Saturn as imaged from above by Cassini last year. Over the next five months, the spacecraft will orbit closer and closer to the planet and will finally plunge into its atmosphere. (NASA)

Seldom has the planned end of a NASA mission brought so much expectation and scientific high drama.

The Cassini mission to Saturn has already been a huge success, sending back iconic images and breakthrough science of the planet and its system.  Included in the haul have been the discovery of plumes of water vapor spurting from the moon Encedalus and the detection of liquid methane seas on Titan.  But as members of the Cassini science team tell it, the end of the 13-year mission at Saturn may well be its most scientifically productive time.

Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) put it this way: “Cassini will make some of its most extraordinary observations at the end of its long life.”

This news was first announced last week, but I thought it would be useful to go back to the story to learn more about what “extraordinary” science might be coming our way, with the help of Spilker and NASA headquarters Cassini program scientist Curt Niebur.

And the very up close encounters with Saturn’s rings and its upper atmosphere — where Cassini is expected to ultimately lose contact with Earth — certainly do offer a trove of scientific riches about the basic composition and workings of the planet, as well as the long-debated age and origin of the rings.  What’s more, everything we learn about Saturn will have implications for, and offer insights into, the vast menagerie of  gas giant exoplanets out there.

“The science potential here is just huge,” Niebur told me.  “I could easily conceive of a billion dollar mission for the science we’ll get from the grand finale alone.”


The Cassini spacecraft will make 22 increasingly tight orbits of Saturn before it disappears into the planet’s atmosphere in mid-September, as shown in this artist rendering.  (NASA/JPL-Caltech)


The 20-year, $3.26 billion Cassini mission, a collaboration of NASA, the European Space Agency and the Italian Space Agency,  is coming to an end because the spacecraft will soon run out of fuel.  The agency could have just waited for that moment and let the spacecraft drift off into space, but decided instead on the taking the big plunge.

This was considered a better choice not only because of those expected scientific returns, but also because letting the dead spacecraft drift meant that theoretically it could be pulled towards Titan or Enceladus — moons that researchers now believe just might support life.

Because the spacecraft wasn’t sterilized before launch, scientists didn’t want to take the chance that it might carry some earthly bacteria that could possibly contaminate the moons with our life.

So instead Cassini will be sent on 22 closer and closer passes around Saturn, into the region between the innermost ring and the atmosphere where no spacecraft has ever gone.  On April 26, Cassini will make the first of those dives through a 1,500-mile-wide  gap between Saturn and its rings as part of the mission’s grand finale.

As it makes those terminal orbits, the spacecraft will have to be maneuvered with precision so it doesn’t actually fly into one of the rings.  They consist of water ice, small meteorites and dust, and are sufficiently dense to fatally damage Cassini.

“Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft. But we’re also being cautious by using our large antenna as a shield on the first pass, as we determine whether it’s safe to expose the science instruments to that environment on future passes,” said Earl Maize, Cassini project manager at the NASA Jet Propulsion Lab. “Certainly there are some unknowns, but that’s one of the reasons we’re doing this kind of daring exploration at the end of the mission.”

Then in mid-September, following a distant encounter with Titan and its gravity, the spacecraft’s path will be bent so that it dives into the planet itself.  The final descent will occur in mid September, when Cassini enters the atmosphere where it will soon begin to spin and tumble, lose radio contact with Earth, and then ultimately explode due to pressures created by the enormous planet.

All the while it will be taking pioneering measurements, and sending back images predicted to be spectacular.


The age and origin of the rings of Saturn remains a subject of a great debate that may soon come to an end. Ring particle sizes range from tiny, dust-sized icy grains to a few particles as large as mountains. Two tiny moons orbit in gaps (Encke and Keeler gaps) in the rings and keep the gaps open. (NASA)


While the Cassini team has to keep clear of the rings, the spacecraft is expected to get close enough to most likely answer one of the most long-debated questions about Saturn:  how old are those grand features, unique in our solar system?

One school of thought says they date from the earliest formation of the planet, some 4.6 billion years ago.  In other words, they’ve been there as long as the planet has been there.

But another school says they are a potentially much newer addition.  They could potentially be the result of the break-up of a moon (of which Saturn has 53-plus) or a comet, or perhaps of several moons at different times.  In this scenario, Saturn may have been ring-less for eons.

As Niebur explained it, the key to dating the rings is a close view of, essentially, how dirty they are.  Because small meteorites and dust are a ubiquitous feature of space, the rings would have significantly more mass if they have been there 4.6 billion years.  But if they are determined to be relatively clean, then the age is likely younger, and perhaps much younger.

“Space is a very dirty place, with dust and micro-meteorites hitting everything.  Over significant time scales this stuff coats things.  So if the rings the rings are old, we should find very dirty ice.  If there is little covering of the ice, then the rings must be young.  We may well be coming to the end of a great debate.”

A corollary of the question of the age of Saturn’s rings is, naturally, how stable they are.

Curt Neibur, lead program scientist at NASA headquarters for the Cassini mission. (NASA)

If they turn out to be as old as the planet, then they are certainly very stable.  But if they are not old, then it is entirely plausible that they could be a passing phenomenon and will some day disappear — to perhaps re-appear after another moon is shattered or comet arrives.

Another way of looking at the rings is that they may well have been formed at different times.

As Cassini Project Scientist Linda Spilker explained in an email, Cassini’s measurements of the mass of the rings will be key.  “More massive rings could be as old as Saturn itself while less massive rings must be young.  Perhaps a moon or comet got too close and was torn apart by Saturn’s gravity.”

The voyage between the rings will also potentially provide some new insights into the workings of the disks present at the formation of all solar systems.

“The rings can teach us about the physics of disks, which are huge rings floating majestically and with synchronicity  around the new sun,” Niebur said.  “That said, the rings of Saturn have a very active regime, with particles and meteorites and micrometeorites smacking into each other.  It’s an amazing environment and has direct relevance to the nebular model of planetary formation.”


This recently released Cassini image show’s moon Daphnis, which is embedded within a ring.  The moon
kicks up waves as it orbits within what is called the Keeler gap. This mosaic combines several previous images to show more waves in the gap edges. (NASA/JPL-Caltech)


Another open question that scientists hope will be answered during the plunge is how long, precisely, is a day on Saturn.

The saturnine day is often given as between 10.5 and 11 hours, but that lack of precision is unique in our solar system.

The usual way to determine a planet’s rotation is to look for a distinctive point and watch to see how long it takes to reappear.   But Saturn has thousands of miles of thick clouds between the rings and the core, and so no distinctive points have been found.

The planet’s inner rocky core and outer core of metallic hydrogen create magnetic fields that potentially could be traced to measure a full rotation. But competing magnetic fields in the complex Saturn ring and moon system make that also difficult.

“The truth is that we don’t know how long a day is on Saturn,” Niebur said.  “But after the finale, we will finally know.”

The answer will hopefully come by measuring the expected “wobble” of the magnetic field inside the rings. Since Cassini will pass beyond the magnetic interference of those rings, the probe should get the most precise magnetic readings ever taken.

Project scientist Spilker is optimistic.  “With the magnetic field we’ll be able to get, for the first time, the length of day for the interior of Saturn. If there’s just a slight tilt to the magnetic field, then it will wobble around and give us the length of a day.”

Artist rendering of Cassini over Saturn’s north pole, with it huge hexagon-shaped storm. (NASA/JPL-Caltech)

Perhaps the most consequential findings to come out of the Cassini finale are expected to involve the planet’s internal structure and composition.

The atmosphere is known to contain hydrogen, helium, ammonia and methane, but Niebur said that other important trace elements are expected to be present.  The probe will use its mass spectrometer to “taste” the chemistry of the gases on the outermost edge of Saturn’s atmosphere and return the most detailed information ever about Saturn’s high-altitude clouds, as well as about the ring material.

Instruments will also measure Saturn’s powerful winds (which blow up to 1,000 miles an hour), and determine how deep they go in the atmosphere.  Like much about Saturn, that basic fact falls in the “unknown” category.

For both Spilker and Niebur, the biggest prize is probably determining the size and mass of Saturn’s rocky core, made up largely of iron and nickel.  That core is estimated to be 9 to 22 times the mass of the Earth, and to have a diameter of perhaps 18,000 miles. 

Cassini project scientist Linda Spilker of JPL was on the Voyager team in the 1970s. She has a long-standing research interest in Saturn’s rings. (Bill Youngblood, Caltech)

But these are broad estimates, and neither the size nor mass is really known.  Those thousands of miles of thick clouds atop the atmosphere and the planet’s chaotic magnetic fields have made the necessary readings impossible.

The Cassini instruments, however, are expected to make those measurements during its final months.  As Cassini makes its close-in passes and then enters the atmosphere for the final plunge, it will send back the data needed to make detailed maps of Saturn’s inner magnetic and gravitational fields.  These are what scientists need to understand the core and other structures that lay beneath the planet’s atmosphere.

This work will compliment the parallel efforts underway at Jupiter, where the Juno mission is collecting data on that planet’s core as well.  If scientists can measure the sizes and masses of both cores, they will be able to use that new information to answer many other questions about our solar system and beyond.

“A better understanding Saturn’s interior, coupled with what Juno mission learns about the interior of Jupiter, will lead to (new insights into) how the planets in our solar system formed, and how our solar system itself formed,”  Spilker said in an email.

“This is then related to how exoplanets form around other stars.  Studying our own giant planets will help us understand giant planets around other stars.”

In other words, Saturn and Jupiter are planetary types expected to be found across the galaxies.  And it’s our good fortune to be able to touch and learn from them, and to use that information to analyze distant planets that we can only indirectly detect or just barely see.


An animated video about Cassini’s final chapter is available here.