Highly inspired by How would we fare against an interstellar RKKV Attack?

Lets repeat the setup of the original question:

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.

The original question gave 10 days to humanity to react. Top voted answer right now is, that in that short amount of time, all you can do is to panic.

So, it brings me to the question set in the title:

What is the shortest amount of time for humans to come up with any saving scenario?

Assume "current day" scenario

  • $\begingroup$ I am truly sorry but this question is confusing: in 10 days this unstoppable thing creates havoc on earth, after that is either try and go to the stars or try and save what's left of humankind with massive global efforts. Either case will take not less than decades, so I really can't understand what you're asking $\endgroup$ Jun 27, 2018 at 8:18
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    $\begingroup$ I am asking the question in bold font $\endgroup$ Jun 27, 2018 at 8:26
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    $\begingroup$ @ValerioPastore The original question gave 10 days to humanity to react. My question asks how much time is needed. Understood now? $\endgroup$ Jun 27, 2018 at 8:43
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    $\begingroup$ It depends on the political situation. Currently, humanity couldn't agree on the time of day in ten years. $\endgroup$
    – Aganju
    Jun 27, 2018 at 13:33
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    $\begingroup$ Given the alacrity humankind is showing in dealing with climate change, I'm tempted to say that any amount of time would be insufficient. Give us 30 years to respond, and we'd spend the first 10 years debating whether the attack is a hoax or not, then next 10 years arguing over who should pay for the countermeasures, and then the final 10 years arguing that it's too late to do anything about it anyway, so why bother... $\endgroup$ Jun 27, 2018 at 14:55

10 Answers 10


This started as a comment so apologies if it reads like one

Maybe I'm misunderstanding things here, but Earth is a very small target to hit, (in the grand scheme of things) if they knew the trajectory, (which yes would take a lot of time) then used a sufficiently high yield series of nukes, in a Radial in/out normal/anti-normal direction, then the $Delta V$ requirements to miss Earth are tiny by comparison to destroying the projectile or slowly it down

I ran it through Kerbal Space Program, using the real solar system and realism overhaul. A 100 tonne object travelling at 40000 m/s (a lot lower speeds and smaller object i admit but KSP is just a game, it can't really handle factors that high), but when the correction maneuver was done as it passed Neptune's orbit, it required a mere 0.6 m/s Anti-Normal to go from hitting earth dead center to missing completely, a series of nukes detonated along its axis could provide enough Anti-Normal or Normal directional $\Delta V$ to alter the RKKV's path to completely miss earth...

However... the Voyager 2 probe took off from earth in 1977, and arrived at Neptune in 1989, now in theory we could get there quicker if the relevant planets were in the right places to allow gravity assists, but we wouldn't be knocking it down to a few months it could knock it down to 10 years and that's just travel time...

Not including time to design and build and test the rocket used for this which my gut says (yes pulling a number out my stomach) would be about 3-5 years.

So probably 15 years total, and assuming they'd probably launch several rockets with different collision positions through the solar system in case one or more launches failed.

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    $\begingroup$ In case one or more launches failed? Not if, when. $\endgroup$
    – Davidmh
    Jun 27, 2018 at 11:03
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    $\begingroup$ @Davidmh, how very pessimistic of you... or realistic... or more likely both. fair point well made. $\endgroup$ Jun 27, 2018 at 11:05
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    $\begingroup$ @James The Moon has a significant effect on Earth; its presence controls the tides in Earth's oceans, alters the length of Earth's day, and affects the Earth's axial tilt. Messing about with the Moon's orbit is probably a Very Bad Idea with some huge and hard-to-predict consequences. Also it's a lot easier to alter the orbit of a spacecraft than the whole Moon. The Moon is approximately $10^{18}$ times more massive than this spacecraft, which means that any shove you can apply to the Moon would have an effect 1,000,000,000,000,000,000 times larger if applied to the spacecraft. $\endgroup$ Jun 27, 2018 at 15:21
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    $\begingroup$ Your simulation drops a lot of orders of magnitude from the problem to say it is not a lot of work. $\endgroup$
    – user25818
    Jun 27, 2018 at 18:34
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    $\begingroup$ While I haven't run the numbers, I suspect that if 1Mton at 0.035c hit the moon then the side of Earth with the moon in the sky would be immediately toasted, and anyone on the other side wouldn't live much longer. $\endgroup$ Jun 28, 2018 at 9:10

3.5% of light speed is too much for current technology to prepare effective countermeasures, no matter how much lead time we are going to have.

No engine that we currently can build is capable of providing enough thrust for an intercepting missile. "Project Orion" builds speeds that high over the course of months and years.

The only thing that we can do - and only if we know attacker's trajectory well in advance - is to deploy massive amounts of sand and pebbles on this trajectory. The idea is that at 3.5% light speed, collision with a few pebbles will make attacking vessel disintegrate, and further collisions will disperse plasma cloud enough so Earth will survive.

My estimate is that we need to deliver at least 10,000 tons of material to interplanetary orbit. That's 67 SpaceX's BFRs. This may be doable within 10 years, but, as I stated before, we need to know well (months) in advance the exact trajectory of the attacker.

  • $\begingroup$ 10 k tons to destroy a 1 M tons object? $\endgroup$
    – L.Dutch
    Jun 27, 2018 at 8:16
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    $\begingroup$ @L.Dutch - at 3.5% c, kinetic energy differential (E=mv2) is more than sufficient to turn 1 M object to plasma. $\endgroup$
    – Alexander
    Jun 27, 2018 at 8:19
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    $\begingroup$ Maybe i'm misunderstanding things here, but Earth is a very small target to it, (in the grand scheme of things) if they knew the trajectory, (which yes would take a lot of time) then used a sufficiently high yild series of nukes, in a Radial in/out normal/anti-normal direction, then the DetlaV requirements to miss Earth are tiny by comparison to detroying/plasmafying the RKKV, it would still take a couple of years at least to get the defence weapon out to a point it would require such a small change $\endgroup$ Jun 27, 2018 at 8:59
  • $\begingroup$ @BladeWraith I agree, but bear in mind that's only true if the RKKV is unguided. $\endgroup$
    – Rekesoft
    Jun 27, 2018 at 10:19
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    $\begingroup$ @Rekesoft, this is true, but if i were designing a RKKV and had the forethought to make it guided i would also have fitted it with countermeasures, therefore any and all attempts to deflect it would be useless, therefore we're back to the only option being... PANIC!!! $\endgroup$ Jun 27, 2018 at 10:46

How long we need to prepare very much depends on the target.

A few key points:
The RKKV is under power. Meaning its nuclear pulse engine is still active and it's presumably on automatic guidance to hit Earth. It has been adjusting its trajectory during its approach to the solar system and is likely already on its final course.

We cannot accelerate anything substantial to those velocities, that's simply not something we have the tech for, maybe if we survive the strike we might be able to build something like the RKKV ourselves, but it'd be an extraordinary venture.

In the current description, we have about a month before impact, that's nowhere near enough to do anything useful beyond hurling a "black box" of sorts containing information and maybe biological samples into orbit.

But assuming we have prior warning, some time-traveller or whathaveyou has told us it's coming and we know where and when.

We have a few options.

  1. Evade it
  2. Hunker Down
  3. Destroy it
  4. Divert it

Evading is self-evidently impossible, if we could move our planet out of the way we would laugh at the problem of stopping a weapon like this. However this option does encompass escaping using every spacecraft available and attempting to restart humanity in space or on another planet, not infeasible, and if we were in a real desperate money-no-object hurry we could conceivably put enough material and people into space for a sustainable community.

Hunkering down, our current most reasonable option, build bunkers, stockpile resources, batten down the hatches and pray. Even something is catastrophic as an RKKV is unlikely to kill everyone, survivors with enough resources and time can rebuild the world.

Destroying it is not possible, not in isolation, we're talking about a million tons in a vacuum. However its velocity gives us some options, at 3.5% of the speed of light it has a hell of a lot of energy behind it. So we can put something reasonably chunky in front of it and watch it produce an explosion bigger than anything we've ever made before. No nukes required. The only question is whether that will appreciably disperse its mass before it reaches us.


Our better option is to Divert it using the same method. Instead of planting something big in its path, we instead smack something big into it at an angle, Clip it edge-on with an asteroid perhaps. The blast will be off-center from the RKKV's center of mass and therefore divert its course. Alternately we could outright hit it in the side with our asteroid, assuming we could nudge one hard enough and accurately enough to hit such a fast moving target.

We would need to set this in motion a long time in advance and be prepared for the target to make minor course-corrections to avoid the asteroid.

That last point is one possible weak-point though.
The RKKV is able to adjust course, it has to be. Nobody wants to launch a weapon, wait 100 years only to realise it smacked into an asteroid they didn't realise was there.
While it undoubtedly did its final course corrections before we even saw it, the hardware is still there. Meaning that if its control-systems detect an obstacle it should automatically take steps to evade. So we could potentially mess with it by pushing asteroids into its path and forcing it to recalculate its course until it's too late and it misses.

More directly, if we can establish any kind of communications with its guidance systems (or spoof its telemetry) we could force it off course using its own thrusters. It wouldn't take much to make it miss.

Evade - The more time, the better, but as things stand, we could put a crate of frozen embryos and seeds into space within a few weeks if we had to.

Hunker Down - The more time, the better. Right now the main goal would be to stock up and shift our infrastructure to be more robust, years ideally, but we could make do with the few weeks given in the initial question.

Destroy it/Divert it - hitting it at a distance that would make a difference requires literal years of flight-time for any projectile, assuming it can't change course to avoid. Whether the goal is to divert it or not. We'd want to hit it outside the orbit of Jupiter at least. for comparison, the Galileo space probe made the flight to jupiter in Six Years. Call it ten years because we need time to build and test the mission beforehand.

Spoofing using asteroids to force course-corrections would be a huge project, we're talking a decade at minimum to fit asteroids with the engines and guidance systems to do what we'd need. This is also not a surefire method because we're assuming the RKKV is going to try and alter its course at all, it may simply smash through any obstacle we're able to move.

However the plans to electronically spoof it or directly gain control over its telemetry have some promise, ideally a couple months or more should be enough to do it via radio signals and experimentation. Though it does very much rely on the RKKV being able to listen for telemetry instructions, which it may not be doing. If this is the plan, it's a gamble and should be done in parallel to other options.
This also relies on it being within communication's range, something we will struggle to achieve much beyond the heliosphere. so that sets a hard limit on our time-frame for this at about three weeks.

Dunno about you, but the prospect of hacking into an alien flight-telemetry system using a ping-time of upwards of a day, figuring out any encryption and security systems and working out how to send legitimate commands..all in the space of about a week and a half (to do it before the RKKV passes through jupiter's orbit) utterly terrifying. I mean, no pressure right? it's only the planet :P
That'd be a hell of a movie though.

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    $\begingroup$ So youre saying hack it and send elon musk into space with some seeds in the back of his tesla? $\endgroup$
    – user22106
    Jun 28, 2018 at 1:03

Slap Dash super Weaponry

Supposedly after the movie independance day came out some formerly soviet physicists were drinking and talking about how to actually stop such a force. They did some good old fashioned drunken napkin math and came up with a really cool idea.

STEP 1: Begin evacuating a very very large area. This thing is gonna hurt when you fire it.

STEP 2: Dig a hole into bedrock as close to perfectly cylyndrical as you possibly can. It will need to be a several hundred meters in diameter and about a kilometer or more deep.

STEP 3: fill it with water about 1/3rd to 1/2 of the way up.

STEP 4: place a thermonuclear warhead of atleast 300 megatons suspended pefrectly in the center of the water's mass. A Full yield tsar bomba in other words.

STEP 5: Plug the hole with copper a few dozen meters deep.


When the nuke is detonated the bedrock will contain the massive detonation for just the briefest amount of time. The heat and pressure will instantly vaporise the massive volume of water into steam. This will fire the several ton copper plug out of the hole. It will be moving at least a small percentage of the speed of light ("only" a mere 2 to 3 percent). Obviously you will need to do some very serious math to ensure the weapons "barrel" is pointed in the right direction. It is very, VERY difficult to aim, and relies on the target following a predictable path and not employing any evasive manuevers. Also theres the small problem of irradiating an area the size of north america when you fire it, but honestly thats a plus. Aliens who might survive will think twice about invading, afterall if we're willing to do that to ourselves imagine what we might do to them!

Its just a thought experiment, the point of which was to deliver enough energy to a massive object to be garunteed of destroying it. A blob of near plasma copper moving at a percentage of C impacting another object moving at a percentage of C is going to deliver so much energy upon impact that no known matierial in the universe could hope to stop it.

Im not really sure how long it would take to do. Atleast a few years if the various world powers all chipped in. Also somebody has to agree to be ground zero cuz firing this thing is going to throw fallout that covers an area bigger than most continents.

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    $\begingroup$ You are actually Jerry Pournelle writing from an undisclosed location and I claim my 5 pounds. $\endgroup$ Jun 27, 2018 at 22:11
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    $\begingroup$ The water won't flash into steam, the intense x-ray pulse will turn the water into plasma (which actually makes coupling the energy from the bomb to the projectile much more efficient). I also suspect that the device providing the energy might not have to be so energetic, although I can't really find a good way to calculate how much energy you would actually need. $\endgroup$
    – Thucydides
    Jun 27, 2018 at 23:12

Bringing the man on the Moon starting from scratch, or better from a speech, took about 10 years.

Considering that the challenge is, in a 0-th order estimate, comparable, I would start with that number in mind. Then I would double it with a "safety coefficient dictated by engineering common sense".

What is the shortest amount of time for humans to come up with any saving scenario?

20 years at least, if the funding, the support and the drive are similar or higher to those experienced by the Apollo program.

  • $\begingroup$ I kind of disagree. We have already plans against asteroid impact avoidance, so it's not really a new project from scratch $\endgroup$
    – Kepotx
    Jun 27, 2018 at 7:52
  • $\begingroup$ @Kepotx, your very same link states "Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project" $\endgroup$
    – L.Dutch
    Jun 27, 2018 at 7:54
  • $\begingroup$ @L.Dutch I'm not saying that we have a ready-to-go rocket, just that we already have theorical solutions for this. I don't have enough knowledge to know how different current rockets are from rockets used against asteroids, either by impact or gravity tractor, and I'm maybe too optimistic, but "20 years at least" seems a little bit to much for me. Also, Apollo was a NASA only project, currently, even if there are lower budget, there are collaboration with ESA and other space agencies $\endgroup$
    – Kepotx
    Jun 27, 2018 at 8:04

I'll stand by my an answer in the previous thread:

Intercepting an ICBM is hard because you get very little time to react, and most interceptors will pass the ICBM's trajectory in a splitsecond. This isn't a problem with the RKKV as its going straight for earth, the only hardships are getting something traveling along the trajectory and getting enough stuff there.

As this converted ICBM shows, to get it to the moon you can only carry a maximum of 500kg of cargo... But the rest of the missile is still there. So what you do is empty as much of your ICBM's as possible and launch as many as possible in orchestrated launches. The goal is to get as many missiles as possible in the RKKV's trajectory simultaneously so it connects with as much mass as possible, and increases the chance of a hit.

Upon connecting the RKKV is likely shattered and partially vaporized. This does not automatically save earth as a 1M ton plasmasphere at 3,5% of light is still going to be catastrophic and carry almost the same destructive power. That's why you want as much mass and countervelocity to impact with the RKKV before impact with earth.

This is one point where the high velocity is both a blessing and a curse. The sphere wont have spread out much between waves of ICBM's, meaning that all waves after the first hit will get hit by the shattered RKKV pieces and plasma. It also means that you want to hit the RKKV as far away from earth as possible to spread the pieces out as much as possible. Regardless, between the RKKV impacting their ICBM's and the RKKV hitting earth, there's likely just minutes if not seconds due to the immense speed and the relative low speed of our ICBM's.

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    $\begingroup$ The problem is delta-V. No ICBM in production has the capacity to go much further than lunar orbit. Most of them are a lot less. Any RKKV will flash past that engagement radius in a matter of seconds and the plasma-bubble of any missile that strikes it in that window won't have time to appreciably expand. You'll want to have something hit it out by jupiter's orbit so the plasma cloud has time to expand and reduce the overall impact enough to be survivable. $\endgroup$
    – Ruadhan
    Jun 27, 2018 at 11:48
  • $\begingroup$ @Ruadhan2300 thats basically what I mentioned isnt it? $\endgroup$
    – Demigan
    Jun 27, 2018 at 12:31
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    $\begingroup$ This does not provide an answer to the given question. Please edit so that it includes a reasoned estimate for the shortest amount of time we could react to this issue with a save scenario. $\endgroup$
    – Korthalion
    Jun 27, 2018 at 14:51
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    $\begingroup$ I'm pointing out that no ICBM ever built has an engagement range long enough to make a difference, so the plan to launch "as many as possible in orchestrated launches" is pretty much a write-off before it begins. You can't get your warheads further than the moon using an existing ICBM platform, not enough Delta-V. Hitting the target within 10 seconds of it hitting the earth will not help appreciably either. You need something that can hit it somewhere in the vicinity of jupiter's orbit. The Galileo probe did something comparable using multiple gravity slingshots and a 6 year flight time. $\endgroup$
    – Ruadhan
    Jun 27, 2018 at 15:00
  • $\begingroup$ @Ruadhan2300 using this: space.stackexchange.com/questions/26281/…, there is a difference between TLI and TMI, but it doesnt seem unsurpassable. I also saw a fuel difference of 18% on another site. Possible additional boosters could help. And having mass in front to absorb the blast is going to help. Theres about 400 minutemen missiles in America weighing 35.000 tons and about 90% is fuel. That alone is 1.400.000 tons going 19.000km/h (based on Genesis launched from a relatively similar rocket) $\endgroup$
    – Demigan
    Jun 27, 2018 at 17:41

Don't use physical projectiles

Physical projectiles take a long time to get there, are slow to build and expensive to launch. they also need proper timing in their launch phase and you literally need the planets to align for them.

Use light instead.

Science has come up with some really powerful lasers. I believe the most powerful current laser is around 5.3 petawatts, and the scientists who created that one are working on a 10 PW model by the end of this year. There are also plans for a 100 and even 180 PW model, although those are created by different scientist groups.

Ablative heat shielding means that if it gets hot enough, it breaks away. After enough time, this ablative effect can cause the ship to change direction similar to how an impact or an explosion nearby would. This technique is called laser ablation, and besides being the end result of a what-if XKCD, it is also a promising method for space propulsion.


The main problems to solve here are:

  1. A powerful laser;
  2. A way to target the projectile with the laser that keeps most of the energy intact;
  3. A way to power the laser;
  4. an orbital platform so we don't ignite the atmosphere.

the 10 PW laser is scheduled for the end of this year. the 100 Petawatt laser is scheduled for 2023, but with extra funding we might be able to shorten that. The other stuff can be done mostly in parallel.

Assuming we can prove the threat and put enough money into the project, we might be able to get the entire system (laser, targeting system, orbital platform, energy source) ready and functional in about 5 to 10 years. That of course depends on technological advancement speed and the effort put into the endeavour.

  • $\begingroup$ I really like this answer, however it does not answer the stated question. OP is asking for a time scale first and foremost. If you could edit one in this would be a great answer :) $\endgroup$
    – Korthalion
    Jun 28, 2018 at 9:08
  • $\begingroup$ I dont think ablating the heatshielding would give a significant problem for the projectile. You need a focused laser for that. We cannot get a converging beam that far away into space. You'd want the waist of your beam really far away, but that really stops somewhere in the meters, couple hundred is extremely long. From that point, your laser is divergent. Your source sadly doesn't give waist distance, so I can't get into specifics $\endgroup$
    – Gloweye
    Jun 28, 2018 at 11:21
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    $\begingroup$ @Korthalion I added a timescale to my answer $\endgroup$
    – Nzall
    Jun 28, 2018 at 11:50
  • $\begingroup$ Unfortunately lazers are bad to use at long distances. Atomic Rockets for example explains how lasers are hard to focus at long distances, which is why railguns and missiles are considered the long-range option. projectrho.com/public_html/rocket/spacegunconvent.php. Theres also the question of how effective the laser would be in-atmoshphere. It would likely create plasmachannels and expend a lot of energy (so do rockets ofcourse), so you might want to build it in space. $\endgroup$
    – Demigan
    Jun 28, 2018 at 13:06

I'm going to ignore the political aspects -- I'm tired of political snark, anyway -- and look at the time it would take to come up with a technical solution assuming the political will was there. I will also assume that it's on a purely ballistic trajectory, because if it can dodge, I would suggest prayer as the only efficacious course left.

We can't rendezvous with it with any projected technology. OTOH, we do have the ability to intercept it as far out as the orbit of Mars without developing any new technology. So I see two potentially workable schemes.

One is the Load of Crap plan suggested above: put a lot of crap in the way and let the collision turn a solid object into a spreading cloud of debris. This ought to work pretty well as long as the collision takes place far enough from Earth that the debris cloud has expanded a lot (ideally to much larger than the Earth by the time it gets here.

There's a trade-off where the sweet spot isn't obvious. Closer to earth we can hit it with more mass and hit it quicker and hit it more accurately, but further away gives more time for the mess to disperse into harmless pieces. Working out the optimal distance is something you'd pay a first-rate research firm $10M to figure out.

The second approach is to nudge it so that it misses the Earth, by exploding nukes next to it -- basically doing an Orion drive course correction. Again, this would have to be done a long way away to be effective, but it looks like the energetics are pretty good. The hard part would be triggering the nukes at just the right time.

We have dozens of rockets completed or within a year of completion that could launch a ton or so out to 100,000,000 miles and get there pretty quickly. We have many more that could get a small mass there more slowly. So, use salvos.

The first salvo is the hottest rockets launched about a year after the decision to go. In parallel, ICBMs and the like are converted and launched to hit it later. (If it's diverted enough so that it misses, who cares if they're wasted. And if not, they'll be where they need to be.)

So I'm guessing 1-2 years from the Go signal to modify existing rockets and launch them and then for them to get to the rendezvous. Salvos continuing as it gets closer until the object hits or misses.

The closer the intercept to Earth, the quicker the response, but there's still an unavoidable slug of time needed to develop the new program and sensors, ready and launch the rockets with their new payload. My gut feeling is that it would be very hard to come in under a year, though for a rocket that was ready to go, the time might be halved.

It's hard to see how we could detect the object far enough away to have a year or two to prepare.



OK, if we were dealing with a much slower object, we could start planning a feasible solution within an acceptable timespan. After all, heck, this comes from Proxima Centauri!

BUT! This things moves at such a speed that all calculation for a barrier or interception are thrown out of the windows as soon as you're done with calculation to interception in a given area. And that because we do not have the resources to create an equally fast mean to respond. We don't have the tech.

So, all in all, we have only the time to brace for impact. The advantage of having this kind of object coming at that speed makes it easier to calculate the impact area.

This said, we have about 15 year to prepare for the worst. Building a space station or a moon colony would just deplete precious resourcers and time and help spread panic as everyone would want to embark for the stars. Not to mention that after Earth is hit there would be no one coming anymore for a long time.

And since the impact, for how much serious it will be, will not destroy life wholesale, there's better hope staying on Earth to rebuild.


I think if we had ten years the chances would still be against us. I am assuming the large object headed our way was moving as fast as our fastest rocket in space today maybe elon musk big fucking rocket style.

If it was moving any percentage of the speed of light. I think we would need 30 years. By then we could have a sustainable post on Mars as a very sad backup.

Then we would have to mine what’s left of Earth for our resources from homebase Mars.

In that case I want to be an archeologist space dude.


How cool would it be to rescue the Bull from wall street in orbital debree, or fragments of the statue of liberty. One of the faces from mount rushmore, or a bent half of the Eiffel tower stuck onto half of the smokestack from a nuclear power plant in california. Or even a ginormous old landfill site floating stable like around the debris cloud from earth impact

How would something like this affect the moon?


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