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Most of us know the Death Star from Star Wars - a huge planet like structure built in order to destroy planets (160 km in diameter Google says). Now building something that big seems like an overkill.

I could imagine that a civilisation, somewhat more advanced than ours (like Type II advanced) could build something less bulky. With the idea of planet destruction in mind, I would propose a mesh of small (like a big house small) satellites that would form a net over the planet. Then they would simultaneously hit the planet with the energy beams the draw from the Sun, causing it to overheat and well, just crack and fall apart.

How scientifically plausible is this idea? How many satellites would be plausibly needed? If you think this idea, scientifically thinking, is crap, how would your planet killing machine of a small to medium size look like? (we are talking about like big starships at most,nothing Moon-big)

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    $\begingroup$ Planets won't just "fall apart", they are held together by gravity, not glue. Please have a look at the vast number of "how do I blow up a planet" questions that we have previously had on this site and see if any of them answer your question, especially worldbuilding.stackexchange.com/questions/4679/… $\endgroup$ Mar 6 at 22:56
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    $\begingroup$ "Energy beams that draw from the sun" is a bit vague, but it seems unlikely that a set of satellites in orbit around a planet could focus significantly more of the sun's energy on it than it would already receive. (Plus, at least some of the satellites will be eclipsed by the planet and in shadow, and what good will they do?) $\endgroup$
    – Cadence
    Mar 6 at 22:58
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    $\begingroup$ (a) You are allowed one and only one question. Asking multiple questions is a literal reason to close a question ("Needs More Focus"). (b) Per the tour, Stack Exchange is not a discussion forum. Inviting a discussion is off-topic. (c) We know of nothing with enough energy density to provide the firepower you're looking for, which is good, because hitting said battery with anything would vaporize a good chunk of a solar system. (d) Movies like SW do things to look cool, not to be scientifically accurate. (e) Scientific reality is boring - it almost never allows fantasy, which this is. $\endgroup$
    – JBH
    Mar 7 at 16:13

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Frame challenge: there is really no way to make a small Death Star.

a mesh of small (like a big house small) satellites that would form a net over the planet. Then they would simultaneously hit the planet with the energy beams the draw from the Sun, causing it to overheat

A planet doesn't overheat nor fall apart because of the radiation it gets from its star, unless the star is being a supernova. By putting a shell of satellites around the planet to collect that same energy, you won't get it either.

To put some numbers with the help of Wolfram Alpha:

  • Earth gravitational binding energy is $2\cdot 10^{32} \space J$.
  • The solar constant for Earth is 1370 $W/m^2$.

This means you would need a $10^{29} \space m^2$ to harvest that energy around Earth in 1 second, which would be an emisphere with radius about 840 au, or 0.013 light years.

Even adding batteries to store the energy would not help: Earth receives $1.74 \cdot 10^{14} \space W$ from the Sun, so you would need to store that flux for $10^{15} \space s$ to accumulate the needed energy, which is about 32 million years.

If you want to vaporize a planet beyond its gravitational binding point, you would probably be better off by getting the entire output of the star and throwing it on the planet.

So, you would need to surround the star with a shell of satellites and then rely this energy to the beam delivery system. This won't look anyway close to what you describe, though.

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No net of satellites would be enough

Your suggestion has a big flaw. Each planet is already bombarded by direct rays of the star. Even for planets close to the star this isn't enough to disintegrate them. Planets fall apart by the gravity of the star before they succumb to the direct starlight. Light simply isn't so strong.

Putting "a net" of satellites around the planet will not increase the amount of light input much. It will just have the light input of a planet slightly bigger than the current one. Worse is that if any part of the net is in the direct starlight, you'll actually reduce the amount of light falling on the planet.

What you would need is an amount of satellites that spans a large part of the solar system, redirecting as much energy as possible towards the target. Even then I'm not sure if it'll destroy the planet, even if the whole crust will he scorched. Redirected light can never heat something to warmer than where it came from. Heating the whole surface of the planet to the temperature of the star will definitely destroy anything living on it, but it will not destroy it as the Death Star would.

You could store some of the energy and give a big pulse with all of the satellites all at once. This is probably your biggest bet. But again, the amount of satellites you need in the solar system is immense. They need to soak up as much of the starlight as possible. At that point it seems best to think of Dyson Swarms. They float around the star, collecting energy and beaming it where needed.

Even if the mass of a planet is spread out around the sun I doubt they can store and fire enough energy to destroy a planet. Suddenly the Death Star does seem to be economical. A space station the size of a moon. For just the materials of half a planet you can destroy another. Instead of putting it apart into billions of satellites that do not get the job done.

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Obviously, you need the Darksaber!

But seriously, it depends on what you want. If you want to shatter a planet, something as big as a death star might actually be too small. Remember, mass=energy times the speed of light. In practice, this means a planet already is more energy than a star outputs in a reasonable amount of time. If you want to break that up, moon sized rocks are probably a better bet. Stick some massive engines on a moon, that will teach those rebels! Though if you want to be economically with you planet busting, you only need a rock the size of Everest. Way less work than building all those satellites. You don't even need to leave engines on the rock. You can set up the engines long enough to lunch your extinction level asteroid at the planet, than pack them up and be gone a before those poor rebels even know anything is wrong. It's not like the asteroid is gonna slow down on the way.

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Not a satellite swarm

A solar powered satellite swarm capable of blowing up a planet would have to be much bigger than the death star, not smaller. It takes up a LOT of power to blow up a planet. So, let's look instead at something much more power dense. This closely related question https://worldbuilding.stackexchange.com/a/4684/57832 tells us you need about 1.3 trillion tons of antimatter to blow up an Earth sized world, but does not quite tell us the smallest possible kill vehicle for destroying a planet. If you are going for a minimally small world destroyer, you need a bomb, not a giant space gun. The good news is if you are technologically advanced enough to make this much antimatter, you may also be advanced enough to make antimatter into complex elements.

Isolated from normal matter, anti-matter and matter behave exactly the same; so, you could in theory create a ship or missile entirely out of antimatter complete with engines, optics, deflector shields, computers, etc. such that you don't actually need extra weight for a giant anti-matter containment system making it theoretically possible for your ship to weight only 1.3 trillion tons.

Now density gets a little tricky here, because we can not say for sure how dense a space ship designed to be a kill vehicle would be. The best approximation I can think of is that it should be somewhere between a normal missile, and a solid lump or iron (not considering denser that iron because you can't use found elements and any element more dense than iron becomes much more expensive to manufacture). This puts the total density of your antimatter kill vehicle somewhere in the 1.7-7.8g/cm^3 range

Using a basic volume of a sphere calculation divided by density we can determine that an antimatter bomb of this size could very believably have a diameter somewhere in the range of 6.82-11.34 km.

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