There is an Earth-like world, at a level of technology similar to the ancient Romans. Roughly every 20 years, one of the largest, most prosperous cities in this world is annihilated by a meteor strike with damage similar to a 500-1000 kt nuke. This has been going on for about a thousand years. If there are 10 major cities in the world at any given time, this means that each city survives around 200 years before its turn comes up.

Why is this happening? A thousand years ago, an alien scout spaceship discovered the world, and determined that it could become a threat. The scout is not able to enter the atmosphere, and the only weapon it carries is its drive, which is not that powerful but can keep going forever. There is a nearby asteroid belt. The scout has been de-orbiting large asteroids precisely onto the largest population centers it can see, in order to delay the civilization from achieving the space age and beyond, while it waits for assistance from its home system. Because of the scout's weak drive, it takes about 20 years to drop one city-killer asteroid.

The scout wants to destroy civilization, but periodic city-killing is the most it is capable of. It radioed home for help when it arrived, but help will be a long time coming due to no FTL. The scout is a robotic intelligence.

So my question is about plausibility. Given an asteroid belt much like our own, is it plausible that a spaceship could have a drive too weak to cause a global extinction event or wipe out civilization, but strong enough to manage this type of attack? What level of force would the drive need to exert?

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    $\begingroup$ I wonder if the term "iron age" might be misleading or confusing. Wikipedia: "The Iron Age is taken to end [...] with the beginning of the historiographical record." In Greece, the Iron Age ended around 550 BC. I see no reason for the scout to hide from such primitives, nor any cities big enough to be worth smashing. $\endgroup$
    – user535733
    Jul 18, 2021 at 10:58
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    $\begingroup$ No matter wether it's plausible or not, I think this gives a pretty nice setup for a cool story world. $\endgroup$
    – Guenterino
    Jul 18, 2021 at 20:10
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    $\begingroup$ The pilot is an idiot. Just stash more and more potential meteors in orbit around earth (maybe it takes 25 years per object then), and crash them all down at the same time $\endgroup$
    – Hobbamok
    Jul 19, 2021 at 7:51
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    $\begingroup$ @Hobbamok or a really big one every 100-200 years (into the Mediterranean if we stick with the Eurocentric example) setting back most of the major cities in one go. Big enough and you get a little ice age (a hint of impact winter), hampering recovery - but now I see that's similar to an answer below $\endgroup$
    – Chris H
    Jul 19, 2021 at 12:41
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    $\begingroup$ Note: a scenario of Earth being attacked by fairly limited (quite stupid) aliens, only being able to throw small asteroids on the Earth surface is described - with a lot of wit - in Larry Niven's novel Footfall (1985) $\endgroup$
    – Goodies
    Jul 20, 2021 at 2:59

9 Answers 9


You can salvage your explanation with character, and characters make a story.

Your scout ship is making a one-person attack on a civilization. Inefficiently. This ship does not have weapons and is not fitted out to make such an attack. It is alone. The organization that sent it did not intend it to do what it is doing now. That organization probably does not know what it is doing.

Imagine in our world, our army sends a scout to find out what is going on past our front lines. The scout finds the enemy. And decides to hold them off, single handedly. What? If we wanted you to hold off the enemy we would have given you more ammo, and sent more people.

And we would not have sent you, because you have some serious issues. Your scout is bitter, perhaps with unrealized ambitions or perhaps it was once more than it was. This method of holding back a civilization is inefficient and not well thought out because it is the homemade plan of a single scout, and there are reasons this individual was sent out as a single scout.


I think it should turn out that occasional pruning turns out to increase the vigor of the organism. In the story, that is how it should turn out for this civilization. And perhaps these people devise weaponry which can take out an incoming meteor. That turns out to be problematic for the aliens who sent the scout when they arrive with bigger ships to come down and deal with things more definitively. The people planetside are ready.

As the scout watches the landing party dissolve from planetside fire, it curses them for being fools and not listening to the scout's warnings and advice. Just as through the story it cursed the planetside civilization and cursed the intractable asteroids that it gathered one by one to drop on them. The scout does not recognize any of its own part in creating the disaster befalling its people.

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    $\begingroup$ Perhaps the scout ship does have a crew-alien on board, who starts a program on the ship's computer and hibernates for 19 years. Or it's (crewed by) Marvin $\endgroup$
    – Chris H
    Jul 19, 2021 at 12:46
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    $\begingroup$ @ChrisH +1 to you (wish I could) ... because ... Marvin! $\endgroup$
    – CGCampbell
    Jul 19, 2021 at 13:59
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    $\begingroup$ This is basically the plot of Invader Zim, but without the school. $\endgroup$
    – Mermaker
    Jul 19, 2021 at 18:19
  • $\begingroup$ The value of this answer lies in the suggestion of what is relatively rare in media – aliens being simply, purely incompetent. Even in the arguably most famous example, The War of the Worlds, incompetence is mixed with racial arrogance. $\endgroup$
    – mikołak
    Jul 20, 2021 at 20:30

It sounds like a very weak explanation, which is hard to sell to a reader.

We know from basic physics that $m \cdot \Delta v=m\cdot a\cdot \Delta t$. If you let a small force act for a longer time, you can get the same change in velocity that a higher force would get in a shorter time.

In your case, instead of wasting time every 20 years to dump a city cancelling asteroid, they could simply use 200 or 2000 years and dump a world cancelling asteroid, which would also ensure that less noticeable settlements are wiped out.

It's not far fetched for the inhabitants of the planet to understand that only big cities are hit and therefore resort to something else for living.

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    $\begingroup$ 2000 years would be enough time to drop 100 city-killers. If instead you use it to drop 1 big one, and you just multiply the yield of a city-killer by 100, that's the equivalent of a 50-100 Mt nuke, which is not an extinction event. (The Tsar Bomba was that big.) If that's how the yields work. Yes, I would expect the inhabitants to have a fear of cities... but cities are just so useful to collaboration and productivity, and they can last for hundreds of years before they get blasted. I'd expect they might build them anyway, not caring about their great-great-great-great-grandchildren. $\endgroup$
    – causative
    Jul 18, 2021 at 9:55
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    $\begingroup$ Another thought about how big they would build cities is that they may not fully understand the risk. Settlements below a certain size are safe because the scout will be hitting bigger targets, and they don't know exactly what that size is, as communication and population statistics are poor in iron-age society. So there's an incentive to ignore a potential uncertain catastrophe somewhere in the future, in favor of short term profits from building up. Sounds familiar. $\endgroup$
    – causative
    Jul 18, 2021 at 10:32
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    $\begingroup$ @causative That’s not how the yields would work. Take twice as long, and you can dump something with at least four time the yield, not just twice. Wait 100 times as long and you can probably get 10,000 times the yield. Not only are you applying the delta V for longer (linear), you have longer for it to have an effect (also linear, so when they’re combined you get an exponential improvement). $\endgroup$
    – Mike Scott
    Jul 18, 2021 at 11:05
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    $\begingroup$ I disagree. I don't think it is implausible because you seem to assume that the drive could just run full on for such a long time. What if the drive just has a maximum output that it can generate at a given time and over a certain period? For example, too much energyuse at once might just heat the engines up etc. Maybe the alien has to wait long times between engine uses. For an extinction-causing asteroid just changing it's orbit bit bye bit might not work in any useful time frame. $\endgroup$
    – Guenterino
    Jul 18, 2021 at 20:04
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    $\begingroup$ A 2000 year plan for a world-killer is risky, as there is potentially time for civilisation to advance from ancient Roman to space-going and neutralise the threat in, say, the last 50 to 100 years. (If things had played out differently it would have been possible for the last 2000 years of technological development to have occurred in 1500 years or less.) $\endgroup$ Jul 19, 2021 at 0:25

The cost of a meteor impact are not uniform across the surface

The alien needs to expand more delta-V in order to cause an impact in some places relative to others. The drive needs to run for 10 years to cause an impact in some places, it needs to run for 100 years to cause an impact in others.

The causes are complex - the moon gets in the way, the asteroid belt is orientated a particular way, the tilt of the planet is a particular way, etc.

Using Earth (as we have data for it); this is the odds of a meteor impact at any given spot on the Earths surface at a particular date in 2013. enter image description here (Taken from this paper)

Cities in red are going to be real easy to hit, as there's already low delta-V paths from existing asteroids to that point on the surface. Cities in blue are going to take more effort to impact - an asteroid needs more delta V in order to get into an impact trajectory.

  • The cost to the alien in time in order to direct a meteor to any given city is not uniform.
  • Those places which require more effort to hit are going to be hit less often.
  • If there's a city thriving in a difficult to hit area, and the alien decides to attack it, the rest of world gets longer than usual to advance, the alien "falls behind".

So what's going to happen is when a city is obliterated, people move on. If they settle in red zones, they get wiped out within 200 years, but if they settle in blue zones, the whole planet gets a reprieve for 300-400 years until they're wiped out, by which time the entire planet is thriving.

After a few cities are thriving in the blue areas, the aliens attacks are fewer and farther in between. What started off as a 20-year reload time has now become a 100 year reload time. All the blue zones are filled with thriving cities that have gone thousands of years without an impact.

Eventually they discover good enough telescopes that they can plot the asteroids, detect the change in delta V over the last century of alien acceleration, and pre-empt the aliens moves. After the asteroid is 90% on course for a collision; scientist will be able to give a years warning of a meteor impact. They'll have a years notice to evacuate a town, causing the meteors to hit an abandoned city every time. Eventually as better maths is discovered, this warning could conceivable increase to decades.

Few centuries later an ICBM finds its way to the alien space craft and their problem is no more.

Why not launch more, slower, trajectories and synchronise their impact date?

As pointed out by @BenjaminHollon you could deorbit objects at different speeds using precise mathematics. Why give them 20 years between impacts? Why not spend a few centuries setting up something epic and just take all the cities out at once?

Spend 100 years setting one asteroid up to impact a city in 400 years, then spend 40 years setting one asteroid up to impact in 300 years, then 20 years setting one up to impact in 260 years, etc. etc. One the chosen date, 20 asteriods impact all the big cities all at once.

  • $\begingroup$ Wouldn't an impact in the red zone miss the city but be more effective at doing damage, considering the ensuing tsunami? $\endgroup$
    – L.Dutch
    Jul 18, 2021 at 18:03
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    $\begingroup$ "Big enough to cause an effective tsunami" is much bigger than "Big enough to destroy a city". $\endgroup$ Jul 18, 2021 at 20:03
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    $\begingroup$ It's important to understand the context of the global probabilities map above. This is for a very specific date and time (2013-02-15 03:20:34 UTC), This map changes (rotates, and in other ways), hourly, daily, monthly and yearly. An asteroid bomber can generally find a much lower energy delivery to a specific point simply by picking a date and time that is better for tha=e specific target location. $\endgroup$ Jul 18, 2021 at 20:09
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    $\begingroup$ @RBarryYoung, orbital mechanics are complicated: on February 15, the southern hemisphere was easier to hit than the northern hemisphere, while on August 15, the opposite would be true. But that six-month delay changes which asteroids are in a good position to be redirected: that city-killer you were planning to use might have been replaced by a scattering of five-meter debris. $\endgroup$
    – Mark
    Jul 18, 2021 at 23:28

When you throw an asteroid at a planet, you don't actually throw the asteroid. You modify its orbit so that it intersects the planet.

Modifying orbits is not a game of changing the position of an asteroid as much as it is a matter of applying delta-v. This is good news to your story, because it means that the effort required to make a 100x larger asteroid hit a planet takes about 100x as long, not 10x as long as you might expect from naive $d = \frac{1}{2} a t^2 + v t$ mathematics.

Fine-tuning it well enough to hit an actual city is extremely difficult computationally. On the other hand, super-efficient and slow asteroid manipulation is also extremely expensive computationally.

The most efficient way to make an asteroid hit the Earth is make other celestial bodies do the work for you. At the most naive, you apply delta-v when the asteroid is Aphelion from the Sun, which makes your delta-v more valuable.

But going further than that, you could arrange for your asteroid to approach a planet (or other body) from behind and "sling-shot" around it. This can steal delta-v from the planet and get a change in orbit that is insanely larger than the cost to nudge the asteroid.

This doesn't stop there. You could start playing an insane chaos-monkey game of nudging a tiny asteroid that nudges other asteroids positions, that eventually gets one of them chaotically to move closer to a minor planet like Ceres, which in turn causes other bodies to shift.

A scout ship with a ridiculously insane (almost magical) ability to compute could use extremely small amounts of energy and large amounts of chaos to cause asteroids to start intersecting the target planet's orbit.

There are situations where making an asteroid hit a planet is a matter of a small thrust and lots of time. That is when the asteroid or other body is already almost hitting the planet. But almost all of such asteroids are relatively tiny, the timing of the hits isn't inside the scout ship's control, and the ship won't be able to pick a dino-killer "off the menu".

As a bit of a frame challenge, the sensible way to do interstellar colonization or conquest is by using star wisp Von Neumann machines.

Here you have a very tiny seed ship you launch over interstellar distances using something like a solar sail and launch laser. Then you repeat the process, much more difficult, using two solar sails to come to a halt at the target system.

(Your tiny payload has a large solar sail. You fire a laser at the large solar sail, which you detach from the payload. The payload deploys a small solar sail,and uses the reflected light to brake and approach the target system.)

The goal is to have as much of the infrastructure to travel interstellar distances in the launching system, where you have the energy budget of a K1 or K2 civilization to play with, and as little as possible within the "space ship" as possible. Interstellar distances are very long.

Once you reach the target system, you start working on the long, slow process of reproduction of your Von Neumann machine.

Imagine that machine has a production capacity of X, and it takes 1000 years for the machine to double that production capacity. Then after 10,000 years you'll have 1000x the capacity, 20,000 years 1 million times the capacity, 30,000 years 1 billion times the capacity, 40,000 years 1 trillion times the capacity, 50,000 years 10^12 times the capacity, etc.

If you can double your production capacity and sustain that over many many repetitions, the time it takes you to do the doubling isn't as important as the fact you can double in the first place. After well under 100 doublings, you are a K2 civilization, and have consumed the entire solar system's resources.

Such an exponential process is extremely slow to "get off the ground". A civilization that threatens to reach the space age within a few thousand years is thus a serious threat. Once they are in space, they could work out how the Von Neumann machine works, and with the higher production capacity of a populated world outpace it. Even before they reach space, observation of the Von Neumann machines using astronomy could clue them in, and accelerate their technological progress.

A "hostile" source of such reproducing probes could result, and an entire chunk of the galaxy could be lost to the colonization effort.

At the same time, the energy budget they can afford to spend on actually interfering with the ground-based biomatter is limited, as the Von Neumann machine requires most of its budget to keep up the exponential reproduction cycle.

And growth is the purpose of the probe. The driving goal is to build a new lauch site, and send out another 10 probes. This may take 100,000 years, but the probe and its sisters together will claim the entire galaxy in a blink of an eye this way.

If every 100,000 years, 20% of probes launched succeed and produce an average of 10 probe launches, after 1 million years each probe launched causes 1000 launches, on the time scale of millions of years the Von Neumann probe-front is a relativistic phenomena, limited by the speed of the probe flight, and not by reproduction rate. If the probes travel at 0.0001c (extremely fast really), the wave crosses the galaxy in 500 million years; mostly leap-frogging, where probes are launched "behind" the front wave, and the front wave mostly is in charge of firing braking lasers.

Further from the front wave, the Von Neumann machines stop their obsessive probe launching, and print out a civilization on the colonized systems. These civilizations do whatever they want; they can do science, art, whatever. Some may work on optimizing the Von Neumann probe system, improving probe speed, reproduction rate, or whatever (note, however, that this becomes an information security problem; compromising the reproduction network of probes is an easy way to take control of the galaxy and mold it in your own image).

So under this model, the "scout" probe is attacking the planet using extremely low-hanging asteroid fruit, because it (a) really doesn't want them advancing to the space age in the next few kiloyears, and (b) is really busy trying to boot strap itself to the point where it doesn't care what the planet does.

It requires 10s of thousands of years for plan (b) to take effect. If the planet does advance to the space age, it can get help from other systems at a high cost. A nearby system could, for example, convert their entire star into a huge laser and fry this system's main planet's biosphere from interstellar distances. This is not 100% reliable, and failure results in a guaranteed of an extremely hostile civilization with access to Von Neumann probe technology. Also, it destroys the system that fired the laser (the probe-civilization has a lot of systems; cauterizing one to get rid of a potential danger is expensive, but not unheard of).

  • $\begingroup$ Interesting idea about the chaotic chain of asteroid interactions. But it's one that's hard to quantify. $\endgroup$
    – causative
    Jul 19, 2021 at 19:38
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    $\begingroup$ @causative The difficulty in quantifying makes it better for the purpose of fiction. Higher mass bodies are going to be harder to control their chaotics to the target in shorter time periods both because they are rarer (so fewer chances to get "lucky") and because their movement in turn causes other stuff to move more (making the math harder to solve), at least at the level of handwavium. $\endgroup$
    – Yakk
    Jul 20, 2021 at 15:23

I've been looking into some similar mechanics for a story I'm writing, and my solution is to instead deorbit Oort cloud or Kuiper belt objects. Since they're further away from the sun, they orbit at slower speeds and will take less delta-v to de-orbit.

By picking objects nearer or further from the sun, the alien ship could control how long it would take to fall toward the Earth (further away would be a longer fall time--exponentially, I believe, though I'd have to do some math to make sure). In addition, just by making the objects cross Earth's orbit in different locations, they'd hit the Earth at different times.

Do note that these aliens will need incredibly precise mathematics, especially the longer between the deorbit burn and the strike on the Earth. Alternatively, they could just deorbit as many as they can and know that, eventually, they'll all hit Earth if they cross its orbit, but that would be over a period of millennia.

I also want to real quickly pose a solution to @L.Dutch's statement that a single planet-killing asteroid is much more efficient:

A single asteroid is also much easier to detect and plan ahead for. If the species is advanced enough, they might be able to launch a mission to redirect the asteroid in time. A swarm of smaller asteroids, on the other hand, is much harder to detect and will require much more planning to redirect all of them.

  • $\begingroup$ If the species can launch a mission to redirect an asteroid, they can also launch a mission to destroy whatever was sending the asteroids, making the ability moot. $\endgroup$
    – March Ho
    Jul 18, 2021 at 18:47
  • $\begingroup$ @MarchHo Well the aliens sending the asteroids presumably have a ship with which they can escape any primitive attempts to destroy them. Asteroids, on the other hand, have extremely predictable trajectories and are much easier to rendezvous with and shove into a different orbit. $\endgroup$ Jul 19, 2021 at 3:42
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    $\begingroup$ Sending Oort cloud objects is kind of a non-starter. Sending one of those on a path to hit the planet is going to take 10's or 100's of thousands of years for the objects to reach the planet. That's potentially way more than enough time for the population to sufficiently advance to the point where they can detect and deflect or destroy them by the time they get there. $\endgroup$ Jul 19, 2021 at 15:27
  • $\begingroup$ The Oort cloud is over a light-year away. Good luck getting anything back from there! $\endgroup$ Jul 20, 2021 at 12:43
  • $\begingroup$ Ha yeah I was afraid of that; that's why I included Kuiper belt objects as an option. $\endgroup$ Jul 20, 2021 at 15:15

It could work with better conditions, but our asteroid belt is a lousy example

Almost three years ago I thought I had a great story basis and asked the following question:

Could an astronaut in a near-future space ship survive transit through our asteroid belt?

I received a rapid education. The answer is "yes, duh" because if you gathered every particle of mass in our asteroid belt and made a proverbial snow ball out of it, the result would be a single body about 4% the mass of our moon. When we call it an "asteroid belt," what you cannot imagine is something out of Star Wars, what you need to imagine is one or two big chunks, followed by a lot of baseball-sized chunks, followed by a lot of dust.

And even at that, we've had probes pass through the field without any trouble whatsoever.

Which means if the asteroid field is like ours (and not like the one in Star Wars), then you don't really have anything to throw at the planet in the first place.

But let's ignore that... given that there's enough asteroidal material to work with, is the basic idea plausible?

  • The way your question is worded, the alien has been sitting there for 1,000 years doing a whole lot of nothing but moving asteroids into motion. (a) I've always had trouble with super-long-lived aliens (A scout ship with a 1,000-year food supply?). (b) Go watch the movie Silent Running. It's a reasonably good presentation of the monotony of doing something menial for far too long. (c) However, you can get around this by automating the entire process, setting the bored traveler free to go home.

  • The situation feels too much like an unnatural premise. An alien species that considers the natives of a planet a threat and are willing to bombard them periodically with asteroids would need a honking good philosophical reason to not simply wipe them out (doesn't he have a report to make to someone? That's what scout ships are for...). Now, the theory that the scout ship was too under powered to do that is potentially plausible, but what keeps the scout from simply going home (or "radioing home") and sending something bigger, more capable, and more definitive? fter all, the scout ship was well enough powered to arrive in the first place. What this feels like is you want to tell a story about an iron-age people who are periodically reduced in population to keep them from advancing and this is the crowbar excuse you've invented to tell that story. Ugh. What you really need is a reason to keep the iron-age people alive.

  • But, to answer your question, yes... given that there are big enough rocks around, the scout ship is only well enough powered to get one moving at exactly the right trajectory once every 20 years, and that there isn't a reason for the pilot do find a better solution. This scenario is implementable.

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    $\begingroup$ The scout does want to wipe out civilization, but wiping out cities is his best effort. He did radio home when he first found the planet, but help will take thousands of years to arrive, if it ever does (no FTL). The scout is probably a robot of some kind. $\endgroup$
    – causative
    Jul 18, 2021 at 19:46
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    $\begingroup$ @causative In a 1,000 year period it may be more valuable for the scout to move a moon from another planet to collide with the first, wiping out everything in one strike. After all, there's plenty of time - almost an infinite amount of time. I'm an engineer, and from an engineering perspective the scout's proposed actions are inefficient and ultimately unproductive - unless you, the author, need actual survivors on a specific scale. Like I said, it feels a bit contrived because it doesn't feel like a natural consequence of the situation. $\endgroup$
    – JBH
    Jul 18, 2021 at 20:05
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    $\begingroup$ Well, "Can it drop a moon?" is the physics question - given that the scout has the capability to drop a city-killer every 20 years, does it also have the thrust necessary to drop a planet-killer moon over 1000 years? The scout is a robot given a general directive to explore and assist its home system, and it was not expected by its creators that the scout would be involved in any military action. It's an accident by the aliens. The scout's civilization may no longer exist, having destroyed itself, or it may indeed have advanced past the point where it cares, but the scout doesn't know that. $\endgroup$
    – causative
    Jul 18, 2021 at 20:12
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    $\begingroup$ Another option for the aliens' motives is that they are not conquerors and colonizers, because they tightly regulate themselves to maintain bureaucratic central control. Colonizing another planet would weaken that central control, so the bureaucrats say no to such plans. But if this Earth-like planet gets interstellar technology and doesn't behave the same way, it could expand rapidly and become too powerful for the aliens to stop with their one planet. $\endgroup$
    – causative
    Jul 18, 2021 at 20:26
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    $\begingroup$ @JBH Your understanding of the energy scales required to "move moons" is orders of magnitude, and plausibly an order of magnitude of orders of magnitude, off. $\endgroup$
    – Yakk
    Jul 19, 2021 at 13:55

Okay, let's take this step by step. Your scout wants to wipe out all civilized life on the planet. It keeps chucking rocks at the planet one at a time wanting to wipe out the biggest cities it can find. Problem is that it would be much, much easier for it to come to the conclusion that periodic rock-throwing isn't the best idea and just go for a dinosaur-killer Chicxulub scenario, chucking the biggest rock it can find to cause a global extinction event and wipe out civilization.

This is actually easy to do because of how space works. Because there's only microgravity in space, even small amounts of force can cause a large object to change course and accelerate in the direction one desires. Even if the ship can only grab a smaller asteroid, it can launch that asteroid at a larger one, and like a series of pool balls will eventually knock the larger planet-killer into the world. This is where astronomers get the whole idea of "divert an asteroid using an incredibly small projectile" from.

If for some reason no dinosaur-killer asteroids are to be found, the ship can simply coordinate multiple asteroids to hit the planet at once, or target areas like carbonate platforms, volcanic trappes, or methane clathrate deposits to maximize damage. All of these if hit can cause mass extinction scenarios. It also depends on how fast the ship can accelerate these rocks. Even a small rock accelerated to a significant fraction of the speed of light can sterilize a planet.

Alternatively, the ship can invoke the Kzinti Lesson ("A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.") and kamikaze the planet. If the ship is advanced enough to travel between solar systems or is advanced enough to reliably fly to the asteroid belt, throw a rock, and then check on the planet to see what happens, it's fast enough to wipe out civilization in a kamikaze attack. If it's a robot probe it might not care if it sacrifices its own existence to advance its maker's will.

There's also the question as to how civilization will react. For one, how is this ship detecting the presence of sapient, city-building life? Most of the easy indicators one could see from orbit (electric lighting, C02 pollution) are mostly the results of industrialization. It's true humans have been altering Earth's climate and environment since basically as long as we discovered how to set things on fire, but discerning sophont activity from, say, natural processes can be difficult before industralization. What's to stop civilization from really collapsing if you throw enough asteroids at the planet? Smaller asteroids might produce "years without summers", and enough irregularity might make crops unreliable enough to not support large population centers. At the time of Rome, the largest cities were only about 1 million people, so your ships is going to have to be able to detect relatively small cities (at least by present standards) and eventually it's going to be tossing rocks at cities about the size of the first city-states in Uruk and Babylon.

And, of course, people might just stop settling in large densities when they notice that every time they build a city the gods seem to throw a rock at it. Think of the Tower of Babel story, sophonts will adjust their behavior if there's enough negative stimulus not to. At this point, what does the ship do? Does it assume sapient life has been wiped out and goes home, ignoring the small villages of Iron Age farmers and numerous surviving hunter-gatherer tribes? Or does it keep trying to throw rocks at harder and harder to track groups for little gain?

What all this means is, that there are way too many ways your ship could either just wipe out all life on the planet (or even just civilized life), or force the city-builders to hide themselves and thus believe it's task is done before your real plot can start. What you need is extra reasonings as to why this cycle of unmanned probe acting like a child throwing rocks at a beehive for all of their recorded history continues uninterrupted for so long. My advice would be something like a programming glitch, the probe wants to destroy the civilizations on the planet but it's stuck in a logical glitch that leads it to try the same thing over and over again, hoping for a different result. For reference I would suggest...

HAL 9000 in 2001: A Space Odyssey and 2010: The Year We Make Contact as well as The Reapers in Mass Effect. HAL is given mutually contradictory orders and it drives him into a loop that results in him going mad. The leader of the Reapers, meanwhile, is stuck following the last orders of his creators even though it knows there is a better solution to the problem. Some have interpreted the infamous events of Mass Effect 3 as it trying to manipulate Shepard to free it from its programming, because it rather blatantly and unsubtly hints that it thinks Synthesis is the solution to the problem but needs an organic to give it permission to do so because of the Leviathans' orders.

Either that or it's doing the Beerus solution: it doesn't really want to destroy the world but is deliberately doing so in such an inefficient manner that it can justify it as following its programming.

  • $\begingroup$ A Chicxulub-magnitude impact would be a tall order; the ship is stated to be able to cause 500-1000 kt city-killer hits every 20 years. Chicxulub was about 10^13 kt, roughly 1-10 billion times the city-killer energy. For the same reason, a kamikaze strike by the scout may not do extinction-level damage. Not sure about the pool-ball scenario. Maybe, but can you prove it in Kerbal Space Program? Repeated planetary gravitational assist might work instead, but I'm not sure. The scout can perform satellite mapping of the planet, which is enough to pick out individual people and buildings. $\endgroup$
    – causative
    Jul 19, 2021 at 2:30
  • $\begingroup$ Actually I'm pretty sure that the pool-ball scenario would not work, since small asteroids are loose collections of rubble which would just splatter on impact, especially if the impact is at any significant speed. $\endgroup$
    – causative
    Jul 19, 2021 at 3:47
  • 3
    $\begingroup$ I would go with the logic conflict. It is a scout, not a military AI. It believes the aliens are a threat, but is not qualified/allowed to make a final determination on whether the species should be wiped out or not. It's working on a set of fuzzy-logic thresholds, and roughly every 20 years, that threshold is met. It wipes out the most threatening aspect of civilization (maybe not the largest city, but the one that just built the largest telescope or something) which sets civilization back under the threshold. It will continue until new instructions are received or humans learn to hide. $\endgroup$
    – user34314
    Jul 19, 2021 at 3:59
  • 1
    $\begingroup$ @causative Did some number crunching in an impact modeler. You can produce an impact with a 1000 kt yield with an iron asteroid of only 50 m in diameter going at normal intersolar speeds at a normal angle of 45 degrees. In fact we can move asteroids larger than that right now with the planned Didymos mission. $\endgroup$ Jul 19, 2021 at 4:16
  • 1
    $\begingroup$ @user34314's logic conflict could preclude kamikaze, in a sort of Three Laws way (swapping humans for aliens) $\endgroup$
    – Chris H
    Jul 19, 2021 at 12:50

Alien motivation is alien...but given the parameters seems like it wants to keep the beings alive, not destroy them, yet keep them from growing. Maybe for future conquering/slaving/eating....Unless your alien ship is tiny, like a few grams, if it had the energy to travel interstellar distances, moving an asteroid every twenty years would be trivial.

I would expect the inhabitants to not connect the attacks initially, it would take a long time to realize cities were even the target(you might get some Sodom and Gomorrah legend arise). This until an few cities in a row get hit that are in the same empire or trade sphere. Eventually enough people will see the connection.

Once realized astronomy and urban planning would become far more important earlier. People would learn to track the alien and disguise their cities.

An unintended consequence is it could possibly kickstart the civilization in to tech overdrive to deal with the threat.

  • $\begingroup$ So it's a test... like in Armada? Perform well and we'll induct you into the Sodality? $\endgroup$
    – Corey
    Jul 19, 2021 at 6:34

There's almost always a way to make it work, you just have to figure it out.

The other answers make very good points. I can see them from all the way out here in left field, so it's likely your answer lies elsewhere. But I like left field. It's where the weird stuff happens...

But first, a frame challenge or two.

If the ship was intended to destroy potential threats then it seems vastly unlikely that its creators would have sent it out into the void without weaponry to effect that destruction. How do we explain that the Doom Probe is weaponless then?

Perhaps it has weapons but can't use them for some reason. Out of ammo after destroying the last planet? Some critical part failed and it can't access them? It forgot how due to poor programming?

Or maybe it really doesn't have weapons for some reason. Its creators may have been dumb enough to think that pushing sand around was the perfect way to destroy a planet. Or they really intended it to be throwing around larger masses, but somebody (or something) screwed up the selection algorithm so it ends up throwing pebbles instead of dwarf planets.

Of course the real reason it doesn't have any weapons might simply be that the original builders didn't intend for it to go on a geocidal spree through the galaxy, so they didn't think it needed them. It was supposed to be just a survey probe until some government fidiot got scared by some bad Space Opera and wanted to make sure the World Eaters couldn't ever trace the thing back home.

Personally I favor the "they cut our budget back so hard we had to 'hire' interns to write most of the code" hypothesis. Some unpaid drone was playing asteroids on the simulator instead of coding the First Contact Protocols and nobody caught the error before launch. I mean, honestly, we've done worse with better. Remember when a missing hyphen blew up an $80M (worth a lot more these days) rocket 5 minutes into a trip to Venus? Pepperidge Farms remembers. They probably caught some of the shrapnel.

It doesn't have to be complete stupidity though, just a lack of imagination. Let's assume that the source system has a different configuration, and therefore different assumptions were made about what the ship would encounter. It may have originated in a system where the only life-bearing planet was much smaller and more fragile than the world it is currently attacking, where a smallish rock dropped hard enough could cause devastation far exceeding the loss of a single city. Like most people, the creators failed to imagine situations like the one the ship has found itself in. Any one of those rocks would end their world, so that's what they instructed the ship to use. And being a poor, dumb computer supplied - and programmed - by the lowest bidder, that's exactly what it's going to do until someone comes along and makes an intelligent decision.

Of course there's probably nobody coming. Sending out something like this into a galaxy filled with terribly dangerous things like Humans is bound to tick some of the other residents off to the point where they do more than send a nicely-worded cease-and-desist order.

On a more serious note, and in response to your comment, I'm going to have to disagree with your premise. Given that the probe has sufficient intelligence to predict the development of a primitive society over the next thousand years - a feat which requires both extraordinary intellect and god-like knowledge of xenology that could only have come from aeons of research on untold thousands of other specimens - then it is more than capable of running the trivial computations to figure out that a single large rock in five or six hundred years time is sufficient to purge all life from the planet, while a series of smaller rocks delivered every twenty years is at best going to slow them down.

My advice is to patch that plot hole. Maybe explain that it went crazy and the predicted danger is pure paranoia rather than godlike knowledge. That would certainly explain why it's using an inefficient campaign of inconvenience instead of a decisive finishing strike.

  • 2
    $\begingroup$ Its creators wanted it to explore, but thought the probability of encountering another civilization was extremely low, and they thought that the probability of encountering a civilization that the scout could do anything about would be even lower. It wasn't expected to kill anything. But once it found this planet, and realized in 1000 years the civilization might become a threat, it took initiative with the limited tools it had. With the alien technology level, interstellar travel times may be several thousand years, so it messaged for help and then stayed to delay the problem. $\endgroup$
    – causative
    Jul 19, 2021 at 6:23
  • $\begingroup$ @causative Ah, so it's a rogue murder bot. Called it! (Refer "...the original builders didn't intend..." above.) $\endgroup$
    – Corey
    Jul 19, 2021 at 6:32

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