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SYFY WIRE Bad Astronomy

What If Saturn Paid Us a Very Close Visit?

By Phil Plait

Over the weekend, Saturn reached opposition in the sky. This means it’s opposite the Sun—literally, the Earth is physically between Saturn and the Sun—so it rises when the Sun sets and sets when the Sun rises. That means it’s up all night, making it easier to see … with the added bonus that it’s as close to the Earth as it’ll get all year, so it looks bigger and brighter, too.

This is the best time to see Saturn, so I highly recommend getting out and taking a look, especially if you have binoculars or a small telescope. EarthSky has great directions on how to find it. It doesn’t have to be right away; it’ll provide excellent viewing for weeks.

Still and all, Saturn is a long way off: Even at its closest, it’s still more than 1.3 billion kilometers (830 million miles) away. Saturn’s a big planet, nine times wider than Earth, but from that distance it’s reduced to a point-like dot to your naked eye.

But what if it were closer? Like, a lot closer? What would it look like if it were, say, as close as Mars… or were to even swing by the Earth, passing between us and the Moon?

Mind you, it can’t; its orbit is huge and (very nearly) circular, so it’ll never get any closer than about a billion kilometers. That’s a tremendous distance. Still, it’s fun to wonder what it would look like if, for some reason, Saturn somehow took a left turn and headed for Earth.

Animator yeti dynamics—whose real name is Nick—decided to find out. Using images of Saturn from Cassini and Voyager, he created a photorealistic animation of the giant ringed planet depicting a very, very close encounter with Earth. And it’s stunning. You absolutely want to make this hi-res and full screen.

WOW! That’s so cool. I did a bit of math, and he’s right; the way he shows it, at the distance of Mars, Saturn would be about as bright as the full Moon, even though it’s 150 times farther away! That’s pretty astonishing. You could read by Saturnlight, a romantic thought. The planet’s disk would be about one-forth of the size of the full Moon or so, and the rings would be stretch to about two-thirds the Moon’s size. What a sight that would be!

But if it were to get closer, well, things would get interesting. Mind you, Nick wasn’t going for physical accuracy, just visual accuracy. In other words, keep everything looking the same, but let Saturn get close. His depiction does that very well.

But it means Nick had to ignore gravity. Why? Because Saturn has a mass nearly 100 times that of Earth, so it has an immense gravitational attraction. As it drew near, Saturn’s gravity would create utter havoc here on Earth, and vice versa. Nick couldn’t really depict that in the video, but we can think through the ramifications here. Hint: It doesn’t end well for us.

The Gravity of the Situation

For example, long before it even got here, Saturn would toss the Moon right out of Earth's orbit. Its gravity would influence the Moon more than Earth does, and in any three-object encounter like this, the lowest mass object loses. The Moon would be ejected, probably to go into a highly elliptical orbit around the Sun … which could eventually recross the Earth’s orbit, leading to a potentially very bad future scenario (see here for a taste). Not that it would matter, really ... but I get ahead of myself.

Before we get to what happens on Earth, let’s take a moment and see what’s happening on the Saturn side of things during the approach. How would the rings fare if the situation in the video came to be? Well, they’d be obliterated, one way or another. Saturn’s rings are made of countless tiny particles of ice, and Earth’s gravity would pull on them, more and more as they got closer. In the video we see the Earth punching a hole through the rings, but that’s not what would actually happen (Nick told me the simulator he was using wouldn’t model that accurately).

In reality the Earth’s gravity would actually pull the particles out of the rings, creating a long plume first reaching toward the Earth, then dragging after it once it was through (much like what happens when galaxies collide). I suspect the majority of the particles would continue to orbit Saturn—Earth lacks the gravitational pull to completely strip them from the planet—though they would be on elongated orbits. The gorgeous ringlets and delicate structures we see now in Saturn’s rings would be destroyed. However, over time, the particles would collide with the other parts of the ring, and eventually the system would settle down once again.

Except I doubt they’d get the chance. Even if the rings could survive their encounter with Earth they wouldn’t have a very long future. Nick put Saturn on a course that takes it nearer to the Sun. As Saturn approached our star, it would of course feel increased heat. The ice particles in the ring would warm up, eventually going directly from a solid into a gas (a process called sublimation). Instead of a spectacular ring system, we’d see Saturn get enshrouded in a vast cloud of water vapor, streaming behind it as sunlight and the solar wind blew it away from the planet. It would be the gods’ own comet.

In fact, many of Saturn's moons are icy, and the same thing would happen to them. Saturn would be King Comet, surrounded by dozens of attendants.

That would be a pretty amazing sight, too. But there’d be nobody left to see it.

Ringing in the Apocalypse

As Nick says in the video, the Earth would never survive this encounter with Saturn. A large object like Earth getting too close to a much larger one like Saturn is a recipe for doom. And that’s because of gravity … or, more specifically, the fact that gravity gets weaker with distance.

For example, think of the Earth and Moon. The side of the Earth facing the Moon gets pulled by the Moon’s gravity a little bit harder than the far side of the Earth. The difference isn’t much, but it’s enough to stretch the Earth a little bit (it’s actually more complicated than that, of course, but for now that’s close enough). That’s what we call the tidal force.

The net result is that it stretches the object upon which it’s acting. Right now, the Moon’s tidal force on Earth pulls water up and down by roughly a meter or two between high and low tide as the Earth rotates under the Moon every day.

The tidal force depends on the mass of the object doing the pulling, the distance the second object is from it, and the size of that second object. For a very massive object pulling on a smaller but still large object getting very close to it, the tidal force can get enormous. Really really enormous.

You may see where this is going.

Imagine the Earth passing close to Saturn, like in the video. One side of Earth is closer to Saturn than the other, by the diameter of the Earth: about 12,740 km (7900 miles). Saturn has a lot of mass, so its gravity is quite strong, and as the Earth gets closer to it, the stretching force grows and grows.

When Saturn is still 20 times farther away than the Moon—well over 7 million kilometers distant—the tidal force it exerts on Earth is equal to the Moon’s. By the time it’s a million kilometers away that force has increased by a factor of more than 400! That would cause disaster on a global scale, mostly in the form of apocalyptic floods as huge tidal waves marched across the planet, destroying everything in their path.

By the time Saturn is at the same distance as the Moon, its tides would be many thousands of times stronger than the Moon’s. Fault lines would rupture, volcanoes would blow their tops, and anything left on the surface of the Earth would be wiped out.

And then comes the final blow. Judging from where we hit the rings, Nick depicts Saturn passing about 130,000 km from Earth. At closest approach, the tidal force Saturn wreaks on Earth would be a staggering 200,000 times the Moon’s! This distance is well inside Saturn’s Roche Limit, the distance from Saturn where the bigger planet’s tidal force is so strong it literally tears our planet apart.

That would be that for Earth. The sextillions of tons of vaporized rock that used to be our blue-green world would be stretched out along a line, like Silly Putty drawn out by a vengeful god. The pieces would then be flung away by Saturn’s gravity as it passed us, dispersing them across the inner solar system. The Sun would get a new asteroid belt, while Saturn would fly on, hardly the worse for wear after its Galactus-level swath of destruction.

See for Yourself

Phew! That’s quite the barn burner of a story. Happily, it’s entirely fictional; in reality Saturn stays in its part of the solar system, and we stay in ours.

Even if the video perforce ignores gravity, it’s still pretty. And like I said, it’s fun (for a sufficiently broad definition of “fun”) to think about what would happen if this were to occur. But all in all, I’m glad the events in the video can’t happen!

However, back in reality, on Earth in the here and now, Saturn is presenting itself beautifully in our sky. If you go out over the next few weeks, the view of it you’ll get through a telescope is incredible, so please please please go out and look. Tell your friends, tell your family. If there’s an observatory (or a friend with a telescope) nearby, find out if you can take a peek. You will not regret it.

Saturn is the most beautiful planet in the solar system (other than our own), and seeing it through a telescope yourself can be life-changing. It literally was for me.

Tip o’ the dew shield to Maksim Kakitsev for alerting me to this video!

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