In September, the NASA/JPL Cassini mission began the last two years of the Solstice Mission, the final stretch of its explorations of Saturn, its rings, and its moons—including the giant Titan, a haze-enshrouded satellite with Earth-like features and complicated organic chemistry, and small, icy, and surprisingly active Enceladus.
Launched in October 1997, after a decade and a half of planning, design, and construction, the Cassini spacecraft may be one of JPL's missions that is most well remembered, says JPL director Charles Elachi (MS '69, PhD '71). Elachi, who is also a Caltech professor of electrical engineering and planetary science, may be forgiven a small amount of bias toward Cassini, as he has served since the mid-1980s as the team leader for the spacecraft's radar experiment—the instrument responsible for mapping the previously hidden surface of Titan.
We recently spoke with Elachi to gain his unique perspective on Cassini's achievements—and what will come next.
How did you first get involved in the Cassini mission?
When the mission concept was being developed in the early 1980s, JPL worked with the science community to define the mission, and one of the key instruments that the community wanted was radar. At that time, all that we knew about Titan, Saturn's largest satellite, was that it is a ball. It is completely haze covered, and you cannot see the surface. Radar was an ideal instrument because of its capability to see through the haze to map the surface.
At the time, I was the lead scientist at JPL in radar activity. I was involved in some of the earlier radar missions like Seasat, one of the first orbiting radar satellites. I was the principal investigator of a series of shuttle imaging radar missions (SIR-A, SIR-B, SIR-C). I was a member of the Venus radar team for NASA's Magellan mission. So I decided to propose that type of instrument for Cassini. It was selected, and I was selected as the team leader. Caltech professor of planetary science Duane (Dewey) Muhleman, who is now retired, joined me on the radar team.
Other Caltech faculty and alumni include Andrew Ingersoll, professor of planetary science, who was a member of the Cassini imaging team, as was Torrence Johnson (PhD '70); imaging team leader Carolyn Porco (PhD '83), who is also the director of the Cassini Imaging Central Laboratory for Operations (CICLOPS); and Dennis Matson (PhD '72), who was the Cassini project scientist from the beginning of the mission through early part of the orbiting phase.
Do you recall what your hopes were for the mission when it first started?
At the time, some people had the theory that Titan is a water ball—with an ocean across the whole satellite. Other people were saying there is no ocean. It would be too arid. We had no idea what to expect. For me, the most exciting thing was that it was going to be a complete surprise. It was different from the case with Mars, for example, where we had some idea of what the planet looked like from ground telescopes.
My goal was to map as much of Titan as possible. The radar instrument maps Titan by serial flybys, where, with every flyby, we image a wide strip of the surface. My hope was to map at least 50 percent of Titan. So far, we have mapped almost 60 percent.
What was the biggest surprise?
In my mind, the two biggest surprises were, first, that it has lakes with rivers coming into them—all made of hydrocarbons. The lakes are roughly the same size as the Great Lakes in the U.S. It looks like Earth to some extent. The other big surprise was the sand dunes. We did not expect that there would be fields of sand dunes.
What are the sand dunes made of?
We don't know. They could be made of hydrocarbon particles or frozen grains of snow or ice. We cannot tell what their composition is from the radar or other instruments. That's for the next mission.
We know they are extensive. All around the equatorial region on Titan, you see sand dunes of different sizes and with different structures.
They look very similar to the sand dunes in Namibia and Saudi Arabia. The phenomenology is very similar, with the wind blowing particles around hills and mountains to create the patterns.
Have you seen changes in the radar imaging over the last 11 years?
We see changes in a couple of places, and we are very puzzled about the reason for those changes. In the lakes, some small islands have appeared a couple of times. When you see things like this you debate, is it some anomaly in the instrument or is it real? It's perfectly conceivable it could be real, that the level of the liquid could be moving up and down, like what happens in the winter or the summer in lakes here on Earth.
Is there anything that you still hope to learn over the next two years?
I'm always ready for some surprises. A few passes that we will be doing will cover some new areas. One thing we have been looking for is if there are lakes on the other side of Titan, the other pole, now that we know there are lakes in the north. If we detect lakes over areas that we haven't covered, that might give us some hint about why the lakes are there. Also, because Saturn's seasons are progressing, but very slowly, we keep looking for changes as the spacecraft goes over the same areas—changes in the lakes or changes of volcanic flow, changes in sand dune patterns, anything that gives us an indication that something dynamic is happening.
How will the Cassini mission end?
It is going to be a dramatic end. We are planning, on purpose, to have the spacecraft enter Saturn's atmosphere and burn up before it completely depletes its fuel. In order to do that, we have to do a number of orbits that come very close to the rings. In fact, we'll be going through the gap between the rings and the planet, so we might find something new that we haven't seen before now.
Now, you may say, "Why are you crashing into Saturn?" The reason is there are rules for planetary protection. In the long term, once we lose control of the spacecraft, we want to ensure that it doesn't end crashing into Titan, or crashing into Enceladus, to keep the satellites pristine.
How did the team pick Cassini's final day, September 15, 2017?
I think it came from the orbital dynamics. We needed to do it before we completely use the control fuel. The orbit guys came up with a number of scenarios, and then the science team collectively sat down and decided on one of the scenarios.
Have you thought about how you are going to feel on September 16?
On one hand, we will be thinking, "Gee, we are losing one of our great missions."
But this mission has been so amazing. It made so many discoveries—finding the lakes and the sand dunes on Titan; the geysers on Enceladus; details of the hexagonal hurricane in the northern hemisphere of Saturn, which is allowing us to understand the planet's atmospheric dynamics. Cassini's discoveries have completely changed our thinking about the whole Saturnian system. It is changing the textbooks.
The way I think about it is that Voyager gave us snapshots of all the outer planets—Jupiter, Saturn, Uranus, and Neptune. It triggered our curiosity about them. Cassini gave us an in-depth understanding of the whole Saturnian system, which is almost like a mini solar system.
It's like this with every scientific exploration. You answer a certain question, and it raises new questions. What are the sand dunes made of? Do they change? What is the liquid made of? How deep are the different lakes? We're starting to think about the next mission for Titan. Some people are looking at possibly dropping boats in the lakes. Some people are looking at rovers. That will be a different technological challenge at that very low temperature.
We also are looking at possibly landing on Enceladus or sending a spacecraft to fly through its plume, capture samples, and either analyze them in a mass spectrometer on the spacecraft or bring a capsule all the way back to Earth.
After Cassini's mission ends, how much longer will you still be analyzing the data?
The mission is funded at least through 2018. Beyond that, I think people will be analyzing the data at least for a decade, if not longer.
Time brings new perspective. As you learn more about how planets form and about the tectonic activities or the atmosphere, you come up with new ideas. Then you go and look at the data and see. Does that fit with the result of the measurements that were made using a different instrument? Plus, people now are getting much more knowledgeable about analyzing data from multiple instruments that complement each other.
Will you be involved in another mission?
If I'm still alive, maybe. The next big mission is to Jupiter's moon, Europa. I'm a member of the team doing the radar sounder to measure the thickness of the ice.
Did you ever consider stepping away from the science as your other responsibilities increased after becoming director of JPL in 2001?
I always keep a finger on the science. I have found that an important thing for the director at JPL is to stay involved in the science at the team level, so we understand what the institution is doing and understand the issues of the community. It's part of my management style.
For the Europa mission, I felt we needed a younger person to be the team leader. I am only a team member. We structured the team so that half the team is … let me call them "mature." More experienced. People like me. The other half is relatively younger people. We teamed each senior person with a young person. A person from the University of Texas who is in his early 30s is kind of my understudy, if you want. He will be working with me, so I will transfer my experience.
In addition, I am a team member on a very exciting Discovery Venus radar mission, which will be a major advance beyond Magellan.
Is it bittersweet, handing off the reins like this?
I tell people I'm envious of them, because there will be so many discoveries happening in the next 30 years. I wish I were 20 years younger so I could see those discoveries. I'm sure when I was young, people who were older were envious of us.
Hopefully, if I live long enough, I will see some of these results. It's an incentive to stay in good health.
In our business, you have to be patient. It takes a long time, particularly for the outer planets. With Cassini, we had seven years to sell it to Congress, seven years to build it, seven years to get to Saturn. That's even before we started getting the data. But, once you start getting the data, the excitement is worth every bit of that patience. I would say my career was worth it.