Relatavistic Projectiles

[Ariphaos]

In a hard sci-fi setting, the scariness of relativistic projectiles comes from the near impossibility of detecting them in time to perform any kind of interception. Tossing an asteroid at 0.01c gives the target a chance (not necessarily a good chance - it depends on how prepared their defences are) to detect, intercept and deflect the projectile.

In a hard sci-fi setting, relativistic projectiles are in no case weapons, any more than building a board and nail big enough to smash the Earth is a weapon. They are fundamentally futile and vastly inferior to the capabilities directing the raw energy of the parent star provides.

Even if you magic up a projectile material that a coil array can bring to ~.86c in less than a solid fraction of a light-year, there is still plenty of time for a defending system of sufficient but still lesser technology to react.

[Darth Hoth]

I thought the point was that if your relativistic projectile travels at high fractions of c, no realistic (i.e., propagating at c) detection system will offer enough of an early warning to allow for any kind of meaningful response?

Oh, and 2001 is, if the book is in any way even tangentially related to the film, absolute crap. Several unrelated segments with characters that have absolutely nothing to do with each other, monkeys, rocks, a cliché evil computer going crazy for no apparent reason, and an acid trip of an ending that sends the viewer into "What the Hell?!" Mode. It has no coherent plot, no interesting characters, nothing. And worst of all - slooow (as in positively damn glacial) pacing. Some scenes, such as the monolith at the end, could actually have had an air of mystery or wonder - if one had not been bored to death by it being stretched out for twenty minutes. About its only redeeming feature is the space station docking scene (and I imagine most modern audiences would be bored to hell even by that). I watched it just to see why everyone was wanking to it ("It's so special!"), and that is one of the few films that have made me feel it was a total waste of time.

[Ariphaos]

I thought the point was that if your relativistic projectile travels at high fractions of c, no realistic (i.e., propagating at c) detection system will offer enough of an early warning to allow for any kind of meaningful response?

There is no realistic mechanism for accelerating a solid projectile to a significant fraction of c within any short timeframe. You can sort of do it with light sails, but those things are huge, and it still takes nigh unto forever - light years of travel.

This is ignoring the issues of targeting at such a range.

Even if you did, though, a two-light year radius sensor network with a complete sky coverage of less than a light-second (at that radius) can feasibly be built out of the mass of a single kuiper belt object. They don't need to see anything coming, they just need to see it passing or going.

This of course, is above and beyond the fact that it is both slower than light and inefficient. Rather that pointing your energy straight at a target, you are pointing it at a coil array for several thousand AU and then pointing -that- at them. It takes a special sort of person to think that that is a smart idea.

Oh, and 2001 is, if the book is in any way even tangentially related to the film, absolute crap. *snip irrelevance*

Wait. You are insulting a book you have not read, based on a movie you did not understand.

'Special' indeed.

[xammer99]

Holy smokes, thanks for the information there. Now, a couple of other things.

The reason I asked this is also related to a RPG (Star Wars Saga) that I run. The current story arc that I'm running is based on the turning of Qymaen Jal Sheelal into General Grevious with the end of the Huk-Kalee War. The current situation is that the Yam'rii (the Huk) have been driven off of Kalee after several years of insurgent warfare, and now the Kalee want revenge for everything the Yam'rii did. As the leader of the former insurgency, Qymaen wants to take the fight to the Yam'rii, but lacks the navy to mount an invasion and none of the major powers will give him one to do it with. What he does have are enough to basically mount a possibly suicidal covering attack on the Yam'rii homeworld while something to totally wreck the planet's eco system gets through.

Which is where the relativistic sand thing came in. I'd read about it in the Douglas series and it always struck me as being particularly amusing, and it seemed like it might fit. Sure the Star Wars universe has the power generation capacity of DOOM, but their galaxy is also consists of planets as islands with out much attention being paid to the space in between. You fly off planet jump to your destination, and then land on the next planet (or asteroid, or whatever), with little attention paid to the stuff in between, i.e. interstellar space. So, with a patient mad man with some resources, it sounded like an interesting and perhaps plausible idea to have a couple of freighters jump into interstellar space and then over the course of a month or two, run themselves up to some % of C and fling themselves at the planet to wreck it.

But since folks could detect that coming, and destroy the ships, the idea of the sand cloud behind it became more appealing. So the ships would in fact just be decoys. They release the sand as they enter within the solar system and accelerate slightly ahead of it. The defenders destroy the freighters, but the sand goes on through and wrecks the planet.

Granted, rocks and/or nukes would do the job too, but this just seemed more "horrifying" and harder to stop.

[Alien-Carrot]

How would someone possible stop a cloud of sand moving at even 0.1c? An asteroid could possibly be deflected with a side-strike impact. But a CLOUD of sand...

And as for 2001 that movie/book has exactly one redeeming quality.

GOOD MORNING, DAVE.

EDIT: I have just been informed that HAL never actually says this in the movie or book, so yeah them movie/book sucks all kinds of ass.

[xammer99]

How would someone possible stop a cloud of sand moving at even 0.1c? An asteroid could possibly be deflected with a side-strike impact. But a CLOUD of sand...

That is precisely what I'm going after... you can't stop it, but it would cement Qymaen/Grevious's place as one seriously sick SOB, and in turn bring him to the attention of folks like Dooku/Palpatine as the future general they need. Then the next story arc would center on the clone wars itself.

[Darth Wong]

How would someone possible stop a cloud of sand moving at even 0.1c? An asteroid could possibly be deflected with a side-strike impact. But a CLOUD of sand...

That is precisely what I'm going after... you can't stop it, but it would cement Qymaen/Grevious's place as one seriously sick SOB, and in turn bring him to the attention of folks like Dooku/Palpatine as the future general they need. Then the next story arc would center on the clone wars itself.

Why can't you stop it? If you have the sort of civilization which can build ships capable of hauling 500,000 tons of sand at 0.1c in the first place, why can't you deploy huge cow-catchers to push the sand off course, or detonate nukes in the middle of the cloud to do the same?

Remember: you don't need to slow it down from 0.1c; you just need to nudge it enough so that it will miss.

[Darth Hoth]

Wait. You are insulting a book you have not read, based on a movie you did not understand.

'Special' indeed.

Yes, I did not understand it. I fail to see how that makes me stupid, as there was nothing there to understand. Several unrelated and uninteresting sub-stories stretched out to infinity (just the monkey scene is on the order of, what, half an hour?) capped off by the visual equivalent of a madman's ravings. But hey, Stanley Kubrick and Arthur C. Clarke, both big names, made it, so it is obviously "great art"!

And since Clarke and Kubrick wrote the book together while making the film, I make the assumption that they are fairly closely related.

[Ariphaos]

How would someone possible stop a cloud of sand moving at even 0.1c? An asteroid could possibly be deflected with a side-strike impact. But a CLOUD of sand...

If you can accelerate it to .1c you can vaporize it.

Yes, I did not understand it. I fail to see how that makes me stupid, as there was nothing there to understand. Several unrelated and uninteresting sub-stories stretched out to infinity (just the monkey scene is on the order of, what, half an hour?) capped off by the visual equivalent of a madman's ravings. But hey, Stanley Kubrick and Arthur C. Clarke, both big names, made it, so it is obviously "great art"!

And since Clarke and Kubrick wrote the book together while making the film, I make the assumption that they are fairly closely related.

The movie does not go into the reasons that the book does. The movie takes shortcuts. The ending sequence is much more straightforward, even if the movie's symbolism is more appropriate considering what is going on. When Clarke introduces his 2010 novel, he makes a specific note that it is a sequel to the movie, and not the book. The movie intentionally attempts to leave the audience to its own devices about what and why, while the book is a lot more explicit about what is going on with the 'monkey scene', Tycho, the monolith in general, why HAL goes nuts, they journey to Saturn rather than Jupiter - for a reason - and "My god, it's full of stars!"

[Darth Wong]

If you can accelerate it to .1c you can vaporize it.

Vapourizing it won't necessarily help that much. A sufficiently massive cloud of fast-moving vapour could potentially do serious damage to a planetary biosphere as well. It depends on whether it disperses enough so that most of it avoids the Earth entirely. But your point is well-taken in the sense that many people seem to discount the difficulty of achieving this acceleration in the first place, and act as if the defender would not have similarly impressive technology.

[Darth Hoth]

The movie does not go into the reasons that the book does. The movie takes shortcuts. The ending sequence is much more straightforward, even if the movie's symbolism is more appropriate considering what is going on. When Clarke introduces his 2010 novel, he makes a specific note that it is a sequel to the movie, and not the book. The movie intentionally attempts to leave the audience to its own devices about what and why, while the book is a lot more explicit about what is going on with the 'monkey scene', Tycho, the monolith in general, why HAL goes nuts, they journey to Saturn rather than Jupiter - for a reason - and "My god, it's full of stars!"

Oh, I understood the monkey scene, at least I think I did - it was how the black tombstone catalysed human evolution, or thereabouts. It still does not need to take half an hour without dialogue trying to show that.

Though if the book does explain things better, I withdraw my judgement for the time being.

[Junghalli]

Get the fuel to fuse without slowing it down and you can make a Bussard ramjet with theoretically infinite delta V.

Just in the shower this morning I realized this sounds like it implies acceleration to > c speeds. I phrased that really poorly, didn't I?

I meant, of course, that it could theoretically accelerate forever. It still could never pass c though, of course, as c is infinity in terms of energy needed to get to it.

[Ariphaos]

Vapourizing it won't necessarily help that much. A sufficiently massive cloud of fast-moving vapour could potentially do serious damage to a planetary biosphere as well. It depends on whether it disperses enough so that most of it avoids the Earth entirely. But your point is well-taken in the sense that many people seem to discount the difficulty of achieving this acceleration in the first place, and act as if the defender would not have similarly impressive technology.

Well I generally assume that they took a quasi-realistic amount of time to bring it up to speed. One presumes that the team in charge of the 380 yottawatt power supply a bit over eight light-minutes away is pro-Earth-biosphere.

If not, I would thing a more useful discussion would be what counts as a reasonable localized defense. Stratospheric aerogel?

Oh, I understood the monkey scene, at least I think I did - it was how the black tombstone catalysed human evolution, or thereabouts. It still does not need to take half an hour without dialogue trying to show that.

Though if the book does explain things better, I withdraw my judgement for the time being.

It may not, but a flaw does not necessarily make the movie pointless. Nor is the book perfect - it's easy to feel that Clarke is preaching in a lot of his books - but they do have things to say. My own, personal take is - we are not even infants yet.

[Connor MacLeod]

Samuel, your energy value is wrong. Relativistic kinetic energy is always higher than that obtained using the classical approximation. 500,000 metric tons of sand moving at .99c has a kinetic energy of ~4E26 J, or about 20 times that needed to blow off the atmosphere.

Uh, the atmosphere is some 5e18 kg IIRC, and you need to accelerate it to a MINIMUM of escape velocity to evacuate it from the surface. That's well into the 3e26 joule range. And that's going to be ridiculously conservative as a minimum because it assumes complete efficiency AND that you can directly attack the atmosphere. Some heating will occur when you pass through it but most of it will be indirectly as your projectile hits the planet itself. But that complicates things because you can start throwing up ejecta and water vapour (the latter more likely) which will only increaes the energy requirements to blow off the atmosphere. About the mos tefficient way to attack the atmosphere I could think of would be nuclear airbursts, but even that isn't going to be totally efficient.

So it would probably be a safe bet that the energy requirements will exceed E26 joules and go into the E27 joule range in all probability. E25 joules would probably be enough to introduce substnatial temp changes in the amtosphere though, which will kill off alot of life on the planet fairly rapidly though.

[starslayer]

I didn't actually run the numbers myself; I just used what Nyrath posted. I'll take your word for the energy required. And Starglider got the KE of the sand correct in his first post here (~2.7E26 J); I forgot to square the .99.

[Ariphaos]

5e18 * 11,186^2 / 2 = 3.13e26 joules.

Obviously, there's the problem that nearly all of the energy is being directly absorbed by the planet, which is rather more resilient, but if somehow directed at the atmosphere 4e26 joules is sufficient. It's raising the temperature by forty million degrees Kelvin, to begin with.

[cosmicalstorm]

5e18 * 11,186^2 / 2 = 3.13e26 joules.

Obviously, there's the problem that nearly all of the energy is being directly absorbed by the planet, which is rather more resilient, but if somehow directed at the atmosphere 4e26 joules is sufficient. It's raising the temperature by forty million degrees Kelvin, to begin with.

I often see you mentioning starstrafing but I've never found a proper explanation of what it is despite looking into many of the RKV threads on this forum. Where can I find an explanation?

[Ariphaos]

I think I termed it in my second participation in RKV debates.

It is technically feasible to create lenses of sufficient size and resolution to strike a planetary target out to several hundred light years. This has several advantages over RKVs.

1) Your attack is moving at c, not some fraction close to c. There is no warning of an attack. The interstellar medium begins interfering with RKVs much above .99 of c.
2) Your attack is more accurate. Not just because the target is going to drift less, but because you can guarantee complete coverage of an area.
3) Your attack is infinitely more feasible to pull off. It's easy to discount the resources required to build a linear accelerator of sufficient power to launch a solid projectile at relativistic velocities. In order to bring the length of coil down below light-years, you need to guarantee that effectively 100% of the inefficiency gets dissipated into the coil, not into the projectile. Even then, we are talking about thousands of AU of coils. Targeting sunshine and happiness, on the other hand, is easily done with arrays of kilometer-sized lenses. Remember you are targeting planets - you do not care that beam dispersal is ten thousand times the size of your lenses at such ranges. In fact, that's an advantage. You are tossing immense amounts of energy at your victim, you want to hit the target.

I call it starstrafing because it does not work if it is performed in a predictable manner. In order to be effective, you need to strafe out random patterns with it, and if you are smart you will leave their settled worlds well enough alone. They are nothing - the statite swarm surrounding their star is everything - and even then is going to take a long time to destroy.

All targeting planets is going to do to a competing civilization is to piss them off, and in the logical scenario, vengeance will not come from a single impotent star, but rather thousands. There are measures a civilization can take to defend against this, after all, but it only works to a point.

[Connor MacLeod]

5e18 * 11,186^2 / 2 = 3.13e26 joules.

Obviously, there's the problem that nearly all of the energy is being directly absorbed by the planet, which is rather more resilient, but if somehow directed at the atmosphere 4e26 joules is sufficient. It's raising the temperature by forty million degrees Kelvin, to begin with.

Even if you could target the atmosphere directly, I'd still assume it would be rather inefficient to try to get the atmosphere to evacuate simply by turning it to superhot plasma and the actual figure being much higher than 3e26 joules. Or am I missing something?

[Ariphaos]

Even if you could target the atmosphere directly, I'd still assume it would be rather inefficient to try to get the atmosphere to evacuate simply by turning it to superhot plasma and the actual figure being much higher than 3e26 joules. Or am I missing something?

Obviously it would need to be very specifically targeted to do that instantly, but - I need to hunt down the particle velocity equation - but a thousandth of the 2.7e26 joules quoted above is enough to raise atmospheric temperature to around 27,000 Kelvin. At that temperature no molecule remains, and if my math is right the average particle velocity is going to be 25 kps. The atmosphere does not leave immediately, but it boils away pretty fast. I can't find the equation on-line and I'm remembering from Uni, though : /

[avatarxprime]

Now if it were a 500,000 ton nickel-iron asteroid moving at .99c, then turn off the lights...it's over. I think a cloud of sand, however, would have a harder time imparting that immense energy in as useful a fashion

You fail physics forever. What do you think happens when the sand vaporizes in the atmosphere? It transfers its kenetic energy into heat energy.

Well there is the potential for the detonation of the sand as it hits the atmosphere to blow away the mass of the sand behind it and therefore decrease the efficiency of the attack. Whether or not it would be enough to make this a less than biosphere destroying attack I don't know. Based on a bit of searching to find the mass of a grain of sand (.23 to .67mg) each grain is equivalent to a 0.03-0.09 kiloton explosive, provided I did all my math right. Now that is significantly less than any high altitude nuclear tests I could find conducted by the US or the USSR (1.2 kt was the smallest) so depending on the shape of the sand cloud and the effect of the blowback it might be possible to not kill everyone.

[Surlethe]

Well there is the potential for the detonation of the sand as it hits the atmosphere to blow away the mass of the sand behind it and therefore decrease the efficiency of the attack. Whether or not it would be enough to make this a less than biosphere destroying attack I don't know. Based on a bit of searching to find the mass of a grain of sand (.23 to .67mg) each grain is equivalent to a 0.03-0.09 kiloton explosive, provided I did all my math right. Now that is significantly less than any high altitude nuclear tests I could find conducted by the US or the USSR (1.2 kt was the smallest) so depending on the shape of the sand cloud and the effect of the blowback it might be possible to not kill everyone.

What are you talking about? What conceivable mechanism could the first impacts possibly use to reverse the incredible momentum of nearly 500,000 tons of sand moving at 0.99c?

Silly question time: would 500,000 tons' worth of sand have the effect predicted? I'm assuming that it would not hit the atmosphere all at once, but would inevitably disperse from being dumped out a freighter or two or three, and from hitting interstellar or near-planet dust and orbital objects on the way in. In addition, the individual grains of sand are unlikely to actually survive to hit the ground, instead burning up in the atmosphere like far larger meteors do today on Earth.

Now if it were a 500,000 ton nickel-iron asteroid moving at .99c, then turn off the lights...it's over. I think a cloud of sand, however, would have a harder time imparting that immense energy in as useful a fashion.

The only difference is that the 500,000 tons' worth of sand will impact in a much greater area, so the damage mechanisms will be slightly different. Instead of one giant fuck-all explosion ripping the atmosphere off with a huge shockwave and vaporizing a gigantic crater in the crust, you'll end up literally blanketing a continent-sized region with explosions. Imagine 50 tons of TNT detonating for every square millimeter of surface area, and you'll get some idea. The damage will be more evenly spread out and less singularly catastrophic (but everything on the planet is fucked anyway).

[avatarxprime]

Well there is the potential for the detonation of the sand as it hits the atmosphere to blow away the mass of the sand behind it and therefore decrease the efficiency of the attack. Whether or not it would be enough to make this a less than biosphere destroying attack I don't know. Based on a bit of searching to find the mass of a grain of sand (.23 to .67mg) each grain is equivalent to a 0.03-0.09 kiloton explosive, provided I did all my math right. Now that is significantly less than any high altitude nuclear tests I could find conducted by the US or the USSR (1.2 kt was the smallest) so depending on the shape of the sand cloud and the effect of the blowback it might be possible to not kill everyone.

What are you talking about? What conceivable mechanism could the first impacts possibly use to reverse the incredible momentum of nearly 500,000 tons of sand moving at 0.99c?

I'm not saying it would reverse the course of the sand, but as the first grains hit the atmosphere the resulting explosions could start slowing down the successive grains of sand and possibly incinerate some of the sand above the atmosphere as well thereby reducing the overall effectiveness of the attack. I don't know how large an effect those initial detonations would have but I thought I would raise the issue as being something different than if it was a single large impactor. Or am I simply overestimating the effect of the initial detonations or underestimating the momentum of each grain of sand?

[erik_t]

A thought experiment:

Let us suppose a highly diffuse sand cloud, a cylindrical cloud with a radius of 5000km and a depth of 1000km (how to disperse the sand in this way after 0.1c delta-v is left as an exercise to the reader). Note that Earth's radius is is ~6400km, so this entire cloud will strike the atmosphere, and indeed will do so over the span of about 33 microseconds. But no matter.

The density of silicon is about 2300kg/m^3, so our cloud has a volume fraction of about 2.8e-8 silicon. Even a 1cm thick cloud will be 99.72% empty space. Note also that the grains of sand have very high surface area per volume, so will not retain any significant heat from their acceleration event.

Now, when this cloud is 5min away from the planet, it will subtend a solid angle of 9.67e-9 steradians, about 1e-4 times the solid angle of the Moon or Sun. For those solid-angle-challenged, the apparent diameter of the cloud (at 5min away) will be 0.06 degrees. And it's several nines of empty space.

Detecting this cloud (especially at a worthwhile distance!) is likely to be nontrivial.

[Darth Wong]

It would probably be easier to detect the space vessels while they spent years accelerating it to that velocity. It helps to know that there's a hostile out there. If he knows where you are but you don't know he exists, there are plenty of ways he could hurt you. But if you know he exists and you have eyes on him, it would be pretty hard for him to do a lot of these grandiose things we talk about without detection.