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Obviously future tech is allowed, but I'm looking for a solution using technology as near as possible.

I would like to send a moon the size of Earth's moon crashing into a planet Earth's size. I would appreciate it if the victim planet, who has technology the same level as mine, cannot stop this event. That is, if you have a solution requiring technology hundreds of years in the future, the victim planet will also have this level of technology.

Extra points if the victim planet doesn't even notice until the moon starts crashing down.

I'm looking for a) lack of detection by victim planet and b) closeness of technology to modern day.

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    $\begingroup$ So, do you want "a moon" or "a large object like a moon"? There's a huge difference, so you should point out which one you want. If the latter: Why not just build a ship with the size of said moon and navigate it right to the surface of the targeted planet? $\endgroup$
    – Stegax Khenacc
    Commented May 31, 2017 at 19:12
  • $\begingroup$ I'm reading your question as you want to crash an object already in orbit into a planet, correct? This makes a huge difference. How long should the crashing down take? A century or a couple of minutes? $\endgroup$
    – Raditz_35
    Commented May 31, 2017 at 19:15
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    $\begingroup$ "A moon." No suicidal Death Stars here. $\endgroup$
    – Pointy cat
    Commented May 31, 2017 at 19:15
  • $\begingroup$ The crashing down can take any amount of time as long as the victims won't be able to stop it. $\endgroup$
    – Pointy cat
    Commented May 31, 2017 at 19:17
  • $\begingroup$ You should edit the title of your question, if you only want a moon to be legit. $\endgroup$
    – Stegax Khenacc
    Commented May 31, 2017 at 19:39

7 Answers 7

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Using self-replicating nanotechnology, plant weeds that will cover the entire surface. The trailing hemisphere becomes vantablack, absorbing all the sunlight hitting it. The leading hemisphere becomes mirrored, reflecting all light.

This will cause a thrust that slows the body along its orbit, so it will spiral in.

In general, about 1016 watts of power is available by harvesting all the sunlight that reaches the moon. Note that using this much power is a world’s definition of the Kardashev Type I! The moon is smaller, so this is 7.4% of Earth’s K-I. This is still 700× the current energy consumption of the entire human civilization.

Even so, The moon's orbital motion has a kinetic energy of about 7×1028 J, so it would take 250 thousand years to bring it to a complete stop.

The problem is that when it reaches its Roche distance, it will break up and you end up with rings, not a crash. That is true with any slow gradual method.

See also this older question.

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  • $\begingroup$ You might want to apply the relationship between energy and momentum to the power supplied by the sun to something like the moon, then factor in the mass of the moon, and then calculate how long the process will take. In other words, just no. $\endgroup$
    – WhatRoughBeast
    Commented Jun 1, 2017 at 2:55
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    $\begingroup$ @WhatRoughBeast This (about 10^16 watts) outclasses the energy of other answers which just say “use (nuclear) rockets” but you didn’t say anything about them. I'm getting into Type K-1 civ territory here! $\endgroup$
    – JDługosz
    Commented Jun 1, 2017 at 3:38
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    $\begingroup$ The good thing is this answer has a realistic time frame for dropping the moon on the planet. It takes into account the kinetic energy of the moon too. The only way this technique would surprise the planet's inhabitants would be if they lived several million times more slowly than life on Earth. Good to see an answer that considers the technical requirements of the problem. $\endgroup$
    – a4android
    Commented Jun 1, 2017 at 8:48
  • $\begingroup$ This is a great answer! Maybe the Roche Limit could be defeated in this case by the nano-weeds holding the moon together. $\endgroup$
    – Thriggle
    Commented Jun 1, 2017 at 12:09
  • $\begingroup$ @a4android - Alas, the time frame is not accurate. Try about 70 billion years. $\endgroup$
    – WhatRoughBeast
    Commented Jun 3, 2017 at 14:19
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If sci-fi tech is allowed: Tunnel into the Moon and build massive Star Trek style impulse engines to slow down the Moon. I picked the ST version because they have no reaction mass output and would be invisible to modern day scanners. If you wanted it to be invisible to advanced scanners, borrow a Romulan cloaking device for the engines.

The more you can slow its orbital velocity the faster it will fall in.

If you want to go the ridiculous tech version: take a page out of the Lensman tactical guide and use massive inertialess drives to position any handy moon to the point where the moon's initial velocity intersects with the planet's motion. Then turn off the inertialess field. Better yet, with some planning, accelerate the moon to a significant fraction of c and then pop the inertialess field in place.

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  • $\begingroup$ Star Trek impulse engines are also a type of warp technology, and don't impart real lasting motion to the bodies within the field. You can tell because the Enterprise is not towing a “coal car” full of antimatter that it empties to reach half-light speed. So you would have to fly it into the Earth, not gradually slow it. $\endgroup$
    – JDługosz
    Commented Jun 1, 2017 at 4:01
  • $\begingroup$ If you can accelerate a moon to a significant fraction of c, you didn't need the inertialess field. Simply aim the acceleration vector of the moon at the planet. This will be followed by a planet-shattering KA-BOOM!!! NB: the rate of acceleration will be remarkably high to crash a moon at a significant fraction of c. $\endgroup$
    – a4android
    Commented Jun 1, 2017 at 7:40
  • $\begingroup$ @a4android The purpose of the inertialess drive is so you can accelerate the moon somewhere else and then transport it to the target. Think of an area far away from anything else where you have a bunch of small planetary bodies accelerating, waiting for a time when you have a planet that needs to go BOOM. $\endgroup$
    – ShadoCat
    Commented Jun 1, 2017 at 18:07
  • $\begingroup$ @JDługosz, I didn't think that was how impulse engines worked. They are always shown as a separate system from the warp drive. The writers generally didn't know what Newtonian Physics was. The many Technical Manuals and Star Fleet Battles treated it as a separate, sub-light, reactionless drive system. However, flying it into the planet would work too. $\endgroup$
    – ShadoCat
    Commented Jun 1, 2017 at 18:13
  • $\begingroup$ @ShadoCat Ah! Now the light dawns. That makes sense. That goes from: "where did our moon go?" to later: "It's coming back at a significant fraction of c! We're doomed!" $\endgroup$
    – a4android
    Commented Jun 2, 2017 at 4:17
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Divert a large enough asteroid (I'm talking really large) to pass slowly between the planet and moon, attracting them towards each other. The asteroid could then escape the system after setting things in motion.

Even if possessing similar technology and resources, the target planet's inhabitants might just fail to detect it until it's too late.

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    $\begingroup$ The problem with this method is that if you are able to divert a large enough asteroid then you are probably better off just diverting the moon $\endgroup$
    – Miguel Bartelsman
    Commented Jun 7, 2017 at 8:58
  • $\begingroup$ @Miguel A relatively slight nudge far enough in advance can do disproportional diverting in an asteroid that already had a large magnitude speed. No such cheap trick for a moon. $\endgroup$
    – Emilio M Bumachar
    Commented Jun 7, 2017 at 15:36
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Try smashing the planet with a moon very tiny compared to the Earth's moon but larger than the asteroid that killed the dinosaurs. I know that's not what you want but your victims will be just as dead.

Suppose that your target planet has a very small moon orbiting at a great though realistic distance. Suppose that the people of the target planet don't have many facilities on that small moon or consider it worth defending.

Possibly they have a lot of important facilities on closer moons and orbital space stations a lot closer to their planet. And they might have super force fields around their entire planet and those close orbital facilities to defend against any attack. And giant orbital ray guns to blast any space ship that comes close to the planet and its orbital facilities.

So the attacking space navy comes and blasts at the planetary defense shields at a safe range and the planetary defense ray guns keep them covered, firing whenever they get too close. And many ships duck behind the outer moon and pop out from behind to fire at the planet and then duck back into cover. And the planetary defenders think that's all they're doing.

But some of the space ships are unloading parts for a giant space drive, a giant rocket or anti gravity engine or whatever. It took them months or years to design and build an engine so much bigger than any built before. And using tractor beams they are assembling it on the back side of the moon but pointed forward so that its trust will slow down the orbital speed of the moon.

So they turn on the giant engine and it quickly slows the moon's orbit so that it soon has no speed relative to the planet and falls toward the planet under the gravity of the planet.

It will take just a few days to smash into the planet, just as Apollo space craft took about three Earth days to "fall" from Lunar orbit to low Earth orbit.

The defenders try blasting it with their giant space ray guns, but they can't vaporize away enough of the moon's mass in a few days. It will still have enough mass and velocity to cause an extinction level impact.

They can't build a giant space engine to take to the moon and change its velocity and trajectory in the time they have left.

They send ships with giant atomic bombs to try to reduce the mass and/or change the velocity of the falling moon, but those are all blasted by the attacking space armada once they pass outside the defense force fields.

They're doomed.

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The path of least resistance is to reduce the moon's speed. This will eventually upset its orbit so that it falls into the planet.

The challenge then is coming up with a way to reasonably accelerate the moon in the opposite direction of its rotation around the earth.

Conceivably, one could apply nuclear explosives or nuclear rockets to slow the moon's orbit. Alternatively, large, high-velocity asteroids or other massive bodies could be set on a collision course for the moon. Maybe the inhabitants would believe they'd avoided certain disaster thanks to the moon blocking the rogue asteroid, only to discover later that the impact has caused the moon's orbit to decay.

Explosions and asteroids seem pretty dramatic, but nuclear rockets installed on the far side of a tidally locked moon could be surreptitious enough to go unnoticed.

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  • $\begingroup$ It will not be beyond the horizon. See libration. Pointy: no, you want to thrust against its orbital motion. You slow to reduce your altitude. Maybe you misunderstand “leading” hemisphere? $\endgroup$
    – JDługosz
    Commented May 31, 2017 at 23:32
  • $\begingroup$ Ah, I see what you mean. But wouldn't half the rockets then be visible? $\endgroup$
    – Pointy cat
    Commented Jun 1, 2017 at 0:53
  • $\begingroup$ @Pointycat, yeah, I think that was J's point. So you might need to bury the rockets to conceal their eruptions, unless you can place them far enough beyond the horizon that they aren't visible but not so far that they'd push it into an elliptical orbit instead of slowing it down. $\endgroup$
    – Thriggle
    Commented Jun 1, 2017 at 1:18
  • $\begingroup$ You might want to show how much Uranium you need (more than exists in the solar system?) for a given thrust and how long that will take. I'm guessing more reaction mass than the moon’s original size, and billions of years for any concevable “burried rocket” design. $\endgroup$
    – JDługosz
    Commented Jun 1, 2017 at 4:13
  • $\begingroup$ @JDługosz this question isn't tagged hard science or even science based; are such mathematical implementation details as necessary as the concept, given the asker's openness to "future tech" (which nuclear rockets, having been untested in realistic conditions, and asteroid/mass manipulation arguably are)? I do appreciate constructive criticism, and I value the new things I often learn from your comments. $\endgroup$
    – Thriggle
    Commented Jun 1, 2017 at 11:58
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In gravity simulators, you can crash moons by flying other bodies very near them / slingshotting around them. The gravitational attraction of the passing smaller body can slow the moon enough to destabilize its orbit.

Benefits:

1: Fussing around with some asteroid at a great distance will not be that noticeable. Over a long period you can incrementally add energy with your rockets to get the thing up to speed. Maybe you could use a gravitational slingshot off of something massive to get some more speed.

2: Rapid, impact-free flyby of asteroid (turn off the rockets or they will get wise!) might not be noticed; if it is noticed but target planet will be pleased it was not itself hit.

3: Target planet may not realize until later that orbit of moon was destabilized.

Risks:

  1. You need really good math up front to calculate trajectories and make sure your asteroid is going to fly through the correct path to destabilize the moon, because there is no good way to steer it.

  2. Maybe you should just crash that asteroid into the moon and slow it down that way.

  3. Maybe you should just crash that asteroid into the target planet and call it a day.

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  • $\begingroup$ I don’t know what you mean by destabilize its orbit. The pass will slow the moon slightly, lowering the opposite end of the orbit. Since WhatRoughBeast is insisting, you might want to calculate how many billions of passes would be needed, and how many × more energy that would take compared to just thrusting the moon directly. $\endgroup$
    – JDługosz
    Commented Jun 1, 2017 at 4:09
  • $\begingroup$ @JDlugosz: by destabilize I mean convert from a stable orbit to one that culminates in the moon hitting the planet, as requested in the OP. One could potentially impart a greater vector force to the moon with a slingshot flyby than one could with a nonelastic collision. The calculation requires knowing the mass of your asteroid(s). Billions if asteroids are small. Fewer if they are large. $\endgroup$
    – Willk
    Commented Jun 3, 2017 at 17:32
  • $\begingroup$ Small or large: can go with total mass of idealised flytbys. Then compare that to the moon and planet involved. $\endgroup$
    – JDługosz
    Commented Jun 4, 2017 at 1:29
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Well its a hard one. You need other bodies to interact/crash into the planet/moon. An other solution is to create a magnetic field (super strong) to time the moment when the north and respetivly south pole of the planet in question are the closest. So a powerful positive magnetic field would start when close to the north pole - to start an attraction, when its nearer to the south pole, the magnetic field becomes negative. Then it would start to accelerat inwards.

I also think there is a physical problem of doing all this. The energy created to move the moon inwards - will be the impact energy inwards. (However there will be alot of friction going on - but it would have been there by itself in x years, assumig the moon is not orbiting away from the planet). But it really does sound evil to do it.

About the events of the planet - you would experience more powerful tide waves, and compasses might go strange. Also I do not know the demagntication about it would lead to. Probably the planet´s poles would be weaker and weaker.

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    $\begingroup$ Sorry, but you haven't the faintest idea what you're talking about. You don't understand magnetism, you don't understand gravity, you don't understand "friction", and it's just not clear that there is any part of the issue which you do understand. So, just no. $\endgroup$
    – WhatRoughBeast
    Commented Jun 3, 2017 at 16:19
  • $\begingroup$ (+ -) attracts. (- and -) detracts. If the planet in question have magnetic poles - you could in theory create a magnetic field on that moon that communicates with the planet. About friction (my point is that it would be almost impossible for the moon to crash in with a 90 degree angle) - it will scrape the planets surface. Gravity is created by the moon and planets masses... $\endgroup$
    – Lealo
    Commented Jun 3, 2017 at 16:31
  • $\begingroup$ So you time the planet´s pole with (+ -), (- +), (+ -). By altering a magnetic field on that moon. And you will get closer to the planet. $\endgroup$
    – Lealo
    Commented Jun 3, 2017 at 16:35
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    $\begingroup$ You keep making my point. What you think you know is wrong, and so fundamentally wrong that I have no way to educate you within the constraints of this site. $\endgroup$
    – WhatRoughBeast
    Commented Jun 3, 2017 at 16:38
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    $\begingroup$ Sigh. A moon is not a monopole - it has both + and - poles ON THE SAME BODY, so the moon does not experience either net attraction or repulsion. Most moons are in something close to an equatorial orbit, so they don't approach either pole. Look - you continue to embarrass yourself, but don't even realize it. Just give it up. $\endgroup$
    – WhatRoughBeast
    Commented Jun 3, 2017 at 16:52

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