I'm working on a story about Earth or an Earth like planet with current or very near future levels of civilisation and technology drifting or speeding out of the solar system. Currently my method for achieving this is the gravitational effects of a rogue object passing nearby (a bit more research to be done on how big a rogue object is needed).

I understand that were this to happen it is pretty much impossible for the population of the planet to do anything about it. However there is no doubt that they will try.

So my question is what might they consider or try to do to prevent the home planet going rogue? Anything they could consider trying right from the moment of discovering the incoming rogue object onwards is admissible.

I'm not asking about how they might try to survive for as long as possible on the rapidly cooling world. Nor about attempts to set up colonies on other planets etc, just about attempts to stop Earth/Earth analogy being sped off to an uninhabitable place.

However if for example there was a way to try to get Earth captured in orbit around a gas giant on the way out of the solar system and this would leave Earth somehow habitable then this would be admissible.

To be clear - I know that they are doomed, I just want to know what they might consider and try anyway to provide background to my main narrative.

  • $\begingroup$ I'm fairly new here and this is my first question on Worldbuilding so constructive feedback would be appreciated. Thanks. $\endgroup$ Commented Oct 19, 2019 at 16:22
  • $\begingroup$ How much time do they have left when discovering the rouge object? $\endgroup$ Commented Oct 19, 2019 at 16:35
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    $\begingroup$ @Mephistopheles According to this quora.com/… up to 10 years. If someone has a possible solution that contradicts this I'm willing to consdier it. I'm happy to allow the population to be lucky and spot the incoming at earliest opportunity if that helps the plot. I'd like for it to be at least a few years warning at the least and a couple of decades at most. $\endgroup$ Commented Oct 19, 2019 at 16:48
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    $\begingroup$ @011358smell The "somehow aiming earth at Jupiter's orbit" bit was just an example of the kind of whacky idea that would be considered. $\endgroup$ Commented Oct 19, 2019 at 16:50
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    $\begingroup$ We should first stop national talk like a pirate day en.m.wikipedia.org/wiki/International_Talk_Like_a_Pirate_Day. Oh different kind of rogue? $\endgroup$
    – Demigan
    Commented Oct 20, 2019 at 6:59

5 Answers 5


Call your shot.

Your incoming mass is near the size of earth - too big to move except with something else of comparable mass. Hmmm...

Your world has an AI at its disposal that knows the site, mass and velocity of all objects bigger than 10 kg in its immediate vicinity. There is a mass of comparable size which could be made to intercept and gravitationally deflect the incoming mass. But the interceptor mass is itself very large. To move it will require a different mass of comparable size. A smaller such mass is available and correctly positioned. Although smaller, this mass is also not easy to move. A mass smaller yet is also in the correct position...

Your AI presents a series of 12 successively larger gravitational interactions of which the smallest and first is within the ability of humans to achieve with rockets. In a planetary billiards chain reaction, an array of progressively larger objects will change course, with the end result the gravitational deflection of the incoming impactor.

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    $\begingroup$ I like the sound of this! Any tough estimate how long the process might take once set in motion? $\endgroup$ Commented Oct 19, 2019 at 20:25
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    $\begingroup$ @WiggotheWookie - /how long/ - The AI can model several scenarios each with different time courses and other ramifications. You can make it as long or as short as the story demands. But obviously it has to happen in time to deflect the impactor. $\endgroup$
    – Willk
    Commented Oct 19, 2019 at 21:41
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    $\begingroup$ @WiggotheWookie As many as comically needed. Sources. But there is the curse of space being ridiculously big to ruin our fun $\endgroup$ Commented Oct 20, 2019 at 17:05

There are two things that would help:

Firstly this rogue planet needs to be detected a long time before it approaches. With a few hundred years warning, they might consider sending fleets of ships to set up at network of installations on the surface for setting off thousands of hydrogen bombs one at a time over a year or so. Although almost insignificant, very large numbers of such weapons in conjunction with a very great distance might just be enough to deflect the object by a wide enough margin to minimise the bad effects.

The second thing that would help would be for the rogue planet to be as small as possible and be generating its effects by virtue of a very close approach imparting a rapid change rather than a more massive object generating its effects via a more prolonger encounter. Because it would be easier to deflect a smaller object.

Another option that is unlikely but possible would be to deflect the orbit of an asteroid into the path of the planet. Such an impact sufficiently far in advance might have some effect.

Finaly thought assuming the rogue planet had a frozen atmosphere, nuclear power stations and nuclear rocket engines could be set up on the surface to convert some of the frozen atmosphere into the gas phase and then use a large number of nuclear thermal or nuclear electric rocket motors to generate massive amounts of thrust.

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    $\begingroup$ I disagree on how you're using the nukes. You're going to expend a lot of delta-v to match and land, there's no need for that. Don't send ships, send bombs with radar fuses. $\endgroup$ Commented Oct 20, 2019 at 6:44
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    $\begingroup$ @Loren Pechtel Yes that might be a good idea depending on circumstances of size of planet approach velocity and the size and density of the atmosphere. Best if there is no atmosphere. $\endgroup$
    – Slarty
    Commented Oct 20, 2019 at 9:04
  • $\begingroup$ I heard somewhere that trying to blow up something like that would cause huge numbers of meteors (or meteorites or asteroids, IDK the exact terminology) to smash into the earth. Could be made a part of the story, though. $\endgroup$ Commented Oct 20, 2019 at 16:54
  • $\begingroup$ True if it were a small object like a asteroid <1km. But we are talking about a much larger object. It would have to be much larger in order to significantly affect the orbit of another planet more like a Pluto, Mars or Venus. No way this size of object would break up. $\endgroup$
    – Slarty
    Commented Oct 20, 2019 at 21:13
  • $\begingroup$ An extremely close approach would decrease the amount of mass needed to eject Earth from its orbit, but if the object passes too closely, the resulting tidal forces will likely kill everyone before the main plot has a chance to start. Alternately, it could pass that close, and the plans could be ways to save as many people as possible from the worldwide floods, rather than ways to deflect the other planet. $\endgroup$
    – Ray
    Commented Oct 21, 2019 at 2:42

Nukes are the only sane thing to try, you get far more energy per pound than with anything else. However, you deliver them by missile. Their usefulness at planet-moving is basically unaffected by their velocity, all you care about is aim (you want them to come down at the point you want the planet moving away from) and detonation altitude. This is a planet, you don't have death-star level energies available, there's no danger of disrupting it so there's no upper limit on the boom. You're going to throw the biggest bombs your rockets can deliver. (Larger bombs are going to be cheaper per megaton than small ones.)

However, I don't think they'll do more than run the numbers and determine this is hopeless. We can't muster anything like the energy to produce a meaningful deflection of a planet. Instead, the effort will go into survival--building bases dug in deep that will remain habitable after ejection.



Given the ridiculous distances involved in space, if they discover it soon enough, they could attempt to redirect it.

Normally, the only use for nukes in space is to detonate them in the Van Allen Belt and watch Fortnite players cry as the telecommunication systems of an entire planet goes down in a blaze of glory. That is because there is no matter nearby that the nuke's radiation could heat up.

Depending on your planet's civilization, and plot convenience, they might have a mass driver lying around. Such thing would squarely be future, but if the planet's society was ideal, they could have built one before. These handy inventions would allow for the transportation of nukes and other components into space where they'd be assembled.

Mephisto's BOOM Box

In order to harness the punch of your nukes, you have to slap a solid cap on them that would be plasmified when the fission happens.

Since we've already landed a delicate probe on a speeding asteroid, a big chunk of metal shouldn't be a problem. They should be aimed so that they make contact with the object and explode, loading most of their kinetic energy into it and also sending the caps' remains into God knows where.


If you have similar YT reccomendations to mines, you probably recognized that the previous proposal could be turned around to make the famous Hyperspace manhole cover


As I understand it,

1. An object is approaching Earth...

Fair enough so far.

A common sci-fi scenario is, an object is approaching Earth that is - say - a few miles or even 100 miles in size.

That object is on course to hit Earth, obviously causing an insane amount of damage.

Usually in such stories, we push it off path using nukes.

So, such an object hitting Earth would obviously crush continents, etc.

Note however that it is inconceivable such a small object would actually knock Earth out of the solar system.

In your story,

2. .. the object is so big it will actually swing Earth out of the solar system (!)

Thus to be clear, your approaching object is, indeed, "Earth sized", it's a massive planet-size object.

3 Your question, what can be done?

The answer is very simple:

4. Absolutely nothing - at all - whatsoever! unless you invoke (absolutely magical) sci-fi powers.

Note that unfortunately the task at hand would be as difficult as ..... moving Venus or Mars. "!"

"with current or very near future levels of civilisation and technology" there is just utterly nothing, whatsoever, like within something like 15 (!!!!!!!) orders of magnitude that we could do about "moving Venus or Mars".

Note that even within say the Star Trek universe (transporters! totally novel physics! etc.) it would be utterly impossible for them to move a planet.

Note that annoyingly, even if we created a man-made black hole (a big one) and sent it along to Eat the incoming planet, tragically that new black-hole object would have exactly the same gravitational effect as the original incoming planet! No more or less.

We can conceivably move asteroid-size stuff, but it's totally inconceivable to move planet-size stuff (sadly!)

Great question that highlights the scales involved!

  • $\begingroup$ As I said in the question, I'm not looking for things that would necessarily work as I'm fairly sure there isn't anything. But I'm fairly sure that the good folks of the planet will at least try something, so was asking what they might try, probably out of total desperation. $\endgroup$ Commented Oct 20, 2019 at 15:20
  • $\begingroup$ He knows that there is nothing that can be done and is asking about what crazy schemes the inhabitants could try to stop (and fail). $\endgroup$
    – Geronimo
    Commented Oct 21, 2019 at 14:07
  • $\begingroup$ There is literally nothing that could be done. There is no "crazy scheme". Consider this - we have often set off big nuclear bombs here on Earth. Did it "move the Earth"? There is literally not even a "crazy scheme". $\endgroup$
    – Fattie
    Commented Oct 23, 2019 at 11:48

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