Bob Mitchell of NASA’s Jet Propulsion Laboratory is the Program Manager for the Cassini-Huygens Saturn mission, whose flight team took home this year's National Air and Space Museum Trophy for Current Achievement. Mitchell spoke to Air & Space Associate Editor Heather Goss in February. 

Air & Space: During the eight years or so that the Cassini mission has been studying the Saturn system, what has been the most thrilling discovery or biggest surprise?

Mitchell: I’d say the most thrilling discovery and the biggest surprise are probably two different things. I think the most thrilling thing was seeing the surface of Titan. This wasn’t such a surprise because we knew that we didn’t know what was there; we knew we were going to see something different, but we didn’t really have much of an idea what we were going to see. So to actually see it, I think that was the biggest thrill.

The biggest surprise had to be the plumes on [Saturn’s moon] Enceladus. We fully expected Enceladus to be another icy satellite like several others that orbit in that vicinity. So to get there and find the plumes, that was a very pleasant surprise. And we’re still working on understanding exactly what’s happening there: what it is that produces them and the effects that the material has on the rest of the Saturn system.

What else has Cassini discovered about Enceladus? NASA scientists have talked about Enceladus having “astrobiological potential.” Do you have hopes that Cassini will uncover evidence of life there?

There’s still a lot we don’t know, but we hope to learn a fair amount more before we’re done. We’ve developed some models and ideas about it. Some of the questions are: What is the energy source that drives the plumes? The scientists think that Enceladus should be too small to produce such energy. But obviously it does somehow. Another question is whether there’s a reservoir of liquid water under the surface. And, do the plumes vary with time? You might think the mechanism is something like what drives Old Faithful in Yellowstone, but every time we’ve looked for the plumes they’ve been there, always more or less the same density and height; whereas at Yellowstone, Old Faithful only goes up about once every hour. So it’s quite different from what’s happening at Enceladus.

The question about the possibility of life is always a very exciting one. We have found, at Enceladus, organic compounds. There’s obviously an energy source. And the scientists think very likely there’s liquid water beneath the surface. So with those three things, that’s everything you need for life as we know it on Earth. We haven’t found life on Enceladus, and we don’t really expect to [with Cassini] since we aren’t really instrumented or in a location to be able to do that. But Enceladus would certainly be an exciting place to go look for life.

You mentioned that getting a look at Titan’s surface was a thrilling moment. Would you describe the first glimpse from the Huygens probe?

It was so Earth-like that it was eerie. There were mountains and dry river beds filled with rocks that were polished smooth, just like in river beds here on Earth; clear evidence of erosion, weather, flowing liquids, and even smog – and by smog I mean hydrocarbon haze, but it’s made up of many of the same things as smog here on Earth. Huygens didn’t see flowing liquid, but it was very clear that there had been something flowing before. It wouldn’t have been water, because they found the temperature to be around 90 Kelvin, so water would have been very solid ice. Rather, it was liquid methane. So there’s a weather cycle that’s very much like Earth’s happening on Titan, but with liquid methane instead of water.

There had been a lot of speculation before we arrived that Titan might be covered by an ocean of liquid hydrocarbons. Huygens certainly disproved that, but the Cassini orbiter subsequently found that there are very large lakes – larger than our own Great Lakes – full of liquid hydrocarbons on Titan, mostly at the poles.

How does Cassini study Saturn’s rings, and what have you learned?

We use cameras and spectrometers, and with those we can take pictures and measure composition, measure temperatures, and study the dynamics. When the spacecraft passes behind the rings, as seen from Earth, that means the radio signal from Cassini goes through the rings, and it’s really quite impressive what the scientists can deduce about the size and density of the particles and even the composition just from how the rings affected the radio signal.

We’ve learned a lot about the dynamics: how the ring particles tend to clump under the influence of their own gravity, and then disperse and break up due to Saturn’s gravity. We’ve learned a lot about how ring particles interact with nearby moons that orbit in ring gaps, and how they shape and affect the edge of the gap from their mutual gravity. You can also see waves that propagate clear across the rings – this is an interesting phenomenon – and we can use them to study general properties of the particles, particle size distribution, and have uncovered a wealth of information that we didn’t have pre-Cassini. 

What other contributions to space science has Cassini provided?

A couple of things that we’ve done relate to the theory of relativity. One is that we looked for gravitational waves during the cruise phase from Earth to Saturn. We had radio equipment very sensitive to perturbations of the kind a gravitational wave would make. The scientists are still processing the data that we got from that, but so far they have not found anything. I’m not sure what their expectations were, but gravitational waves are, presumably, not real common occurrences, and so to be listening at exactly the right time is perhaps a bit of a long shot.

Another thing that we do fairly regularly, about once a year, is a conjunction experiment when the spacecraft is on the opposite side of the sun as seen from Earth.  When the radio signal comes from Cassini to Earth, that signal passes close by the sun, and the scientists analyze it to look for confirmation of the theory of relativity, which predicts that the signal should be affected by the sun’s gravity. And that, in fact, has worked and they have confirmed Einstein’s theory to a greater level of precision than had been done before.

We also had two Venus, one Earth, and one Jupiter flyby en route to Saturn. We didn’t do very much at Venus or Earth; we just didn’t have the resources to be able to, in addition to all the other preparation work we were doing for when we got to Saturn. And a lot of the software we would have had to have, including spacecraft flight software, was not complete and available at that time.

At Jupiter we did quite a bit: We ran a ‘full-up sequence,’ motivated partly by the dry-run opportunity to be ready to go when we got to Saturn, but we got quite a bit of scientific information at Jupiter, as well. The closest we got was about 10 million km, because that’s what the gravity assist requirement dictated. We had the Galileo spacecraft still functioning and orbiting inside of Jupiter’s magnetic field, and we had Cassini passing by outside of the magnetosphere, so the scientists were able to study how the solar wind variations as measured by Cassini affected the magnetosphere of Jupiter as measured by Galileo. That was a unique opportunity that had the scientists rather excited. And we got some global coverage of Jupiter that Galileo wasn’t able to do because of its antenna problem.

What has been the most difficult challenge in running the Cassini mission?

There really haven’t been any significant long-term challenges. It’s all gone remarkably well. The spacecraft has done excellently for us – a few glitches here and there, but nothing unexpected for a spacecraft that is this complex and operating for this long. Another part of it is that the team, which is made up of a few hundred people, is really just a great team and very capable.

Probably the single most challenging thing that I had to deal with was maintaining and defending our operating budget. We’re a rather large program, with a correspondingly large budget, so whenever NASA needed to find money, we were a very visible target. This was a bigger problem back during our cruise from Earth to Saturn because there had been, by design, a lot of development work postponed until after launch to be done during the cruise phase. And I think, particularly now in hindsight, that was a very good decision. The result of this was that we had a team that was fully on-board and trained and up-to-speed when we got to Saturn. The problem was that it was difficult to convince people of the scope of what had to be done. So I’d say preserving the budget during the cruise phase was probably the biggest challenge. But once we arrived at Saturn and we were getting science data back and everything was going remarkably well, and it was clear we were going to be successful, then things kind of let up and we’ve been adequately funded ever since.

With 16 European countries involved in the Cassini program, has that been difficult to coordinate?

I’ve been very happy with how it’s worked out. Although I should say that my role on the project started just shortly after launch; we had a different project manager for the pre-launch development phase, so I didn’t see some of the interactions there. But since launch, we’ve obviously had a lot of interactions with the Europeans.

We have about 250 or 260 scientists that were selected by NASA; about half are U.S. and half are European, and so we have a lot of interaction with them. They’re just a great group of people who get along together very well. One thing that helped is that there is no exchange of funds across the ocean, so there’s no issue of allocating money between the U.S. team members and the Europeans. Another thing that makes this not be a nationalistic issue is that the allocation of science resources – observing time, data memory allocation, data downlink time to Earth – is done among the instrument teams and not country to country. And the teams are all a mix of U.S. and European scientists, so there’s just not a distinction like that.

Can you explain ‘Solstice,’ the name of your second mission extension, and what you hope to achieve during this phase?

After the primary mission was done, which took four years, we had the Equinox mission; that was our first extension that went for 27 months. During that mission, in August 2009, the Saturn equinox crossing occurred, where the sun passed from below the equator to above the equator. At that time the sun was directly edge-on to the rings. That was a very interesting geometry for the scientists, since any little vertical displacement in the rings created a shadow, and those shadows told us a lot about the vertical structure of the rings.

For the Solstice mission, our current plan, which we have proposed to NASA and they have tentatively approved, is that Cassini will go to the northern hemisphere summer solstice that occurs in 2017. That will let us see seasonal variations, primarily on Saturn and Titan. Saturn’s seasonal variations get magnified by the rings, so that when it’s winter in one hemisphere, the atmosphere is colder just because it’s winter, but it’s even colder yet because the rings are creating very sizable shadows on that hemisphere. So one of the key things that we want to study in this second extended mission is the seasonal variations of Saturn and Titan, and to whatever extent there are variations on any of the other moons. And this is further opportunity to study things that we haven’t been able to get worked out prior to now in the mission. It sounds like a long time, but 13 years is not even quite half of one full Saturn year – one Saturn seasonal cycle – so there’s a lot to be learned, a lot to be studied at Saturn.

How many fly-bys will Cassini have made of Saturn, Titan and Enceladus by the time it’s finished?

We will have done 293 orbits and fly-bys around Saturn, 127 fly-bys of Titan, and 22 fly-bys of Enceladus. For Titan and Enceladus, these are the ones where we navigated to a specific target point, but there have been others, usually at greater distances, where we didn’t control the trajectory to a specific point. That would amount to tens more fly-bys that the scientists in fact use for observations, but we tend not to count them because we didn’t actively navigate to them.

Can you describe the mission’s planned end in 2017?

We use the gravitational effect of Titan almost exclusively to shape Cassini’s trajectory – the onboard propulsion system we use only for small navigational corrections – and so around November 2016 we’re planning a close fly-by of Titan that is targeted to bring periapsis down to just outside the F ring of Saturn. The F ring is a small ring that is just outside the main rings, and that’ll be the closest we’ve been to Saturn yet. We’ll spend about 20 orbits in this geometry. Then we’ll have another close fly-by of Titan around April 2017 that will change Cassini’s orbit enough that periapsis will take a kind of hop over the rings, passing inside of the rings at just above the atmosphere. There’s a gap there [between Saturn and its rings] of about 3,000 km, which the spacecraft will pass through on every orbit. This is obviously by far the closest we’ve ever been to Saturn and it gives us quite a number of unique science observing and data collection opportunities, things that just physically weren’t possible previously.

We’ll stay in that geometry for 23 orbits, and then there is a rather distant Titan fly-by. This encounter will perturb the orbit just enough that Cassini will enter Saturn’s atmosphere, where the spacecraft will vaporize. The primary reason for disposing of the spacecraft this way, in addition to the unique science data collection opportunities it will give us, is that it ensures that we will never have to worry about the spacecraft impacting a satellite or any target of biological interest – in particular, Enceladus. But the other thing is that by this time in the mission we’re going to be getting very low on propellant and would not be able to continue it for any appreciable amount of time beyond this current planned end anyway.

Cassini is well known for the tremendous images it returns. How does the Cassini team handle all of these images?

When the images first come down, they go to two places. One is a site we maintain here that has all of the unprocessed raw images – they come down to the ground and go straight to this site. At the same time they go to a server that’s maintained by the imaging team, so that the imaging team scientists all have access to them. These raw images, some of which are quite spectacular, all go to this publicly accessible site. And then, as the imaging team begins to process them, they will come up with significant, interesting or spectacular images; we release some of those to the news media and quite a lot of them are posted on our Photojournal site, which is also publicly accessible. And as the scientists work with the images and give them a first-level of processing and image calibration, they all are archived on the NASA Planetary Data System, used primarily by scientists, but available to anybody. That’s where they will be kept for long-term permanent storage and availability.

Do you have a personal favorite?

It would be hard to pick between two of them. One of them you’ve undoubtedly seen, it’s where the sun is on the other side of Saturn and you can see the sun’s rays coming through Saturn’s atmosphere and the rings. That’s probably about everybody’s favorite, and is just a spectacular image.

But the other is one that we took on approach to Saturn, when we were still about a month and a half out and looking at Saturn overtaking the spacecraft. It’s just a beautiful, global image; it shows the rings, their shadow on Saturn, and the shadow of Saturn on the rings. It is probably more meaningful to me just because of what it represented when we took it, “Wow, look at this, we’re just about there and this is what we’re going to get to deal with.”

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