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Cassini's awesomeness fully funded through mission's dramatic end in 2017

Posted by Emily Lakdawalla

03-09-2014 19:54 CDT

Topics: Cassini, Titan, mission status, Saturn's moons, Saturn

Last year, rumors swirled that NASA may be so pinched for dollars that the agency might end the Cassini mission early. Today, Cassini received the welcome news (PDF) that it has formally been funded through the planned end of its extended-extended mission in 2017, to the tune of about $60 million per year. Not only that, but they were the highest-rated mission of the seven being considered for extended-mission funding. A huge congratulations to the Cassini mission and all its team members for the assurance that you'll get to perform the awesome science you've promised to us!

Cassini-Huygens at Saturn


Cassini-Huygens at Saturn

What exactly is Cassini going to do during its final three years? NASA doesn't usually extend missions to do more of the same of what the missions already did. To justify extended missions, NASA requires teams to identify new scientific questions, and explain how their aging spacecraft can answer those questions. In effect, the teams are being asked to propose new missions. It makes sense, because the amount of money we're talking about here is not insignificant: all told, it will require a bit more than $200 million to operate Cassini for its final three-and-a-little-bit years (including the final archiving of its data). That is almost half the cost of a Discovery-class mission, and in an era of limited funds, that comparison is important. We love our active missions, but love isn't a good enough reason to fund them: can we get more science out of an old spacecraft than we could out of building a new one?

The Cassini mission answered that question with a resounding "yes." Here is a list of some of the great, new and different science they plan to do in Cassini's final years.

Observe Saturn's atmosphere and magnetosphere at exactly the same time as another spacecraft observes Jupiter's atmosphere and magnetosphere. Having the flagship Cassini mission observing Saturn at the same time that the New Frontiers mission Juno is orbiting Jupiter is going to multiply the value of the results from both missions. By observing how the two giant planets respond in their different ways at different seasons to the same solar activity, we'll get a better chance to form a more general understanding of how giant planets interact with stars. Honestly, this alone could justify the expense of Cassini's extended mission. The rest is just gravy.


NASA / JPL-Caltech

Launched from Earth in 2011, the Juno spacecraft will arrive at Jupiter in 2016 to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, coming only 5,000 kilometers from the cloud tops at closest approach.

Observe seasons never before witnessed on Saturn and its moons. Cassini's prime and extended missions included one quarter of a Saturn year; the spacecraft witnessed late winter through the arrival of spring to the north pole, and the opposite seasons in the south. Both the Voyager and Pioneer flybys took place at similar season.

Saturn's seasons

Ralph Lorenz

Saturn's seasons
This diagram shows the main events of Saturn's year, and where in the Saturnian year the Voyager 1 and Cassini missions occurred.

By surviving past the summer solstice at Saturn's north pole in May of 2017, Cassini will be the first spacecraft ever to witness a solstice at Saturn, and will have seen nearly half a Saturn year -- meaning that it will have witnessed every possible season at Saturn (because while it's summer in the north, it's winter in the south). Jupiter doesn't have seasons like Saturn; this is our last chance to see seasonal evolution on a giant planet up close until a future mission is launched to Saturn, Uranus, or Neptune. Seasonal shifts in lighting also drive weather and rain on Titan. In fact, that leads us to another science goal:

Study the summer season at the north pole of Titan. So far, Cassini  has mostly studied Titan's lake district in winter polar night, with radar. The recent arrival of spring has made the lakes increasingly visible to the cameras. Cassini has just witnessed its first-ever clouds at the north pole of Titan, a possible beginning to exciting new summer weather. Will there be more storms? Will the lakes shrink in the summer sun? We'll have to watch and see! Cassini has also recently developed a new technique to measure the depths of Titan's lakes, but only just deployed that on Titan's largest lake, Kraken Mare, and plans to test depths of the other lakes to see if they are all interconnected.

Summer clouds drift across Titan's north pole


Summer clouds drift across Titan's north pole
On July 21, 2014, Cassini spied a streak of methane clouds over the large methane sea known as Ligeia Mare. These are the first clouds Cassini scientists have spotted on Titan since a large storm in 2010. They planned to continue to watch to see if the clouds' appearance signals the beginning of summer weather patterns, or if it is an isolated occurrence.

Watch to see how Enceladus' geysers change with time. During its prime and extended missions, Cassini discovered Enceladus' plumes, and then showed that the plumes vary with time as Saturn's gravity squeezes and pulls at the shape of the moon. In the final three years, Cassini will have three ultra-close flybys of Enceladus, using several of them to sample the plumes directly to see if their composition has changed. Also, armed with their new understanding of when the plumes are most active, they plan to fly through the plumes at the point in Enceladus' orbit where they're at their most active. The Enceladus flybys are on October 14 and 28 and December 19, 2015.

Map the north poles of Saturn's icy moons. With the Sun only recently risen on the north poles of everything in the Saturn system, global maps are not quite complete. There has not been an opportunity to fill in these gaps lately, because the most recent phase of Cassini's mission has been focused on the dynamics of atmosphere and rings, with Cassini in a steeply inclined orbit that afforded few opportunities for moon flybys. But beginning in March 2015, Cassini will return to the ring plane, allowing it to perform repeated Voyager-class flybys (by which I mean flybys of 10,000 to 100,000 kilometers distance) of most of the moons, closing up the last gaps in imaging. This is particularly important for Enceladus, where we need to understand why the north pole behaves so differently from the south pole. Some of these flybys will be the closest-ever of some of Saturn's smaller moons. There are two extremely close flybys of Dione on June 16 and August 17, wrapping up work on surveying its gravity. Some of this is "more of the same" type science as in earlier parts of the mission, but with so many targets in the Saturn system, there is a lot of real estate on the mid-sized and smaller moons that is not yet thoroughly explored.

Focus on Saturn's ring "propellers" to help us understand what they can tell us about how planetary systems form. Cassini discovered these "propeller" objects in Saturn's rings, litttle mass concentrations that migrate and shift in ways we don't yet understand. In general, the rings are an ever-changing system. Figuring out why they behave the way they do could help us understand how our solar system transitioned from a disk of disorganized material to nucleating planets.



Giant "propeller" in the A ring
"Propeller" features were first spotted in close-up images of the ring taken by Cassini during its orbit insertion. Scientists determined that they indicate the presence of isolated 100-meter-size objects within the rings -- bodies intermediate in size between things that are named as moons and the innumerable individual particles that make up the rings themselves. This propeller, spotted just after equinox on August 13, 2009, is very bright because it sticks up above the ring plane at a time when the sunlight was coming in at a low angle, resulting in dim illumination of the rings.

But after all of this comes the pièce de résistance. Cassini must end its mission by deliberately plunging into Saturn's atmosphere, thus preventing it from impacting the intriguing prebiotic worlds of Enceladus or Titan. They'll do that by dropping the close-approach point on its elliptical orbit, the periapsis, until the periapsis intersects Saturn. But on the way down they are going to have two very exciting mission phases, each of which, Cassini scientists have told me, is like a Discovery-class mission on its own.

On November 29, 2016, Cassini's periapsis will drop to a position only 10,000 kilometers beyond the F ring. They'll be so close to the rings that they'll be able to do radar backscatter measurements on them for the first time. The in-situ instruments will sample much more ring material, getting direct measurements of its composition. And they'll do sensitive measurements of the mass of Saturn and its rings.

But it gets better: on April 22, 2017, the final Titan flyby will abruptly change Cassini's periapse distance to fall in between Saturn's innermost D ring and the planet. Cassini will travel less than 4000 kilometers above the tops of the clouds. This part of the mission used to be called the "proximal orbits" but has now been renamed the "Grand Finale." The death-defying plunge past the cloud tops is awesome, to be sure, but what's the science? The combination of the Grand Finale and the F-ring orbits is the key. By orbiting very close to Saturn first outside, and then inside the rings, they will, for the first time ever, be able to measure the mass of Saturn's rings, and separate the gravitational effects of Saturn from its rings. We'll learn -- as Juno will be doing at the same time -- about the internal structure of Saturn. The gravity and magnetic field measurements Cassini will get at this time may finally tell us what Saturn's rotation rate is (something we still don't know to great precision).

Cassini's F ring and proximal (


Cassini's F ring and proximal ("Grand Finale") orbits
Near the end of its mission, Cassini will reduce its periapsis to come close to the F ring for 20 orbits (green) and then dive between Saturn and the D ring (blue) in the F ring and "Grand Finale" phases of the mission. Periapse happens at local noon on Saturn, providing the best possible gravity measurements, in order for Cassini to study the mass of the rings and the internal structure of Saturn, much as Juno will be doing at Jupiter at the same time.

When Cassini finally flies low enough to fall in to Saturn's atmosphere on September 15, 2017, it will be a day not to mourn it, but rather to celebrate its achievements. The day for mourning will come a month or few later, when Juno's mission likewise comes to an end. On that day, for the first time since the 1970s, Earth will have no active missions exploring any of the giant planets. There won't even be a mission on the way. The Voyagers and New Horizons will (hopefully) still be active way beyond Neptune, but Jupiter, Saturn, Uranus, and Neptune will only be visible through telescopes on Earth. This is the part where I should tell you to write to your representatives to support space exploration, and to support The Planetary Society's efforts to advocate for more funding for planetary science. But even that won't prevent the hiatus; it takes many years to develop an outer planets mission, and many more years to cruise to a target.

That's one reason why it's such important news that Cassini has won funding through the end of its planned mission. It's worth every penny, and more. Congratulations again to the Cassini team, and thanks for keeping the light on in the outer solar system for as long as you can.

See other posts from September 2014


Or read more blog entries about: Cassini, Titan, mission status, Saturn's moons, Saturn


Adolf Schaller: 09/04/2014 12:42 CDT

"The greatest hazard to space exploration turns out to be NASA's cost constraints." (From the Washington Post) - that adequately describes it in the nutshell. Not to belabor the obvious, but the problem is the WHY behind those 'cost constraints' - and more to the point, how those 'constraints' are characterized. And when we learn about how NASA science advisory panels come to judge how missions are being conducted (check Casey Drier's report here: we are left to wonder whether the people within the NASA community are actually all on the same page. Bureaucracy never looked more spectacularly colorful in all its prismatic glory...and very few have the fortitude to point out that this state of affairs cannot be sustained. I must thank you Emily and Casey and the Planetary Society for getting that most important word out.

Adolf Schaller: 09/04/2014 01:34 CDT

Per the NASA 'science' panel that judged Curiosity, may I humbly submit this line from the Washington Post article in illustration of their grave error: "Science can veer into esoteric territory." That is spot on. You send a machine out to explore another world, you better expect to alter your previous expectations and be prepared to figure out what is going on there as you learn more. If a machine designed to drive on the surface of another planet encounters anything not previously anticipated by science, it is obviously up to mission scientists to figure out how to explore it. It is NOT within the province of some 'science panel' to second-guess what that mission is in actual fact busy trying to accomplish. I am appalled that this NASA gives any weight to this so-called science panel. They get miffed because the mission chief is busy doing the science they declare isn't being done? They are annoyed because he doesn't show up personally to validate his reason for being? And then they characterize the mission as deficient in science return and suggesting that the rover should NOT ACCOMPLISH ITS GOAL??? What's their problem? Man oh man, there's an awful lot worse wrong in NASA than what the NASA officials dream.

Adolf Schaller: 09/04/2014 01:40 CDT

pardon the opening sentence of the seventh paragraph - I meant to say: I am appalled that NASA gives any weight to this so-called science panel.

kenmay139: 09/04/2014 02:02 CDT

Will the Cassini spacecraft be visiting the any of the Moon bases and Mars colonies? Oh, that's right there aren't any yet. And what century is this? The 21st century? What happened to the future we were promised? Was it stolen or was it a lie?

Adolf Schaller: 09/04/2014 02:38 CDT

@kenmay: you should start to learn about our solar system of planets orbiting around a star which we call our 'sun' and what spacecraft are as our remote extension of our human senses in our effort to understand our greater natural environment.

Adolf Schaller: 09/04/2014 02:45 CDT

@kenmay: and by the way: nature never lies. Science, a method by which we humble humans strive to understand nature, makes the best job of understanding it.

Arbitrary: 09/04/2014 04:50 CDT

I wonder if the Deep Space Network is becoming a bottle neck and a reason for this discussion about terminating missions early. Now and next year must have a record number of active interplanetary probes.

Anonymous: 09/04/2014 06:31 CDT

Hopefully, sometime in 2016, they'll figure out how to extend the mission to 2048....

Rochus Boerner: 09/04/2014 09:27 CDT

There is at least one ray of hope. In the early 2020s, several thirty-meter class telescopes should see first light, such as TMT and E-ELT. At the diffraction limit and 550 nm wavelength, E-ELT would resolve roughly 10 km at Jupiter distance, 20km at Saturn, 43km at Uranus and 73km at Neptune. I don't know how close to the diffraction limit the new telescopes will get, but it's safe to assume that the view from Earth will get considerably better compared to Keck.

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