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NASA's Cassini Begins Its Final Mission Before Self-Destruction


In 1997, the spacecraft Cassini left Earth. And seven years later, it reached Saturn. It has been orbiting the ringed planet ever since. Well, now Cassini is low on fuel, and NASA has decided to end the mission in dramatic fashion. Come September, Cassini will plunge into Saturn. First it'll carry out what scientists hope will be its most fruitful work to date. Cassini project scientist Linda Spilker joins us now from NASA's Jet Propulsion Lab in Pasadena, Calif. Welcome to the program.

LINDA SPILKER: I'm happy to be here, Robert.

SIEGEL: Why end the mission in this fashion?

SPILKER: We had a lot of options for how to end the mission. But we knew that based on Cassini's discoveries, we didn't want to risk running into two very important moons - Enceladus, with its icy jets out of the south pole, and Titan. Both have liquid water oceans underneath their surface. And Titan has lakes of liquid methane at its north pole.

In case there might be a possible environment for life, we had to find a way to dispose of Cassini that would not risk contaminating these two worlds. So in looking at it from a scientist's point of view, hey, let's dive into the gap between the planet and the rings. And then in that very final orbit, plunge into Saturn and end the mission that way.

SIEGEL: When we say that it's going to do some of its most fruitful work in this last phase of the mission, what will we learn in Cassini's last few months that we haven't known already?

SPILKER: Well, by getting so close to Saturn and also to the rings, we'll learn a great deal more about the planet itself and also the rings. For the first time, we'll get the mass of the rings. That will tell us, are the rings young or old? We'll also directly sample the ring particles to find out what that little bit of non-icy material might be. Then, of course, the planet itself - how is it structured? How deeply do the winds go? Where does the magnetic field begin to be generated? And how long is a Saturn day?

SIEGEL: You have been with this project since the beginning. After 20 years, what are you feeling as the final chapter begins?

SPILKER: Well, it's a time of tremendous excitement to think about the potential new discoveries. And I'm finding myself feeling very proud of being part of this incredible international effort. Scientists from around the world have been working on Cassini. And at the same time, I'm feeling a sense of sadness to realize I'm going to be saying goodbye to this plucky little spacecraft that has returned this incredible data and saying goodbye to my Cassini family.

SIEGEL: Is there a single big thing that you've learned from Cassini, or have you learned hundreds of little things?

SPILKER: We've learned a tremendous amount from Cassini, and two things in particular stand out. That's that this tiny icy moon - Enceladus, only 300 miles across - has jets and plume of material coming out of these cracks at the south pole, a liquid water ocean and the possibility that perhaps conditions might be right for life.

Then, of course, there's the giant world Titan, has this thick atmosphere that we couldn't see through until we actually landed a probe, the first human object to land on the surface of this distant world, and then used radar to pierce through the clouds to see just what Titan might look like.

SIEGEL: Well, thank you very much for talking with us about what's up for Cassini.

SPILKER: I'm very happy to be here.

SIEGEL: That's Linda Spilker, project scientist for the Cassini spacecraft which will end its 20-year mission to Saturn in September.