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View Full Version : Near-light-speed weaponry: what would it do, what would it look like?

dvon5000
2010-Jun-10, 08:30 PM
I am interested in finding out as much as possible about what objects traveling at near-light speeds (.9c or greater) might look like from various angles and distances. I am putting together a short "space opera" sci-fi film, and although I will in part be going for retro, stylized visuals, I would like to try and inform these visuals with some real-life physics and ideas (e.g., even if I have funky ships that look like they're out of an old sci-fi serial, I would like to depict appropriate distances for outer space-- a "nearby" ship might be visible as just a speck of light, or only by the occlusion of stars). I'm no physicist, so please straighten out any crazy assumptions I may have picked up. I'm willing to learn!

From my understanding, even a small object traveling at near-light-speed would acquire tremendous kinetic energy upon impact with an object at rest. Hence my assumption that such objects would make pretty scary weapons. How much damage would a grain of sand do versus, say, a golf ball? What sizes and speeds would be more appropriate for destroying a ship, a space station or even a planet?

Would such an approaching object be easily detectable from great distances once launched? How about visible to the naked eye?

Let's say I'm floating in the void ("I" being the movie camera's field of view), and a "golf ball" traveling at .99c passes within a few kilometers from me, traveling straight "down" across my field of vision as I look straight ahead. What does it look like as it passes me as opposed to when it is approaching (looking up before it passes) and when it is receding (looking down after it passes)? Is it visible only for a split-second as a streak of light, or do I not notice it at all? How does this change if we increase my distance to the object by a few hundred kilometers, or a few thousand kilometers? Or bring me right next to it! Would blue-shifting and red-shifting make a noticeable difference to the eye, or is it subtle enough to only be detectable by equipment?

And of course, what would happen to a ship unfortunate enough to be in its path? Instant space dust, big visible burst of energy, or something else...? I am trying to think visually about it. Also, would such a collision create a cloud of debris also traveling at comparable speeds?

OK, I could go on forever asking questions, but I'll leave it at this for now. Thanks for your time!

IsaacKuo
2010-Jun-10, 09:15 PM
From my understanding, even a small object traveling at near-light-speed would acquire tremendous kinetic energy upon impact with an object at rest.
Yes. It would instantly vaporize at the surface of the target and result in an incredibly powerful surface explosion (for a golf ball, comparable to a nuclear bomb). Unless the target is very large, like a small city, the result is total destruction.

Hence my assumption that such objects would make pretty scary weapons.
Well, the problem with using such an object as a weapon is that it's ridiculously difficult to accelerate a solid object up to those velocities. I am aware of a few serious possibilities, but they're all very...umm..."futuristic".

Would such an approaching object be easily detectable from great distances once launched? How about visible to the naked eye?
They would be invisible to the naked eye, and something the size of a golf ball would be practically invisible to any plausible sensors.

However, it's actually something that can be defended against. The incredible velocity of the object works against it if it hits even the thinnest of "shields". Such a shield could be a thin sheet of film, or an extremely thin field of smoke particles. Even the thinnest of practicable shields would instantly vaporize and ionize the golf ball, converting it into a shower of charged particles which can then be deflected by a modest magnetic field.

Let's say I'm floating in the void ("I" being the movie camera's field of view), and a "golf ball" traveling at .99c passes within a few kilometers from me, traveling straight "down" across my field of vision as I look straight ahead. What does it look like as it passes me as opposed to when it is approaching (looking up before it passes) and when it is receding (looking down after it passes)? Is it visible only for a split-second as a streak of light, or do I not notice it at all?
You see nothing. The only camera view which would be useful at all would be a "missile-cam" flying alongside the golf ball. You could rotate the view around, to see all sides of the missile, and show the stars in the background blue-shifted and red-shifted.

And of course, what would happen to a ship unfortunate enough to be in its path?
If you break it down into super slow motion, the result is essentially a spherical explosion centered just underneath the impact point. In the first moments, the golf ball splashes into the impact point affecting a depth of about a golf ball's diameter. This turns this impact point into ridiculously hot plasma radiating away awesome amounts of thermal x-rays. Maybe half of the energy is invisibly "vented" into outer space while the other half vaporizes its way through the immediate radius at hypersonic speeds.

After vaporizing on the order of a meter radius, the plasma fireball has cooled down enough to no longer be glowing in thermal x-rays. This is when things really start to change, because plasma isn't transparent to UV. This marks the end of the creation of the fireball...maybe a few meters wide hemisphere from the impact point. Unless you're using ridiculously super-slow-mo, everything up until this point is instantaneous.

From then on, the primary destruction mechanism is from the expansion of this hot gaseous fireball. The fireball itself mostly blasts out of the blast crater, but it also pushes outward against the crater's sides, sending that stuff outward at unstoppable supersonic speeds. If you're using slow-motion, it's an unstoppable expanding cloud of glowing molten debris and hot glowing smoke.

swampyankee
2010-Jun-10, 09:57 PM
For the code, there is a relativistic ray-tracing site, here (http://hexadecimal.uoregon.edu/relativity/index.html). For some images, you could try here (http://www.anu.edu.au/physics/Searle/) and here (http://www.fourmilab.ch/cship/).

dvon5000
2010-Jun-10, 10:04 PM
Thanks, lots of very useful stuff in your post.

They would be invisible to the naked eye, and something the size of a golf ball would be practically invisible to any plausible sensors.

Interesting! Around what speed approximately would the object become invisible or start to become invisible to the naked eye? If I was to depict an object accelerating away from me, starting at .7c and moving to .99c, would it gradually vanish as it redshifts, or just blink out at some point?

Murphy
2010-Jun-11, 12:35 AM
IsaacKuo has already explained quite a lot, so I'll just try to show you how to calculate it.

From my understanding, even a small object traveling at near-light-speed would acquire tremendous kinetic energy upon impact with an object at rest. Hence my assumption that such objects would make pretty scary weapons. How much damage would a grain of sand do versus, say, a golf ball? What sizes and speeds would be more appropriate for destroying a ship, a space station or even a planet?

Well all these questions can be answered by the Relativistic Kinetic Energy Formula (You'll find the details on the Wiki page (http://en.wikipedia.org/wiki/Kinetic_energy)). The Normal Kinetic Energy Formula is of course K.E. = (1/2) (mv^2) (where m is mass, and v is velocity), when you're talking about things travelling at near light speed you need to use a somewhat more complex formula.

That is K.E. = mc^2 (gamma - 1). c is, of course, the speed of light and gamma is a more complicated thing. gamma = 1/sqrt(1-(v/c)^2) (sqrt meaning "square root of", as I'm not sure how to type that).

Anyway, to take your examples: a grain of sand, let's say it's 1 mm in diameter and has a density of 3,000 kg/m^3 (which I think is about the density of most rocks), so a mass of 1.5707963267948966192313216916397e-6 kg (or 1.57 mg). If it's going at 95% c, then...
(BTW sorry if these are ridiculously big numbers, I'm just pasting in what Windows Calculator spits out).

gamma = 1/sqrt(1-(v/c)^2)
gamma = 1/sqrt(1-(285,000,000/300,000,000)^2)
gamma = 1/sqrt(1-(0.95)^2)
gamma = 1/sqrt(1-0.9025)
gamma = 1/sqrt(0.0975)
gamma = 1/0.31224989991991991029234465604699
gamma = 3.2025630761017426696650733953537

K.E. = mc^2 (gamma - 1)
K.E. = (1.5707963267948966192313216916397e-6)(300,000,000)^2 (3.2025630761017426696650733953537 - 1)
K.E. = (1.5707963267948966192313216916397e-6)(90,000,000,000,000,000) (2.2025630761017426696650733953537)
K.E. = (141,371,669,411.54069573081895224757) (2.2025630761017426696650733953537)
K.E. = 311,380,019,052.72171574899100542963 joules

K.E. = 3.1138e+11 joules

If you want to convert this into everybody's favourite measure of explosions; the Tons of TNT equivalent, then divide it by 4.184e+9 joules. So we get 74.42 Tons of TNT, the same as a very small nuclear warhead, but certainly very powerful.

Ok these calculations a very tedious, but luckily there are several online calculators for Relativistic Kinetic Energy. This one is probably the simplest to use; Relativity Calculator (http://www.stardestroyer.net/Empire/Science/Relativity.html) (it also handily tells you the amount of time dilation experienced).

Let's see, if the sand grain were travelling at 99% the speed of light it would be even more powerful, with 8.608e+11 joules of energy, or about 206 tons of TNT.

If the same grain is going at 99.9% of the speed of light then it has 3.021e+12 joules of energy, or 722 tons of TNT. As you can see the energy raises exponentially as you approach light speed.

A Golf Ball? Well according to Wikipedia a Golf Ball weighs 45.93 grams. So...

At 95% c the Kinetic Energy is 9.105e+15 joules, or 2,176,147 tons of TNT, or about 2.2 Megatons (now we're talking!).
At 99% c the Kinetic Energy is 2.517e+16 joules, or 6,015,774 tons of TNT, or about 6 Megatons.
At 99.9% c the Kinetic Energy is 8.832e+16 joules, or 21,108,987 tons of TNT, or about 21.1 Megatons.

These kind of things are good enough to destroy spacecraft and stations (although good old nuclear bombs would probably be more practical), but if you want to destroy planets (and who doesn't? Muahhahaha!...;)), then we'll need something a bit bigger.

Read my post in this thread (http://www.bautforum.com/showthread.php/104396-how-disastrous-would-this-be) for an exposition of what relativistic kill Vehicles might do when hitting a planet. The atmosphere of a planet like Earth will matter a lot, small objects will burn up very quickly in the air and not reach the ground, no matter how fast they're going (check out The Earth Impact Effects Program (http://impact.ese.ic.ac.uk/ImpactEffects/), although it's not setup to deal with relativistic speeds). Although a great deal of energy is released in the burn up and that may be enough to cause huge atmospheric heating and cook the inhabitants of the planet (though I don't know how to calculate that kind of effect).

For planet killers, they will need to be big (so most of their structure survives the atmospheric friction and they actually hit), several tons at least, and a few hundred to a few thousand tons to be really sure. You want energies in the high Teraton range to wipe out a civilization. For instance, 100 tons traveling at 95% c produces 4.74 teratons of explosion, about enough to devastate half a continent. 1,000 tons will produce 47.37 teratons, this is getting there but, it's still not as power for instance as the asteroid that wiped out the Dinosaurs (which was apparently over 100 Teratons). Let's go for 10,000 tons, that's 473.71 teratons! Now that, I think, would wipe the planet clean, no chance for survival.

Would such an approaching object be easily detectable from great distances once launched? How about visible to the naked eye?

As IsaacKuo has already said, there wouldn't really be any effects from a relativistic object that can be detected, it would just look like a normal object travelling fast. One interesting (and very dangerous) thing about stuff travelling near light speed is that even if you do detect them they're coming so fast that they will arrive shortly after you notice them.

For example; if say, a large relativistic kinetic Kill vehicle is travelling towards the Earth at 99% of the speed of light and we detect it by some method (say a highly sensitive space telescope), it will already arrive before we can do anything much. Suppose we detected it at the edge of our Solar system, 10 billion kilometres away, the light that we detect coming from it will take 33,333 seconds (approximately 9.26 hours) to reach us. But in that time the Kill vehicle will have travelled 9.9 billion km, and will be only 100 million km from Earth. It will arrive in just 337 seconds, about 6 minutes. So we have only 6 minutes to save the Earth! Not much you can do it that time.

And of course, what would happen to a ship unfortunate enough to be in its path? Instant space dust, big visible burst of energy, or something else...? I am trying to think visually about it. Also, would such a collision create a cloud of debris also traveling at comparable speeds?

A collision with anything would result in an almighty explosion that would definitely be visible from far away. It would look similar to a Nuclear blast (and have similar effects). You would see a blinding flash of light (how long it lasts depends on the size of the explosion, bigger equals longer lived flash) and a fireball of Plasma that would expand out at a tremendous speed turning any space ship into glowing atoms. The matter would then disperse in all directions (although there might be a significant push from the impact to favour the forward direction of the explosion) and then the vaporised debris would cool down to the ambient temperature (a few degrees K if we're in deep space) and no longer be visible. It's possible that fragments from the explosion (grains of sand sized stuff) would be flung out at relativistic speeds and become a hazard to other ships in the area. A kind of deadly shotgun spray.

IsaacKuo
2010-Jun-11, 01:58 AM
Interesting! Around what speed approximately would the object become invisible or start to become invisible to the naked eye?
Something the size of a golf ball? I'm not sure, but I'd guess somewhere around the speed of sound (in other words, very very very slow, compared to lightspeed). Pistol bullets move at about the speed of sound, and that's already way to fast to see with the naked eye. However, if we assume a white bullet glowing in 1AU sunlight against the dark background of outer space, then it may be visible as a faint streak if it's moving almost straight toward or away from the viewer. Tracers are definitely visible, but they glow VERY brightly.

Delvo
2010-Jun-11, 03:14 AM
Whatever method you had for getting the ammo up to that speed in the first place would probably be relatively easy to see, analogous to a gun's muzzle flare or a rocket's exhaust plume being more easily visible from farther away than the bullet or rocket. Even magnetic propulsion would heat up the launcher's magnets, and more heat leads to more radiation in one wavelength or another. People/sensors in one ship would watch for whatever kind of glow or flash the relevant launching/firing mechanism was known to create, and immediately calculate from it, if they could, what direction the projectile was initially launched at and how soon it would either hit or miss them. Those who fire the weapons would try to counter this by firing from far enough away to give away as little of that kind of information as possible, but close enough to get the ammo to the target as soon as possible.

The difference between projectiles with and without their own on-board propulsion systems would be huge. Those without it would be nearly impossible to detect in flight, but could miss much more easily if the target moved in a way that the gunners didn't anticipate (and then coast right on out of the battlefield to end up hitting something else that isn't a target, a hundred thousand years later and ninety thousand light-years away). Those with it, while trying to adjust course to keep up with a maneuvering target, would make themselves easy to see and try to counteract as soon as they used their own engines.

Visual representation of such a long-distance, high-speed engagement would rely on computers representing objects with symbols. Maybe the symbols could even look like the things they represented, but they'd have to be shown much larger than they really were relative to the area of space within which they're interacting, in order for more than one thing to be seen at the same time on the same screen/hologram. (The larger represented size also conveniently reduces their apparent speeds to something you could actually watch moving around, too.) Otherwise, things would just be too far away to see at all except maybe as just a tiny point, and things coming near you would be gone so soon you didn't really get to watch them go by. (At best, a close enough flyby would only let you see a blurred line where the object's flight path is, all at once, for a tiny fraction of a second.)

RalofTyr
2010-Jun-11, 05:17 AM
One problem is accelerating it to near light-speed. Say, if you had a particle accelerator that run around the Moon, it might take some time, even years to get it up to speed.

However, it's actually something that can be defended against. The incredible velocity of the object works against it if it hits even the thinnest of "shields". Such a shield could be a thin sheet of film, or an extremely thin field of smoke particles. Even the thinnest of practicable shields would instantly vaporize and ionize the golf ball, converting it into a shower of charged particles which can then be deflected by a modest magnetic field.

Did you take in the account of time-dilation? It could be that since times slows, the ball itself might remain non-ionized (still a golf ball) long enough to impact. If fact, the ball could well be deep with in the Earth before it slowed down enough to catch of with impact time to have the ball ionize.

It don't want to go crazy with the sci-fi, but the ball could have enough momentum to go through the Earth before it vaporized.

skep155
2010-Jun-11, 01:04 PM
I've actually heard that any sort of close range space battle would be suicidal for both combatants since if one ship went boom the debris would just fly outward without slowing and potentially cut the other ship to ribbons.

Shaula
2010-Jun-11, 02:45 PM
One of the better relativistic weapons I had read about was a shotgun approach. Basically used a load of very tough shards and a huge shaped explosion to fill an area of space with hundreds of tiny fast moving bits. That got around the aiming issues and acted as an area denial weapon. Trouble is for relativistic shards you'd need enough of an impulse that they'd probably be destroyed. These weapons relied on the ships moving very quickly to add to the damage potential.

IsaacKuo
2010-Jun-11, 03:04 PM
One problem is accelerating it to near light-speed.
Yes. It is HARD. Most realistic methods involve external acceleration by a particle beam or powerful laser over a period of many months. However, Jordin Kare's SailBeam (http://www.niac.usra.edu/files/library/meetings/fellows/oct01/597Kare.pdf) acceleration method might, in principle, be able to accelerate an extremely low absorption diamond film laser sail up to relativistic speeds in mere seconds. I should emphasize that this method is highly speculative and I am incredulous of the idea that acceleration would be so perfectly uniform that the sail doesn't tear apart upon launch (vaporizing the torn remains well before any significant velocity). I prefer a method of acceleration involving an X-ray laser over a long period of time.

Did you take in the account of time-dilation? It could be that since times slows, the ball itself might remain non-ionized (still a golf ball) long enough to impact.
I'm not sure it's even possible for this to happen. However, given sufficiently high gamma it's possible that the charged particles could "muscle" their way past the magnetic field defense simply because they aren't sufficiently deflected by it. This depends on the details of the target and the defensive system, of course.

IsaacKuo
2010-Jun-11, 03:06 PM
One of the better relativistic weapons I had read about was a shotgun approach. Basically used a load of very tough shards and a huge shaped explosion to fill an area of space with hundreds of tiny fast moving bits. That got around the aiming issues and acted as an area denial weapon. Trouble is for relativistic shards you'd need enough of an impulse that they'd probably be destroyed. These weapons relied on the ships moving very quickly to add to the damage potential.
It wouldn't work to provide relativistic shards. As you not, they'd need enough of an impulse that they'd be destroyed (vaporized into charge particles).

Shaula
2010-Jun-11, 03:07 PM
It wouldn't work to provide relativistic shards. As you not, they'd need enough of an impulse that they'd be destroyed (vaporized into charge particles).
ISTR that this was a second stage - should have added that. The weapon was a missile type of weapon that got pretty fast itself then went pop to give the shards a bit more of a boost. Makes it a shade more possible but you'd have to do the math to get a real feel for it. I didn't!

mayapan1942
2012-Dec-02, 09:40 PM
Hi,

I'm new to this forum but I've had a deep interest in cosmology for many years. I have a question that's allied to the idea of light-speed weaponry but which I've not yet seen explained.

Although the information and posts here have helped to clear away some aspects, there still remains a fundamental issue, thus: if a space ship reaches 99.9% light speed, what's the probability that the ship would be vaporized by a star, planet, asteroid or speck of dust that just happens to get in the way?

It seems to me that, given that space is not a void at all, there must be billions – perhaps trillions - of objects that are serious hazards to any solid mass travelling at near light speed. And travelling at such a speed allows no room for hazardous objects.

Perhaps somebody here could provide further clarification?

Nick Theodorakis
2012-Dec-02, 10:18 PM
This seems relevant:

What would happen if you tried to hit a baseball pitched at 90% the speed of light? (http://what-if.xkcd.com/1/)

By Randall Munroe, the author of the webcomic xkcd.

Nick

schlaugh
2012-Dec-03, 01:59 AM
It seems to me that, given that space is not a void at all, there must be billions – perhaps trillions - of objects that are serious hazards to any solid mass travelling at near light speed. And travelling at such a speed allows no room for hazardous objects.

In sci fi the authors often shove a big block of ice or an asteroid out ahead of the relativistic craft but that assumes collisions only with dust between the stars. Anything larger - like the aforementioned baseball and golf ball, would likely destroy or heavily damage the shield. As for larger objects...there's just no telling. Radar won't give you any warning before hitting whatever is in your path (we can assume that planets and stars will be well mapped, although brown dwarfs could be an issue). Small stuff might be vaporized by defensive lasers. Maybe.

The other problem with using relativistic weapons is aiming the darned things. If you're trying to hit something even as large as the Earth plodding around the sun, and if your timing is off by even a millisecond - then you've missed. Which is why the shotgun method is better in theory.

mayapan1942
2012-Dec-03, 05:30 AM
Thanks Nick - very informative and entertaining.:D The difference with the spaceship situation is the vacuum (or near vacuum) of space. So, if such a ship encountered an asteroid plodding along there would be an almighty, but different, explosion. Correct? If so, how can any ship's captain ensure that all asteroids will be avoided?

mayapan1942
2012-Dec-03, 05:39 AM
"Radar won't give you any warning before hitting whatever is in your path (we can assume that planets and stars will be well mapped, although brown dwarfs could be an issue)."

Thanks for that, schlaugh. I can then suggest it would very dangerous for any spaceship at near light speed to attempt any navigation of the cosmos, seeing as how there are millions or billions of things - many of which cannot be detected easily or fast enough - that could vaporize it in a literal flash. Do you see it that way also? Or is my logic off?

caveman1917
2012-Dec-03, 11:03 AM
"Radar won't give you any warning before hitting whatever is in your path (we can assume that planets and stars will be well mapped, although brown dwarfs could be an issue)."

Thanks for that, schlaugh. I can then suggest it would very dangerous for any spaceship at near light speed to attempt any navigation of the cosmos, seeing as how there are millions or billions of things - many of which cannot be detected easily or fast enough - that could vaporize it in a literal flash. Do you see it that way also? Or is my logic off?

You're right, at near light speed velocities even dust particles become a threat. For example, at 99% of the speed of light, a 1 gram particle has a kinetic energy equivalent to about 1 megaton of TNT (ie a good yield nuclear weapon). It will more likely just punch through the hull rather than explode, but you can see the problem. You don't need brown dwarfs or asteroids.

schlaugh
2012-Dec-03, 01:38 PM
I can then suggest it would very dangerous for any spaceship at near light speed to attempt any navigation of the cosmos, seeing as how there are millions or billions of things...

Yah, but space is really quite empty. Dust and gas will be impacted but the chance of hitting anything more solid and larger than molecules is pretty darn small.

I suppose you could launch probes (also moving at the same speed as your ship) to sweep a path a few light hours or light days ahead so that you could gain some data. But even those probes would want their own shielding.

Alastair Reynolds wrote a good treatment in Redemption Ark of how to conduct combat when both ships are moving at near light speed. And John Varley touched on using objects as light-speed missiles in Red Lighting... (I think. It may have been Thunder and Lighting).

IsaacKuo
2012-Dec-03, 05:25 PM
You're right, at near light speed velocities even dust particles become a threat. For example, at 99% of the speed of light, a 1 gram particle has a kinetic energy equivalent to about 1 megaton of TNT (ie a good yield nuclear weapon). It will more likely just punch through the hull rather than explode, but you can see the problem. You don't need brown dwarfs or asteroids.

It will most definitely explode. But this is not such a problem. In fact, if you turn around your perspective it's an advantage.

All you need for protection is have a thin disposable shield well in front of the ship. This shield could be a cloud of dust particles. It could be a cloud of ice flakes, created simply by squirting a bit of water in front of the ship.

Even a large obstacle would be utterly obliterated by such a shield, resulting an explosion of plasma. Such plasma could then be deflected by a magnetic field, so it doesn't even touch the ship. The amount of plasma which needs to be deflected can be quite minimal, depending on how far ahead of the ship the shield is.

There is no need for any sort of "laser" to shoot at incoming obstacles. A low mass disposable shield is far more powerful than a laser, due to the extreme kinetic energy, and unlike a laser it just waits there for an unlimited time until an incoming threat hits it. No need to detect the incoming threats beforehand. It just plows a safe path for the starship, acting as a minesweeper.

neilzero
2012-Dec-03, 08:19 PM
Assuming the craft can reach a speed of 0.1 c with respect to the average nearby particles, they are flying blind. Suppose a million drone craft travel approxinately ahead of the manned craft. The drone craft can detect pea size particles up to about 10 kilometers away, and send the data to the manned ship at c via micro waves or laser. Travel time for the data is one second if the drone is 300,000 kilometers ahead of the manned craft which can then aim a high powered laser at the particle, converting it to plasma. The problem is the manned craft approched the partical by 30,000 kilometers in that 1 second, and there are at least millisecond delays in the radar electronics, the radar to data link interface, and in the microwave receiver at the manned ship and in the aiming process for the laser which will vaporize the partical and the travel time for the laser beam (100 milliseconds) that will vaporize the particle. If the particle begins vaporizing 0.1 seconds before the manned ship runs into the still tiny plasma cloud, then gamma rays are produced by the impact of the plasma and manned craft or the film shield ahead of the manned craft. These very energetic gamma rays are at least a minor hazard to the crew and the manned ships electronics. I think you can see that much over 0.1 c there is not time for the plasma cloud to exceed the diameter of the manned craft (S= 1/2 at squared = 1,000,000 times 0.000001 = one meter side movement of the plasma if the acceleration is 1,000,000 meters per second per second and the time is one milisecond) and the film shield needs to be something like 3000 kilometers ahead of the manned craft for the plasma to mostly miss the craft. Even an extreme magnetic or electrostatic field can only deflect the plasma a few degrees in a few milliseconds, because the plasma is traveling about 0.1 c with respect to the manned craft. The manned craft cannot change direction nor speed significantly in one second, unless the inertial dampers (not invented) can handle 1000 g of artificial gravity. Neil

IsaacKuo
2012-Dec-03, 08:39 PM
Assuming the craft can reach a speed of 0.1 c with respect to the average nearby particles, they are flying blind.
No it isn't, but so what if it were? The defense system I describe does not require any sensors.

I think you can see that much over 0.1 c there is not time to vaporize the particle
Nonsense. The time required to vaporize the particle is proportional to the impact velocity. It's practically instant, at these impact speeds.

and the film shield needs to be something like 30,000 kilometers ahead of the manned craft for the plasma to mostly miss the craft.
Actually this is not true at all. Even without a magnetic field, a huge 100m diameter spacecraft would be mostly missed if the shield is only 10km ahead. That would result in roughly 1/10,000th of the explosion hitting the spacecraft. The sideways velocity imparted to the explosion products is proportional to the impact velocity.

Depending on the method of deploying the shield, it could end up much further ahead of the spacecraft. The simplest method of deployment would be to emit it forward through some nozzles. This will impart a certain amount of forward velocity relative to the spaceship, so it will continue to get further and further ahead. So, even if it's 10km ahead now, it could end up 100km ahead or even 1,000km ahead by the time the destination is reached.

Swift
2012-Dec-03, 08:48 PM
It will most definitely explode. But this is not such a problem. In fact, if you turn around your perspective it's an advantage.

All you need for protection is have a thin disposable shield well in front of the ship. This shield could be a cloud of dust particles. It could be a cloud of ice flakes, created simply by squirting a bit of water in front of the ship.

Even a large obstacle would be utterly obliterated by such a shield, resulting an explosion of plasma.
In one of David Brin's Uplift novels, the Dolphins destroyed a ship that was chasing them by dumping a load of water in that ship's path.

mayapan1942
2012-Dec-04, 01:48 AM
Thanks, schlaugh. But, I thought all space had at least one molecule of hydrogen per cubic meter? If so, space is not really "quite empty", as you say. Or am I wrong?

mayapan1942
2012-Dec-04, 01:54 AM
Thaks, Isaac. But, how far ahead must a shield be to keep a ship safe from hazards? And, what would happen if a hazard is planet-sized?

mayapan1942
2012-Dec-04, 03:06 AM
Isaac - your additional explanation and minesweeper analogy I can understand, although I'm not good at the math. What intrigues me most, though, is this: what's the probability that a star ship will ever reach its destination, anyway? For example, how many times would a ship be required to eject successive water shields in a ten year journey? Is the water produced on the ship? Is it probable that every small and large hazard could be identified and avoided, given the velocity involved?

This situation is surely not comparable to tracking and avoiding icebergs in the Atlantic; or even satellites in orbit around this planet.

Solfe
2012-Dec-04, 04:44 AM
There was short story in Asimov about Bussard Ramjet attack. The aliens miss Earth but the message is clear, such a device is possible. Part of the story revolves around the ethics of shooting back, because the weapon hits something in the solar system so the aliens believe Earth is has been hit.

If I can find the issue, I will post the author's name and story title.

AlexG
2012-Dec-04, 06:45 AM
In Larry Niven's book Protector there's an excellent long range, relativistic space battle, using high powered lasers and a hunting rifle.

Also in Niven's Tales Of Known Space there's a story about one Bussard Ramjet chasing another.

IsaacKuo
2012-Dec-06, 04:36 PM
Thanks, schlaugh. But, I thought all space had at least one molecule of hydrogen per cubic meter? If so, space is not really "quite empty", as you say. Or am I wrong?
This is true, but it doesn't really change things. I don't have a reference handy, but I attended a presentation at last year's 100 Year Starship Symposium which calculated the effects of the interstellar medium on flimsy light sails at relativistic speeds. Even those thin films were barely worn down by the interstellar medium. Hydrogen molecules are really tiny!

Thaks, Isaac. But, how far ahead must a shield be to keep a ship safe from hazards? And, what would happen if a hazard is planet-sized?
This depends on the speed and mass of the charged particles ejected, which in turn depends on the mass of the hazard compared to the amount of shield it impacts, and it also depends on the size/strength of the ship's magnetic field. I like to assume a magnetic field which is also suitable for use for starship propulsion (either relativistic particle beam deflection, or pulsed plasma propulsion). Such magnetic fields tend to be somewhat compact but strong. These are massive overkill for protection against collision hazards, but the marginal cost is zero since you need it anyway. In this case, the shield only needs to be maybe 200m ahead of the ship--at minimum. In practice, the shield probably gets further and further away ahead of the ship anyway, depending on how precisely it is deployed.

An alternative magnetic field could be weak but big. Such a large magnetic field might be useful after reaching the destination system as a magsail. Such a magnetic field interacts with the solar wind for low--but continuous--thrust. In this case, the shield would need to be ahead of the ship by something similar to its diameter. Maybe on the order of a hundred kilometers.

Another alternative magnetic field could be weak but very big. Such a magnetic field could be useful for interstellar medium magnetic braking. It's possible that no shield might be necessary, with the ISM itself continuously feeding more plasma into it. This field might be so big that it actually slows the starship down by a serious amount.

None of these possibilities offer a credible defense against a planet-sized hazard. In theory, the shield might be layered in order to successively dig a temporary tunnel through the planet for the starship, but this tunnel will be filled with dense plasma with nowhere to go when the magnetic field attempts to deflect it. It'll just splash back when it hits the sides of the tunnel. Ouch!

Isaac - your additional explanation and minesweeper analogy I can understand, although I'm not good at the math. What intrigues me most, though, is this: what's the probability that a star ship will ever reach its destination, anyway? For example, how many times would a ship be required to eject successive water shields in a ten year journey? Is the water produced on the ship? Is it probable that every small and large hazard could be identified and avoided, given the velocity involved?

This situation is surely not comparable to tracking and avoiding icebergs in the Atlantic; or even satellites in orbit around this planet.
That's right, it's not comparable. The probability that a starship will reach its destination is very very good. Interstellar space is just incredibly empty--so empty that photons sail right through it for thousands of light years without hitting anything. The fact that we can see far away stars at all, rather than the sky looking like pea soup, is a testament to how stupendously empty interstellar space is.

So, you can't know 100% that any particular path through interstellar space doesn't hit a planet. But you can know that very nearly 100% of them don't hit a planet for many thousands of light years. The chances of an unlucky encounter with one during a 4.3 light year journey are negligible. You're not going to run into a planet. You're not even going to run into a significantly sized dust particle.

schlaugh
2012-Dec-06, 04:41 PM
Thanks, schlaugh. But, I thought all space had at least one molecule of hydrogen per cubic meter? If so, space is not really "quite empty", as you say. Or am I wrong?
Well that's a derived "average" which does not equate to what's really in your path - or not. :) As Issac said, space is quite empty.

mayapan1942
2012-Dec-06, 07:07 PM
Many thanks, Isaac. You have provided much to think about. I'll get back to you.:)

Xibalba
2012-Dec-06, 08:13 PM
Well, there was a proton sighted at 0.9999999999999999999999951c which packed the kinetic energy of a 100mp/h baseball. I guess that a proton hitting you with such force could kill you if the shot is well aimed.

Xibalba
2012-Dec-06, 08:36 PM
Would such an approaching object be easily detectable from great distances once launched? How about visible to the naked eye?

If a bullet or missile is travelling at 0,9c, and let's say the target is one light-year away (which hardly makes sense, I know), their best detectors might see it coming 36,5 days before impact, at a distance of 9,45 x 10^10 kilometers (it's farther than Voyager 1), and good luckk noticing such a small object so far away. But there's a chance that it emits brightly in the gamma or x-ray spectrum.

Jens
2012-Dec-07, 02:24 AM
Well, there was a proton sighted at 0.9999999999999999999999951c which packed the kinetic energy of a 100mp/h baseball. I guess that a proton hitting you with such force could kill you if the shot is well aimed.

Wouldn't a proton going that fast tend to just pass through you without hitting anything along the way?

Solfe
2012-Dec-07, 03:57 AM
Wouldn't a proton going that fast tend to just pass through you without hitting anything along the way?

I've been wearing this batter helmet for years for no reason at all?

caveman1917
2012-Dec-07, 04:04 AM
I've been wearing this batter helmet for years for no reason at all?

No, it still goes fashionably well with the tinfoil hat.

neilzero
2012-Dec-07, 06:21 AM
One hydrogen atom per cubic meter may be high for half way to the Centari system, but let's use that number, and 100 square meters for the cross sectional area of the Star ship at 0.1c, which sweeps up 30,000,000,000 cubic meters per second = 30,000,000,000 hydrogen atoms per second for 43 years. Neil

IsaacKuo
2012-Dec-07, 02:58 PM
Yes, but hydrogen molecules are extremely small, and 100 square meters is very big. 30,000,000,000 atoms per second over 43 years still only adds up to 70 micrograms! It turns out that even thin light sails can handle this fine.

Xibalba
2012-Dec-07, 03:05 PM
Wouldn't a proton going that fast tend to just pass through you without hitting anything along the way?

Guess how they detected it. It hit another particle and the shower of particles it generated was detected.

If the battle is in outer space, there's a good chance it won't hit anything until it reaches you, or your ship, but anywhere with some kind of atmosphere, or at least some place filled with particles, the weapon would have terribly short range. For direct impact. You would still probably get hit by rays and near-lightspeed particles formed by the collision of this proton with another particle.

If you want some sort of source for that, Wikipedia is the best I got yet : http://en.wikipedia.org/wiki/Oh-My-God_particle

2014-Jan-12, 11:40 AM
Actually, a ball shaped bullet would look flattened like a coin, due to relativistic contraction.

If after crashing the plasma explosion expanded at near the speed of light, the explosion would tear a 45 degrees cone out of the ship. But the plasma would not expand at the speed of light perpendicular to the bullet movement due to 2 reasons:

1. Moving at near the speed of light perpendicular to the bullet would require additional energy comparable to the total energy of the ball. There is no source of energy in such scale.
2. Time dilation would slow down the perpendicular explosion to 14%, so a 45° cone would be reduced to a 8 degree cone.

So, the ship would be cleanly crossed, in nanoseconds, by the plasma ball at near the speed of light, matter would not have time to get out of the way, so it would be cleanly dragged out of the ship, leaving a clean hole.

If the ball rips and drag 10 times his mass out of the ship, his speed would be reduced significantly, but the plasma expansion would still occur between thousands to hundred of thousands of kilometers behind of the ship. The explosion would be shaped like a disk, not as as sphere, due to relativistic contraction, and since it moves at near the speed of light, it would not bounce back to the ship.

So the ship would not be destroyed, it would just get a golf ball sized hole.

On other side, if the ball were crashing as a plasma against a magnetic shield, and his momentum were stopped, the entire momentum would be transferred to the magnetic shield generators. They would be ripped apart with the power of a thermonuclear bomb, and the ship would be destroyed.