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In this hypothetical scenario, in the present day, astronomers detect an object entering the heliosphere, at a distance of roughly 90AU, from the direction of the Proxima Centauri system. Observations indicate that this is an interstellar alien vehicle, featuring a fairly primitive (well within present-day humanity's capacity to build) Project Orion style nuclear pulse rocket design, with a velocity of roughly 3.5% of the speed of light, and a mass of roughly 1M tonnes- but that it's also unmanned, made largely of solid metal and coated in ablative heat shielding, and heading in on a direct collision course with the Earth.

At its speed and its trajectory, the scientists swiftly confirm, to their horror, that this relativistic kinetic kill vehicle (or RKKV for short) is projected to impact the Earth in only ten days' time, and will slam into the ocean in the (randomly chosen, using a random location generator) vicinity of the Kerguelen Plateau with a force equivalent to over 13 Teratonnes of TNT. For reference, that's roughly 1/10th that of the Chicxulub impactor which killed off the dinosaurs, and roughly 2000 times the net yield of the world's entire nuclear arsenal.

What, if anything, could be done in such a scenario? What kind of public reaction would there be once word got out about the impending impact? Could humanity have any hope of stopping it, or deflecting it? And if not, how devastating would the immediate and long term effects of such an impact be- could civilization, or indeed humanity as a species, survive such an attack? And if so, how long would it take to recover to the extent where we'd have any chance of mustering any sort of response to retaliate against the alien aggressors who launched it at us?

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    $\begingroup$ What, if anything, could be done in such a scenario? Well, you can panic. Off the cuff - the world-outside-your-window modern humanity probably couldn't launch an intercept mission of any description, let alone actually stop it. Space missions take a lot of careful pre-planning, especially for extra-orbital operations. $\endgroup$ – Cadence Jun 26 '18 at 2:09
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    $\begingroup$ pick one question, there are several distinct questions in your last paragraph that each would be bordering on too broad question in and of themselves. $\endgroup$ – John Jun 26 '18 at 4:21
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    $\begingroup$ Since the 'R' in RKKV is relativistic, at 3.5 percent speed of light (psol) this isn't exactly relativistic. Extremely fast, most definitely, and packing an enormous amount of kinetic energy, but not properly relativistic. A quick calculation indicates it should strike 14.3 days. Certainly on its initial discovery there is a 12 hour signal delay due to distance and the speed of light. So that shaves the impact time down a bit to, say, thirteen days. Sorry about the nitpicking, otherwise it's a nifty, thought through question. $\endgroup$ – a4android Jun 26 '18 at 9:20
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    $\begingroup$ Please clarify - is the object travelling on a ballistic trajectory or is it still accelerating using its nuclear pulse rocket? If it is not accelerating then how was it detected? If it is accelerating then at 90AU out how do we know it is aiming for an impact vs a flyby, let alone where on Earth it is going to hit? $\endgroup$ – KerrAvon2055 Jun 26 '18 at 9:22
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    $\begingroup$ @ValerioPastore Not correct. There are enough parameters for a reasonably smart person to think through and devise a sensible answer. Not too broad, on-topic, & it's worldbuilding. $\endgroup$ – a4android Jun 26 '18 at 9:22
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Ten days are barely enough to arrange a preliminary meeting between some space agency executives and the politicians to start talking about funding of some projects. And we never attempted stopping something going to impact Earth (except in some movies).

On the space side you can basically do nothing...

On the Earth side you can probably start storing supplies and prepare for the emergency, which will end in a definitely partial preparation.

And of course one can start praying, panicking or hope for a space hitchhiking (if having a towel)...

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We're all doomed anyway, so we may as well try it.

Massive Nuclear Response.

Have everyone at NASA stop what they're doing.

Arm every single nuclear weapon in humanity's arsenal.

Get the NASA guys to work out the best place to fire the missiles to at least try to nudge this thing off course.

That's really the only hope for stopping it, and if that fails, see @L.Dutch's answer.

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    $\begingroup$ You cannot prepare, launch, travel and hit in 10 days. $\endgroup$ – user3106 Jun 26 '18 at 7:23
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    $\begingroup$ I think he wants to use the military nuclear weapons - which ARE capable of being launched within an hour or less - and want NASA to calculate a concentrated attack against the RKKV. $\endgroup$ – Julian Egner Jun 26 '18 at 7:40
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    $\begingroup$ @Kepotx They don't need to be. Collision with an object moving at 3.5 percent of lightspeed will be a mighty bang. Distance is the big problem. And accuracy too. $\endgroup$ – a4android Jun 26 '18 at 9:13
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    $\begingroup$ @a4android: As you correctly point out, you don't care about the energy of your interceptor. A hit is sufficient; the target itself supplies the energy. That means you don't actually need a warhead at all. In fact, even a small ball bearing hitting the target at 0.035c (relative) will evaporate a good-sized chunk of the target. Who cares about accuracy, then? Load the rockets with shotgun shells. $\endgroup$ – MSalters Jun 26 '18 at 13:03
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    $\begingroup$ @Cadence Quite right. ICBMs are capable of suborbital flight. Intercepting a deep space target moving at 3.5 psol would be effectively impossible. $\endgroup$ – a4android Jun 27 '18 at 5:35
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(UPDATE) - while checking out the numbers in the 'Why?' section below, I've begun to think that, just maybe, the damage might not be so extreme as to force us to do, well, anything except prepare for a sizeable tsunami.

Ten days is not enough to organize and launch any kind of space mission. It can be tried, but in all likelihood it will fail abysmally.

It is not even enough to prepare and launch a missile defense.

Aiming lasers into space cannot be done from Earth, and it wouldn't avail much anyway with the available power.

The only last-ditch, partial and insufficient defense that I can envision is preparing several high-altitude, hopefully unmanned (probably manned by volunteers) jets, and deploy and launch them so that they are between the Earth and the incoming RKKV (actually HVKKV) projectile. Ignoring most safeties and flying a suicide mission with no regard for fuel or engine endurance, a MiG-25 or MiG-29 can reach from 30 to 50 kilometers ceiling.

As an alternative, stratospheric balloons can be sent even higher (and it could be done in addition to the jets), even if they would be much more difficult to aim reliably. However, quantity has a quality all of its own.

Once the projectile is in active range of the balloons and/or jets (for the double mission, there are EMP considerations), onboard fusion bombs are simultaneously detonated through a radio command.

Even more EMP-hardened (okay, okay. Make it "hastily wrapped in aluminum foil and copper mesh") fusion devices will be deployed on the surface, aboard large barges.

The damage to the ecosphere will be negligible anyway against a Chicxulub event, and there is the possibility that the projectile will be fragmented or disrupted enough that the last 30-50 kilometers of air (and the last kilometer or so of water) will be the death of it.

After all, the projectile is more or less just as big as the USS Gerald R. Ford, even if it's ten times denser due to it being solid metal. Just as well, I don't see it faring so well against a multiple multimegaton warhead impact. Granted, the timing will be hairy since the projectile covers fifteen kilometers in a single millisecond, but we do have very precise timers, and very fast proximity detectors.

The projectile will still impact with the same energy, but if the force is distributed enough, perhaps a catastrophic tsunami is "all" that will happen. We'll lose a good part of South Africa, Madagascar and Western Australia, and I don't want to think about the climate or the effect on the Antarctica ice sheets, and the seismic aftershocks will be a nightmare - I expect several faults to go active all at once. But it might still be better than nothing.

Why?

The weakness of the Kinetic Kill Weapon is that its penetration depth isn't really related to its speed, but mainly to its density and size. At that speed, rock behaves like a liquid and the resistance it offers is purely its inertia.

That's why "rods from the Gods" are, actually, rods instead of spheres.

If the thing is a kilometer-length rod of depleted uranium, density 19 against about 3 for rock, it will penetrate twenty kilometers of water (or six kilometers of rock) and create a conical explosion that will, in turn, open its way a little deeper and devastate a circular area of the upper crust.

If we turn the thing in two half-kilometer fragments, tumbled 30° from the vertical, the impact will penetrate only at most two kilometers of rock. There will be two smaller explosions, but the destructive effect should be more than halved.

If we succeed in fragmenting and weakening the impactor to the point that the following tenth of a seconds sees the fragment further explode against the atmosphere, there will be a colossal fireball and more widespread, but less intense destruction.

Another reason

While absolutely devastating, the KKV is still going against a planet. So it is unlikely to wreak even a tenth of the havoc of the Chicxulub impactor, because as said above, it hasn't the necessary size, shape, or volume.

This begs the question, why did whoever sent the KKV send it at all? Why not send a probe to deflect a suitable asteroid, or at least a fleet of smaller impactors, to spread the very localized overkill into a wider "sufficient kill"?

It is still possible that some time before impact, and still out of range of any possible Earth defenses, the KKV will separate into a couple thousand "rods from the Gods" designed to knock a whole emisphere back to the Stone Age; or half of them might slow somewhat, and give their regards to the other emisphere as well.

If that is what's going to happen, then that's what it's going to happen - there is no time or resources to do much of anything, and any evacuation of coastal areas will be useless (Kerguelen isn't going to be the sole major impact point).

But another possibility is that the KKV is just a delivery vector, and that it is designed to do something more than blast a small section of the crust all the way to high orbit. Whatever it might be, it would just have to be something that's not to be accomplished through simple kinetic energy; there must be another component inside the object.

After all, either some guys expended a frightful amount of energy and resources to deliver a blow that in all likelihood won't do very much at all, or they believe it will be enough, which means they know something we don't.

Again, whatever more it is the thing is meant to do, it's not at all certain that it won't do it from very much out of range.

Still, delivering some megatons of nuclear blast at close range might make the difference between a disastrous but limited impact and something far more ominous.

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  • $\begingroup$ If the object is moving .3 light speed the last 30-50 km will be the last ten of a second. How would this be hepful $\endgroup$ – user22106 Jun 26 '18 at 17:23
  • $\begingroup$ @user22106 the damage from a KKV isn't only related to its speed. I added some information on that. $\endgroup$ – LSerni Jun 26 '18 at 17:40
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    $\begingroup$ You do not want to disperse the energy. Ground zero is going to be obliterated no matter what, you want the maximum energy concentrated in digging deep rather than spread out into the atmosphere. $\endgroup$ – Loren Pechtel Jun 26 '18 at 19:20
  • $\begingroup$ @LorenPechtel I am not sure. The long range damage might be less if the blow is more dispersed. $\endgroup$ – LSerni Jun 26 '18 at 19:25
  • $\begingroup$ It's the same thing that limits the size of nuclear weapons--the blast expends it's energy on a volume, thus the radius goes at the third root of the energy. The area of damage goes at the square of the radius. Thus damage goes at the radius^(2/3). Energy expended on overkill is energy not expended elsewhere. $\endgroup$ – Loren Pechtel Jun 27 '18 at 20:32
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Basically a planetary aimed whipple shield?

An RKKV would similarily to micrometeorites impacting a space shuttle shatter on impact, creating a shotgun pattern while each individual piece is easier to stop than the entire piece.Having the RKKV pieces burn in the atmosphere still means it delivers its energy to the earth in the end, so that should be avoided. Still, any impact will vaporize a portion of the RKKV and absorb some of its energy.

So what you do is check what you have in space. The trajectory of the RKKV is going to be relatively easy to calculate as with that velocity few things will noticeably affect the trajectory. See if anything can intersect the RKKV's trajectory to make it break up and cause less damage as hopefully some parts miss earth or hit the moon.

This isnt your only plan. Commercial sattelites are being launched a lot more often these days. Most of the time is spend on the sattelite rather than the rocket (although that too is painstackingly build). Repurpose the rocket for your ends. You might even want to remove mass from the missile, as hitting the RKKV as far away from earth as possible is more important than the amount of mass.

Another plan is repurposing ICBM's. Looking at this one: https://en.m.wikipedia.org/wiki/Dnepr_(rocket) an ICBM is only capable of reaching space properly with a lot less mass inside, so the likelyhood of firing it with a nuclear head attached is small. Still, this is going to be a lot of mass (211tons, although the fuel is likely most of the weight). Now orchestrate that these missiles are fired in a sequence so that they all arrive at the RKKV simultaneously, or since theres not enough launchpads let them arrive in waves. This increases the chance on a hit with each wave and increases the chance you have as much mass connect to the RKKV as possible to reduce the impact of the pieces.

Simultaneously do L.Dutch's plan. Also evacuate people who would be "near" the impact site, Evacuate people near coasts for ineviteable tidalwaves, prepare for foodshortages, look towards options for storing seeds for after the global winter aftermath (depending on how much the pieces stir up the atmosphere or the ground) and towards alternative food solutions (apparently grasshoppers can be bred for tons of food with little resources and space needed).

If all else fails, ask Elon Musk to try and catch it with his robotic landing platforms, he could do it!

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NASA, Roscosmos, JAXA, ESA, etc. all confirm the probe is on a collision course with Earth and will impact in 10 days.

Scientists and militaries around the world offer ideas (such as those in other answers and comments). Can we send ICBMs? Not enough fuel. Launch a Falcon Heavy? Not in 10 days. Etc.

All hope is lost until a junior scientist from NOAA points out that after passing the Sun the probe will nearly go through (Sun-Earth) LaGrange Point 1 on the way to Earth. She suggests using the Deep Space Climate Observatory as a kinetic impactor to deflect the probe while it is still billions of kilometers away.

The probe is just barely nudged out of the way and while it misses Earth it impacts the moon and creates a huge visible crater. For years small pieces of the moon create spectacular 'meteor' showers for Earthlings to see. The public story was a meteor impact.

But for those in the know a dark shadow looms. We're not alone. The Centaurians are aggressive and they launched more than 120 years ago. They're most likely on the way...

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