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(NASA/JPL-Caltech)

Kudos for Cassini

The U.S./European Saturn mission takes home a trophy.

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(Continued from page 1)

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.

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