So links go first.
Today's main point is a continuation of thoughts
on Ewoks from six years ago. Earlier this week, I saw this image in my RSS stuff:
And searching online for it again led me to this, which is
an overly long discussion of a bunch of nonsense about how Endor would have been incinerated by the debris from the Death Star. After thinking about it for a few minutes, I came to the conclusion that that didn't make much sense, as pre-explosion, the DS had to have been moving sufficiently to not crash, so after the explosion, only a small fraction should hit the planet. Right?
So let's throw physics at the problem, and see. According to the Star Wars wiki page,
Endor has a radius of 2450km radius. If it has the same average density as the Earth, this gives it a mass of 3.39e23 kg. We're trying to slam stuff into it, however, so let's go with an alternate calculation that the surface gravity is the same as on Earth (the Earth density Endor has about half-Earth gravity). This gives about twice the mass at 8.82e23 kg.
Now we know what the target looks like, but what about the DS? It apparently
has a radius of 80km. What does that mean in terms of mass? No idea. It's made of "quadanium steel," so we have to make something up. Is it steel? That's 7.75 g/cm^3. Maybe quadanium makes it lighter, so maybe something like aluminum at 2.70 g/cm^3? Plus, a lot of the DS is empty, because otherwise you couldn't do stuff inside it. So there's a fill factor to deal with. Let's say all the rooms look like this one:
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| This one. |
That's an
imperial shuttle, and those are 20m long, and roughly square-ish. Stamp that footprint around, and I come up with something like 120m * 120m * 60m = 864000 m^3. If the walls are 5m thick, the fill factor for the DS made up of these rooms is 33%. In the first movie, the walls tend to look thin like a regular wall. This yields a fill factor of about 1%. This results in less mass for the DS, which turns out to be super important. The high end mass is steel with fill factor=1.0, or 1.66e19 kg. The low end is aluminum with a 1% fill factor, at 5.8832e16 kg.
Why? The DS blows up. It's a good "kablooey" kind of explosion. Assuming this is going to completely disperse all the mass to infinity, this means the explosion is comparable to the gravitational binding energy, U = 3/5 * GM^2 / R. For our 80km radius, and the high and low mass estimates give binding energies of 1.38e23 and 1.73e18 MJ. This has to be converted to kinetic energy of all the debris particles, so assuming equal mass, each particle gets, U = 1/2 (M/Np) * v^2, so v = sqrt(Np) * [129 7.67] km/s.
What's left to know? Where the DS is located in association to Endor. I remembered this scene:
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| Zap! |
But that turns out to not help. I had planned to measure the length of the chord on the arc of Endor, and use that to work out how far behind the DS it is. However, since this is the CGI reconstruction of two models on a stage, this doesn't work. Using that chord to work out the projected scale of Endor, given the known "real" radii of the two objects means that Endor is in front of the DS, as the model Endor wasn't big enough to create the correct arc.
This isn't a big deal, as I remembered that the DS is orbiting Endor because there's a shield generator that's protecting it. This means the DS has to be in a geostationary orbit. That wiki page for Endor claims it has a period of 18 hours. So the DS orbits at 18433km, and has an orbital velocity of 1.79km/s (which was yesterday's plot).
Put this all together, and run it through the N-body simulator to see what happens, and you get:
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| It freaking explodes. |
How much does it explode? It explodes all the way. For the randomly placed particles (they all start on the surface of the sphere at 80km), one triggered the "collision" flag in the simulation. This makes sense, as at the distance to Endor, it subtends 0.13radians, which translates to 0.435% of the full sky close to the 1/200 simulation particles hitting. This is still 2.94e14kg being dumped on the planet. That's about a third the size of the
Chicxulub impact, although it'd be spread out somewhat instead of being a single impact.
The other fun thing I tried was reducing the explosion strength by slowing the particle velocity:
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| Red is 100 times smaller and blue is a 1000. |
In these cases, the particles continue to largely follow the original DS orbit. In the lowest energy case, Endor is spared, and the extra energy goes to kicking particles out.
