## Would a Relativistic Kill Vehicle work?

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Technical Ben
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### Would a Relativistic Kill Vehicle work?

I've seen it said on this forums many times that a RKV would be a Mutually Assured Destruction weapon for any space travelling civilisation. This would be impossible to defend against. However, I've wondered about the problems of accelerating such and object this way. Would it not spread out more and start to fall apart as you try to accelerate it so fast? Even hitting interstella medium is going to break it down right? So would it not destroy its self, before it reached it's destination?

PS, I know it was posted in the planet destroying threads, but not found any counter arguments for it...
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Velifer
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### Re: Would a Relativistic Kill Vehicle work?

First, fictional science subforum.

Second, MAD only works when there's a chance to launch a counterstrike before getting annihilated. You don't get that chance with light-speed-death-bricks. Whoever shoots first wins.
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Technical Ben
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### Re: Would a Relativistic Kill Vehicle work?

But is it practical, or would you destroy your payload in attempting to accelerate it?
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thoughtfully
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### Re: Would a Relativistic Kill Vehicle work?

It really depends on what the actual acceleration is. If your launch system is longer, the acceleration required is less. I would say that atmosphere is going to be a problem, but if your launch system is low pressure, then perhaps your time to target will be so short that there's not enough time for the breaking up to become a problem. You'll also need a line of sight target; you won't be able to use parabolic trajectories, and that's going to limit the usefulness a lot.

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HungryHobo
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### Re: Would a Relativistic Kill Vehicle work?

this should really be moved to fictional science.

anyway.
Stross had a lovely version of this in Iron Sunrise.

Sit a half dozen ships out in the ort clouds with enough fuel to accelerate up to a decent fraction of lightspeed.
Anyone attacks you and they're used to take out the attackers planet.
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Glass Fractal
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### Re: Would a Relativistic Kill Vehicle work?

Velifer wrote:Second, MAD only works when there's a chance to launch a counterstrike before getting annihilated. You don't get that chance with light-speed-death-bricks. Whoever shoots first wins.

Depends on how fast it accelerates and how far away your target is. If it's engine takes two years to reach the needed speed and then takes another two year to hit its target they'll have plenty of warning.

Kang
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### Re: Would a Relativistic Kill Vehicle work?

HungryHobo wrote:Sit a half dozen ships out in the ort clouds with enough fuel to accelerate up to a decent fraction of lightspeed.
Anyone attacks you and they're used to take out the attackers planet.

Provided you have a way of keeping them operational long enough, can geometrically target the planet in question properly and you can actually still initiate the order.

HungryHobo
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### Re: Would a Relativistic Kill Vehicle work?

Kang wrote: and you can actually still initiate the order.

Dead mans switch if you're fairly sure who might attack you, live humans in stealthy bases out in the middle of nowhere to give the order if you want a more general purpose deterrent.
Scale, stealth and distance is a great defence once you're thinking on stellar scales.
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### Re: Would a Relativistic Kill Vehicle work?

Glass Fractal wrote:If it's engine takes two years to reach the needed speed and then takes another two year to hit its target they'll have plenty of warning.
Another two years in whose frame of reference? Keep in mind the lightspeed delay, as well.
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p1t1o
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### Re: Would a Relativistic Kill Vehicle work?

In the "Forge of God" and "Anvil of Stars" the weapon of choice is replicating machines.

However, my takeaway is that the MAD is made probable because everyone will see you do it (it will be pretty hard to hide from everyone else after the event, even if it is hard to see coming).

Then your local neighbours will destroy you because it is the safest option for them - it is not safe to live in the same region of space as a race with a track-record of species-o-cide, and if they have a weapon which you can't see coming, you better get them first (but you had better make a reallly good job of it...).

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### Re: Would a Relativistic Kill Vehicle work?

Until the neighbors of those neighbors decide the safest thing to do is to get rid of the neighbors, confirmed species-killers. Hence the MADness!

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Kang
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### Re: Would a Relativistic Kill Vehicle work?

HungryHobo wrote:
Kang wrote: and you can actually still initiate the order.

Dead mans switch if you're fairly sure who might attack you, live humans in stealthy bases out in the middle of nowhere to give the order if you want a more general purpose deterrent.
Scale, stealth and distance is a great defence once you're thinking on stellar scales.

You are right an automatic execution will work if you have the hunch of who'll jump you, but if you need a remote base to give the order to the way more remote ship it doesn't really solve the problem, as you would still have to alert the remote outpost and give a failsafe order to them before your c-cubed turns into rubble. Anyway, I'm certain that's not really that an important point to begin with, just something I noticed.

Tass
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### Re: Would a Relativistic Kill Vehicle work?

Better to just use your Dyson swarm to point you star at them, then they get no warning as the light travels at light speed.

But when we get to this tech level there is no MAD, because every civilization would be spread out across at least a solar system.

JWalker
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:
Glass Fractal wrote:If it's engine takes two years to reach the needed speed and then takes another two year to hit its target they'll have plenty of warning.
Another two years in whose frame of reference? Keep in mind the lightspeed delay, as well.

If you fire a rocket with a constant acceleration a towards a target a distance x away (in units where c=1), they will observe a time delay of
$\delta t = \sqrt{\frac{2x}{a}+x^2}-x$
between when the rocket is fired and when it hits them. In the limit that x is very large, the warning time becomes
$\delta t \approx \frac{1}{a}$

So, if it takes two years to accelerate to nearly light speed in the frame of the target, then the target has roughly two years of warning time.

n2kra
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### Re: Would a Relativistic Kill Vehicle work?

Technical Ben wrote:RKV - However, I've wondered about the problems of accelerating such and object this way. Would it not spread out more and start to fall apart as you try to accelerate it so fast? Even hitting interstella medium is going to break it down right? So would it not destroy its self, before it reached it's destination

If Bussard Ramjets are feasible:
http://en.wikipedia.org/wiki/Bussard_ramjet

Man-Kzin Wars II, The Children's Hour
http://www.larryniven.net/kzin/reviews.shtml
(Mr Niven usually "does his math")

Accelerate all the way, no breaking. Launch it from
the asteroid belt, with "a couple of spare rocks in tow"

HungryHobo
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### Re: Would a Relativistic Kill Vehicle work?

but, if they're say, 10 or 20 light years away then you could also spend 2 years accelerating and coast invisibly almost the whole way there.
if you need to make adjustments near the end then they might get some brief warning but pinning your hopes on seeing drive flames (pointed away from you) 8 light years away from a ship which could have coasted quietly away from it's origin star for some time first is a bad bet.
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### Re: Would a Relativistic Kill Vehicle work?

JWalker wrote:So, if it takes two years to accelerate to nearly light speed in the frame of the target, then the target has roughly two years of warning time.
What's "nearly light speed"? Keep in mind time dilation as well in this case. At a = 1/2 in your units, you're only accelerating at about half a gee. Which means you're definitely not "nearly light speed" after two years.
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Glass Fractal
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:
Glass Fractal wrote:If it's engine takes two years to reach the needed speed and then takes another two year to hit its target they'll have plenty of warning.
Another two years in whose frame of reference? Keep in mind the lightspeed delay, as well.

The target's frame of reference, I think. I'm not sure if it matters.

Ship turns on its engines in the distance. Light (heat) starts traveling at the speed of light toward the target (and in all other directions). The ship starts moving at some lower speed toward the target. Necessarily the light will arrive first. The invincibility of RKVs hinges on the idea that they're traveling fast enough that there's only a delay of a few days or maybe just seconds between seeing them and getting hit. But if they don't accelerate at an enormous rate then there could be months or years during which the RKV is traveling fairly slowly (lets say at less than .1c from the target's perspective). By my understanding the light will be getting far ahead of them during that time so unless the engine is cold or the target isn't looking for it the time it takes the RKV it get up to speed will make it much less than invisible.

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### Re: Would a Relativistic Kill Vehicle work?

Glass Fractal wrote:Ship turns on its engines in the distance. Light (heat) starts traveling at the speed of light toward the target (and in all other directions). The ship starts moving at some lower speed toward the target. Necessarily the light will arrive first. The invincibility of RKVs hinges on the idea that they're traveling fast enough that there's only a delay of a few days or maybe just seconds between seeing them and getting hit. But if they don't accelerate at an enormous rate then there could be months or years during which the RKV is traveling fairly slowly (lets say at less than .1c from the target's perspective). By my understanding the light will be getting far ahead of them during that time so unless the engine is cold or the target isn't looking for it the time it takes the RKV it get up to speed will make it much less than invisible.

But an important part of the idea of a RKV is that it doesn't have to be very big, and therefore it's very hard to see when it's very far away. At .99c an object of a few dozen tons rest mass would strike with more energy than all the nuclear weapons ever produced. I'm sure someone here with a better grasp of what it would take to end life on a planet can correct me as to the exact amount of energy needed, but an object of a million or so tons would be comparable to the Chixculub impact. A cube of iron a hundred meters wide would do the trick quite nicely. Good luck trying to pick that lump out of the sky from a light year or more away.

Sure, a technology that can see an object a hundred meters wide from light years away is no more far-fetched than a technology that can accelerate a million tons of iron to .99c. But presuming the future existence of one doesn't imply the other.

idobox
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### Re: Would a Relativistic Kill Vehicle work?

Also, a device capable of accelerating a projectile to relativistic speed will look nothing like a chemical rocket.
The power plant spewing insane power is going to be difficult to hide, but the actual acceleration device could be very discreet and easy to hide.
Also, if your "gun" is accurate enough,and if you have a power plant big enough, you won't need to rectify the trajectory, so you can send a lump of metal, and give it an insane acceleration. How much time would it require at 50 or 100 g to accelerate to .99 or .95 c?
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### Re: Would a Relativistic Kill Vehicle work?

And you have even less time from when you see it then you might think you do, because it will be racing against it's own light to reach you. Say for example an object moving at .99c is heading toward you from 1000 light-seconds out. Now, the light from 1000ls out, will take 1000s to reach you. Meanwhile, in 1000s, the object will have traveled 990ls. So, by the time that you see that it's 1000ls out, it'll already be at only 10ls out. The effect is such, that without the time it takes light to travel considered, it will appear to be coming at you at 100c. And your reaction times will be proportional to what you could classically expect for an object moving at you at 100c.

Although on the other hand, whatever is boosting this thing up to speed will likely be massively visible if you're looking for it when it's boosting. Remember that a kinetic projectile is only energy storage. If it impacts with more energy than all the nuclear weapons that Earth has ever produced, then it's going to take more energy than all the nuclear weapons that Earth has ever produced to get it up to speed.

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### Re: Would a Relativistic Kill Vehicle work?

As I said in the planet-buster thread, if we could somehow accelerate something at a constant 10g to Alpha Centauri, light from the launch would get there only 35 days before the vehicle.
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### Re: Would a Relativistic Kill Vehicle work?

Here's what I've never seen explained satisfactorily about the "RKV": If it's going so fast, why doesn't it just go straight through the planet? Everyone just does the calculations of how much energy it would be carrying and then says "that's sufficient to blow up everyone", completely glossing over how that is supposed to actually happen.

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### Re: Would a Relativistic Kill Vehicle work?

Because unless you make the RKV out of something really really dense the energy would still dissipate into the planet.
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### Re: Would a Relativistic Kill Vehicle work?

So make it disintegrate before impact or something. Compared to accelerating something that fast in the first place, it's not actually that difficult to figure out ways to make sure all the energy gets deposited in/on the target.
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### Re: Would a Relativistic Kill Vehicle work?

Goplat wrote:Here's what I've never seen explained satisfactorily about the "RKV": If it's going so fast, why doesn't it just go straight through the planet? Everyone just does the calculations of how much energy it would be carrying and then says "that's sufficient to blow up everyone", completely glossing over how that is supposed to actually happen.

Because you're accelerating it in a near perfect vacuum, and you can just make it any shape you want, like flat and really really wide. Think belly flop over a swan dive. You could even have it thin side forwards for most of the trip to reduce drag from the interstellar medium, turning towards the end of the journey to face the target flat side forward.
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:
JWalker wrote:So, if it takes two years to accelerate to nearly light speed in the frame of the target, then the target has roughly two years of warning time.
What's "nearly light speed"? Keep in mind time dilation as well in this case. At a = 1/2 in your units, you're only accelerating at about half a gee. Which means you're definitely not "nearly light speed" after two years.

It doesn't matter what 'nearly' light speed is or that I randomly made up a two year time window, what matters is there is a warning time that is inversely dependent on how you accelerate your vehicle. No matter how far away the target is, or how fast you accelerate, there is still a nonzero warning time. This means you have to choose between accelerating rapidly to reduce warning time at the cost of having a very easily detected launch, or accelerating slower and having a harder to detect launch but potentially alerting the target sooner. You can't have both. You can avoid this problem entirely by simply using a Nicoll-Dyson Laser. Not only will your target have no warning, it is also far easier to build and requires far less energy to fire.

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### Re: Would a Relativistic Kill Vehicle work?

JWalker wrote:This means you have to choose between accelerating rapidly to reduce warning time at the cost of having a very easily detected launch
We could send them a news broadcast about the launch if we wanted: one month isn't enough time to do anything about an RKV.

And yeah, a laser would be great, if the enemies who are so good at detecting launches fail to notice us building a great big shell of satellites around our star...
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:And yeah, a laser would be great, if the enemies who are so good at detecting launches fail to notice us building a great big shell of satellites around our star...

Like we can build an RKV before we have that anyway.

Technical Ben
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### Re: Would a Relativistic Kill Vehicle work?

I was thinking more along the lines of particle accelerators. They create a massively powerful beam. But it's too concentrated to cause wide spread damage. Is time dilation not going to compress your payload anyhow?
I suppose the main thing is, anything able to accelerate a projectile to a high fraction of c, will be a large power source. Anything that is a large power source can be a weapon. So it's not necessarily the fact you have an object moving a c, rather you have a massive amount of energy. The speed just makes defence or reaction more difficult. But this is always the case in one sided battles. A set of "detectors" theoretically could primitively warn you of a RKV. So in the same way a nuclear weapon has load of energy. However, if it's moving ridiculously fast, it's going to be more trouble.

PS, sorry for not putting it in the fiction forum. I was originally wanting to discuss the physical limits of such a device. As it would be a very big limiting factor to space travel in the future. Just as nuclear weapons are banned, could space craft be banned because of the risk of total destruction?
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### Re: Would a Relativistic Kill Vehicle work?

By the way, no one really answered the OP.
What would be the effect of the intersideral medium on an object moving at that kind of speed?
If the heat makes it explode early, most of the mass won't even touch the target, greatly reducing the destructive power.

The main issue with beam weapons is that you need a large lens/reflector to focus your beam, while a projectile tends to keep its size. Particle beams are a nice compromise I think.
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### Re: Would a Relativistic Kill Vehicle work?

Yeah, if it's moving at a really sizable fraction of light speed, the interstellar gas and dust will create a huge amount of friction and drag.
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### Re: Would a Relativistic Kill Vehicle work?

The computation doesn't look that difficult for someone who knows how to figure the kinetic energy of a relativistic particle.
Interstellar medium has between 1e-4 and 1e6 molecules /cm^3, so 1e2 and 1e12 per m^3. But the sun is currently in the local interstellar cloud, with a density of 1e5 molecules/m3, so I think we can take this number.
The temperature is 6000K, if I didn't do anything wrong, it gives a velocity of 8km/s, negligible in regard on the speed of the projectile.

Since I don't know how to do the math for relativistic speeds, I can't compute the kinetic energy of particles hitting the projectile, and I feear the number of particles hitting the projectile per second might not be density*surface*speed, because of time dilatation.
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### Re: Would a Relativistic Kill Vehicle work?

idobox wrote:Since I don't know how to do the math for relativistic speeds, I can't compute the kinetic energy of particles hitting the projectile, and I feear the number of particles hitting the projectile per second might not be density*surface*speed, because of time dilatation.
It pretty much is, we just have to compute the speed two different ways.

Figure 10^5 protons per m^3, and a speed within a percent or so of c, and you've got 10^5 protons per meter traveled. At 0.99c, a meter takes about 1/300000000 s rest time, or 14% of that in the frame of the ship. So that's 2e14 protons/s per m^2, at .99c. Which is about 180kW if my math is correct.

This is the same kinetic energy delivered by normal air if you're going through it at 19km/s...
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JWalker
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:
JWalker wrote:This means you have to choose between accelerating rapidly to reduce warning time at the cost of having a very easily detected launch
We could send them a news broadcast about the launch if we wanted: one month isn't enough time to do anything about an RKV.

And yeah, a laser would be great, if the enemies who are so good at detecting launches fail to notice us building a great big shell of satellites around our star...

A month is plenty of time to launch a retaliatory strike. Which is sort of what you want to avoid with a weapon that can cause planet-wide destruction. A great big shell of satellites is much more preferable, since it is likely any civilization will be interested in constructing one for energy harvesting purposes. It just so happens that it can also be used for a giant laser. You can build it without fear of retaliation since it is a predominantly peaceful structure, but if it is used for destruction the victim wont have any idea before its too late.

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### Re: Would a Relativistic Kill Vehicle work?

JWalker wrote:A month is plenty of time to launch a retaliatory strike.
Right, I'd forgotten the warning-time discussion started over the question of mutually assured destruction. In that case, you're right: 35 days is plenty of time to send some kind of retaliatory strike (assuming you've got preparations in place, at least), or at the very least get some of your most important assets out of the way of the thing.

I was thinking more in terms of whether you could do anything to prevent the strike altogether. You can do a lot in 35 days, but diverting a 1000 metric ton object moving at most of the speed of light probably isn't one of them.
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JWalker
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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:I was thinking more in terms of whether you could do anything to prevent the strike altogether. You can do a lot in 35 days, but diverting a 1000 metric ton object moving at most of the speed of light probably isn't one of them.

Hah, you'd probably have better luck moving your entire civilization out of the way, which is to say you're pretty much screwed if one of those is coming for you.

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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:
JWalker wrote:A month is plenty of time to launch a retaliatory strike.
Right, I'd forgotten the warning-time discussion started over the question of mutually assured destruction. In that case, you're right: 35 days is plenty of time to send some kind of retaliatory strike (assuming you've got preparations in place, at least), or at the very least get some of your most important assets out of the way of the thing.

I was thinking more in terms of whether you could do anything to prevent the strike altogether. You can do a lot in 35 days, but diverting a 1000 metric ton object moving at most of the speed of light probably isn't one of them.

If you're on a comparable technology level couldn't you take advantage of the RKV's own speed? Just move an object into its path, slightly to the side. They collide and you get a massive explosion for cheap.

35 days away is 507 billion miles at .9c (randomly chosen speed). If it's targeted at the center of the Earth you only have to move it a fraction of a degree in order to make it miss. Of course if it can maneuver that won't work.

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### Re: Would a Relativistic Kill Vehicle work?

Glass Fractal wrote:If you're on a comparable technology level couldn't you take advantage of the RKV's own speed? Just move an object into its path, slightly to the side. They collide and you get a massive explosion for cheap.

35 days away is 507 billion miles at .9c (randomly chosen speed). If it's targeted at the center of the Earth you only have to move it a fraction of a degree in order to make it miss. Of course if it can maneuver that won't work.

It is very difficult to change the course of an object moving at relativistic speeds. If you did put something in front of it, it would essentially turn both the projectile and the target into a cloud of plasma, but due to the relativistic headlight effect, the debris cloud would still be heading towards the target in a tightly collimated beam. Since the momentum of a highly relativistic object is gigantic, it takes a correspondingly gigantic collision to alter the course by even a fraction of a degree. I suppose you could try to shoot your own RKV at it, and that might do the trick, but anything short of that or sticking a planetoid in front of it isn't going to help too much.

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### Re: Would a Relativistic Kill Vehicle work?

gmalivuk wrote:Yeah, if it's moving at a really sizable fraction of light speed, the interstellar gas and dust will create a huge amount of friction and drag.

I feel like I read somewhere that even the CMB would be blue-shifted into ionizing radiation at that kind of speed. I can see that being a bit of a problem as well.
"I think nature's imagination is so much greater than man's, she's never gonna let us relax."