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Alan Stahler: End of mission

Looking down on the north pole of Saturn. Directly over the pole, a hurricane the size of the lower 48. Surrounding the pole, the mysterious hexagon.
Courtesy of NASA |

The Cassini spacecraft launched in 1997, and entered orbit around Saturn in 2004 — a billion miles away, as the crow flies.

Shortly thereafter, Cassini discovered geysers — plumes of water — erupting through the ice of one of Saturn’s many moons. That early discovery would seal Cassini’s fate.

Now, thirteen years later, Cassini is preparing to self-destruct, by diving into Saturn.



To see the rings of Saturn with your own eyes, you’ve got to look at the right time — according to early analysis of very recent data, no earlier than a hundred million years ago. Before that, there might have been no rings to see.

A hundred million years ago, it’s hypothesized, a rubbly object — perhaps a comet, perhaps an asteroid — flew too close to Saturn. With its near parts “wanting” to go faster, far parts “wanting” to go slower, gravity ripped it apart. The rubble spread out to form the rings.

Orbits




Throw a ball as hard and as straight as you can.

The moment it leaves your hand, it begins to fall.

But Earth is round; throw the ball hard enough, and Earth’s surface will fall away as fast as the ball falls, and the ball will fall around the Earth forever. That ball’s in orbit.

Just as heat drops off as you back away from the campfire, gravity grows weaker as you move away from Earth.

To launch a ball into orbit from the roof of a skyscraper, a quarter mile above the surface, you wouldn’t have to throw it quite so hard, quite so fast.

250 miles up, the International Space Station circles Earth at just over 17,000 miles an hour.

22,000 miles up, weather satellites move less than seven thousand miles an hour.

The moon orbits 240,000 miles from Earth’s surface, at a leisurely 2000-something miles an hour.

But the moon is 2,000 miles across — the part of the moon facing Earth is two thousand miles closer to us than the part facing away. The near edge feels Earth’s gravity more strongly, and “wants” to go faster, while the far edge “wants” to go slower.

These different “desires” of near and far parts of the moon cause the moon to stretch. But moonrock is tough, tough enough to endure that stretch.

Moons

Not all moons are that tough, though — some moons could actually be piles of rubble, chunks of ice and rock, held together only by their own gravity.

A hundred million years ago, it’s hypothesized, a rubbly object — perhaps a comet, perhaps an asteroid — flew too close to Saturn. With its near parts “wanting” to go faster, far parts “wanting” to go slower, gravity ripped it apart. The rubble spread out to form the rings.

Like Earth, Saturn has a jet stream blowing around its north pole. But instead of flowing in smooth curves (as on Earth), Saturn’s north polar jet stream forms a hexagon.

Saturn has sixty-plus moons. One of those moons is larger than the planet Mercury, with an atmosphere thicker than Earth’s, where it rains and forms lakes of liquid methane — liquid natural gas.

Having seen geysers spurting from one of Saturn’s moons, Cassini has since found evidence for a liquid ocean beneath that moon’s icy surface. An ocean suggests the possibility of life — or, at least, chemical processes that pre-date life.

Were Cassini to one day crash into that moon, it would contaminate the moon with Earth bugs — Earthly bacteria — which would play havoc with any future search for ET life on this moon.

Cassini is now low on fuel … once out of fuel, the spacecraft will be uncontrollable. To avoid future collisions, Cassini is on a collision course with Saturn.

As it dives in a week from Friday, the spacecraft will radio back what it finds in Saturn’s atmosphere, and then it will disintegrate.

At 8 p.m. on Sept. 16 — the day after Cassini’s plunge — local amateur astronomers will invite the public to the junction of the old Downieville Highway and State Route 49, to look at Saturn, and much else, through the scopes.

Kids are welcome; bring a sweater.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages, and may be reached at stahler@kvmr.org


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