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Mounted in the middle of the Colonist main military base is the Massive Energized Particle Railgun. A BFG (Big Freaking Gun) able to draw on an effectively endless amount of power in order to charge up, and which point it is capable of firing a 1,000 ton “bullet” out of the atmosphere at relativistic velocity and into space. Obviously, once in space the bullet can keep its trajectory forever without slowing. Since the bullet is really an RKV (relativistic kill vehicle, or a big thing going REALLY fast), whatever it hits will be destroyed, as the impact will release more energy than a matter/antimatter reaction with the same mass.

Hang on though, wouldn’t this mean that this gun could destroy any planet in the galaxy if given enough time and accurate information about said planet’s orbit? That’s not really conflict that’s holding a gun to the collective head of the rest of the galaxy. So is there some reason that this BFG could not hit another planet? If not, is there some way that a planet could defend against this galactic scale weapon of mass destruction?

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closed as primarily opinion-based by Aify, Azuaron, Richard U, sphennings, SPavel Feb 8 '18 at 14:27

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Yes, destroy the whole planet the Colonist military base is located on. Only in Star Wars you have secrety secret shadowy secret base that no one know about that have the same power as just droping a 100km wide rock on planet. $\endgroup$ – SZCZERZO KŁY Feb 7 '18 at 16:17
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    $\begingroup$ "firing a 1,000 ton “bullet” out of the atmosphere at relativistic velocity and into space" At 0.9c that's about 10^23 J, roughly the equivalent energy of a "dinosaur killer" meteor impact plowing through and heating your atmosphere. The first victim of the BFG will be the ones who fire it. Put it on a moon. $\endgroup$ – Schwern Feb 7 '18 at 19:33
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    $\begingroup$ I was really hoping this was going to be about a sequel to The BFG by Roald Dahl. (Big Friendly Giant) $\endgroup$ – Todd Wilcox Feb 7 '18 at 19:45
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    $\begingroup$ @Schwern Even if there was no atmosphere, just due to Newton's 3rd law of motion (an equal reaction for every reaction), if the bullet is capable of taking out a planet, the same energy to take out a planet will have to be dissipated in the recoil. Seems like the only safe way to do this is for your launcher to be in space, and be one time use that results in a considerable portion of your launcher being launched at a large fraction of the speed of light in the other direction. $\endgroup$ – Shufflepants Feb 7 '18 at 19:58
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    $\begingroup$ An interstellar projectile that travels at sub light speeds (even if perfectly accurate) seems completely useless. At best, you're shooting at the nearest system to yours and it takes 2-10 years to get there, at worst you're shooting at the other side of the galaxy, and 100,000 years pass before it gets there. Possibly thousands of generations have come and past since the war ended and the two civilizations don't even remember that they were ever enemies if they still exist at all. $\endgroup$ – Shufflepants Feb 7 '18 at 20:09

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First, some problems.

Your BFG isn't Big enough.

Wouldn’t this mean that this gun could destroy any planet in the galaxy?

Depends on what you mean by "destroy".

A 1,000 ton projectile at 0.9c is about 1e23 J. This is a lot, it's about 1/4 the energy of the meteor that killed the dinosaurs, but the impact will not "release more energy than a matter/antimatter reaction with the same mass". It's roughly the same.

Getting smacked by a dinosaur killer is bad, but it won't destroy the planet. A civilization advanced enough to be worth expending 1e23 J on can recover and fire back. We'll get to that.

You can, of course, pump this thing up with as much energy as you like. At 0.999c it's 2e24 J, but it still isn't a planet cracker. More like a major ecological disaster.

Sorry about your atmosphere.

firing a 1,000 ton “bullet” out of the atmosphere at relativistic velocity and into space

This 1000 ton bullet has to plow through your atmosphere adding drag, reducing it's final velocity, and greatly increasing the amount of energy you need to fire it.

Worse, it will compress and heat the atmosphere creating an enormous fireball at the launch site doing a lot of damage to your own planet.

1e23J is roughly a week's worth of sunlight for Earth. One shot will disrupt weather patterns. Fire a few of these and you'll cook your planet.

The first victim of the BFG will be the ones who fire it. This needs to go on a moon with no atmosphere, or in space.

How do you get it that fast?

Assuming you have the energy, and we're talking "Type II Civilization" levels of energy, how do you apply it to the projectile without obliterating it? This is the basic problem of a rail gun, how do you accelerate an object to relativistic speeds before it leaves the "barrel"?

You could make the barrel longer and longer and longer, but at 0.9c this thing will go the diameter of the Earth in 47 ms. Not a lot of time to apply energy.

Better to use a ring accelerator. Basically a giant cyclotron particle accelerator. The projectile spins around the ring, going faster and faster, held in the ring by powerful magnets. When it's reached its final velocity, the magnets release and it goes flying off.

This would take even more energy to hold the projectile in the ring while its accelerating. The bigger the ring, the less energy necessary. So let's build a big ring.

Everyone Will Fire Back

The problem with a purely kinetic is whoever gets hit can trace it right back to you. And they're going to be pissed. And they'll throw whatever they have left at you. As before, this is Type II Civilization levels of energy. Presumably your enemies are also Type II Civilizations to be worthy of such an investment which means they have more than one planet, and ways at striking back at you.

Even if they don't, everyone else is likely to be pissed and come to destroy this threat to the galaxy.

The Big Falcon Ring

This can't be in an atmosphere. It has to be huge. It needs enormous amounts of energy. It needs time between shots to gather energy and accelerate the projectile.

Pick a large moon with no atmosphere and build the ring on the surface (turns out this isn't going to work, see below). As others have pointed out, you'll need several rings to be able to fire at all points in the sky. Putting it in orbit doesn't help. Mine the moon itself for material. Put solar arrays in orbit to power it all, possibly even a Dyson Swarm around the star.

The amount of energy needed to keep the projectile in the ring path is related to its centripetal force. The formula for relativistic centripetal force is $$F = y m v^2/r$$.

y is The Lorentz factor, $1/\sqrt{1 - v^2/c^2}$. This accounts for relativistic velocities.

  • v = velocity, ultimately 0.9c
  • m = mass of the projectile, 1000 kg
  • y = the Lorentz factor, ~2.3 at 0.9c
  • r = radius of the ring

The force necessary to keep the projectile on a circular path is inversely proportional to the size of the ring. Double the size of the ring, halve the force necessary to keep the projectile in a circular path.

Starting with the radius of Earth's moon, 1.7e6 m, we get $F = 1000 kg * (0.9c)^2 / 1.7e6m$ or about 1e14 N. This is a lot. A Saturn V rocket puts out about 1e7 N. However, we can scale this down by scaling up the ring. Put it at Earth's orbit, 1 AU, and we're down to 1e9 N. Assuming the force is exerted over 1 m^2, this is 1 GPa inside the yield strength of steel.

Stronger and more exotic materials, like graphene with a tensile strength of about 100 GPa, would allow a 0.01 AU ring or a "mere" 1.5e6 km. Sci-fi materials would make it even smaller.

Put it in a system that isn't terribly important to you, because that's where everyone will trace your shot back to, and that's where everyone is going to direct their ire. The ring need not be orbiting the Sun, it could be at a stable Lagrange point.

I can't underscore enough just how large a project this is, even for a Type II Civilization.

The BFG needs Big Falcon Power.

We can calculate the forces involved for the cyclotron gun at maximum velocity. The force required to keep a non-relativistic projectile in the ring is F = mv^2/r. For your 1000 tonne projectile at 0.9c on something like the Earth's moon it's about 5e16 N: a lot. To put this in perspective, 5e16 N is the equivalent of lifting everything humanity has ever made 1 meter up in 1 second. And that's before relativistic considerations.

Putting aside the question of how you'll build a moon sided cyclotron that can withstand 5e16 N of force, something very, very large will have to power this. Not only is this a major investment by your civilization, but it's vulnerable to attack, and vulnerable while its being built. You'd need to pretend its some sort of civilian project.

But once it's fired your cover is blown and it is vulnerable. Like a nuclear weapon its role is a balance of terror. Once you use it its value to protect yourself is lost.

How Fast Can You Fire Again?

This thing needs enormous amounts of energy. And it needs time to accelerate the projectile. And the apparatus might be damaged in the firing. How often can you fire it?

This is largely up to you. You can tweak the numbers for your story. If it's once a year, then they can get maybe 4 or 5 shots off at a close neighbor before anyone realizes what's happened. Maybe it's longer. Maybe its shorter.

Narratively it gives time for the attacked civilization to react before another shot can be fired. Even better if they know they're doomed. The first shot has already landed, maybe somewhere relatively unoccupied to limit the immediate damage. They know more are already on the way and they can't stop them. But they can try to destroy BFG before it harms anyone else, and launch themselves at it knowing by the time they get there their planet will probably be wrecked.

Time On Target

If your civilization wants to be really smart, it conducts a Time On Target bombardment. It fires at its furthest targets first, then closer and closer ones. The end result is every target is hit simultaneously. Nobody can see it coming. Nobody gets any warning.

How Do You Know Who You're Firing At?

Is there some way that a planet could defend against this galactic scale weapon of mass destruction?

"Galactic scale" is a bit of a problem for your targeting. We're talking 100,000+ light years end-to-end. Being able to fire a relativistic kinetic projectile and hit a moving target 100,000 ly away is crazy complex and requires information you literally cannot have.

At the time of firing you'll be seeing your target as it was 100,000 years ago. While you can do rough calculations to determine its motion, you can't do this with sufficient accuracy to hit a planet both because of the crazy complicated and chaotic math involved, and because you cannot have sufficient detail at that range. You probably can't even see the planet.

There's also the question of how you got into a fight with someone it takes 200,000 years to communicate with (100,000 years out, 100,000 years back). When your projectile falls, who will even be on that planet? Will it even be the same species? It's like firing at us for something the Neanderthals did.

Either you're doing some extremely grand space opera, or you should scale it back. A lot. 20 light years offers you about 100 or so systems to play with and time scales that are inside a lifetime.

Do we even need a BFG?

What if we put a motor on this 1000 kg mass? What would that take?

Running the numbers we can use the rocket equation to find an ideal solution of how much reaction mass we'd need for a theoretically near perfect sci-fi space motor.

$$m0 = m1*e^{dv/ve}$$

  • m0 = starting mass
  • m1 = final mass, 1000kg
  • dv = total change in velocity, 0.9c
  • ve = exhaust velocity

The tech details don't matter because ultimately space motors throw mass out the back as fast as possible and rely on Newton's second law to be thrusted forward. It all depends on how much (reaction mass) and how fast (exhaust velocity) we throw it out the back. The higher the exhaust velocity the less mass we need. Less mass means a lighter spacecraft which requires less thrust for the same acceleration and the Tyranny Of The Rocket Equation works in our favor.

The exhaust velocity of a very, very efficient ion thruster is 210km/s. So that's $1000kg * e^{0.9c/210km/s}$ or $1000kg * e^{1286}$. $e^{1286}$ is so large even Wolfram Alpha won't give me an answer. So much for known tech.

If ve = 0.1c that's $1000kg * e^9$, 8e6 kg or 8000 tonnes of reaction mass. Not infeasible! Probably about as much mass as your average sci-fi space cruiser. Let's go faster!

Let's say we can throw mass out the back of this engine at ve = 0.9c! $1000kg * e$ is a mere 1700 kg of reaction mass. Great! We're in business... maybe.

What about the energies involved in throwing all this mass at relativistic velocities? How much mass are we throwing out the back and how much energy does it take? The mass is easy enough to calculate since we're at constant acceleration, $reaction mass / time$. How long do we need to be accelerating? Depends on how far the target it. The worst case is a nearby star system at about 4.5 ly. It's accelerating constantly to 0.9c, so the average projectile velocity is 0.45c. It'll be accelerating (hopefully) tiny fractions of this mass for roughly 10 years.

If ve = 0.1c, that's $8e6 kg / 10 years$ or 25g/s. This is a lot. The kinetic energy of 25g at 0.1c is about 1.1e13 J. This is a lot of energy. This means our engine must produce 11 TW for 10 years: 3.2e21J. Assuming the most mass-efficient generator possible, a matter/anti-matter reaction, and using $e = mc^2$ and $m = e/c^2$ that would require 35,600 kg adding significantly to our projectile's mass and throwing off the rocket equation. ve = 0.1c won't cut it.

If ve = the ludicrous 0.9c that's only $1700 kg / 10 years$ or 5.4mg/second. At 0.9 it has a kinetic energy of 6.3e11 J. That's still a lot of energy requiring 0.6 TW over 10 years: 2e20J or the matter/anti-matter reaction of over 2000 kg again throwing off our rocket equation, but not unrecoverably. I'm not sure on the math, but I'd estimate we'd wind up with something like 5000 kg of reaction mass and 5000 kg of matter/anti-matter.

What about beaming it power from a laser? If you can focus a 1 TW laser on a moving target 5 light years away and sustain it for 10 years, why are you messing around with throwing rocks? Just cook them.

As assuming we can come up with some lightweight matter/anti-matter power source, and a lightweight engine that can fire milligrams of matter at 0.9c, this can be done inside known physics... but not known engineering.

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    $\begingroup$ Very nice! The more answers I get the more I see the need for some handwavium and applied phlebotinum with a bit of unobtainium on the side. The numbers were really helpful and gave me a good idea of the damage one of these could do. $\endgroup$ – Nick Feb 7 '18 at 20:46
  • $\begingroup$ @Nick Thanks. I also added a bit about the "galactic" scale just now. $\endgroup$ – Schwern Feb 7 '18 at 20:50
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    $\begingroup$ Also, at such speeds the drag from interstellar hydrogen becomes a significant factor, ablating your projectile away or significantly slowing it down. $\endgroup$ – vsz Feb 8 '18 at 5:55
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    $\begingroup$ "I can't underscore enough just how large a project this is, even for a Type II Civilization." Not to understate that this is a big project but by definition a Type II Civilization already has access to ALL energy available from their star (we are Type ~0.7) so they have a dyson sphere + some other sources (e.g. a small artifal extra sun to get to 100%). I don't think building an accelerator on the much bigger existing dyson sphere is a big afford for them. A Type II civilisation produces 10^26 W so they could fire 1000 such projectiles per second. $\endgroup$ – Christoph Feb 8 '18 at 9:40
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    $\begingroup$ @Schwern If you have to use a ring accelerator (I don't think that is neccessary if you have type II power limits and technology) then you have to compensate for the Bremsstrahlung. Indeeded the energy usage in that case would be a lot higher if you have a tiny ring (few km) but on solar system scale 0.9c wouldn't require much additional energy. If you have a ring at sun earth distance you would need ~1600s for a single round ... at the speed of light ... I doubt accelerating would take that long. $\endgroup$ – Christoph Feb 9 '18 at 7:42
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As Phillip said, this is viable for within-system warfare, but not for interstellar.

Accuracy is a major problem. Even at relativistic speeds, projectile will take many years to reach another star system, so you need to predict where the planet will be then. You cannot adjust your aim as it will take years between shots, and your target will notice the near-misses, and find a way to detect and deflect or destroy further shots.

You can try to install guidance system, but then you need to go slower than light to give it time to react, which also gives target time to respond. And it is a technical challenge to make a engine that is powerfull enough to alter trajectory, and robust enough to survive being shot from a railgun.

If impact destroys the target planet, the recoil will do major damage to launch planet, or at least alter its orbit. You can use planet's own spin and orbital momentum to help accelerate the projectile, but those are a fraction of relativistic speeds. Impact is shorter than acceleration, but not by a whole lot (unless your railgun is miles and miles long).

"firing a 1,000 ton bullet out of the atmosphere at relativistic velocity" will likely obliterate all surrounding area. Here is the a 10kg bullet being fired
But you could put your BFG on a moon without atmosphere.

Defense:
If it is coming at less than full speed of light, you can detect it. Then you hit it with your own impactor. It does not have to be heavy, 1T will do enough to deflect it, or destroy whatever guidance mechanism is there.

If the projectile is coming at the speed of light, I am not sure if guidance system is viable, so defenders they can just alter the orbit of their planet slightly, in random direction, every few years. Your projectile takes more than a few years to get there, and the planet will be out of the way by then. Technology to alter the orbit will be similar to your BFG: launch heavy rocks.

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  • $\begingroup$ I had figured that I would put in on a very dense moon with low atmosphere. You also made a good point about the size of the weapon, it would have to be immense. $\endgroup$ – Nick Feb 7 '18 at 16:31
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    $\begingroup$ The orbit alteration of the launching planet could be fixed by firing a second bullet in the opposite direction. $\endgroup$ – Philipp Feb 7 '18 at 16:36
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    $\begingroup$ @Philipp That's just replacing recoil with the problem of "my planet is being hammered on a giant anvil". $\endgroup$ – chepner Feb 7 '18 at 17:52
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    $\begingroup$ You could launch two objects more-or-less simultaneously, in more-or-less opposite directions, in order to minimize the effect on the launcher's rotation and orbit. $\endgroup$ – Jasper Feb 7 '18 at 18:35
  • $\begingroup$ "but then you need to go slower than light to give it time to react, which also gives target time to respond" Relativistic speeds are already slower than light. $\endgroup$ – jpmc26 Feb 7 '18 at 22:51
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You said in a comment that your universe has FTL travel and communication, while your railgun is firing sublight-speed projectiles.

That makes it a useful weapon for interplanetary warfare, but not so much for interstellar warfare. It will take years to hit something in a neighbor star system and up to 100,000 years to hit a target at the other end of the galaxy. Military deterrent doesn't really work on those timescales. When the civilization living on the targeted planet gets the news that there is a RKV heading their way, they have plenty of time to slightly change the orbit of their planet so it will miss (which might not even be necessary if you forget to account for some unknown mass changing the target orbit or your projectile's trajectory), change its course by hitting it with their own relativistic mass or just relocate elsewhere. And they will likely leave this challenge to future generations while concentrating their current effort on retaliation against you.

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    $\begingroup$ If the round could be put into “hyperspace” so that it’s travel time is greatly reduced, then comes back into normal space before impact, would that make the weapon more useful over interstellar distances? $\endgroup$ – Nick Feb 7 '18 at 16:30
  • $\begingroup$ @Nick That depends on how accurate your hyperspace travel is over interstellar distances and if there is a way to interfere with it. $\endgroup$ – Philipp Feb 7 '18 at 16:31
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    $\begingroup$ @Nick: if you have the ability to manipulate energy on this scale and also the ability to drop things in and out of hyperspace easily you’d probably be better off just dumping the energy straight into the target via hyperspace. Why bother with all those unfeasibly huge accelerations when you can just reach through hyperspace and superheat the bedrock? $\endgroup$ – Joe Bloggs Feb 7 '18 at 17:22
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    $\begingroup$ @Nick If you're talking about hyperspace, that's hardly "reality-check" or "physics". $\endgroup$ – Shufflepants Feb 7 '18 at 19:59
  • $\begingroup$ @Shufflepants the hyperspace part is kind of an afterthought really, which is why it isn’t in the question. The central thing here is could this weapon hit anywhere it wanted, and if so could it be defended against. $\endgroup$ – Nick Feb 7 '18 at 20:06
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There are so many reasons this won't work I'm not sure if I'll be able to list all, but let's give it a try:

Space is big, part 1

Passive missile has big flaw. Little errors on the launch can change into big ones at the target. And we are talking interstellar distances. Radius of planetary orbit is negligible compared to distance traveled. Radius of a planet does not even count.

Space is big, part 2

Three body problem is not solved analitycally yet, and probably will never be — it was proven impossible within our current understanding of mathematics. This means you need to calculate effect of each gravitational mass on your missile, and on each other. Each asteroid that can pull it even so slightly sideways. Each tiny impact. This is a lot of numerical calculations. And lot of numerical errors. And a lot of things you will have to measure, a lot of measurement errors. On the interstellar distance it will make it miss.

Countermeasures are easy

All you need to do is to hit it slightly, a little from the side. You have years to prepare, decades maybe, because impacts with interstellar gas will release detectable amounts of energy. If you have FTL means of knowing it is coming, you have literally dozens of millennia. You can retry few times. Side of missile will explode, rest will fly on different course.

Retaliation

If such missiles could hit, you can be pretty sure that firing one will result in revenge launch. Use one and you are playing with mutual kill. Or, with FTL, they can prepare some nasty surprises for your descendants and make them stop the missile or die,with thousands of years to try and stop it on their own if this fails.

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    $\begingroup$ The three body problem isn't just not solved analytically, it's been proven that there can be no analytic solution for non-degenerate cases. But really, that doesn't really matter, because getting a numeric solution arbitrarily accurate is not ruled out. The real problem of course is physical measurement accuracy and unknowns en route. $\endgroup$ – Shufflepants Feb 7 '18 at 20:00
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    $\begingroup$ @Shufflepants I'm very, very cautious about possible / impossible when talking future this far. $\endgroup$ – Mołot Feb 8 '18 at 7:30
  • $\begingroup$ It's not a matter of likelihood, or not having the technology. It has already been proven mathematically impossible. There is as much chance of finding a general analytic solution to the 3-body problem as there is of proving 2+2=5. Technologically, we could get to the point where our numeric approximation is so good that our calculations wouldn't be measurably improved by having an analytic solution (if such a thing existed), but there cannot exist a closed analytic solution in the general case. $\endgroup$ – Shufflepants Feb 8 '18 at 15:42
  • $\begingroup$ Its not solved, and probably not relevant either. Planetary Orbits over a span of a bunch of thousands of years are pretty stable, and at the speeds of a relativistic missile the effect of gravity is almost negligible to the point that even at the margins of errors mentioned here, its just not going to factor in. If you where shooting at something where a gravitational field WOULD be sufficient to counter the massive amount of intertia, sure, but earth like planets around your regular main sequence stars, I just can't see it being a huge issue at those speeds. $\endgroup$ – Shayne Jan 23 at 2:54
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wouldn’t this mean that this gun could destroy any planet in the galaxy if given enough time and accurate information about said planet’s orbit?

No, because there's a vast amount of space which is completely inaccessible from the BFG. For example, in this image, only that area of space in which the w-axis has positive values are visible. The planet rotates, so other parts of the Universe become visible as others are shielded by the planet, but there are still huge swaths that will never be visible.

For this same reason, the only people in the Northern Hemisphere who can see southern stars are those very near the Equator.

For guns on the equator: visualize what the "tangent plane" would look like, rotating around the planet. (As an example, inflate a beach ball, and then slap a piece of cardboard at the equator.) There's no way that it can hit anything around the Poles.

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  • $\begingroup$ shoot from the equator towards the tangent or lower angles, then use the planet's own gravity to correct the projectile course. That's called an escapw trajectory over a polar orbit. Absolurely doable. $\endgroup$ – Renan Feb 8 '18 at 2:46
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    $\begingroup$ @Renan It takes time for gravity to overcome the momentum of the projectile traveling it's aimed path (which is why current launch vehicles slowly tip over), and with the acceleration needed to get a (big) bullet slug to relativistic (87%+ of c) speeds, the kill vehicle will be too far from the planet happens. (Maybe using the Sun, but I'd want to see the math.) Just build one gun at 45N and another at it's antipode. $\endgroup$ – RonJohn Feb 8 '18 at 7:25
  • $\begingroup$ Launch vehicles do a "gravity turn" to gain sideways speed to enter orbit, not to "overcome momentum". $\endgroup$ – a CVn Feb 8 '18 at 10:36
  • $\begingroup$ @MichaelKjörling Throw a golf ball up (like Alan Shepard did during Apollo 14) while standing on the Moon. Does gravity instantly pull it down? No. It slows the ball down at 1.6m/s/s. Likewise, once you tip the spaceship over, gravity will only pull it over at 9.8m/s/s. That's why a vehicle traveling at 0.87*c* won't be tipped over by planetary gravity. $\endgroup$ – RonJohn Feb 8 '18 at 14:13
  • $\begingroup$ @MichaelKjörling also, there's no way for you to gain angular velocity due to gravity, since gravity is always forcing you down. Gravity changes your direction, though. $\endgroup$ – RonJohn Feb 8 '18 at 14:17
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Space is huge (as other answers have stated) and it's also tremendously unknown. Remember that a 0.000000001% degree error on these calculations with a projectile travelling 1000 light years @ light speed will miss by a few million km. The calculations required for this multi-year weapon to strike a target must include:

  • Projectile can't hit where the planet is now, it needs to project where the target planet will be when the strike occurs. Anything that effects the motion of your projectile could also effect the target.
  • Gravitational effects of all bodies the weapon (and your target) might pass. Remember those bodies that could gravitational effect the bullet are also in motion, so you'll need to know each bodies movement along the path to know where they will be when they interact with the projectile. Remember these factors will also affect the position of your target.
  • Space weather. The Heliosphere protects most of the solar system from some interesting weather that we barely get a glimpse of on Earth...but there is matter travelling in all sorts of directions at near speeds of light that will be knocking your projectile around. Not a major amount by any measure, but it doesn't need to change the trajectory much to cause a miss. I'd be curious what would happen if your speed of light projectile hit a proton travelling at nearly the speed of light in the opposite direction...I'm sure the people at CERN would like to know, and I'd speculate it'd be enough to adjust the trajectory of your bullet.
  • Dark Matter. What is big and unknown, and able to impact the trajectory of your bullet?

You would need a nearly omniscient level of knowledge to strike a target at that range.

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Suppose you describe WW1 16" gun to a civilization that has bows and arrows.

They say "what if picked one up and shot it at a mammoth?"

This seems to be your problem here. The energy involved in pushing 1000T up to a decent fraction of c means you aren't doing it on an inhabited planet. The energy budgets are wrong for that.

That kind of weapon isn't mounted on an inhabited planet. Similarly, a 16" gun isn't something you carry around.

Using this weapon on a civilization that isn't on a similar energy level is like using a 16" gun on a mammoth. Sure you kill it, but why are you using a 16" gun to kill a mammoth? Mammoth's aren't dangerous to someone with 16" gun technology, and anything of value will be destroyed by using it. It is ridiculous overkill.

Your weapon could probably hit a planet within a few light years, assuming you solve the aiming problem, but beyond that the travel time is long enough that chaos in planetary orbits and gravitational tug on the projectile will make the target not be where your bullet is.

We used 16" guns in WWI to destroy fortifications designed to defeat 1800s military guns. If this weapon is built, you'd use it in a similar sense, to defeat something a slightly lower tier of technology cannot defeat. Quite possibly you wouldn't shoot solid projectiles, or you'd somehow mount guidance systems.

It wouldn't be on a planet. Maybe it would be a set of solar-sail based accellerators which in turn are moved around by ridiculously high energy lasers mounted on other platforms. Each shot might involve gigatonnes of mirrors blasted off to infinity and the entire solar system lighting up with reflected laser light.

The weapon you describe is a type 1.5 civilization weapon. At relativistic speeds, the KE of matter is approximetally its rest mass. Type II civilizations have 4E26 Watts of power.

A Type I civilization could fire one every 26 days using the entire civilization power output. A Type II civilization can fire something like that a few 1000s of times per second. A Type 1.5 civilization could fire one using 22 seconds of civilization power budget.

Storing 26 days of your civilizations' power budget is unreasonable; Tsar Bomba is about 3 hours of our civilizations' power budget.

A Type 1.5-2 civilization isn't planet-bound; no significant fraction of their economy is in a Earth-depth gravity well. This is simply a side effect of heat budgets (their energy budget is too large for an Earth-sized planet to handle).

Type 1.5 might be able to have a significant fraction of their economy in a Jupiter-sized body, but by Type 2 that isn't reasonable (other than having disassembled Jupiter-sized bodies for raw materials).

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  • $\begingroup$ I had envisioned that this weapon would be used to destroy space stations and military vessels, so you read my mind on that one, however I wanted to take it to the very endpoint of “what if I shoot it at a planet?” $\endgroup$ – Nick Feb 7 '18 at 20:54
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Putting inside an atmosphere will cut one of its balls off.

Lobbing your bullet out of the atmosphere, it will start to burn up. It will also add some imprecision - as it passes through wind, clouds and turbulence, its course will change. Not a lot, but it's got a long way to go.

It could also damage the planet, and certainly the colony. How loud is the boom?

It would be really hard to hit things that it can't see in the sky - a ship on the other side of the planet, for example.

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  • $\begingroup$ I think this is the best option. The gun may be "free" to fire but has significant costs to the plane that fires it. They can fire it, but recognizing the cost they won't do so unless there's a really, really good reason to. This is similar to the mutually assured destruction tactic. $\endgroup$ – Adam Davis Feb 7 '18 at 19:30
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This is somewhat more of a comment than an answer, but the closest thing to a worked example was in the book "The Millennial Project: Colonizing the Galaxy in Eight Easy Steps" by Marshall T. Savage.

Near the end of the book, he speaks of a galaxy spanning civilization using massive mass drivers to send pods between star systems at .9 c in order to maintain trade and communications between the various star systems (with "reasonable" time delays, especially for a traveller inside the pod). The traveller is encased in some sort of gel or fluid since they will be undergoing about 10 g acceleration for a month in a mass driver which spans a large fraction of the solar system In the book, a similar device in the receiving solar system is used to decelerate the pods, and to launch a return pod.

This may not be entirely practical since it is such a huge construct, and waiting for things to line up to fire the shot will also take a great deal of time.

Unlike many of the other commenters, I do believe that this is a viable system, since the pod (or warhead) can be outfitted with a guidance system and is large enough for thrusters to provide the fine tuning of the orbit needed to strike a planet. Moving at .9 c it is coming very close behind its own light cone, so the target has very little real warning or time to deploy any countermeasures. Increasing the energy density at the launch end cam push the pod to almost any arbitrary speed, at .99 c you likely do have a planet buster, and the warning time is virtually nil.

Needless to say, ownership of such device will be strongly opposed by everyone in the vicinity, and the knowledge that such a thing exists will be seen as an existential threat by almost any civilization in range. The likely response is to not just build a counter weapon in your solar system, but to preemptively carry out a first strike to neutralize the threat.

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A giant space bullet is never going to be able to hit a random planet somewhere in the galaxy.

Even assuming you could build such a weapon, the slightest micrometeor or gravity wave would knock it off course enough to miss the whole solar system.

Imagine a sniper hitting a nickle two miles away in a class five hurricane only harder.

Anything like this would have to have a guidance system. If you have a guidance system, you might as well stick and engine on it and have it fire itself in space AKA a kamikaze spaceship or guided asteroid.

Finally is the time frame. Star systems are light years apart. When you fire the bullet, you'll be wiping out anything from the target's grandchildren to hitting a dead planet because the race already went extinct.

Giant space bullets will never be a cross galaxy threat.

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There are many very cheap deterrents to that weapon.

Some space federation or whatever equivalent to a country there is in that fictional world may simply shoot a very powerful laser at the cannon. Just melt a few millimeters off of it, and the whole thing will explode the next time it fires.

If the cannon is very articulate, you can move a few Phobos-sized asteroids onto the orbit of the planet. Should a projectile hit one of these asteroids on its way through a target, not only will it be deflected, you will also end up spreading a lot of large-sized particles in orbit. This will ❤❤❤❤ artificial satellites, space stations and ships that get hit by the particles. Say goodbye to all your space efforts and forces.

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    $\begingroup$ Don't forget the cheapest deterrent to this weapon: who the heck cares about it? Even if they shoot at you, it'll potentially be thousands of years before it arrives depending on where in the galaxy you are in relation to them. Even if you're right next door, you've still got a couple of years to figure out what to do about it. $\endgroup$ – Shufflepants Feb 7 '18 at 20:06
  • $\begingroup$ That powerful laser beam travels at the speed of light and will take just as long to reach the BFG as the BFG does to reach it, during which time the BFG might have fired more rocks. And it somehow has to remain coherent and accurate enough over light years of space and dust to have an effect. As for moving asteroids to deflect, since the BFG fires at projectiles relativistic speeds you will have little or no warning before the shot impacts. You literally cannot see it coming. $\endgroup$ – Schwern Feb 7 '18 at 20:54
  • $\begingroup$ @schwern the idea with asteroids is to have them there even before a decision to fire the bfg is taken. They are a statement that says "fire that weapon and fuck all your satelites". As for the laser, it may be orbital-based. $\endgroup$ – Renan Feb 7 '18 at 21:04
  • $\begingroup$ @Renan Since you can't know where the shot will land, you have to try to cover as much of the planet's surface with orbiting asteroids as possible to give the highest probability of an intercept. The surface area of the Earth is 500 million sq km. There's a limit to how many things you can have in stable orbits. A 1000 tonne steel sphere has a diameter of about 6 meters. Moving at 0.9c it will cross Low Earth Orbit in about 1 ms. The probability of intercept is very, very, very low. $\endgroup$ – Schwern Feb 7 '18 at 21:16
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    $\begingroup$ @Renan I think there's a clever short story idea here. Someone, somehow, possibly their own people, figure out the BFG is being built. They can't sabotage the BFG, but they instead place an object in deep space in the predicted path of the BFG. Small enough to be undetectable, but large enough to throw off the course of the BFG projectile in spectacular fashion. $\endgroup$ – Schwern Feb 7 '18 at 21:22
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In addition to the other answers, it's worth noting yet another reason the weapon should not be launched from the atmosphere. Not only will it be inaccurate and slow down, but Randall Munroe of XKCD fame can give some insight on a new problem:

3,000 kilometers per second = 0.01c:

This would be pretty bad. The thing that makes this a little unpredictable is the fact that at speeds in the range of hundreds of kilometers per second, the air begins to undergo nuclear fusion (Google that paper title to find the full text). This, combined with plain old plasma heating, may manage to eat apart the diamond sphere completely before it reaches the ground.

In this quote he's talking about a 100-foot meteor; it would start to break apart, but even if your big bullet doesn't stay together, most of the kinetic energy will still hit the target planet. The real problem here is the fusion part. There's a LOT of air in the way of a typical launch, and at relativistic speeds (keep in mind the example given is for only 0.01c!) this would cause the meteor to travel upwards through the atmosphere, causing fusion of certain compounds in the air it encounters on its way. You know what else causes fusion? Thermonuclear bombs.

TL;DR: Put it on a moon or something, as suggested elsewhere.

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