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I'm trying to gauge the efficacy of using cone-of-sand or cone-of-buckshot weapons against a horde of tiny alien spacecraft as an area-denial weapon.

That is, can I rain metal hail on them from far away, or do I have to engage them close up and use my buckshot as a "don't have to aim much" sort of weapon.

Some facts:

  • Aliens occupy a sphere 238,900 miles in radius. Sphere is thus approximately $5.71×10^{16} mi^3$ in volume

  • There are 10,000 alien vessels in this sphere. So each vessel has $5.71x10^{12} mi^3$ of space. Um, space is big. Wow.

  • Okay, let's look at the cross section of the sphere. A circle of that radius has an area of $1.79×10^{11} mi^3$. Assume each ship shows a profile of 1 square mile (which is not tiny!), the ships cover 10,000 square mile, meaning a straight shot through the mass has a 0.0000056% chance of hitting something

Given all that ... seems like I'd have to expend tens of thousands of bearings to get a decent chance of one hit.

So ... am I missing something, or is space just too big for area-denial weapons?

This is a worldbuilding question because I've got some aliens who want to defend their planet, and they're worried about getting their ... area denied. ;D If area-denial is impractical, they're more likely to use a swarm instead of several larger vessels.

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  • $\begingroup$ On the other hand 1E5 grains of sand mass only about 1 kg... $\endgroup$ – AlexP Sep 10 '17 at 22:17
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    $\begingroup$ The space-age equivalent of buckshot to take down 10,000 ships is probably hundreds of thousands of small self-guided missiles. This would solve the volume-of-space issue as they could hone in on their targets once in the general vicinity. $\endgroup$ – Hutch Sep 10 '17 at 22:40
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    $\begingroup$ Um, space is big. Wow. Obligatory quote: "Space is big. Really big. You just won't believe how vastly, hugely mind-bogglingly big it is! I mean, you may thing it's a long way down the street to the chemist but that's just peanuts to space! Listen...". — Douglas Adams $\endgroup$ – MichaelK Sep 11 '17 at 7:57
  • $\begingroup$ @MichaelK thank you for cleaning up my figures, looks much nicer now! Also, you might be underestimating what a pain in the rear it is getting to the chemist during working hours... ;D $\endgroup$ – akaioi Sep 11 '17 at 15:49
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This is a question i ran through some time ago while working in some roleplaying diversions with a few buddies, we have decided area denial weapons would not be effective weapons in space, for a decent range of reasons.

We can start with the pellets/bullets/bearings, assuming that the alien vessels are able to sustain some amount of damage and won't be easily damaged by stray space dust, which in itself can move at ludicrous speeds, it won't also be damaged by these small projectiles, So you would need to have massive pellets so that their mass can carry a significative momentum so that they could damage the vessels. Furthermore, the weapon that would carry these projectiles would be incredibly difficult to design, since for such a large area and assuming the resistance of the vessels you would need to find a balance between the propulsion of the projectiles and the radius of its line of fire. Another problem arises in the barrel of such a weapon: How would these thousands of pellets be shot? Any form of explosive such as gunpowder is instantly eliminated, other chemical fuels wouldn't be fairly effective in that situation, a mechanical sling of sorts could work, but the force needed to propel such a device would be massive and would probably tear it apart, another way is to use a magnetic device, which is a fair choice for futuristic weapons, but that brings forth another problem: What will hold the ship that is shooting in place? Assuming still that the pellets are large spherical or cylindrical objects made of a material with high mass, since smaller projectiles would be mostly ineffective, so the ship that is carrying it will have to hold on to those shots, which will slow down the ship and make its flight less effective, and when they are shot, newton's third law applies, and whichever is the method of propulsion, the ship most likely will be shot back too. Another thing that we can look at is reloading, and again, if the pellets are small, making for shells that have a good size, can be easily stored, and easily reloaded, it simply won't damage the enemy ships, and larger shells wouldn't have the desired radius, would be much more difficult to shoot, and would be that much more difficult to load.

In short, Space shotguns aren't all that good, considering the area that you're giving us to work with, If the number of enemy ships is significantly reduced and put under a much smaller radius, such a weapon could be designed in a way that it would be effective, although i have to say that its design wouldn't be widely used due to the fairly small chance that a scenario in which its use is necessary. I'd say stick with ballistic machineguns, energy blasters and EMP's, they are a fairly better choice than space shotguns when fighting a large number of opponents in open space.

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Call your shot

billiards break

Consider that each alien ship probably contains at least tens of thousands of bearings worth of solid material. If you can hit 3 or 4 at the perimeter of the sphere with a wide (so it does not just penetrate through and thru and keep moving) impactor moving at relativistic speeds, you can convert the material of the enemy ship itself into your cones of shrapnel. The vector of the initial impactor should be towards the center of the sphere so the resulting shrapnel moves this direction also.

Ideally enough of your initial momentum is transferred to these first order pieces of shrapnel that they go on to make second order pieces of shrapnel from the ships they hit, and so on. Ships towards the middle of the sphere will be at risk from first, second and third order pieces of shrapnel.

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The main issue is conservation of momentum. Anything you can shoot at the enemy pushes you backwards with equal and opposite force. As others have stated, you need huge amounts of weight or velocity to cause any damage.

Instead of physical objects like buckshot/rocks/sand, why not use something else? Sources of electricity/power for other ideas might be able to be created with futuristic power supplies (fusion, harnessing stars, etc)

A specific frequency/intensity of light - aliens may not have defences against [insert light category here]. Probably need to make sure it's a frequency that's not naturally emitted by close-by light sources, but is definitely something you could fire as a "cone", with an effective (i.e. not infinite) range.

What about an orb that contains lots of very powerful lasers, that you shoot into the area, and it tracks individual enemy ships and hits them with concentrated lasers? What if the orb gave off a powerful electro-magnetic pulse that could destroy ships' computers within a given (large, but not infinite) range? Or perhaps a communications/"wifi" jammer? Without effective communications, the enemies lose track of each other in the vastness of space and cannot attack you effectively, ultimately drifting off course and "evaporating into the vastness of space"

A massive (in terms of actual mass) device that implodes to form a black hole that in turn opens a worm-hole that jumps from ship to ship, "sucking" them into the black hole, which then evaporates once the majority of the fleet is destroyed?

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First thing first, if you have time, you should check this website, which is a big resource collection for the needs of making consistent hard-SF: http://www.projectrho.com/public_html/rocket/spacewarintro.php

Specifically here, it depends on what technology they have access to. You could probably fill the zone with sand with technology not that much more advanced than what we have today. Essentially, use a fragmentation bomb. If detonated at the right distance, by the time the shrapnel cloud arrives, it will cover the entire area. If the bomb was big enough and shrapnel small enough, you should have a high probability to hit something.

The sand would impact at orbital speed, that is a few km/s, or even up to a few tens of km/s if you are directly attacking from an interplanetary trajectory or using a giant advanced railgun or coilgun to launch the bomb. At those speeds, sand pack an enormous punch! If the crafts aren't adequately armoured, many of them will be crippled or even destroyed.

However, if they expect you to use this weapon, or if they are especially concerned with micrometeorites (for example if those are long-endurance crafts that expect to get hit by micrometeorites at some point due to how long they stay out there), then there are very effective defence against it. The simplest and cheapest (mass-wise) one is whipple shields. They put multiple thin plates in front of their hull, each separated by some dozens of cm of vacuum (or possibly low density material like aerogel). The first plate will break the sand grain into expanding plasma, the second or at worst the third one will stop the now expanded plasma.

However, even then, such weapon could still be useful: it can damage any exposed equipment (antennas, exposed docking ports...) and force them to retract them if they can. This could give you an edge in the later phases of combat. This would require denser sand clouds, but frag bombs are rather cheap anyway.

Note that if you are fighting in orbit, all that sand is still in orbit afterwards, so only use it if you don't care about Kessler syndrome. Then again, causing a full Kessler cascade could work at your advantage.

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There are a number of issues to deal with here.

First of all, the volume of space you need to cover is incredibly huge. If you consider there are something like 500,000 objects orbiting the Earth alone, yet no one is particularly worried about collisions with space debris when launching a satellite, going to the ISS on a Soyuz capsule or bringing supplies up with a Dragon spacecraft (or spending months at a time on the ISS itself....). There is enough warning of oncoming satellites, used booster stages and space debris to plot orbits to avoid them or do burns that manoeuvre the spacecraft out of range.

enter image description here

Representation of the objects and debris orbiting the Earth

Military spacecraft will have similar or greater abilities to sense incoming space debris and manoeuvre to avoid. The only advantages you would get from that is to force the spacecraft to manoeuvre out of station and perhaps create gaps in their orbital coverage, and force them to use their manoeuvre fuel or reaction mass, and be forced to leave station to refuel and refit.

What you really want is a way to strike enemy spacecraft at long range, and with sufficient speed that they have a very low chance of evading the strike. Based on the question, we are going to ignore the idea of laser or particle beam weapons reaching out at the speed of light. A sufficiently powerful laser weapon could become a "Ravening Beam of Death" (RBoD) and capable of vapourizing metals, ceramics and carbon fibre in milliseconds at ranges of one light second (300,000Km), which would certainly be a valid way to clear a path through swarms of spacecraft far enough away that they can't fire back with kinetic energy weapons (A RBoD would need an accelerator ring @ 1Km in diameter to power an X-ray laser, so this isn't a small, swarm weapon.....) The Atomic Rockets website discusses laser weapons in the Conventional Weapons section.

What would be easier to use would be a compact weapon capable of projecting matter or energy at high velocity. This can be done through the use of nuclear devices to power weapons. Tough SF has two very good and detailed posts: The Nuclear Spear, which discusses the CASABA Howitzer, and Nuclear EFP and HEAT which discuss using the energy to drive explosively forged projectiles, streams of liquid metal or even clouds of pellets in a "shotgun" type weapon.

enter image description here

Theoretical reconstruction of an ORION pulse unit. Nuclear powered space weapons are derived from this basic design

Experiments done in the 1980's as part of the Strategic Demesne Initiative suggested that a small nuclear weapon could provide the energy to send a cloud of pellets at a target at velocities up to 100Km/sec. These clouds of pellets were actually meant to strip away low mass decoys and expose the warheads of an incoming ICBM strike, so the military issue is actually similar.

When you consider orbital velocity around the Earth is a "mere" 7Km/sec, this means that each pellet has vastly more energy than a simple ball bearing in earth orbit, and the enemy spacecraft has only a fraction of the time to identify the range and bearing of the incoming cloud of pellets and make a burn to evade. And this is on the "low" end of the scale. CASABA Howitzers send a spear of star hot plasma at a target at an appreciable fraction of c compounding the problems of evading the incoming fire.

So the best way to clear an area in space combat is to use nuclear powered "shotgun" shells to strip away small spacecraft by striking them hard and fast enough to prevent them from manoeuvring out of the way. "Shotgun" rounds will fill enough volume of space that the pellets that don't directly strike the initial target will remain very dangerous to any ships close by, but have the added advantage that they are moving af faster than solar escape velocity, so the firing units don't have to worry about them remaining in orbit once they have passed through the target area.

Nuclear shotgun rounds are relatively small and cheap compared to RBoD class lasers, and don't require the launching spaceship to need mirrors, cooling systems or radiator arrays, just launching racks or cells to house the missiles, or large mass drivers to shoot the warheads in the direction of the enemy. Large ships can carry more warheads, larger missiles or bigger mass drivers, so will have an advantage over small ships, even if the smaller ships can carry the same weapons. Large ships will have range advantages and overwhelm smaller ships with sheer volume of fire).

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Depending on the speed you can send out your pellets, they can be quite small. This will reduce the amount of material that you need to expel. Say, if you can accelerate your particles to 99.9% of the speed of the light, they will pack 70 times more punch than their weight's worth and at those speeds they will be extremely hard to stop.

Micrometeoroids cause great damage to spacecraft, they are at the fraction of aforementioned speed. If your aliens do not have any shielding, they can be ripped apart by these micro bullets. Also dodging the bullets will be impossible as there won't be enough time between seeing bullets and bullets hitting the ship.

If they are 10 milligrams (which will make it around 2-3 orders of magnitude heavier than micrometeoroids), 10^5 of them will make a kilogram of pellets. That is not too much. For 10000 vessels you will need 10 metric tons of pellets, assuming every shot destroys one ship. Compare that to the weight of bullets spent per year in Iraq war, which is roughly 50000 tons.

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