An alien species has been conquering planet after planet around the Milky Way for some time. They detected intelligent life on Earth a while back and now they decided it’s time to add us to their collection. To capture us, they send one of their massive acquisition ships.

They want it to take as little time as possible, so they decide to make an immediate show of power and demand our surrender. To accomplish this, the ship will massively decelerate and stop just before hitting the Earth.
The force provided from this will cause massive natural disasters, and possibly kill a proportion of the Earth's residents, without destroying the planet.

Is it possible to cause such effects by deceleration?
and/or what would the effects actually be?

Additional Info:
The ship would be something like a moon-sized battle station (or possibly a giant head…)
The maximum amount of population they’d wish to kill would be roughly 5%.
They can travel faster than light if necessary.
They have the potential to go light speed to 0 in roughly 0.2s, but don’t worry about the how.
I'm not asking about the effects of another moon in orbit, rather just the initial appearance.

Any figures of distance, speed or deceleration would be appreciated!

  • $\begingroup$ Sorry I should have noticed this in the Sandbox "moon-sized" diameter or mass? It doesn't make much difference to option one in my answer but it does change option two a bit if it's feather light but the same diameter as the moon. $\endgroup$
    – Ash
    Commented Aug 6, 2018 at 16:57
  • $\begingroup$ @Ash Was think diameter mostly but I will say both just for the sake of simplicity $\endgroup$
    – BMS21
    Commented Aug 6, 2018 at 16:58
  • $\begingroup$ Sweet my answer stands as is then. $\endgroup$
    – Ash
    Commented Aug 6, 2018 at 16:59
  • $\begingroup$ If the spaceship's deceleration is felt by the earth, stopping "just before hitting the earth" isn't really any different than just hitting the earth. The ship's kinetic energy is still transferred to the earth and its atmosphere. $\endgroup$ Commented Aug 6, 2018 at 17:41
  • 1
    $\begingroup$ Real aliens don't bother to intimidate the Earthlings, they just tractor-beam the entire planet out of orbit and tow it to wherever they want it to be kept. The humans can protest, or not, but there isn't much they can do about it in any case. $\endgroup$ Commented Aug 7, 2018 at 2:31

8 Answers 8


How do they decelerate?

If they do it by action and reaction, then they decelerate by sending mass (rocket fuel etc) ahead of them so they slow down. Mass times acceleration. Stop half the mass by accelerating the other half to twice the velocity? (Ignoring relativity.) Or stop most of it by accelerating a little bit a whole lot faster....

The kinetic energy proportional to mv^2. A whole lot of kinetic energy there to transfer to the earth....

Well, but since they're aliens with unknown technology they don't have to do it that way. They could do the equal and opposite reaction with a giant burst of neutrinos headed our way. Would that affect us? Not much.

Say it's mostly neutrinos plus a little bit of whatever they want. They could hit us with a burst of gamma rays and cook everything a bit on one side of the planet.

They could hit us with a gigantic EMP burst and destroy a whole lot of our electronics.

Maybe they could do anything else they wanted and hit us with any combination of electromagnetic frequencies. If they already know human physiology, they might hit us with maser frequencies at just the particular combinations of infrared that get best absorbed by one particular protein. Everybody's blood gets a little bit hotter, but one particular protein gets very hot and maybe gets denatured. They might kill 5% of the people, and make most of the rest sick to varying degrees. Also affect other species at random, depending on how they are affected by those particular frequencies.

I liked the discussion of tidal effects. If it's moon-heavy and closer than the moon, it will cause bigger tides. Which depending on the time of arrival might reinforce the lunar tides. A tide at New York City only 20 feet higher than the highest tide so far, would cause a great big emergency.

Bigger stone tides could cause earthquakes, but those would be hard to predict -- especially from many light-years away. Also once the stone is stressed it could increase earthquakes for decades to come. That would matter for invaders who intended to use the planet later. They might have enough experience from previous attacks on similar planets to guesstimate how likely they'd get more damage than they wanted. Or they might want to limit the potential effects to the point the water tides weren't very destructive either.

  • 2
    $\begingroup$ I like the neutrino brake option! $\endgroup$
    – Willk
    Commented Aug 6, 2018 at 19:37
  • $\begingroup$ I like both this and Rob's answer but I'm going to wait till at least a day before accepting one. $\endgroup$
    – BMS21
    Commented Aug 7, 2018 at 8:11

You can only cause damage with this scenario in one, or both, of two ways:

  1. the ship actually hits the atmosphere. This is problematic for two reasons, deep space vessels tend not to be designed for atmospheric entry, (it's expensive of mass and thus thrust energy) and you end up very far into Earth's gravity well with added atmospheric drag to contend with as well, getting back into orbit is going to be a difficult proposition. Damage from the atmospheric shock wave will be relatively localised making it slightly more controllable.

  2. the tidal forces from a such a massive (having much mass) object making a very near approach to the planet disrupt the normal flow. This is a difficult one to qualify with any certainty but the ship has to be gee tolerant to pull off the braking maneuver described so it should be fine, Earth would experience major tsunamis and land tides triggering worldwide earthquakes. The widespread nature of the damage will make the total magnitude of the disaster much harder to control.

In either case the ship will be interacting with a chaotic system, the Earth, putting hard limits on casualties under those conditions will be impossible; the full consequences of either a gravitational disrupt or an atmospheric shock wave are too unpredictable. With a good enough engine system you could dial in the initial parameters to a very fine tolerance, and get a pretty accurate initial casualty estimate with enough computation but the long-term effects can't be mapped.


Wikipedia already has one solution for you - using an Alcubierre drive to go FTL could create a sonic-boom like burst of particles in front of the ship when it exits FTL. Normally you'd aim slightly away from your destination so that you'd be ready to go into orbit around it and not bombard it. In this case the bombardment is exactly what they want.

As for how to kill no more than 5% of the world's population - the moon is about the size of the United States, which has just under 5% of the world's population. As long as the bombardment won't kill 100% of the people in it's path, you won't hit 5% of the world's population.


If you park a moon-sized station in geosynchronous orbit it will wreck all sorts of havoc.

  1. The gravity pull of the station will cause coastal flooding.
  2. If they move the ship back and forth from the planet they could cause tsunami.
  3. Moving such a big mass so close to the planet will certainly destroy all of our satellites (by messing with their orbits)

It might be difficult to limit the number of casualties, but given they are advanced aliens, maybe they ran complex simulations.

If they time their actions correctly they could flood South-East Asia and kill hundreds of millions fairly easily. Without satellites (GPS, weather, communications) rescue operations will be extremely limited thus making the death toll much higher.


The atmosphere doesn't stop all cosmic rays, just a % of them.

If the ship in question were to exhaust lots of such particles as part of its propulsion system (not outside the realm of possibility), it could well cause some really nasty effects directly underneath the exhaust. These could include:

  • Direct radiation exposure for anyone who happens to be underneath at the time (particularly those outdoors). Effect 10x as great on anyone in a commercial airliner.
  • Ozone depletion.
  • Creation of radioactive elements.
  • Failures of electronic equipment (particularly the newer stuff with smaller transistors, and anything with RAM).
  • An ungodly amount of lightning.

It would have to be orders of magnitude more than the amount hitting the affected area all the time as background radiation for it to be noticeable as being caused by the ship. Depending on the propulsion system used, that may be feasible.

But for me, the lightning alone might do the job. Nothing quite says to the primitives "Your god has arrived!" quite like a hundreds of miles wide hurricane of lightning.


You doing it wrong, man. Let's see why.

  • If you wish to intimidate a civilization, you do not do it like a small child on a child bike intimidating a girl he is in love with.

First of all, it like in a bad comedy about aliens - people didn't notice. I mean not that they won't notice potential chaos and destruction, happening, but generally, it will be attributed to a 1/10th of dinosaur asteroid.

And the whole braking right before collision thing, the whole point of it is one see it coming, that time between of noticing and its end is the when all the fears are boiling.

So people won't see a fast incoming thing even if its size of the moon. That's the sad truth of space explorations and the current situation. So you need to make a significant effort to be noticed and give some time for detection and for fearmongering.

If you wish to intimidate you have to make a credible threat and have sufficient time for them to notice you, give them time to boil in their fears and hopelessness.

Credible intimidation

The ship itself is quite a threat, it does not have to do anything to impress people, and there is basically no chances of military retaliation against it. So I guess you may hit any type of deal, without killing anyone(I understand plot demand evil aliens, yeah, okay)

If you think humans that stupid and really need some reinforcement and confirmation that those aliens are worthy and humans won't be able to wait out the stuff on the surface of the planet, playing seek and hide with aliens in jungles, the only thing the ship has to do is to be a second moon, just a little bit a special one - one which creates a perpetual Eclipse.

and if it is not enough, get in the L1 Sun-Earth and roll out the shiny foil, and make a real eclipse on the whole planet.

The valuable aspect of that sort of intimidation, effects are more precisely controlled.

P.S. however, not sure, what happened with a usual shot from orbit to the pole of the planet? Why it is not good enough.

  • $\begingroup$ I wasn't asking for another method, or if its a good idea, rather purely possibility and effects. $\endgroup$
    – BMS21
    Commented Aug 7, 2018 at 7:58

So, generally the force caused by quickly decelerating right before you hit something comes from the gasses around you. For example, if you swing your hand towards the desk and stop just short of hitting it, a gust of wind will push things away from your hand. The deceleration doesn't actually cause the damage; the air displaced by the quick motion in advance causes it.

So if it entered atmosphere very quickly and then decelerated short of touching the surface, then it would definitely cause a lot of damage by displacing a lot of air very quickly; possibly even creating a sort of crater below it (look up microbursts for something similar to this effect; only yours would be much much bigger).

The displaced air may be the least of our worries, though. A ship of that size entering the atmosphere quickly enough would also compress all of the air below it, heating the surface to the point that things would pretty much combust.

It's worth mentioning that just the presence of a moon-sized battleship near earth would still cause significant natural disasters (storms, hurricanes, tidal waves, and maybe even volcanic eruptions and earthquakes), just because of its gravitational pull on the planet.

  • 1
    $\begingroup$ If it entered fast enough displaced air would be the least of our worries: air compression would heat the surface to the point that things would pretty much just combust. $\endgroup$
    – Joe Bloggs
    Commented Aug 6, 2018 at 16:54
  • $\begingroup$ Excellent point. I'll edit to add that information. $\endgroup$ Commented Aug 6, 2018 at 17:29

There are a couple of major problems with this scenario.

  1. Please do not have your villains twirl their mustaches! Immediately killing a lot of people (and animals) is a really bad cliche used to show just how evil and uncaring your villain is! Many writers don't want to put the thought into creating a villain through plot, so they use cliches like this. In reality, if you injured 5% of the population of the US, you would probably bring almost all of society down, except for the military, which would probably have an immediate bad response.
  2. Perhaps just as importantly, if the ship can decelerate that quickly and violently, which is protecting the crew of the ship from being turned into jelly by these forces?
  3. There is a reason that earth's moon doesn't orbit much closer than it does now. When it gets close enough, the tidal forces produced by the earth to the ship will tear the ship apart.

One good book to read for the possible outcomes of something like this is "High Crusade" by Poul Anderson. This is on a much smaller scale. It is set in the middle ages and an alien scout/slaver ship lands at an English village. The goal is just to awe the people with their ship. It was working, and then the aliens shot a few people with energy weapons, and the bowmen responded by reflex, killing a couple of the demonic-looking aliens. This resulting in the human villagers mobbing the ship and killing all but one of the aliens. Had the aliens just stood there, the village might have surrendered. But once the villagers knew the aliens could be killed, they did it.

Having a ship the size of a moon orbiting earth would probably cause us to surrender quickly. Yes, I'm sure somebody would try to nuke it, but if the aliens destroyed the missiles with no apparent effort, there would be a lot of pressure to surrender.

Destroying or hurting society is counterproductive. For one thing, if you destroy the government,there will be no one to accept a surrender!


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .