Either the currently envisioned “rods of god” concept of using orbital satellites capable of dropping inert projectiles, or the sci-fi hypothetical of accelerating objects to near-light speed and pointing them at planets for complete destruction.
As in, if you knew about their location beforehand could you possibly shoot them down or deflect their course with explosives? Or even create some sort of shield against them?
I’m asking because a few months ago I was reading a website that claimed that kinetic bombardment was the perfect future weapon since it was basically uninterceptable and the moment any civilization gains the ability to accelerate things to a degree of light-speed is the moment the civilization is finished since they have no practical means of stopping anybody from in the future using it to end the world.
It depends on the size of the object, how close to the target it is launched, and how fast it is moving. An object traveling at a reasonable fraction of light speed (for the sake of a weapon, I’d call 1 percent a reasonable fraction) would have to start accelerating a long way from the target. If you happened to have a convenient object between the target and the weapon near enough to be pushed into it’s path, and the weapon was small enough, you could destroy the weapon. You still may be shotgunned by some amount of fragments and plasma, though.
A “rod from god” launched from Earth orbit towards the Earth isn’t going to have anything anyone could reasonably call “close to light speed” but would be almost impossible to stop in time.
There is centuries or millennia of technical progress needed to get from “rods from god” to near-lightspeed artificial meteors. Your question is so broad as to defy a single answer.
We could build a “rod from god” launcher with current tech. See Kinetic bombardment - Wikipedia for some excerpts from a 2013 USAF report in the topic.
It would have impact results about like a conventional MOAB dropped from a B-2. IOW pitiful compared to a modern nuclear warhead.
The impact speed is not unlike that of an ICBM. So whatever tech can be used to do terminal defense against ICBMs would have some effect. By far the best results come from frying the guidance system out near the space/atmosphere boundary. That won’t prevent a heavy high speed hunk-o-junk from landing in a nearby zipcode from where it was aimed. But it will probably keep the thing from hitting the high value target it was presumably aimed at.
Nature already has planet-killers out there in the form of asteroids. And we’re working towards the tech to detect and deflect them years in advance of their impact. Nobody thinks the problem is insoluble; the only question is political: will we try hard enough to build a functional system before Nature surprises us with the first live test case? Hard to say.
One of the things about space is that it’s really, really big. It takes comets a month or more to get from Jupiter’s orbit to the vicinity of the Sun. And they are “falling” from around 1 to 10,000x the distance to Pluto.
If we assume some sufficiently advanced tech to take an asteroid belt object and accelerate it to a velocity (both speed and direction) to have a one week time of fall to Earth, well the tech to stop it would be a few decades or centuries old by then. You’re talking about truly insane amounts of deltaV = energy. Which if you could apply that much energy that quickly to a tumbling rock, you could probably as easily apply the energy directly to the target.
Bottom line:
Dropping asteroids on planets sounds really SF cool. But us thinking about that possibility is the equivalent of Romans speculating on what sort of catapults the people of their far future, say 2000CE, will have. Funny enough, our tech did not just build a bigger better Onager.
Long before today we gained the ability to harness energy more directly than in twisted rope and to apply it to something better than randomly shaped hunks 'o rock. And when we did, all the onagers went first to war surplus outlets and later to museums.
I feel like I must be missing the premise of the question, because it seems too obvious to state that the defense is destroying the satellite carrying these weapons.
I mean, nobody bothers trying to destroy the bombs after the leave the airplane. They just try to destroy the airplane before it drops the bombs.
The issue with that approach is that it requires committing the act of war of destroying the satellite when its’ first deployed.
It’d be the equivalent of going out and sinking somebody’s navy because at some future date that navy might be used to launch weapons at your country.
To be sure, if you were about to start a shooting war with a country having orbital weapons, those would be real high on your list of stuff to destroy first.
Which, like ICBMs, leads to crisis instability, where the incentive is to use your weapons in a surprise attack *before *the enemy whacks them. And conversely to destroy your enemy’s weapons before hostilities start to preclude them being the first wave sent against you.
Both of which motivations tends to start wars rather than deter them.
Aren’t the (astro)physics of the “rods from God” quite shaky? When I read about them, people were taking the rods’ orbital velocity as an input in their kinetic energy equation.
If a rfG retains its orbital velocity, it’ll stay in orbit. If it comes in at a shallow angle, it’ll go through lots of friction which will be though on its electronics and lower accuracy. If it comes in at a steep angle, it’ll mainly have the velocity it’ll gain from falling, not anywhere near 7km/s.
Launching them would cost hundreds of millions or billions of dollars and their deployment would be inflexible.
Plus, satellites don’t have much in the way of power or antenna size so sensor/comms jamming may not be that difficult.
If there aren’t any purpose-made means of defense against it, it’s because the idea is so poorly thought out.
It has a cool name and the idea of it involves being very speedy but that doesn’t make it a good idea outside Michael Bay movies.
You’d have to put an awful lot of them in orbit or wherever, and people would notice. I imagine you’d get a Cuban Missile Crisis before it went too far. “Launch that platform and we nuke you now.”
Physics make those pretty implausible, however. Due to the energy involved even a tiny amount of inefficiency will destroy the projectile along with whatever is accelerating it, for example.
Also, a society where that was a problem would move all its assets to mobile platforms like space stations. An RKKV can’t destroy something that isn’t where it was aimed at. There’s also the problem that using those things to destroy another civilization - the usual scenario in this argument - creates bright lines visible for light years pointing to their origin announcing “genocidal aliens here, get them before they get you”.
You can get most of the orbital velocity to the surface. Remember, for all useful effects, orbits are about the centre of the Earth. The location of the surface is irrelevant. If you jump in the air you are in orbit. It is just that your orbit intersects the surface. Same deal with the rods from God. You only need change the orbit so that it intersects the surface at a desired point. That is not the same as wiping out its orbital velocity. You would only need to remove all the velocity if you wanted a trajectory that intersected the centre of the Earth.
You’re right.
In that case, however, it would get an awful lot of friction. Any educated guess as to what would happen to such projectiles are their electronics if the perigee were lowered to intersect with the surface, coming into the atmosphere at a shallow angle?
Accelerating projectiles with significant mass to a significant fraction of C takes significant time and energy with any realistic technology. It would be easy to detect from a long way off, so you could intercept the projectile somewhere in the middle presuming you have space travel. Dropping rods from orbit are about as dangerous as cruise missiles but are more expensive (and likely will always be more expensive) and have a longer travel time, plus are currently banned by treaty.
If you have typical science fiction drives, then you have something extremely dangerous that’s usually glossed over in SF. Any society that has shuttles and private craft that can move at high velocity is letting shuttle pilots and anyone rich enough to own a private craft have nuke-level destructive power. Most SF just treats their ship drives as no more dangerous than a car, however.
If we have the technology to accelerate large masses to a significant portion of c (even 1 percent), then presumably we’ve used that technology to colonize many planets and moons in the solar system and maybe even sent generation or sleep / stasis ships out to other stars. In short our civilization is not going to be stuck on one sphere of dirt that can be taken out with a kinetic weapon.
Why the restriction to one week? If we’re allowed several centuries to destroy the Earth, couldn’t we find just the right asteroid, adjust its velocity only slightly, slingshot it around Jupiter after several orbits, and head it straight for Kansas?
My “one week” was about making the attack quick enough that it couldn’t be countered.
If country A tried to destroy country B with an asteroid drop and it takes centuries to get there, well that gives country B centuries to detect the attempt and put up countermeasures.
Heck, if evil aliens actually did launch such as attack on Earth back in 1700CE we’ll almost certainly detect and deflect it before it gets here in, say, 2200CE.
Century timescales and “weapon” just don’t go in the same sentence.
To avoid this sort of smoking gun, an aggressor could use an object in the target systems’ own Oort cloud. You attach a drive system to an Oort cloud object (or several), then accelerate towards the world(s) you wish to hit, using the ice of the object for fuel and propellant. If your drive system uses a rocket there is a good chance that the target system will spot you; the incoming RKKV (Relativistic Kinetic Kill Vehicle) could then be targeted with sufficiently powerful lasers (assuming you have these to hand).
Laser beams of course travel at light speed, so should be able to hit even a fast-moving object, although the further away it is, the more uncertainty you have about its true position. If you let it get closer, the more likely you are to hit it, and the tighter the focus of your laser beam - but leave it too late, and the debris will hit the target anyway.
Robert Heinlein used this concept in The Moon Is A Harsh Mistress, when the rebel Lunar colony started launching big rocks from the Moon at Earth. Earth was unable to defend against them, even after Luna told them where the impact points were going to be so people could be evacuated.