Just like the title.

A splinter fleet alien group arrives and long story short some human scientist explains to the president of his country that they might decide to weaponize asteroid against us at some point.

The president is troubled as asks what can we do. This is where you guys come in.

Can humanity now or in the near future, no more than 20 years, find a way to stop even a single asteroid? Provided that in this scenario that we can expect total human coordination and cooperation.

If why can't then why? And if we can then, obviously, what is it?

I mean I thought of ICBM but I'm not sure.

We know little of the aliens tech and that is a part of the problem. But we can expect them to be able to do so. Spoiler, not really but just thought it appropriate, they don't resort to asteroid dropping. But nevertheless I'd like your help in figuring out if it is possible to find a solution or not.


Question asked above, rest is extra fluff

Famous objections:

The aliens might have planet killer type weapons. Why bother?

They don't. This group is like if a bunch of separatists that just got a fleet together and got here. Imagine of a population of a US state, just an example and the US is famous, decided to leave the state and settle on another planet. Do you expect the state to have aircraft carriers, fighter jets, and nuclear weapons?

Why would they destroy the earth, or won't it be more beneficial for them to negotiate, ...etc?

True and true. But this is just to maybe provide an answer to this question. Also way off topic.

Surely they have other weapons. They have FTL which is amazing and far beyond anything we possess.

Again this is off topic. But also no. They just arrive with ships and some "basic" weapons. Their galactic military does not park FTL capable planet killer capable ultimate doom ships with the keys in the ignition.

Can't they weaponize the ships?

Nope. The ships in particular are hardcoded to behave in certain ways. They can't even do the famous divert power from X to X. The ships "AI" treats them like children actually.

That would be a bad PR move. Surely they want to come in peace, and entice us with stuff, ...etc.

Appreciate it. But again, this is to answer this particular question.

You can't expect all the world to pool together. That is unrealistic.

Can I please have people in my story act this way? I mean, sure, we as a species can be stupid sometimes but this is a story and maybe I want smart leaders and global cooperation against our doom.

How do we even know that they plan or even think about if they don't try it?

You be the advisor to the President that says that! Hey, Mr. President. I don't know and I don't care. Tell me to come up with a plan once they drop an asteroid.

I don't mean to be rude or against anything useful. I appreciate all input even if slightly off topic. But sometimes things are way off topic. So please let's keep this to stuff that might actually help the core issue.

And as always a detailed no is as good as a detailed yes.

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    $\begingroup$ This is going to depend entirely on how the aliens divert asteroids and how effectively (i.e. size and velocity of asteroids over time) they can weaponize them. $\endgroup$ – Gene Oct 30 '20 at 20:09
  • $\begingroup$ Could you list any of the standard fictional responses you've already considered and discounted? $\endgroup$ – Robbie Goodwin Oct 30 '20 at 22:45

There's dozens of ideas, each with their own benefits and drawbacks. Generally all our most reliable methods need a few years warning that the asteroid is coming, which assumes that the hostile aliens will need a few years to manoeuvre the asteroid into an attack trajectory.

We could:

  • Nuke it. (Fracturing the rubble pile, or causing it to act like a rocket engine on the asteriod).
  • Nuke next to it (deflecting the asteriod)
  • Hit it with a heavy thing to create a nice crater exactly where we want it, and then nuke the crater (which would create an optimal engine).
  • Hit it with a kinetic impactor in order to deflect it subtly
  • Put a probe next to the asteroid, and let gravity do the rest.
  • Hit it with Ions from a nearby probe.
  • Use mirrors to vaporise part of the asteriod
  • Send a robot onto the asteriod to mine and launch bits of it on a mass driver, changing it's velocity.
  • Land a rocket engine. Turn it on, and use it directly to tweak the path.
  • Use lasers to heat up one side of it, causing expanding gasses to accelerate it in a desired direction.
  • Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film as a solar sail
  • "Painting" or dusting the object with titanium dioxide (white) to alter its trajectory via increased reflected radiation pressure or with soot (black) to alter its trajectory via the Yarkovsky effect.
  • Releasing a cloud of steam in the path of the object
  • Attaching a tether and ballast mass to the asteroid to alter its trajectory by changing its centre of mass.
  • Run a coil of wire in a big loop around a spot where the asteroid is passing through, and use electromagnitism to accelerate or decelerate the asteroid so it doesn't cross orbits with Earth. (Asuming iron core)

If the aliens launch a rock and then run away:

The Kinetic-Impactor-And-Then-Nuke-It approach is probably our best bet with minimal warning. Someone asked a related question previously I was able to calculate that to stop a specific asteroid from impacting with a years notice that we'd have 7 launch windows in a year, but only 1 decent chance with enough time for the deflected asteriod to miss. We'd probably send multiple impactors and nukes just to be safe.

If the aliens launch a rock, and then actively defend it:

Laser ablation is our best chance - and it allows us to keep all our tech away from their ships. If their ship gets in the way to block the laser, they'll be damaged by it. The aliens would need to constantly apply thrust to the asteroid to counter our ablation.


The problem is the Aliens are 'up' there. And we are 'down' here. Its sort of like expecting a man stuck at the bottom of a well to defend himself from an attacker up on the surface. Even if you give him a gun he still can't stop his foe from dropping anything we wants, rocks, grenades, petrol, venomous spiders etc down the hole onto the victim. He can make it harder for his attacker but he can't stop it.

This is especially the case because if the Aliens fail once they can always try again (there are plenty of rocks to choose from) and our defense/s (whatever they are) have to succeed every single time.

So the only way I can see any defense being successful is if one or both of the following holds true;

  1. The attackers are time poor i.e for whatever reason they only have a very limited window of opportunity to launch their attack. e.g. because they have limited resources and have to head 'home' in short order (which given they have crossed interstellar space seems unlikely). Or because they only have a limited amount of whatever technology they are using to capture and deflect asteroids onto a collision course with Earth and can only do this so many times before they run out of X.

  2. Asteroids are the only weapon they have. They came completely unprepared for conflict and have no means of deflecting or nullifying whatever defenses (missiles etc) we decide to use.

  3. They are very few in number i.e a single ship with little or no ability to (force amplify) by building stuff (weapons, other ships etc) once they get here. Note; in this case its a fleet so by default this doesn't apply in the current scenario)

  4. By some miracle Earth has actually already established a fairly extensive presence in space outside of Earths orbit already (also not applicable in this scenario)

Long story short, if the Aliens are persistent we are toast.

  • $\begingroup$ "venomous spiders [dangerous to humans]" - only in 'straya - and then they have plenty of better options :D $\endgroup$ – John Dvorak Oct 31 '20 at 5:02
  • $\begingroup$ I completely agree about the overall conclusion. In the story they won't find a solution. I just like the proposed methods of the "answer" because it covers more grounds and give me more to work with. Anyway thank you. $\endgroup$ – Seallussus Nov 1 '20 at 1:24

Alien fluff apart, this is basically asteroid impact avoidance

Asteroid impact avoidance comprises a number of methods by which near-Earth objects (NEO) could be diverted, preventing destructive impact events. A sufficiently large impact by an asteroid or other NEOs would cause, depending on its impact location, massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of placing large quantities of pulverized rock dust, and other debris, into the stratosphere.

In 2016, a NASA scientist warned that the Earth is unprepared for such an event.

In short, we have just ideas on how we can approach such an event, but none of those ideas has been turned into a plan yet.

These ideas falls under:

  • nuclear explosion
  • kinetic impact
  • rocket engine

and other more fancy approaches.

Considering that anything related to space takes at least a decade for reaching the deployment phase, I would say it would take a lot of joint effort to make any of those ideas mature.

  • 2
    $\begingroup$ But if need be I'm somewhat certain we could get a nuke on a collision course with the thing. We already have nukes that are ready for outer-athmosphere conditions (some ICBMs), and we already have (unrelated) rocket plans that can carry to far off asteroids. Putting both together reliably would take some time, but I'm sure it could be done under 5 years if need be. In the worst case we just send 20 Rockets and hope that one hits. And the closer the asterois the easier to hit $\endgroup$ – Hobbamok Oct 30 '20 at 15:23
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    $\begingroup$ @Hobbamok I think it would be more involved than that, Bruce Willis notwithstanding. Just hitting an asteroid with a nuclear missile is more likely to produce many large-enough-to-still-be-dangerous asteroid fragments, now more radioactive than before, and still on a collision course with Earth than other outcomes. The worst case scenario of launching 20 rockets at a nearby asteroid is a pretty bad scenario indeed! $\endgroup$ – Upper_Case Oct 30 '20 at 16:50
  • $\begingroup$ @Upper_Case An asteroid large enough to cause serious damage to the Earth will not be even significantly damaged by a reasonable-yield nuclear weapon. The point is instead to knock it off its trajectory just a little bit ten or twenty years in advance, such that the deviation adds up over many orbits and it misses the Earth instead of hitting. $\endgroup$ – parasoup Oct 30 '20 at 17:44
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    $\begingroup$ @parasoup I am aware. The comment I was responding to used terms like "collision course" and "just hope one hits", on a short time scale. The planning and calculation of a successful asteroid diversion is (as far as I'm aware) the slowest and most difficult step, which makes the timeframes described in the answer difficult to shorten. The operative issues aren't about whether or not we have nuclear missiles or the ability to launch things into space-- we have those, but we aren't ready to deploy them for a task like this (which is what this answer is getting at, I think). $\endgroup$ – Upper_Case Oct 30 '20 at 17:51
  • $\begingroup$ @Upper_Case, Very perspective of you. You are right. Ultimately the discussion ends with we have little option. Anyway your contributions are great. $\endgroup$ – Seallussus Nov 1 '20 at 1:21
  • They attack with an asteroid. We try to divert it.
    If there was a natural asteroid incoming, mankind could try to divert it if there is enough warning. Definitions of "acceptable risk" and "reasonable expenses" become more elastic if there is a Dinosaur Killer on the way and if the ticking clock shows a couple of months.

    • Don't use ICBMs. Swap the scientific instruments of a space probe for a big nuke, launch it into an orbit next to the ISS, refuel all tanks (and the upper stage?) by space-walking astronauts if necessary, then launch it.
    • Lather rinse, repeat. While the first probe is on the ramp, prepare the second, the third, and so on. If the first explosion fragments the asteroid instead of diverting it, send the follow-up missions after the biggest fragments that are still incoming. If the first mission fails because part of the software is metric and the other part is not, send a patch.
    • There could be separate attempts by different groups. US. Russian. Chinese. Does ESA provide launchers to the Americans or try their own? Again, more baskets for the eggs. If one fails, perhaps one of the others works.
  • But wait, they are going to defend their projectile.
    Mankind would be much harder pressed to come up with something that could get past the alien point defenses, especially on short notice. Will it be possible to saturate their defenses?

  • How far out will they be when we realize foul play?
    If there are alien starships/warships in orbit, sending threatening messages, and they divert an asteroid at the same time, then all attention might be on the ships and ways to knock them out. Concentrate on the rocks only after that battle is fought, and won, with a modification of the jury-rigged anti-ship weaponry.
    That might preclude the "modified probe" approach. An improved ABM system?


1. Keep a tab on all the candidate asteroids.

Fortunately, the more dangerous an asteroid is, the larger it has to be. So we can see where it is and where it might be going. Several candidates have been already mapped. On the other hand, currently we know where the asteroids are simply because we know where to look, i.e., we trust that nobody is putting thrust to them. This assumption immediately fails in our scenario.

So, we need to pimp up our early warning network by several orders of magnitude. This also means placing assets in space: stations large enough to host and power substantial active radar dishes.

Once we have that, and it is not too difficult if everyone pulls in the same direction, we can -

2. Follow the weaponised asteroids.

We need a secondary sensor network - a targeting network - capable of locating an asteroid which might be camouflaged (i.e. sprinkled with VantaBlack or radar-absorbing polymers). This requires integrating infrared sensors, background radiation analyzers, and having the sensor network spread out enough to increase the chances of also exploiting random occultations.

3. Hit the asteroids.

This can be realistically done in only two ways: masers, to vaporize the surface of the asteroid and generate enough thrust to put it off target; and/or high speed asteroid killers. We know how to build those, even if they're expensive, and we need to build several of them because the first prototypes are more likely to fail than not.

A large depleted uranium impactor rod is accelerated - possibly, using nuclear propulsion: we need the highest acceleration possible to reduce the time for the aliens to react - and targeted at the asteroid. The rod is the first part of the weapon, an even larger version of the GBU-57A/B ordnance. Its only purpose is to drill deep enough; against chondritic asteroids, it should achieve about 100-150 meters depth.

On the rear of the penetrator rod, a massive axle and spring mechanism provides a few milliseconds' safe deceleration for the second part of the asteroid killer - a lithium deuteride fusion device attached to a second uranium tamper. The purpose of this assembly is to direct the main force of the explosion inwards, to fracture the asteroid, while the massive backblast will supply a powerful thrust to disrupt the asteroid's trajectory. We might also try to "salt" the casing with appropriate elements - cobalt, zinc, sodium - to ensure that the whole area will become intensely radioactive, hampering any attempt to recover the asteroid.

4. Asteroid choice

The aliens should probably choose an asteroid in the two to twenty kilometer diameter range: the smallest size will cause large but not unrecoverable damage. The larger is in Chicxulub range and would be enough to trigger an extinction-level event.

But the larger an asteroid is, the more difficult it is to move around. The required delta-V may be as small as 0.1 m/s for a period of ten years (see above), but going from a speed of 20,000 m/s to one of 19,999.9 is a difference of around 4000 between the squares of the velocities - and you have to multiply that for a mass of the order of 1e+15 kg, giving an energy total of 1e+18 J. The Saturn V rocket had a power of 1.2e+11 W, or 1.2e+11 J each second. It would need to fire for eight million seconds, or about one hundred days, without interruption, to supply the required delta-V.

It is unlikely that the aliens have more power than that (also consider that they will not possess such asteroid-moving engines; they'll be repurposing their own ships' engines, which aren't designed for that kind of stress. It would be like using a Tesla model S with an earth-moving blade soldered on).

So, the aliens will have to limit themselves to smaller asteroids, and those, even M-type asteroids, simply do not have the cohesion or the density to effectively resist a thermonuclear explosion delivered one hundred meters below their surface. They will likely shatter, with the pieces running away in random orbits with very little chance of representing a threat.

The main risk will be the aliens successfully intercepting the impactors. It might well be the case that they'll have to be actively defended - i.e., manned by suicide crews.


Pre-emptive strike

Don't wait for their action. Nuke their fleet with all you got in a surprise attack.

You did say you didn't want to hear they might want to come in peace...


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