realized that pulp fiction was entertainment: and not particularly educational.

Meanwhile, scientists were studying the real Saturnian moon.

Gerard P. Kuiper used a spectroscope to confirm that in 1944. He figured the pressure might be around 10 kPa. Earth’s sea level pressure is around 101 kPa. That would have made Titan’s atmosphere significant, but much lower than our planet’s.

Still, a moon with an atmosphere was remarkable. Still is.

By the 1970s, we’d learned that Titan’s atmosphere had a lot more methane than Kuiper detected; and was a great deal thicker. The 1980 Voyager 1 flyby showed that Titan’s surface pressure was about half again as high as Earth’s.3

Life As We Don’t Know It?

Titan and Earth are the only places in the Solar System with thick, nitrogen-rich atmospheres. That makes Titan Earth-like, but not very.

“…Life as we know it is thought to be possible in stable environments that offer liquid water, essential chemical elements, and a source of energy (from sunlight or chemical reactions). Before Cassini launched in 1997, it wasn’t clear that any place in the icy outer solar system (that is, beyond Mars) might have this mix of ingredients. By the next year, NASA’s Galileo mission revealed that Jupiter’s moon Europa likely has a global ocean that could be habitable. Since its 2004 arrival at Saturn, Cassini has shown that Europa isn’t an oddball: Potentially habitable ocean worlds exist even in the Saturn system—10 times farther from the sun than Earth….

“…Saturn’s largest moon, Titan, offered tantalizing hints that it, too, could help us understand whether life could have evolved elsewhere. Cassini and ESA’s Huygens probe (which landed on Titan’s surface) found clear evidence for a global ocean of water beneath Titan’s thick, icy crust and an atmosphere teeming with prebiotic chemicals. Based on modeling studies, some researchers think Titan, too, may have hydrothermal chemistry in its ocean that could provide energy for life. On its frigid surface, which hosts vast seas of liquid hydrocarbons, scientists wonder, could Titan be home to exotic forms of life ‘as we don’t know it’?”
(NASA/JPL)

Reinforcing Aristotelian biases with unflagging devotion to Ussher’s chronology, I could decide that life can’t possibly exist anywhere except Earth: because it’s ‘not in the Bible.’ (June 30, 2017; June 16, 2017; December 2, 2016)

That seems imprudent, since I think I live in Minnesota.

My part of the world isn’t mentioned in the Bible. But I’m quite certain that the town I live in exists. Even if it’s not ‘Biblical.’

I’ve known a few dedicated ‘Bible-believing’ Christians, but none who told me that Minnesota isn’t here. (July 23, 2017; April 21, 2017)

As I keep saying, I take the Bible, Sacred Scripture, very seriously. (Catechism of the Catholic Church, 101133)

I’m not, however, obliged to try believing that science is poetry: or that the Bible reflects a contemporary Western viewpoint. (August 20, 2017; May 7, 2017; March 17, 2017)

I certainly do not have to choose either faith or reason.

Faith, reason, and an honest search for truth get along fine. Or should, for Catholics who understand our faith. (Catechism, 3135, 159; “Fides et Ratio;” “Gaudium et Spes,” 36)

Organic Chemistry

Tholins are what we can get when ultraviolet light shines on organic compounds like methane or ethane.

Scientists think tholins on Earth may have helped life get started here. They’re not around now, and haven’t been since the Great Oxygenation Event. (May 19, 2017)

Tholins won’t form naturally on Earth these days, but there’s enough sunlight — ultraviolet included — in the outer Solar System to make tholins there.

Scientists figure Titan’s orange-red atmosphere and haze get their color at least partly from tholins.

Knowing that tholins could form in Titan’s atmosphere is one thing. Proving that they’re there is another.

What sort of chemistry is happening over Titan is something I think we’ll learn, most likely after more robotic explorers visit the Saturn system.

We may not find life on Titan. If we do, it may be organic: but wouldn’t quite be “life as we know it.”

Scientists have worked out hypothetical biochemistries using Titan’s liquid hydrocarbons the way we use water.

The critters would breathe H2 instead of O2 and acetylene instead of glucose, producing methane instead of carbon dioxide. It’s far from impossible. Earth’s methanogens have similar metabolisms.

If we do find critters living on Titan, one of the big questions will be how they got there.

There’s a chance that microbes hitched a ride in the Huygens lander.

Apparently COSPAR, the Committee on Space Research, decided that Titan was so cold that we didn’t need to be particularly careful.

Life as we know it wouldn’t have evolved there, probably. COSPAR said the moon’s freezing temperatures would kill microorganisms from Earth. That was the idea, anyway.

We’ve learned a bit more about extremophiles since then. I expect some lively scientific debates, if we find life on Titan.

If that happens, I expect even louder debates along much less scientific lines.

Some folks may decide that life can’t be on Titan because it’s not in the Bible. Others may say that God can’t exist because we found critters on Titan. We’ve been through this sort of thing before.

Looking at how the post-Darwin ‘science versus religion’ brouhaha developed, my guess is that ‘Bible-believers’ will inadvertently lend credence to the “God doesn’t exist’ folks.

I don’t see the ‘are we alone’ question that way, and I’ll get back to that.

Answers and New Questions

(From NASA/JPL-Caltech/ASI/Cornell, used w/o permission.)
(Ligeia Mare, second largest body of liquid on Titan, near the north pole.)

Titan’s seas, lakes and rivers look much like Earth’s; at least in images from Cassini’s mapping sensors.

That’s natural enough, since they’re probably formed by similar processes. Either that, or some other process just happens to produce something like Earth’s drainage networks.

Earth’s rivers hold water. Something else fills Titan’s.

Titan is so cold that water is a mineral, at least on the surface. The ‘water’ there would be liquid hydrocarbons: methane or a methane-ethane mix, probably.

Whatever Titan’s solid surface is made of, it’s almost certainly not the sort of rock we see on Earth. Scientists have several ideas about what it is, but right now that’s another unanswered question.

Titan may have liquid water. It could be a water-ammonia mix, deep underground.

We’re more certain about Saturn’s moon Enceladus, and Jupiter’s Europa and Ganymede. Scientists found strong evidence of liquid water on these moons.

More accurately, in the moons. This was a surprise, raising new questions.

Liquid water doesn’t necessarily mean life. But it’s getting increasingly difficult to rule out that possibility in Enceladus, Europa, and Ganymede.

Many New Questions


(From NASA/JPL, used w/o permission.)
(“This graphic summarizes Cassini’s 13 years orbiting Saturn, with moon flybys grouped into columns for each phase of the mission. The Grand Finale orbits appear as Saturn flybys in 2017. This list of icy satellite flybys includes three additional close encounters (Phoebe, Helene and Epimetheus) not included in the official tally of 12 ‘targeted’ flybys. At bottom, Saturn’s northern hemisphere seasons are indicated from 2004 to 2017.”
(NASA/JPL))

Scientists looked forward to learning quite a bit during the Cassini-Huygens mission.

Instruments on the spacecraft were designed to help them learn more about Saturn’s rings, the planet’s atmosphere down to cloud level and its magnetosphere.

They’d be looking at Saturn’s moons, too. For one thing, we knew that Iapetus was much darker on its leading hemisphere, but not what the surface on that side was.

We’d known that Titan has an atmosphere, but not much about it and even less about its surface.

We got answers, learning that subsurface oceans might support life: which raises more questions. Lots more questions.

We also got weather data for very roughly half of a Saturnian year.

The NASA/JPL website says more about what we’ve learned, and are learning:


God’s Executive Decisions

Thinking that God gave us brains and isn’t offended when we use them makes sense to me. But being comfortable with both faith and science is a tad countercultural at the moment, so I’ll explain why knowledge doesn’t threaten my faith. Again.

We’ve known that Earth is roughly spherical for a long time.

European scholars of the 11th century didn’t think Earth is flat. Some of them were, however, overly-enthusiastic about Aristotle.

The idea that Earth might not be the only world was discussed, again, around that time. Some figured that Anaximander had been on the right track, and that we’d find other worlds.

Some of Aristotle’s fans said that Earth had to be the only world: because Aristotle said so.

That’s when the Bishop of Paris stepped in. I’ve talked about 1277 and making sense before. (December 2, 2016; May 5, 2017)

Basically, God’s God, Aristotle’s not. God’s executive decisions are not subject to expert review.

That works for me. So does learning more about how this universe works.

I think God exists, and sustains this universe. (Catechism, 300310)

I think that humans are rational creatures. Because I am human, I can use by brain. Or not. We have free will. (Catechism, 311, 1704, 17301731)

Whether or not I use my brain is up to me. I’ve learned that it’s better if I think. Acting on impulse is easy, but can lead to suboptimal outcomes. Often did, in my case. And that’s yet another topic.

Truth, Beauty, and Secondary Causes

I think truth and beauty are important.

Both can point me toward God. They’re expressed in words and in the visible world: “the rational expression of the knowledge;” “the order and harmony of the cosmos,” “the greatness and beauty of created things.” (Catechism, 32, 41, 74, 2500)

I keep saying this.

Studying this world, learning what’s happened since it started, is a good idea. (Catechism, 282289)

Fearing knowledge is irrational.

As Leo XIII wrote, “truth cannot contradict truth.” (Catechism, 159, 214217; “Providentissimus Deus“)

That includes knowledge we’ve been uncovering over the last few dozen millennia.

Ancient Mesopotamians did not have all the answers. Neither do we. But we’re learning. And that’s a good thing.

I’m a Catholic, so I think faith is a willing and conscious “assent to the whole truth that God has revealed.” (Catechism, 142150)

What we think is true about how the universe works has changed. That seems to upset some folks, but not me.

I figure God gave us brains, and curiosity, and lets us figure some things out on our own.

I won’t insist on this, but I think we see God’s willingness to let us act like humans in Genesis 2:19. Naming animals isn’t what got us in trouble, and that’s yet again another topic. (July 23, 2017; March 5, 2017)

We’ve known about fire for a long time, and wondered how it works.

Empedocles figured everything was made from four basic elements: earth, air, fire, and water.

That made sense at the time, but wasn’t entirely accurate. It looks like some “elements” are more like processes or states of matter than substances. (May 12, 2017; September 9, 2016)

Phlogiston was a 15th century effort to refine our model for the “fire” element. (March 24, 2017)

We’ve learned a bit more about the physics and chemistry of fire since then.

I’d be surprised if we’ve learned everything there is not know about it. But I think we’re closer to having the full picture.

I also think that understanding God’s creation a bit better is okay.

Thinking that a fire’s light and heat involve electron transitions certainly doesn’t cause a crisis of faith.

Everything we observe reflects a facet of the Creator’s truth, according to its nature. (Catechism, 301308)

All natural processes involve secondary causes: creatures acting in knowable ways, following laws woven into this creation.

Since I believe that God creates everything, and that God is not a liar: learning about this universe gives me more reasons to admire God’s work. (Catechism, 159, 214217, 282283, 294, 341)

Looking For Life


(From NASA, via BBC News, used w/o permission.)

I am quite sure that life exists elsewhere in this universe: or not.

We may find life in the moons of Saturn and Jupiter. Or maybe we will still be looking for extraterrestrial life when our descendants have examined every planet, moon, asteroid, and comet in this galaxy.

By then, some “extraterrestrial life” will almost certainly have been planted by us. It’s hard to imagine folks settling anywhere without planting crops and raising ornamental plants.

Either way, whether or not life started elsewhere is up to God. Part of our job is looking around this universe, learning what’s here and how it works.

The best- or worst-case scenario, depending on viewpoint, is learning that we have neighbors: free-willed beings with bodies. People, but not human. (September 8, 2017)

If we do meet folks who are people, but whose “clay” is on another world,4 I’m quite sure that some of us will panic. Others will react in equally-irrational ways. Thinking takes effort, which may help explain why so many apparently don’t.

Again, I don’t know if we have neighbors or not. If we do, I’m quite sure we will learn that God’s creation is much more interesting than we imagined. (December 23, 2016)

Some of the most clear-headed thinking I’ve seen about extraterrestrial intelligence is this:

“I been readin’ ’bout how maybe they is planets peopled by folks with ad-vanced brains. On the other hand, maybe we got the most brains…maybe our intellects is the universe’s most ad-vanced. Either way, it’s a mighty soberin’ thought.”
(Porky Pine, in Walt Kelly’s Pogo (June 20, 1959) via Wikiquote

More, mostly about life, the universe, and being human:


1 The Saturn system, briefly:

2 Cassini-Huygens:

3 Titan and life:

4 About clay, creation, and all that:

About Brian H. Gill

I'm a sixty-something married guy with six kids, four surviving, in a small central Minnesota town. I mostly write and make digital art. I'm only interested in three things: that which exists within the universe; that which exists beyond; and that which might exist.
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2 Responses to Cassini-Huygens Mission

  1. irishbrigid says:

    Missing article: “about moons of Jupiter, Saturn, and the other outer planets.”

    Repetition: “Europa’s cracks, which it doesn’t have. Which it doesn’t.”

    Extra word: “that I the town I live in exists.”

    The Friendly Neighborhood Proofreader

Thanks for taking time to comment!