Relativistic projectiles release of energy on impact?

[cosmicalstorm]

I recently discovered the idea of relativistic projectiles and it got me thinking.
Basically, what im wondering is; would all the energy really be released on impact with its target or would it just pass right through?

In the opening chapter of The Killing Star they are described as having the effect of very large nuclear detonations. (or maybe even worse)
But in reality, would this actually happen?

Would'nt something moving at this speed (0.9c to 0.99c) just pass right through even a planet the size of Earth without depositing too much of its energy?
(You should be able to fly by some 12 or more Earths in less than a second if you're moving at near c)

Has there been any serious research or simulations done to test this?

The wikipedia page didnt carry too much information and the Atomic Rocket page only mentioned the possibility of this happening when shooting at spacestations briefly.

I hope I posted this in the correct forum, im new here.

[Darth Wong]

The idea of a relativistic projectile passing through 12000 km of rock and pressurized iron without being destroyed strikes me as utterly preposterous.

At the moment of impact, I would expect any such projectile to be converted to plasma by the shock. For that matter, air friction acting upon the projectile would destroy much of it before it even hit the ground, depending on how massive it is.

[Surlethe]

I'm not sure how well this translates to a relativistic problem, but in classical physics air resistance is proportional to velocity. So the faster you're going, the more resistance you run into, and you bleed off speed in an exponential decay toward the terminal velocity. For a near-c object, terminal velocity is practically standing still, so when it enters the atmosphere it will be shedding speed (and hence heat) very, very quickly.

Edit: It's actually closer to the square of the velocity for speeds higher than 1cm/s (IIRC). So the projectile bleeds speed even faster.

[Einhander Sn0m4n]

Think of the nastiness that comes out of large hadron collisions, say, relativisyic metal nuclei against a metal target. Then multiply the effect by a few sextillion.

From The Killing Star by Charles Pelligrino and George Zebrowski (you really should read this book):

All the energy put into achieving that velocity had transformed the Intruder into a kinetic storage device of nightmarish design. If it struck a world, every gram of the vessel’s substance would be received by that world as the target in a linear accelerator receives a spray of relativistic buckshot. Someone, somewhere, had built and was putting to use a relativistic bomb -- a giant, roving atom smasher aimed at worlds...

The gamma-ray shine of the decelerating half was also detectable, but it made no difference. One of the iron rules of relativistic bombardment was that if you could see something approaching at 92 percent of light speed, it was never where you saw it when you saw it, but was practically upon you...

In the forests below, lakes caught the first rays of the rising Sun and threw them back into space. Abandoning the two-dimensional sprawl of twentieth-century cities, Sri Lanka Tower, and others like it, had been erected in the world’s rain forests and farmlands, leaving the countryside virtually uninhabited. Even in Africa, where more than a hundred city arcologies had risen, nature was beginning to renew itself. It was a good day to be alive, she told herself, taking in the peace of the garden. Then, looking east, she saw it coming -- at least her eyes began to register it -- but her optic nerves did not last long enough to transmit what the eyes had seen.

It was quite small for what it could do -- small enough to fit into an average-sized living room -- but it was moving at 92 percent of light speed when it touched Earth’s atmosphere. A spear point of light appeared, so intense that the air below snapped away from it, creating a low-density tunnel through which the object descended. The walls of the tunnel were a plasma boundary layer, six and a half kilometers wide and more than 160 deep -- the flaming spear that Virginia’s eyes began to register -- with every square foot of its surface radiating a trillion watts, and still its destructive potential was but fractionally spent.

Thirty-three kilometers above the Indian Ocean, the point began to encounter too much air. It tunneled down only eight kilometers more, then stalled and detonated, less than two-thousandths of a second after crossing the orbits of Earth’s nearest artificial satellites.

Virginia was more than three hundred kilometers away when the light burst toward her. Every nerve ending in her body began to record a strange, prickling sensation -- the sheer pressure of photons trying to push her backward. No shadows were cast anywhere in the tower, so bright was the glare. It pierced walls, ceramic beams, notepads, and people -- four hundred thousand people. The maglev terminal connecting Sri Lanka Tower to London and Sydney, the waste treatment centers that sustained the lakes and farms, all the shops, theaters, and apartments liquefied instantly. The structure began to slip and crash like a giant waterfall, but gravity could not yank it down fast enough. The Tower became vapor before it could fall half a meter. At the vanished city’s feet, the trees of the forest were no longer able to cast shadows; they had themselves become long shadows of carbonized dust on the ground.

In Kandy and Columbo, where sidewalks steamed, the relativistic onslaught was unfinished. The electromagnetic pulse alone killed every living thing as far away as Bombay and the Maldives. All of India south of the Godavari River became an instant microwave oven. Nearer the epicenter, Demon Rock glowed with a fierce red heat, then fractured down its center, as if to herald the second coming of the tyrant it memorialized. The air blast followed, surging out of the Indian Ocean -- faster than sound -- flattening whatever still stood. As it slashed north through Jaffna and Madurai, the wave front was met and overpowered by shocks rushing out from strikes in central and southern India.

Across the face of the planet, without warning, thousands of flaming swords pierced the sky...

Then out of no where -- out of the deep impersonal nowhere -- came a bombardment that even the science fiction writers had failed to entertain.

Just nine days short of America’s tricentennial celebrations, every inhabited planetary surface in the solar system had been wiped clean by relativistic bombs. Research centers on Mars, Europa, and Ganymede were silent; even tiny Phobos and Moo-kau were silent. Port Chaffee was silent. New York, Colombo, Wellington, the Mercury Power Project and the Asimov Array. Silent. Silent. Silent.

A Valkyrie rocket’s transmission of Mercury’s surface had revealed thousands of saucer-shaped depressions where only hours before had existed a planet-spanning carpet of solar panels. The transmission had lasted only a few seconds -- just long enough for Isak to realize there would be no more of the self-replicating robots that had built the array of panels and accelerators, just long enough for him to understand that humanity no longer possessed a fuel source for its antimatter rockets -- and then the transmission had ceased abruptly as the Valkyrie disappeared in a silent white glare.

Presently, most of the station’s scopes and spectrographs were turning Earthward, and Isak found it impossible to believe what they revealed. The Moon rising over Africa from behind Earth was peppered with new fields of craters. The planet below looked like a ball of cotton stained grayish yellow. The top five meters of ocean had boiled off under the assault, and sea level air was three times denser than the day before -- and twice as hot...

The sobering truth is that relativistic civilizations are a potential nightmare to anyone living within range of them. The problem is that objects traveling at an appreciable fraction of light speed are never where you see them when you see them (i.e., light-speed lag). Relativistic rockets, if their owners turn out to be less than benevolent, are both totally unstoppable and totally destructive. A starship weighing in at 1,500 tons (approximately the weight of a fully fueled space shuttle sitting on the launchpad) impacting an earthlike planet at "only" 30 percent of lightspeed will release 1.5 million megatons of energy -- an explosive force equivalent to 150 times today's global nuclear arsenal... (ed note: this means the freaking thing has about nine hundred mega-Ricks of damage!)

The most humbling feature of the relativistic bomb is that even if you happen to see it coming, its exact motion and position can never be determined; and given a technology even a hundred orders of magnitude above our own, you cannot hope to intercept one of these weapons. It often happens, in these discussions, that an expression from the old west arises: "God made some men bigger and stronger than others, but Mr. Colt made all men equal." Variations on Mr. Colt's weapon are still popular today, even in a society that possesses hydrogen bombs. Similarly, no matter how advanced civilizations grow, the relativistic bomb is not likely to go away...

Sounds just a wee bit nastier than any nuclear weapon I've heard of.

[Admiral Valdemar]

You don't want the RKV flying through your target anyway. You want it to deliver as much energy on target as possible much like a hollowpoint bullet is made to severely harm the target as opposed to going right through uselessly.

[cosmicalstorm]

I was suspecting destruction on impact, but wasnt sure. Thanks for the replies.

[Sea Skimmer]

I’d fully expect the projectile to be explosively converted into vapor upon hitting anything, including say atoms of space dust, but considering how fast the thing is already moving this explosive reaction, at least initially, may be highly directional along the axis of impact.

[Darth Wong]

I’d fully expect the projectile to be explosively converted into vapor upon hitting anything, including say atoms of space dust, but considering how fast the thing is already moving this explosive reaction, at least initially, may be highly directional along the axis of impact.

The moment it struck an object, the shockwave formed at the point of impact would move through the body of the projectile and vapourize it. This reaction would not need to be be particularly directional, but the normal component of the velocity of the vapour cloud would be insignificant compared to the forward velocity, so it would still continue moving forward. Mitigation of the destructive potential depends on how far away the target is, and whether the projectile has sufficient time to disperse.

[Sea Skimmer]

That’s pretty much what I was thinking. Instead of a spherical burst like you’d get with a conventional bomb, when the velocity of the delivery vehicle is normally irrelevant compared to the velocity of the detonation, you’d have a burst more like a cylinder.

I do still wonder about the ability of a very low mass projectile to transmit its energy onto a target though once it as exploded, at least in the near total vacuum of space. Would a very small quantity of very high velocity plasma actually be able to pierce or melt heavy bulkheads or would it just deflect off at incredible speed? Or would the main energy be release be in the form of various photons? Nuclear weapons at least, aren’t well known for being able to destroy steel structures, thus the need for neutron bombs to kill tanks, and thats with the advantage is being able to add millions of pounds of air into the shockwave.

[Ariphaos]

I’d fully expect the projectile to be explosively converted into vapor upon hitting anything, including say atoms of space dust, but considering how fast the thing is already moving this explosive reaction, at least initially, may be highly directional along the axis of impact.

It's not significant at .9c, but it does set a sort of speed limit as to how fast such a projectile can go before ablating. The IPM is of course denser, and solar radiation will vaporize RKVs at significant velocities (Time dilation factor > 30 or so, assuming it doesn't actually cross the Sun's path).

Long distance RKVs are of course the tools of morons in general.

[Surlethe]

That’s pretty much what I was thinking. Instead of a spherical burst like you’d get with a conventional bomb, when the velocity of the delivery vehicle is normally irrelevant compared to the velocity of the detonation, you’d have a burst more like a cylinder.

Depends on the frame of reference. If you're at rest with the RKV, you'll see it blow up like normal. If you're at rest with the target, you'll see it blow up, but you'll see it blow up more slowly and it will travel quite some distance before it dissipates.

[Darth Wong]

That’s pretty much what I was thinking. Instead of a spherical burst like you’d get with a conventional bomb, when the velocity of the delivery vehicle is normally irrelevant compared to the velocity of the detonation, you’d have a burst more like a cylinder.

I do still wonder about the ability of a very low mass projectile to transmit its energy onto a target though once it as exploded, at least in the near total vacuum of space. Would a very small quantity of very high velocity plasma actually be able to pierce or melt heavy bulkheads or would it just deflect off at incredible speed?

If you think of a small quantity of relativistic plasma as a proton particle beam that has been slightly dispersed by your defenses, you might get a better impression of how a physical projectile would behave shortly after being vapourized. It depends on just how fast it's moving of course, but if it is moving at close to light-speed and contains, say, a megaton of kinetic energy, it would still hit with most of this energy after being vapourized unless it had so much time to disperse that much of it missed the target vessel.

Or would the main energy be release be in the form of various photons? Nuclear weapons at least, aren’t well known for being able to destroy steel structures, thus the need for neutron bombs to kill tanks, and thats with the advantage is being able to add millions of pounds of air into the shockwave.

Plasma only radiates strongly when it is relatively dense: a condition which itself will not last long as the vapour cloud disperses. Most of a plasmoid's radiation is actually called "braking radiation" or bremsstrahlung and is caused by the magnetic inter-play of charged ions moving around at high velocity close to another and changing each others' velocities. The rest of the radiation is caused by protons re-acquiring electrons and becoming hydrogen atoms: a condition which won't happen if the plasmoid is sufficiently energetic or dispersed.

In other words, once a very hot plasma begins to disperse, it's just a cloud of protons and electrons moving through space. If it was traveling at close to light-speed before it was converted to plasma, it will still be extremely dangerous to any vessel in its path for a brief period before it has dispersed so far that much of it misses the target vessel. In order to calculate the length of this period, one would have to pick a target temperature and prior velocity for the projectile/plasmoid, then determine the average particle velocity, then account for relativistic time dilation.

PS. Mind you, the cloud might pass through a target vessel and impart only a small fraction of its energy to its structure, depending on what kind of sci-fi defenses it has. The fancy shielding of Star Wars and Star Trek ships would presumably stop the particles, thus forcing the ship's defenses to either absorb or reflect the entire energy of the shot (and if you reflect it, then you have double the reaction forces). If you don't stop the particles and they pass right through your ship, they will either blow a big hole through it or (if sufficiently dispersed) irradiate its occupants.

Designers of real-life spacecraft have to worry about what effect solar wind bursts would have on long-term manned vessels traveling to Mars; this would be a much more severe example of that problem.