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Saturn: ‘Grazing’ the Rings

What the Jet Propulsion Laboratory refers to as ‘the first phase of the mission’s dramatic endgame’ begins tomorrow for the Cassini Saturn orbiter. Having given us an ocean within Enceladus and numerous images of Titan’s lakes and seas (not to mention ring imagery of spectacular beauty), Cassini now enters a phase in which it encounters the rings in a new way, diving past their outer edge every seven days in a series of 20 passes. The spacecraft will be in an elliptical orbit inclined some 60 degrees from the planet’s ring plane.

“We’re calling this phase of the mission Cassini’s Ring-Grazing Orbits, because we’ll be skimming past the outer edge of the rings,” said Linda Spilker, Cassini project scientist (JPL). “In addition, we have two instruments that can sample particles and gases as we cross the ring plane, so in a sense Cassini is also ‘grazing’ on the rings.”


Image: Cassini crosses Saturn’s F ring once on each of its 20 Ring-Grazing Orbits, shown here in tan and lasting from late November 2016 to April 2017. Blue represents the extended solstice mission orbits, which precede the ring-grazing phase. Credit: JPL.

That grazing will include two passes directly through a tenuous ring created by meteor strikes on the small moons Janus and Epimetheus. Each orbit will cross the ring plane just outside the F ring, considered to be the boundary of the main ring system, with Cassini actually moving through the outer edges of the F ring in April. Here the science should be particularly interesting — the 800-kilometer wide F ring is malleable, developing and dispersing filament-like structures, dark channels and streamers over short periods of time.

“Even though we’re flying closer to the F ring than we ever have, we’ll still be more than 4,850 miles (7,800 kilometers) distant. There’s very little concern over dust hazard at that range,” said Earl Maize, Cassini project manager at JPL.

A gravity assist by Titan on the T-125 flyby put the craft into its ring-grazing orbits. The new orbits (‘revs’ or revolutions in JPL parlance) begin when the spacecraft is at apoapse, its most distant position from Saturn, and the ring plane crossings happen when Cassini is at periapse, its closest approach to the planet during that orbit. The first ring-grazing orbit beginning on November 30 will see the ring plane crossing 5 days later. A final Titan flyby (T-126) in April sets up the last phase of the orbiter’s mission.

The new orbital configuration will allow Cassini to make close studies of the A and B rings as well as the F at a high level of detail. The A ring’s so-called ‘propellers’ — features that mark the location of tiny moonlets — should be seen in the best detail yet, offering researchers the chance to examine their structure. We’ll begin to see images from this phase of the mission in December as the spacecraft resolves details smaller than 1 kilometer per pixel.


Image: Saturn’s rings were named alphabetically in the order they were discovered. The narrow F ring marks the outer boundary of the main ring system. Credit: NASA/JPL-Caltech/Space Science Institute.

The distances involved in this phase of the mission are worth noting. The new orbits will take the craft within 90,000 kilometers of the planet’s cloud tops, but the Grand Finale phase, scheduled to begin next April, closes to within 1628 kilometers. This should be breathtaking, for the craft will move again and again through the gap between Saturn and the rings before making its final plunge into the atmosphere on September 15. Preparations for this final phase begin with a main engine burn on December 4. This is an engine that has served us well — this will be its 183rd burn — but the remainder of the mission will be handled with thrusters.

The spacecraft will be making a nine-hour movie of Saturn’s north pole with its Visual and Infrared Mapping Spectrometer, while also measuring (with other instruments) the boundaries of the planet’s upper atmosphere, which will be directly sampled in later orbits. You can find a complete list of Cassini’s ring-grazing orbits on this JPL reference page. Bear in mind as you look at it that there is a wonderful symmetry here. The level of detail we’ll see on these final orbits will be rivaled only by what Cassini saw upon its arrival at Saturn in 2004.


Comments on this entry are closed.

  • Michael Spencer November 30, 2016, 7:53

    Can anyone explain the rationale for destroying this lovely bird by plunging into Saturn? Why not put her into a very wide orbit, leaving the instruments run as long as possible? Galileo, too, was destroyed, I suppose for the same reasons?

    • Paul Gilster November 30, 2016, 9:47

      Planetary protection is the rationale — i.e., make sure Cassini doesn’t one day impact on a world we’ll eventually be studying up close in terms of astrobiology.

      • Michael Spencer November 30, 2016, 11:46

        I understand that, Paul – but seriously, isn’t this a case of going too far? Microbes on the spacecraft making a comfy home on Europa is a bit of a stretch.

        Too, even a parking orbit will decay at some point, plunging the craft into the planet – or (there’s an infinitesimal chance of hitting a moon, one supposes – but with the superb ability to change planes, surely an orbit and plane could be found that brings the chance to near zero.

        I was hoping that an actual planetary scientist would reply ( no offense to our genial host!). Is crashing into Saturn very close to silly, or not?

        In short: NASA is famous for calculations reducing catastrophe to odds. What are the odds?

        • ljk November 30, 2016, 13:02

          Michael Spencer said on November 30, 2016 at 11:46:

          “I understand that, Paul – but seriously, isn’t this a case of going too far? Microbes on the spacecraft making a comfy home on Europa is a bit of a stretch.”

          How so? Europa and Enceladus have LOTS of liquid water, which is ideal for life as we know it. And there have already been experiments showing that microbes can survive the extremes of direct exposure to space for long periods of time, so an alien moon with a global ocean of liquid water would be paradise in comparison.

          You may find this site useful:


        • DJ Kaplan November 30, 2016, 13:14

          You gotta love science. Some day, in say 2025, a probe lands on Europa, discovers microbes and instantly doubt sets in because Cassini had crash-landed there. Prudent habits need no excuse.

          • Michael Spencer December 1, 2016, 7:32

            I meant to say that any Earth microbes would be subject to strong (and deadly?) radiation on the surface, even if Cassini ever did actually crash into Europa.

            But point taken, and thank you both.

            • ljk December 1, 2016, 11:09

              Some microbes just love high doses of radiation:


              Then there are the ones that do just fine in boiling hot springs, acidic pools, under miles of solid rock, and so forth.

            • Moebius December 1, 2016, 11:43

              Cassini won’t crash on Europa (muuuch too far) but more likely on Enceladus, and the radiation environment around Saturn is not as strong as around Jupiter.

              However if crashing an interplanetary probe can contaminate a moon, that also means that if we send a future probe and that it discovers dead microbes or organic chemicals on that moon, the second probe can also have brought them along, so which one is guilty, the crashed probe or the working one?

          • ljk December 1, 2016, 10:10

            This will also bring relief to those who thought that Cassini’s RTGs were the biggest threat to Earth ever. Some of them even thought there was a chance the probe would somehow be flung out of orbit around Saturn and come back home!


            I had trouble taking one particular protestor seriously when he said Cassini should be smashed into Venus – because that planet had NO atmosphere and therefore no life forms to worry about.

        • Moebius December 1, 2016, 11:26

          Europa is a moon of Jupiter, not of Saturn, just saying. So we should be more concerned about Cassini crashing into Dione, Titan or Enceladus than on Europa, even an accidental close encounter with Saturn or one of its moons has infinitesimal chances of slingshooting it back to Jupiter’s system.

  • J. Jason Wentworth November 30, 2016, 10:47

    I’ve always heard–and seen in print–the words for the points of closest and most distant, respectively, approach to a body (when in an orbit around that body) as “periapsis” and “apoapsis,” *not* ‘periapse’ and ‘apoapse.’ They are “generic” to all bodies (including ones whose names would be difficult to “mold” into those terms). Thus, while perigee and apogee refer to orbits around the Earth, and perilune (or pericynthion) and apolune (or apocynthion–take your pick :-) ) refer to orbits around the Moon, periapsis and apoapsis refer to orbits around *any* star, planet, moon, asteroid, or comet. Even in open (escape) orbits, such as the hyperbolic (or very unlikely, parabolic) trajectories of flyby spacecraft past planets, periapsis–the point of closest approach–is still used (but not apoapsis, for the obvious reason that it never stops increasing). In fact, I first read the term “periapsis” in reference to Pioneer 10’s flyby of Jupiter (the Jupiter-specific terms perijove and apojove were used in reports about the Galileo mission). Also:

    It’s a shame that Cassini’s imaging system can’t take clear pictures of close-by ring particles (even if the spacecraft’s velocity didn’t prevent it, the systems’s inability to take in-focus images of even relatively still objects closer than about 30 miles away–the closest picture Cassini could accomplish with its by-then starlike, receding Huygens probe–is a fundamental obstacle). Hopefully, a future probe can investigate the rings from close range as well as within the rings, as such close-up imaging and multi-spectral analysis would teach us much more about the different (particularly differently-colored) rings, as well as examine the super-smooth ellipsoidal moon Methone, which orbits just beyond the outermost rings.

  • Horatio Trobinson November 30, 2016, 17:26

    It’s protection, making sure Cassini doesn’t end up littering a place like Enceladus, for example.

    But couldn’t this also be avoided altering Cassini’s final orbit in such a way that Saturn slingshots it in a trajectory perpendicular to the plane of the ecliptic?

    The second reason for doing it this way is to make Cassini tell us more about Saturn’s atmosphere is it takes its last breath.

    • J. Jason Wentworth December 1, 2016, 6:01

      Are you referring to having Saturn’s gravity (perhaps also with the help of a “set-up” encounter with Titan first) hurl Cassini into an out-of-ecliptic orbit around the Sun? If so, Cassini doesn’t have access to the amount of energy that would be required to enter such a solar orbit. It *could* have done it if it had approached Saturn the way that Pioneer 11 flew by Jupiter in 1974 (the Ulysses high-inclination solar orbiting probe also did this at Jupiter), aiming for a point below (or above) one of Saturn’s poles and letting the planet’s powerful gravity sling it north (or south) out of the ecliptic. But once Cassini had spent much of its high approach velocity of falling toward Saturn by rocket-braking into a closed orbit, it could no longer escape into a very inclined solar orbit (escaping from its closed Saturn-centric orbit into a low-inclination solar orbit with the help of encounters with Titan–and other Saturnian moons–might very well have been possible, but probably not anymore, since Cassini is now low on propellant. But:

      If you’re referring to putting Cassini into a highly-inclined, out-of-ecliptic “graveyard orbit” around Saturn (whose apoapsis and periapsis would be well clear of any moons, particularly Enceladus), that might be impossible due to orbital dynamics and Cassini’s current propellant situation. Titan has already been used to generate such high-inclination orbits, but the periapsis points (periapses?) of such orbits will always be near Titan’s orbit, because that is the injection point of such orbits.

      If Cassini had more onboard propellant than it does now, it could raise the periapsis of such an orbit much farther from Saturn by making a “circularization burn” at apoapsis (just as geosynchronous Earth satellites do to convert their highly-eccentric, elliptical transfer orbits to nearly-circular orbits about 22,300 miles high (in Saturn’s powerful gravitational field, Cassini–even soon after arrival–might not have been able to reach a circular high-inclination orbit, but only a low-eccentricity elliptical one whose periapsis and apoapsis lay well between the most distant prograde-orbiting satellite (Iapetus?) and retrograde-orbiting Phoebe). But:

      Cassini probably couldn’t have spared the propellant to conduct its long mission *and* have enough to enter such a “graveyard orbit” around Saturn, while its “slow descent to destruction” program is affordable in terms of propellant and safeguards any possible life on Enceladus, while also enabling never-before-possible observations to be made. This is a “Win, Win, Win” situation (although a bittersweet one…) all around.

    • Michael Spencer December 1, 2016, 7:38

      Thank you for troubling to answer. Your point about exiting would depend on the amount of fuel remains of course. And I do take the point that there is much to be learned during the plunge.

      But at what cost! Cassini is a priceless bird – and with nothing on the horizon as replacement. Surely by now so much is known about the dynamics of Saturn’s arena that an orbit could be selected with no chance for ‘crashing’ by chance.

  • ljk December 2, 2016, 10:02

    More on Cassini’s final days, including the final Titan flyby and some nice raw images of Enceladus:


  • ljk December 6, 2016, 12:46

    The dive into the ring begins:


    When Pioneer 11 was approaching Saturn in 1979, there was a big debate among the mission team whether or not to send the space probe into the Cassini Division of the planet’s rings. At the time it was thought to be a gap between the rings. Turns out the division was not a void and was instead filled with darker material. Thankfully, Pioneer 11 flew by outside the ring system and lived to eventually become the second artificial satellite of humanity to enter the wider Milky Way galaxy.

    Let us hope Cassini does not encounter such “hidden” particles when it goes between the ring system and the planet. Or I guess that will be an indication that such objects do exist.



  • Mark Zambelli December 7, 2016, 6:52

    I like the poetic aspects of sampling Janus’ dusty ring… as the mission draws to a close I find myself looking both forward to the end and backwards at such a wealth of data and images from such a wonderful mission.

  • ljk December 7, 2016, 10:31

    Cassini’s first images from its new orbit around Saturn:


  • ljk December 7, 2016, 17:03

    Dec. 6, 2016

    Saturn’s bulging core implies moons younger than thought

    By Blaine Friedlander

    Freshly harvested data from NASA’s Cassini mission reveals that Saturn’s bulging core and twisting gravitational forces offer clues to the ages of the planet’s moons. Astronomers now believe that the ringed planet’s moons are younger than previously thought.

    “All of these Cassini mission measurements are changing our view of the Saturnian system, as it turns our old theories upside down. It takes one good spacecraft to tell us how wrong we were in the past,” said Radwan Tajeddine, Cornell research associate in astronomy and a member of the European-based Encelade (pronounced en-CELL-ad) scientific team that pored over the Cassini data and published a paper in the astronomy journal Icarus (January 2017).

    Full article here:


    To quote:

    Tajeddine explains that if Saturn moons actually formed 4.5 billion years ago, their current distances from the home planet should be greater. Thus, this new research suggests, the moons are younger than 4.5 billion years, favoring a theory that the moons formed from Saturn’s rings.

    The team also found that Saturn moon Rhea is moving away 10 times faster than the other moons, which is the first evidence that a planet’s dissipation factor can vary with its distance in relation to the moon. The scientists have no definitive explanation.

  • ljk December 13, 2016, 10:16

    The first images of Saturn from Cassini in its final mission phase prominently display the hexagon-shaped clouds in the gas giant planet’s northern hemisphere:


    To quote:

    Each ring-grazing orbit will last a week and will start with the spacecraft at its most distant position from Saturn. Crossing of the ring plane in every orbit will occur when Cassini is closest to Saturn.

    On April 22, 2017, Cassini will fly by Titan for the final time, using its gravity to make the first of 22 dives between the innermost ring and Saturn itself – a gap of about 1,500 miles (2,400 kilometers) – four days later.

    These 22 plunges will culminate in the September 15 dive into Saturn’s atmosphere. Data on the atmosphere’s composition will be returned until the last moment possible.

  • ljk December 23, 2016, 15:24

    How do we colonize Saturn’s moons, because, why not…


  • ljk March 14, 2017, 22:04

    How Pan was discovered, by the man who discovered it: