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Discovering Saturn through Cassini’s lens

LiveMint logoLiveMint 04-05-2017 Dilip D’Souza

Some months ago, I wrote in this space about a massive hexagon on Saturn (“Earth, We Have A Hexagon”,). Scientists first observed this remarkable celestial formation in the 1980s, in data the Voyager spacecraft sent home. More recently, we’ve seen it—and learnt much more about it—via images taken by the Cassini spacecraft, which was launched in 1997 and has been orbiting Saturn for 13 years now.

While the hexagon was quite a find, we’ve learnt much more about Saturn via Cassini than just an unexpected shape at its north pole. But as I mentioned in that previous column, the doughty little spacefarer is coming to the end of its fuel. So its bosses at the National Aeronautics and Space Administration (Nasa) planned a year-long farewell tour for Cassini. It began one last orbit around the planet last September. A few days ago, it embarked on the first of some final manoeuvres that will last till next September.

Cassini’s swansong is a series of dives through Saturn’s famous rings—that is, between the planet and the rings. The first dive started on 26 April, the second on 2 May, and there will be one every week until September. During each dive, Cassini is doing what it has done so valiantly for two decades now: shooting and sending back images of Saturn and its surrounds. To give you an idea of what I mean, Cassini has sent us nearly 400,000 photographs since it first went into orbit around Saturn in 2004; nearly 1,500 of those are from the first of its final dives.

The photographs Cassini has sent are, as you might expect, spectacular. All right, not every one—and I say “spectacular” not because they are beautifully composed, but really because they give us such a close look at a world totally alien from ours. I feel the awe every time I look at these pictures , every time I think of what we’ve learnt from them.

Like Saturn’s moons. The astronomer Christiaan Huygens was the first to discover a moon orbiting Saturn in 1655. That was called Titan. By 1684, Giovanni Domenico Cassini (thus the name) had found four more. By the time the modern Cassini was launched, in 1997, we knew Saturn had 18 moons. Quite the brood, yes? But in the last 20 years, that count has gone up to 53 confirmed moons and nine are still being examined. A rather larger brood than we had suspected, and much of the credit for finding these bodies whirling around Saturn goes to that spacecraft.

Nor is it just the number of moons.

Hyperion was discovered in 1848, but it was only in 2005 that we realized what an unusual object it is. It is so pockmarked with sharp-edged craters that it looks like a gigantic sponge (take a look at this stunning image). Tiny Polydeuces, with a radius of less than 1km, is a “Trojan” moon. That means it actually trails a larger moon (Dione, in this case) in its path around Saturn. Daphnis tumbles along through Saturn’s rings, visibly carving a path as it goes—much as if you rolled a ball through an expanse of flour. And Enceladus appears to have powerful geysers at its south pole, sending plumes of water crystals several hundred kilometres into space. Mention water, of course, and astronomers get excited—because it suggests a possibility of life. Saturn’s largest moon, Titan, also hints at life. Cassini sent a probe, Huygens, to Titan’s surface in 2005, and found it was covered with lakes and rivers. Only, these aren’t water-bodies as we know them; they are filled with liquid hydrocarbons. Which means life on Titan would be significantly different from anything we know.

Yes, life on one or the other of Saturn’s moons. Courtesy Cassini, we can at least imagine that possibility.

The dive took Cassini to within 3,000km of the tops of Saturn’s clouds. If that seems like a yawning gap, remember that given Saturn’s size—its diameter 120,000km—3,000km is minuscule. And from that close, Saturn is like a hulking, brooding presence in Cassini’s viewfinder. Some blurry images show enormous swirls in the planet’s atmosphere. That’s a storm, but one on a scale hard to comprehend: It’s as big across as our Earth is. Puts our own supercyclones and typhoons in their place, certainly.

Come September, Cassini will be ready for its final blaze of glory, quite literally so. Nasa recognizes that there’s a chance microorganisms from Earth might have piggybacked on Cassini across the solar system, surviving these two decades. If they just let Cassini be after its fuel runs out, it might collide with Titan or Enceladus—and those microorganisms might then contaminate those possibly life-sustaining moons. To avoid that possibility, they will send Cassini plunging through Saturn’s atmosphere, where it will quickly burn up and disintegrate.

That is, Cassini raced through those rings much like you and I might race through a sudden downpour, holding a book or a briefcase over our heads. I just love that mental image.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter Of Numbers explores the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at Read Dilip’s Mint columns at

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