TOPICS > Science

Cassini Enters Saturn’s Orbit

July 1, 2004 at 12:00 AM EDT


JIM LEHRER: Now a new visit to Saturn, the planet of the rings. Ray Suarez is our guide.

RAY SUAREZ: After a journey from Earth lasting nearly seven years and more than 2 billion miles, the Cassini spacecraft became the first to go into orbit around the sixth planet from the sun, Saturn.

Loud applause broke out at NASA last night when the spacecraft moved within 13,000 miles of the top of the planet’s clouds, traveling more than 60,000 miles an hour, Cassini arrived after completing a complicated maneuver. It passed through a gap between two of the planet’s many rings, in this case the F Ring and G Rings. Then Cassini turned around and fired its main engines to break its progress.

Over the next four years Cassini will study the second largest planet in the solar system and some of its 31 moons, and send back pictures of the planet and its many rings, first observed by Galileo in 1610.

The $3 billion mission is a partnership of NASA, the European Space Agency and the Italian Space Agency.

More about Cassini now. It comes from Kevin Grazier, an investigation scientist from the Cassini team.

Welcome to the program. Why go to Saturn in the first place? What was it that Cassini was sent out there to find?

KEVIN GRAZIER: We explore planets in stages. We first looked at planets with the unaided eye thousands of years ago; we developed telescopes; then we sent fly-by spacecraft; the Voyager spacecraft surveyed the planets of the outer solar system in the ’70s and ’80s; in some cases raising more questions than they answered. So now we go back with a mission to answer many of Voyager’s questions. That would be the Cassini mission.

Cassini will go there, spend four years in orbit to look at Saturn, its rings, the IC satellites and especially the moon Titan. There’s a lot to learn there, and this is just the beginning of it.

RAY SUAREZ: Well, what were the major areas of investigation, the things that we really needed to know about Saturn, that Cassini is designed to find out?

KEVIN GRAZIER: The moon Titan — let’s start with Titan. Titan is the only moon in the solar system with an appreciable atmosphere. When I say appreciable, I mean that if you were standing on the surface of Titan, the pressure on you would be the equivalent of one and a half times our atmosphere or the equivalent of being 15 feet below the ocean if you were a scuba diver.

Now, that atmosphere has thick, dense orange clouds that we have yet to be able to see through — at least not well. We’re fairly certain that with the Cassini spacecraft we will see through those clouds to the surface below and now see the largest unmapped solid surface in the solar system. That moon, Titan, has an atmosphere that we think is like Earth’s atmosphere was 3.8-to-4 billion years ago. That atmosphere gave rise to life on Earth, so in some senses we think by understanding Titan we are looking at the early Earth in a deep freeze and studies of Titan can actually have, we believe, implications for the formation of life on Earth.

RAY SUAREZ: Is Titan comparable in size to the Earth?

KEVIN GRAZIER: Titan is actually much smaller than Earth but it is larger than two planets in our solar system. It’s the second largest moon in the solar system. It is larger than the planets Pluto and Mercury; were it orbiting the Sun, it would be considered a planet.

RAY SUAREZ: So Cassini made it out and after what by common consent was a pretty tense night last night. It moved into orbit and moved closer to the rings of Saturn. Does it move into regular orbit the way a moon flies around the planet now?

KEVIN GRAZIER: First thing I’d say the spaceship — I don’t know if I’d say moved into orbit — it rocketed into orbit. At one point we were traveling about 69,000, 70,000 miles an hour, so we were moving pretty quickly last night when we got into orbit; this is why it was so tense around here.

But as far as the orbit trajectory we have dramatically different orbits throughout the span of the mission. Our first orbit is a 116-day orbit. We are in a very long looping leisurely orbit, which we will trim subsequently over time, then we will actually have excursions in inclination, or tilt, relative to Saturn’s ring plain. Different tilts of orbits will get us different science.

So unlike the most recent previous mission to a large planet, Galileo, which stayed in the plane of the equator, which for that mission was completely reasonable to do, we will be at different inclinations throughout the mission, different sizes of orbits, different orientations of orbits.

RAY SUAREZ: Let’s talk a little bit about the pictures that have been coming back from several hundred million miles away. They’re beautiful, but to the untrained eye, we don’t really know what we’re looking at. The scallop ring, for instance, members of your team were very excited about it. Why?

KEVIN GRAZIER: The big surprise about a lot of the imagery we’re seeing is that we have done computational and theoretical studies of what the rings should be like, we think, and what we’re seeing is exactly what computer simulations have said we should see. The surprise is that things are so close to what theory has predicted.

The scalloped edge there is created by a passing moon. And there are density waves, those bands you see. The banding are spiral density waves, also predicted by theory, similar to a gravitational wake from a passing moon. What we’re looking at here is the Enke Gap, and there is a moon in the Enke Gap that has created these features.

RAY SUAREZ: And one that got a lot of excited attention had to do with the density wave and the bending wave.

KEVIN GRAZIER: Right. Again, we see features that we were certain existed. We see density waves, which are waves that are created by gravitational, what we call resonant effects of moons. Bending waves — and I should just point out that since we’re looking at a dark-side image, this is the unlit side of Saturn, dark in this image means more material or it can mean empty space. So dark means more stuff. We’re looking at the unlit side, so we’re seeing these waves of increased density which are, again, similar to gravitational wakes, but we’re also seeing bending waves, whereas the edge of a ring is literally flapping. Like, imagine a flag flapping in the breeze. The edge of this ring is flapping from being excited by the passage of a moon.

RAY SUAREZ: Well, these wisps are very different from the things we saw in the earlier photographs, which are very precise, almost geometric. These wisps are, they’re almost smoky.

KEVIN GRAZIER: The F-Ring is a different type of ring than the main ring system. It is less dense, but also it has what are called shepherding satellites on either side. These two moons keep the F-Ring in place, and their passage, when it passes by the F-Ring, it creates structure within the ring.

We should also see little places that look like something that’s taken a slice out of the ring in these images. Interestingly enough, one of the members of our imaging team, Professor Carl Murray — one of our international members, he’s from London — recently wrote a paper that predicted we would see these things. And now we’re seeing them, and we’re back to again the practical images are showing what we theoretically predicted to an amazing degree.

RAY SUAREZ: Are the rings solid, or are they semisolid? What are they made of, and could you fly through them, for instance?

KEVIN GRAZIER: The rings were made — are made of billions of pieces of ice, ranging from dust-size to house-size roughly. Most of them are about 1-to-2 centimeters, about half an inch to an inch in diameter. And that’s the main ring system, the big, bright rings.

The outer rings, like the G-Ring, you could pass through. The inner rings, if you pass a spacecraft through that, you would stand a very good chance of losing your spacecraft. The G-Ring has actually been transited by three spacecraft. Three spacecraft: Voyager I, Voyager II and Pioneer 11 have passed through the G-Ring unscathed.

Beyond the G-Ring is what we call the E-Ring, which is extremely large in aerial extent. We will pass through that numerous times because the density of particles is so low and the particles are so small we don’t worry about that. But for the main ring system, the particles are fairly large, the density is fairly large, therefore we avoid them because the large rings are a collision hazard and would damage the spacecraft.

RAY SUAREZ: Are the rings flat, disk-like, or more puffy, rounded, like a big space doughnut?

KEVIN GRAZIER: (Laughs) I’ve never heard the term “space doughnut” before. Actually, the rings are — come in different flavors. The main ring system are very flat. We’re talking about 60 to 100 feet thick — very, very thin compared to a very large planet. However, as you move out, we have the G-Ring, which is about 1,000 kilometers thick, but again very rarefied, so there you actually have your space doughnut. And then beyond that, we have the E-Ring, which is just large, very thick, again, 1,000-kilometer, roughly, extent. And it extends thousands and thousands of kilometers out from Saturn.

RAY SUAREZ: Kevin Grazier from the Jet Propulsion Laboratory, thanks for joining us.

KEVIN GRAZIER: It’s been a pleasure.