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SYFY WIRE Bad Astronomy

Star Wars 40th anniversary: That’s no moon ... but what if it were?

By Phil Plait

Forty years ago this week, 12-year-old me sat in a movie theater to see what this “Star Wars” movie thing was all about.

Two hours later, I walked out dazed. Blown away. For that soon-to-be teenaged boy, it was pretty much the apotheosis of my science fiction nerdery. You’ll see a lot of articles about the movie this week, since it’s the 40th anniversary of it hitting the theaters (though how many of those younglings actually saw it when it premiered, I wonder?). And since I write for Syfy Wire, it’s only appropriate for me to join the horde.

But while those other writers will wax on about Jedi and characters and the prequels and and and, I am who I am. So: Let’s talk science*.

Let’s talk Death Star.

Ah yes, the Empire’s superweapon, the spherical mobile planet-killer that has become an icon of evilness. It’s so large, Luke mistakes it for a moon ... well, he says a “small moon,” but of course even a small moon can still be pretty big.

And that got me thinking (as science fiction is supposed to do). What if the Death Star were a moon? How would it stack up? Just how moony (moonish? Mooneriffic?) is it?

First things first. What’s a moon?

“That’s No Moon” — Obi Wan Kenobi, who was technically right

Simply put, a moon is an object orbiting another, bigger object. The obvious example is our own moon, the Moon. It’s about 3450 kilometers in diameter (2160 miles), while the Earth is closer to 13,000 km (8000 miles). So, our Moon is, properly, a moon.

Technically, the Earth is a moon of the Sun, but no one really calls it that. Still, many asteroids have moons. So do many (perhaps even most) of the icy worlds orbiting the Sun out past Neptune. Nature, apparently, likes making moons, even on all scales.

Humans do, too. Back in the dawn of the space age, satellites were sometimes called “artificial moons”, which has a fun retro feel to it, but no one calls them that anymore. Still, the Death Star would count, as long as it’s orbiting a planet. So, to be fair, when Han, Luke, Obi Wan, and Chewie first encounter it, and Obi Wan makes his famous utterance, it wasn’t a moon. But, later in the movie, it orbited Yavin, so for that part, it was.

There’s no real lower limit to the size of a natural moon; at least none we can define. So, how does the Death Star stack up against a real moon?

Death Star battle

“Cut the chatter, Red 2”

How big is the Death Star?

Although it’s never explicitly said in the movies, the battlestation is considered to be about 160 kilometers (100 miles) in diameter, which sounds about right to me given how it’s depicted on screen (though, it may have been bigger). That’s really big for a constructed artifact.

But for a natural satellite? There are at least 30 moons in our solar system bigger than the Death Star. It would be placed just under Uranus’s moon Puck on that list. Vader may think his weapon is impressive (most impressive), but nature easily has him beat.

And I’m no Jedi, but I must point out that, even from a distance, an experienced astronomer would know something is weird about that “small moon”. The problem, you see, is hydrostatic equilibrium. That’s a fancy term meaning that the self-gravity of an object (its own gravity acting on the material making it up) is in balance with the pressure inside an object holding it up.

Think of it this way: If you take a pile of dirt and add more dirt to it, it’ll get bigger until the force from the weight of the dirt overcomes the friction and other forces holding the pile’s shape. Stuff will flow down, or it’ll collapse. If you do this on a much larger scale, like that of a moon or planet, at some point it’ll get big enough that the material making up the object won’t be able to resist its own gravity, and it’ll form itself into a sphere (because gravity pulls everything toward the object’s center).

How big the object has to be to reshape itself due to hydrostatic equilibrium depends on what it’s made of. Ice is less stiff than rock or metal, so it flows more easily, and it turns out an object somewhere between 200 – 400 km in diameter will shape itself into a sphere. If it’s rock it’ll have to be somewhat bigger, maybe 600 km, and for a metallic object it would have to be larger still.

See the problem? As soon as the Millennium Falcon spotted the Death Star when they came out of hyperspace into the rubble debris that used to be Alderaan, an astronomer would’ve known right away that couldn’t be a moon. Instead, they followed the TIE fighter too long and got caught. It worked out well in the end, but if they had had an astronomer on board, a lot of effort could have been saved.

Death Star rebuilt

The Mass x Acceleration Awakens

So fine, the Death Star is obviously artificial, since it’s too small to have formed a sphere on its own (and BTW, a sphere is the most cost-effective shape to make a big ship; it has the most volume for a given amount of material). But how does it stack up against other moons? I mean, you could, properly outfitted, take a stroll on the surface of our Moon. We have proof of that! But could you do that on the Death Star?

First, I’m not sure you’d need to. One thing the Death Star has is room: Remember your high school geometry? The volume of the Death Star is over two million cubic kilometers.

That’s a lot of breathing space. Well, not literally. The Star Wars Wiki says that there’s a “crust” (really, a spherical shell comprising the top layers of the Death Star) several kilometers thick, which is the inhabited region of the station. It’s not specified what “several” means, but let’s say it’s between three and five kilometers deep. That means the volume of just the top habitable layer is 200,000400,000 cubic km.

OK, that’s a lot of breathing space. Even if we assume a crew of a couple of million people (again, as claimed by Wookieepedia), that’s more than enough elbow room. It’s not clear in the movies how they support such a population — replicators aren’t part of the canon, so either they grow food there (plausible) or ship it in. One person eats roughly two kilos of food every day, so we’re talking 4000 tons of just food consumed on the Death Star every day. That may sound like a lot, but, assuming it has the same rough density as water — a ton per cubic mete r— it’s only a cube 16 meters on a side. That amount could be shipped in easily by a freighter daily. Even if we assume they’d want to have a store of food for several months (it’s a warship, after all) that upper shell has nothing but room. They could easily keep food and water in it that would last for months. Decades.

I figure they can recycle water and air indefinitely easily enough; we almost have the tech to do that now. So the Death Star has no problems supporting its huge crew.

Death Star plans

But if you suffer from claustrophobia and want to get out for a while for a stroll on the surface, what happens? They have artificial gravity inside the Death Star, but what about outside? Would there be enough gravity from its mass alone to go for a stroll along the outer shell? I hear the equatorial trench makes for a lovely constitutional, if you can avoid the constant X-wing attacks.

Well, let’s see. The gravity you’d feel pulling standing on the Death Star’s outer hull depends on its size (which we know) and its mass. That’s tougher. We don’t know what it’s made of, or how much of that stuff is in it. If it were a solid metal ball, for example, we could multiply the volume by the average density of the metal to get its mass.

But it’s made of something else (quadanium steel, apparently, though sadly the physical characteristics of that substance are not available). And it’s not clear how much of the station was empty space and how much was solid; a room is mostly empty space even if the walls are thick. In Return of the Jedi, the center is shown as mostly empty and apparently many kilometers across...but that’s a small volume compared to the whole thing.

So, let’s do this three ways: First, assume it’s a solid ball, just as an upper limit to its mass (and we’ll assume quadanium has the same density as steel; 8 tons per cubic meter). Second, assume it’s 10% quadanium by mass. Third — and I thank my friend Randall Munroe for this idea, as we were having some fun discussing this the other night — let’s assume it has the same density overall as another huge fighting metal war machine: An aircraft carrier. I did a bit of fiddling with the math for that, and find that it’s an astonishingly lightweight 200 kg per cubic meter! That may seem low, but it occurred to me that to float, it must be less dense than water (one ton per meter cubed), and a typical carrier floats pretty high. So, that sounds about right to me.

Multiplying it all out, I got some interesting numbers.

If it’s a solid steel ball —t he densest possibility, and therefore the one with the highest gravity — it’ll have a surface gravity of only about 2% Earth’s! If I stood on the surface of the Death Star, I’d weigh about three pounds.

And that’s at best. If it’s 10% quadanium, the surface gravity is about 0.2% of Earth’s. If it’s the density of an aircraft carrier, you’d weigh a minuscule 0.04% what you do on Earth. For me, that’s about an ounce.

It would actually be hard to get traction if you weighed that little. Walking would be hard unless you had magnetic boots, assuming quadanium steel is ferromagnetic.

What’s funny is that even a small moon has more gravity, because they’re so much bigger. With malice aforethought, let’s pick Saturn’s moon Mimas to compare the Death Star with; at 400 km in diameter Mimas is about the smallest moon you can have and still be a sphere. Its density is about 1.15 tons per cubic meter; it’s mostly water ice with some rock mixed in.

On its surface, you’d weigh about 0.65% what you do on Earth. That’s still four times what you’d feel on the surface of a “realistic” Death Star. Ice is less dense than steel, but Mimas is solid and has a lot more mass.

The Death Star’s gravity is so small that if it were to orbit the Earth where the Moon is, we’d hardly notice any difference. The Moon is 40,000 times more massive than the Death Star! So again, nature wins.

My advice: If you want to go for a stroll while visiting the Death Star, stay inside.

And oh, one thing: If the Death Star is 10% quadanium by mass (i.e. a density of about 800 kilos per cubic meter), or the same density as an aircraft carrier, it would float, as well! There would be some odd effects where the waterline would be; the space station would have enough gravity to pull the water up a bit around it. But still, it would float.

Death Star floating

Oops, my mistake. That is a moon.

I want to leave you with a funny thought. The Death Star has a superweapon, what’s called a superlaser, capable of destroying planets. In real life, that won’t work; the energy it takes to completely blow up a planet is really huge, like the amount the Sun emits in over a week (in The Force Awakens the new superweapon sucks a star dry to get all its energy; I’m not sure that might work exactly but that’s the right scale of thinking). Anyway, if you’re interested in that, an astronomer with whom I’m familiar wrote about this topic on this very site a few years back.

This superweapon appears as a gigantic curved dish embedded in the northern (or upper) hemisphere of the Death Star. Here’s the funny bit: Check out this image of the aforementioned Saturn moon Mimas:


Yeah, that looks familiar, doesn’t it? The crater, called Herschel, is 130 km across. It’s an impact crater, and if whatever smacked into Mimas had been a bit bigger, or moving more quickly, it would’ve shattered the moon! Amusingly, perhaps, another moon of Saturn, Tethys, looks very much similar.

I love the irony of this. They both look like the Death Star, a weapon meant to destroy worlds. But the reason they do is because of giant impact craters that almost destroyed the moons, themselves! And in fact, if you really did want to wipe out a planet, pushing a moon into it is a way better idea than using some superweapon. How’s that for irony?

Astronomy humor is weird.

May the 40th be with you

So, there you go. The Death Star may be the most feared weapon in the galaxy, but it’s really actually kinda weak compared to even the smallest, lowest density moon you can find. It’s funny how nature can completely crush our boldest attempts to be dominant. ‘Twas ever thus.

But of course, that’s not the really important thing. What makes the Death Star so iconic is not just its size (which is hard to grasp anyway), but its shape, its design, the implications of who is on board. Heck, I find Darth Vader more threatening than the Death Star, because his menace is more personal, more palpable, than any huge piece of construction. And in the end, that’s what makes for better storytelling, anyway.

So if you decide to rewatch the classic Star Wars original movie today to celebrate the anniversary, do so in the knowledge that nature may have us outmassed and outnumbered, but when it comes to our fiction, it’s the characters and the stories that count. That’s why we’re still watching Star Wars movies after 40 years.

* By the by, I did write about some of the science of Star Wars recently, discussing whether it’s really in a galaxy far far away, or if maybe George Lucas was pulling a fast one and planned to have it take place in our Milky Way all along. Conclusion: Spoilers, sweetie. Click the link and find out.