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UFO's are real and fighting around the Earth!

I have an alien spacecraft in modern Earth's atmosphere, and rival alien spacecraft in Earth's Orbit. Both aliens are capable of maneuvering at great acceleration (100 G's?) equivalent to what is commonly portrayed for UFO's as seen in various videos in the atmosphere (and faster in space). They are aware if any kind of weapon is targeted at them with light speed, and are capable of accelerating any mass from subatomic particles up to a neutron star to near-light speeds in extremely short distances. Assume a spherical hull 100 feet in diameter with strength equivalent to battleship armor and near-infinite heat resistance.

The attackers in space are unwilling to cause massive collateral damage to the Earth (defined as any event causing the death of more than 1000 humans). With advanced notice, the defender aliens are willing to position themselves invisibly over urban centers to increase the odds of collateral damage.

Given the limit of matter, but the choice of ANY matter from particle beams to neutronium slugs, and any velocity, can the aliens in orbit successfully target the aliens near the surface despite their early warning and great speed, or is the atmosphere too dense to allow projectile matter to enter fast enough without causing massive shockwaves or crust-damaging impacts from misses?

The attacker entering the atmosphere more than superficially, or sending smart projectiles to pursue the near-surface craft, violate the limits of the question. They can carry or manufacture any exotic materials (like antimatter or exotic elements) on-board to use as projectiles or particle beams.

Bonus points if you can minimize the conflict being seen by humans and avoid any human casualties.

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  • $\begingroup$ What means "They are aware if any kind of weapon is targeted at them with light speed"? $\endgroup$ Oct 29, 2021 at 0:22
  • $\begingroup$ @ErdelvonMises They detect an incoming projectile using passive sensors detecting the operation for their enemies' weapons, with detection speed at light speed. So a particle beam firing would be detected at light speed from the firing point, not the round trip time of an active sensor. $\endgroup$
    – DWKraus
    Oct 29, 2021 at 1:07
  • $\begingroup$ You computer is no 0-time to compute the origin of the projectile. Unless you want a silly soft sci-fi. $\endgroup$ Oct 29, 2021 at 1:23
  • $\begingroup$ @ErdelvonMises I didn't say there was no computation time, although they are some really advanced systems, and the ship can maneuver evasively before it even knows where the projectile is going. There is some handwavium in being able to accelerate anything at any speed, or accelerating in atmosphere at 100 G's, so I didn't go hard-science. We're assuming some very advanced tech. It sounds like you have thoughts - I'd love to see an answer. $\endgroup$
    – DWKraus
    Oct 29, 2021 at 3:28
  • $\begingroup$ how is posssible to the OVNI to detect the projectole if this does no is dectectable? $\endgroup$ Oct 29, 2021 at 9:22

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Any form of particle beam energetic enough to damage armor will also excite the atoms in the atmosphere to plasma and therefore create a small and brief flash visible to half the planet.

Any solid projectile entering the atmosphere, even if it doesn't ablate away, create a small plasma trail by pushing the atmosphere out of the way, also visible to half the planet. Beyond that, even 1 gram of matter at 99% lightspeed delivers the equivalent of 131 kilotons of energy, which is guaranteed to cause collateral damage even if it hits the target.

To guarantee a hit on the target UFO, the attacker's beam/projectile needs to reach the UFO before it can move out of the way. 100 gees is 981 meters per second per second of acceleration, meaning that the UFO can move ~30 meters, vacating the 30 meter (100 feet) space it was previously occupying, in 0.25 seconds. Therefore, a lightspeed projectile needs to be fired from less than 0.25 light-seconds away from the target, which is 74,948 km. Earth's atmosphere is about 480 km in height, so the attacking UFO can indeed land a hit on the defending UFO without entering or even really approaching the boundaries of the Earth's atmosphere.

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    $\begingroup$ The attacker can be further away if the target cannot detect that it's being fired upon until the muzzle flash. If the projectile is going .99c, that will only give a warning of 1% of the travel time. $\endgroup$
    – Ryan_L
    Oct 29, 2021 at 2:13
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    $\begingroup$ A projectile fired at a significant fraction of lightspeed will not penetrate the upper atmosphere. It will become a nuclear fireball in the first ~100m. See xkcd's "Relativistic Baseball." $\endgroup$
    – codeMonkey
    Oct 29, 2021 at 19:36
  • $\begingroup$ The atmosphere is only a thin, thin layer. Any flash within it will be limited by the horizon, not half the planet. Even something as bright as the sun can be directly visible in one location, yet totally dark a few hundred km away. I remember watching and comparing meteors live with a friend about 200km away and what was overhead for one of us was low in the sky for the other. $\endgroup$
    – Innovine
    Oct 29, 2021 at 20:43
  • $\begingroup$ @codeMonkey Yep, that scenario is exactly what the "if it doesn't ablate away" in my answer refers to. It doesn't apply anyway because the question contains "Assume a spherical hull 100 feet in diameter with strength equivalent to battleship armor and near-infinite heat resistance."; presumably the projectile is also made of the magic infinitely heat resistant material, preventing the xkcd scenario from occurring. $\endgroup$ Oct 30, 2021 at 2:56
  • $\begingroup$ @codeMonkey Did you read about muons, and how they prove General Relativity? $\endgroup$ Oct 30, 2021 at 13:34
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  1. Overload the passive sensors of the target to a level on which the launch of the projectile registers as noise. If you can, make the jamming come from multiple directions

  2. Launch the projectile at a speed that doesn't allow enough for the reaction time

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Part #1: How to don't miss

What your projectile needs to no miss is to travel a certain mean speed $v$ so that the target cannot move fast enough to avoid the impact. This speed is given by our target maximun accelaration $A$, target's detection computing time $\gamma$, the distance to the target $d$ and the target's shape and size.

The target's shape and size determine the value of the distance $\Delta x$ that the vehicle must move to avoid be impacted by a incoming projectile in some certain part of it. To compute the $\Delta x$ of some part of some of some part of the target you must take some point inside the target and draw a line until nearest point that still is part or the target and is desired, the lenght of this line is equal to $\Delta x$.

Finally the required mean veolocity of the projectile is given by the equation $$ v = \frac{d}{\sqrt{\frac{2\Delta x}{A}} + \gamma} $$


Part #2: How to no explode the world

For no produce a unreasonable big explosion we need to the Energy $E$ to be enough to penetrate the tank but less than the needed tomake a explosion able to make collateral damage the we will call $b$.

The Energy of the projectile on impact (assuming the only force that acts in it is gravity) is determined by $$ E = \frac{1}{2}mv^2 + mg\Delta h $$ So, for no explode the world the inequality $$ b \leq \frac{1}{2}mv^2 + mg\Delta h $$ must hold, there is a lot of variables to tweak to archive this, therefore this time no will be a simple equation and good luck with the algebra.


Part 3: How to avoid shock waves

Aerodynamics says that every particle that travels in a medium faster than the speed of sound of that medium will produce a shockwave, and in general it it travels fast it will produce increassly loudest sounds. So, at least the target is really big is likely that it projectile will be very loud.

Except if the projectile is small enough so it can travel in bewteen the atoms (I am no refering to collide/enter inside molecules) of the atmosfere, and therefore no produce shock waves or sound. (Note this probably requires to being computed a projectile path that avoid interactions).


Part #4 How to no leave a visible plasma

First of all follow part #3 instructions, because if some of your subatomic projectiles hits a molecule this will be very exited and will make some visible plasma. Secondly you also have to consider the relevant magnetic interactions that you projectile can experiment in their path. If you acomplish this steps (And I am no mistaken) your projectile should no be noted by anyone.


As last note, depending of the case is possible that the energy required to hit/damage the target is bigger than the limit to no explode the world.

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  • $\begingroup$ So... Neutrinos harmonized to oscillate into interacting with matter all in the same spot? (I know just enough particle physics to be dangerous). THX, BTW for the answer. $\endgroup$
    – DWKraus
    Oct 29, 2021 at 13:24
  • $\begingroup$ @DWKraus I was thinking in quarks accelerated, and shoted in a path that is practically guaranted to no impact any atom or molecule, so they don't make plasma or any thing like shockwaves or sound. But that thing of neutrinos sounds interestly. $\endgroup$ Oct 29, 2021 at 13:29
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    $\begingroup$ The physics of neutrinos are not well understood by us, but would be by the aliens. If you couldn't make them hit matter controllably, perhaps you could create millions of sources for the neutrinos, all converging on a single point like modern radiation therapy. Since there are no limits to mass, each incoming beam has one neutrino hit the craft (times a million equals a lot of neutrino hits inside the hull). A fly-by of the attacker at 0.999 light speed firing a neutrino beam every meter makes for a lot of shots. $\endgroup$
    – DWKraus
    Oct 29, 2021 at 18:00
  • $\begingroup$ @DWKraus If you can't control the neutrinos then, is prettry probable that the neutrinos will do a lot of collateral damage. $\endgroup$ Oct 29, 2021 at 23:59
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    $\begingroup$ @DWKraus If they are nough neutrinos to make a noticeable effect in the target, there will be as many to make a lot of collateral damage. $\endgroup$ Oct 30, 2021 at 13:10
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Radiation Poisoning

You fire highly penetrating radiation, like high energy neutrons or high-energy photons or neutrinos. This radiation will interact with the atmosphere, armor, computers and flesh of stuff along the beam-path very little, but because only a small portion of it interacts with anything it "hits", it doesn't care what is between it and the target.

You fire enough of that its interaction with the flesh of the targets causes lethal radiation poisoning. Biology is more fragile than atmospheric gasses and steel armor.

Your shots may even be able to penetrate the planet, so you will want to ensure there is nothing behind them besides the atmosphere and space, and/or have accurate GIS data and fire through the planet, killing mostly only the humans along the firing path.

You will always hit

Your beam is going to move at basically light speed. The enemy being aware you fire isn't going to be able to dodge; they can, however, move so they aren't where they where when they last saw you.

Their knowledge of your position is going to be about 40 ms off (assuming they are on the far side of the Earth; lower if they are closer). At 100 G, they can vary their location to any spot in a sphere of radius $\frac{1}{2} (40 ms)^2 (1000 \frac{m}{s^2})$, or 80 cm. Dodging is not practical.

Or just Cook them

While you'll deposit some energy in the atmosphere and in the armor, the amount you need to kill a biological creature is so low it won't make much difference.

It is harder if your goal is to instantly kill the target. Suppose we want to deposit enough energy that it cooks them -- increases their temperature by 100 C (to boiling), and that the energy is deposited in proportion to the mass of what the beam goes through.

A cone that is 12,000 km long and 100 m wide (the size of the enemy flying saucer) has a volume of 3.14 * 10^10 m^2. If the density is on average 5 times higher than water (we are firing through the planet), that is 6 * 10^17 J of energy in the beam, or 10^5 KT of TNT.

However that energy is deposited uniformly over the entire length of the cone. As the amount of energy isn't a world-destroying explosion, you won't even have a noticable Earthquake.

The only remaining problem is the waste product of the beam. Highly penetrating radiation (of whatever kind) doesn't cleanly make heat; when the radiation interacts with the matter, it won't produce thermal energy, but instead end up emitting more radiation of whatever kind. If it is high energy photons, deflected almost as high energy photons will spread out from the beam path. Other beam particles will generate other "debris". Due to conservation of momentum, they will still mostly stay within the beam path.

If the Earth is a backdrop, the real danger is on the far side of the planet (when your beam is wider and has generated more near-light-speed "debris" by interacting with the planet). Tuning the beam to cook the target but not penetrate the planet may be possible.

The defending target may do the equivalent of gluing humans to their ship. Fly inside skyscrapers, and keep 1000 humans in the firing line between them and the enemy, and arrange is so when you crash you also knock down a building.

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The ship is a sitting duck.

Neither the aliens nor any missile/construct can enter "the atmosphere". The alien courts have decreed this to begin at precisely 100 km (Karman line) based on Earth precedents. The ship is essentially on the ground, 100 km from the attacker. 1E5 m / (3E8 m/s) = 3E-4 s = 0.3 milliseconds of warning. Accelerating at 100 * 10 m/s^2 = 1E3 m/s^2 for that time, that gives an average velocity of 0.15 m/s for 0.3 ms, which gets the ship ... less than a millimeter. Ouch. If it could get to the speed of light instantly it would be up to 100 km away, but it's just not that fast.

## Cook it with X-rays

Hard to see, highly penetrating, without cosmic ray style light shows all over the night sky. The lethally irradiated aliens will hopefully slink off to someplace civilized and discreet to die.

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  • $\begingroup$ x-ray penetration through the atmosphere is problematic, on account of there being an awful lot of atmosphere. A powerful x-ray source is also likely to a) be quite visible due to atmospheric ionisation and b) extremely dangerous to bystanders on account of scattering. $\endgroup$ Oct 30, 2021 at 8:43
  • $\begingroup$ @StarfishPrime A beam that lethally irradiates someone with X-rays (such as for bone marrow replacement) would not be visible on the ground, and the scattering seems to be relatively manageable. I don't think 15 pounds per square inch of atmosphere should attenuate the beam too terribly - a comparable weight of wood would not be proof against a strong X-ray beam. $\endgroup$ Oct 30, 2021 at 10:41
  • $\begingroup$ The OP askes for particles, no photons. $\endgroup$ Oct 30, 2021 at 13:32
  • $\begingroup$ @MikeSerfas according to the NIST tables of mass attenuation coefficients, a beam of 350keV xrays (as used for bone marrow myeloablation) have about a 70% attenuation over 100m of dry sea-level air. About a quarter of the losses are from absorption, the rest from scattering. This does not bode well for shooting from space to the ground, nor for controlling collateral damage. The absorption will likely present as ionisation and recombination, and for high x-ray fluxes will be a visible glow. $\endgroup$ Nov 1, 2021 at 14:37
  • $\begingroup$ @StarfishPrime OK, I assume this is your source ( nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir5632.pdf ), got sumatrapdf to read it because Mozilla's rendering is like water torture, and see the attenuation is 3000 cm^2/g at 1 keV to about 0.02 cm^2/g at 100 MeV. I have 15 psi = 1060 g/cm^2 of air, and after I multiply I take e^-whatever. Despite my skepticism you seem to have a point - we go down to 1/e^21 = 6e-10! I'm starting to wonder whether it is actually possible to X-ray through random metal items also... $\endgroup$ Nov 1, 2021 at 21:26
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You're Looking for Missiles

The traditional way to kill a fast moving aircraft is to hit it with a faster moving missile. I think you're attempting to rule that out with the "no smart projectiles" restraint, but honestly how can you call them "Advanced" weapons if you exclude concepts that those dumb humans have already built?

Switching to Guns!

If you really are sticking to the no missiles rule, then the most important issue becomes your firing angle.

If you fire straight down, any misses are going to hit whatever is below your target, which could include a city if the enemy positions themselves properly. So you're going to want to pick a different angle to avoid collateral damage.

But the Earth is roughly spherical, and the enemy ships are hovering in mid-air. You can position your ships such that any misses fly off into space.

Don't do this:

          *     (ship)

          |     (bullet)

          *     (ship)
          _
        /   \    (Earth)
        \ _ /
         

Do this!

*   ---   *
          _
        /   \
        \ _ /
          

Now your misses fly off harmlessly into the void, to kill something far away a long time from now. Probably not a human!

Minimum Mass, Maximum Speed

Shoot Lasers. They're technically particles, which means they have mass. They can also reach your targets without spontaneously fusing, which most other forms of matter would when moving through the atmosphere at significant fractions of light speed.

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    $\begingroup$ Light has no mass, but it has momentum. If energetic enough, light will most assuredly interact noticeably with the atmosphere, dumping energy into the atmosphere. Air is nearly opaque to far ultraviolet and above. This is an important component of the damage that occurs during nuclear explosions. $\endgroup$ Oct 29, 2021 at 17:22
  • $\begingroup$ Photons have no mass. Also the definition of particle is something with mass. $\endgroup$ Oct 30, 2021 at 13:28
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Weaponize converging wave forms.

Whenever two waveforms overlap you get either constructive or destructive interferencee based on how the waves line up. What you are looking for is a wave based weapon (like a laser) that packs too little energy to be harmful... and then you want to get a few million of those little guys, point them all at the same spot, and synchronize their wave patterns. Anywhere in front of or behind where all these beams converge will be relatively harmless places to stand, but that tiny place right where they all come together could very quickly become several million degrees.

Since the ship you are targeting is pretty much temperature proof, this will not melt its way through the enemy ship, but what it will do is create a thermal explosion like a tiny nuke that will send a very powerful shockwave through the ship ripping it apart.

Because the beams move at the speed of light, the enemy ship does not have time to dodge. Also, if you miss, the weapon will still create a detonation at the prescribed altitude creating a ball of plasma that will be opaque to the lasers... so there is virtually no chance of the weapon actually reaching the surface.

How is this a mass based weapons?

The beams do not have mass, but they are not the weapon, just the ignition system. The actual weapon is the burst of plasma made up of Earth's own atmosphere where the lasers all converge... basically think of it like a depth charge, but with a much less dense medium and a much more energetic explosion.

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  • $\begingroup$ I don't think this stays inside the official limits of the question. To be fair to the idea, though, I was considering a network of thousands of super-powerful neutrino beams (a bit of an oxymoron) like radiation therapy, where the convergence point would be on the alien ship. Each one only has a few neutrinos hitting matter, but collectively they add up to vaporizing the insides of the ship. $\endgroup$
    – DWKraus
    Oct 29, 2021 at 22:15
  • $\begingroup$ Something like this, but with neutrinos mayoclinic.org/tests-procedures/stereotactic-radiosurgery/about/… $\endgroup$
    – DWKraus
    Oct 30, 2021 at 13:05
  • $\begingroup$ "The beams do not have mass, but they are not the weapon, just the ignition system. The actual weapon is the burst of plasma made up of Earth's own atmosphere where the lasers all converge... basically think of it like a depth charge, but with a much less dense medium and a much more energetic explosion." with this logic every weapon is a mass based one. $\endgroup$ Oct 30, 2021 at 13:59
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Many others have suggested designing a weapon fast enough to hit them before they can move out of the way. That's a very hard task, and leaves you with a projectile that's moving fast enough that misses are very dangerous.

Instead, go with a slower weapon using a projectile that will burn up before it gets to the surface. The enemy ship will be able to dodge it easily, but that's fine because you're not actually aiming at them. You're simultaneously launching thousands of them, creating a ring of fire centered on their current location. If the enemy ship can travel X meters between firing the projectile and impact, than your ring of fire should have a radius about 20% larger than X. Now, maintain your wall of fire, squeezing that ring tighter and tighter over several seconds until the radius is zero.

The falling projectiles will essentially form a cone shape. The enemy ship can't flee without flying into the projectiles raining down around them. As long as you ensure the space between two adjacent projectiles is smaller than the width of the target, they'll have no way to avoid being hit.

A few humans might see the rain of projectiles falling from the sky. If they burn up before getting close to the surface, however, those humans won't think too much of it. Micro-meteorites and other bits of space debris enter the atmosphere and burn up all the time. People really only pay attention to them when they make it to the surface or violently explode and damage something.

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Neutron beam bank shot.

/They are aware if any kind of weapon is targeted at them with light speed/

This does not mean they are aware of all incoming projectiles, only being targeted. Your attacker will fire in the opposite direction. It will send a beam of neutrons at a high fraction of c, banking it around the moon in a tight curve such that it comes back on a path to hit the defender in the atmosphere. The neutrons will be largely unabated when they hit the defender. The neutrons will pass through the targeted ship disrupting electronics and biological systems. They will eventually hit the earth.

The beam will produce the same effect as a neutron bomb. Living things in the path of the neutrons both in the target ship and on the ground will die within a few days from acute radiation toxicity. The beam of neutrons spreads en route and is wide enough to encompass the entire targeted ship. It will be slightly wider when it hits the surface. Odds are good it will hit the ocean. Odds are low it will hit a human.

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    $\begingroup$ "It will send a beam of neutrons at a high fraction of c, banking it around the moon in a tight curve such that it comes back on a path to hit the defender in the atmosphere." What force do they use for the "banking it" part? $\endgroup$ Oct 29, 2021 at 1:40
  • $\begingroup$ Otherwise, ++ for the idea of a close to luminal speed boomerang. $\endgroup$ Oct 29, 2021 at 1:53
  • $\begingroup$ @AdrianColomitchi - /What force do they use for the "banking it" part/ Gravity. The moon has that and is willing to use it. But where is that promised ++? $\endgroup$
    – Willk
    Oct 29, 2021 at 2:18
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    $\begingroup$ @Yakk: Ok the truth. Their speed is really a fraction of the speed of light. But "high fraction" was for the investors because they like to hear stuff like that. $\endgroup$
    – Willk
    Oct 29, 2021 at 21:39
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    $\begingroup$ @Willk Shouldav used "synergistically targeted with a significant consumer penetration and blinding fast time to market". Probably "engagement" may worth mentioning too. $\endgroup$ Oct 29, 2021 at 22:19

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