Assuming fission thrusters as described here: Would a nuclear fission thruster with continuous fuel injection be possible?, and assuming drilling for the planet's molten mettle core as discussed here: Could you cool the earth's mantle and then bore into it?, and also assuming that people have built self-sustaining environments underground, and won't instantly freeze or starve to death the second the planet moves further from the star, would it be possible to add thrusters to a planet and travel to a different star system by using the unstable elements in it's own core for fuel?

Maybe the main thrusters would be built at one of the poles so that the planets spin is not such a problem. The spin would have to be gradually reduced to make navigation and guidance easier.

Is the star's gravity a major problem or can you just gradually widen your orbit?

How far would a planet get given its own fuel reserves?

  • $\begingroup$ What kind of tech do your world use? do your 1 million nuclear fission thrusters with continuous fuel injection able to hold a candle against the total amount of energy from the Sun that hits the Earth's surface? (that's don't even qualify as tickling Gaia) $\endgroup$
    – user6760
    Jul 31, 2015 at 4:49
  • $\begingroup$ Perhaps I should have added the hard-science tag. How many Newtons of force does solar wind actually apply to the earth? How much thrust could be created from the volume of uranium contained in the earth's core. I can't crunch the physics which is why I posted this question. $\endgroup$ Jul 31, 2015 at 4:58
  • $\begingroup$ schlockmercenary.com/2003-08-03 suggests using a gas giant instead of a solid planet to hold the candle, and putting the people on the gas giant's moons. It doesn't provide any calculations, though. $\endgroup$
    – oliver
    Jul 31, 2015 at 11:26
  • $\begingroup$ Metal not mettle! $\endgroup$ Jul 31, 2015 at 16:18
  • $\begingroup$ tardis.wikia.com/wiki/The_Pirate_Planet_(TV_story) $\endgroup$
    – Paul
    Aug 2, 2015 at 9:32

4 Answers 4


So, your question faces two physical problems, even given your assumptions:

  • The energy requirements to move a planet; and
  • The time it takes to get to other planets.

I'll address the former for now.

Could your engines do anything useful?

I found a number online that says how much uranium we mine every year. This number is not accurate (no unclassified number would be!), but I'm going to use it anyway: 50 Gg (giga-grams; or mega-kilograms; how's that for a unit?).

So, let's say this number represents your quantity of uranium for the fuel (to use for the reactor). I'm also assuming the 90% UTB from the linked page. Note that this is an over-estimate because there will be losses and you'll never achieve this, but it'll help this problem.

With that fuel, the 90% UTB reactor can thrust for 1633986.928 seconds (or 18.9 days) on one-year's worth of mined uranium.

Burning for that long with the specs in the link provides 5e19 J of kinetic energy (theoretically). Note that this is also an overestimate because losses will occur.

So, using the 1/2*m*v^2=energy equation and the mass of the earth (thanks, Wolframalpha), this translates into a whopping...

...wait for it...

4.313 mm/s change in velocity!


According to another link I found (which is right up your alley! and is pasted below...), the escape-velocity from the sun's gravitational field in the vicinity of earth is...

...wait for it...

42 km/s

You'd have to be 10,000,000 more productive than we currently are to reach escape velocity.

In terms of joules, it requires 4.457e32 J for earth to escape.

Nuclear fission is particularly good at turning mass into energy (E=mc^2 and all that), but it's still not really good at it. However, if you assume you instead have an engine that IS perfectly good at this, and you were to convert a year's worth of uranium into pure energy to propel earth, you'd get a whopping...

...wait for it...

4.949e24 J

So it would still take almost 100,000,000 more energy than that to get earth to escape velocity.

So no, it is not particularly realistic to move earth to travel between star systems.

But here's the link, which you'll want for some light reading: http://www.quora.com/How-much-energy-would-it-take-to-shift-the-Earth-from-its-orbit-around-the-Sun-and-propel-it-out-of-the-solar-system-and-are-there-any-processes-natural-or-otherwise-that-could-achieve-this

(Also, that's a ridiculously long link.)

One final note: it's amazing how many of these numbers are awfully close to starting with a 5...

Some fun number comparisons:

5e19 J, the amount of energy produced by these engines, is...

  • 38% the energy released by the 2004 Indian Ocean earthquake
  • 48% the energy consumed by the United States in 2001

4.949e24 J, the amount of energy it would take to propel earth to escape velocity, is...

  • 1.3% the energy output of the sun per second
  • 10x the estimated energy released by the Chicxulub meteor impact (whatever that is)

What if...

What would happen if you converted 10% of the earth's mass into pure kinetic energy (E=m*c^2 again)?

Firstly, earth wouldn't survive. But assuming it did...

You'd be traveling at 0.222 c (22% the speed of light).

The nearest star is 4.22 ly away. This means that, after turning 10% of your planet into energy, it'd still take you 19 years to get there. Note that this is NOT a renewable energy source, and is definitely not green.

  • 2
    $\begingroup$ The Chicxulub impact is what caused the crater just off the Yucatán peninsula; it's the one that killed the dinosaurs. $\endgroup$ Jul 31, 2015 at 9:48
  • 2
    $\begingroup$ And once you reach the target star, you would need to slow down to an appropriate orbital velocity around that star. Which needs approximately the same amount of fuel, but now for decelleration. Note that the Earth already travels at about 30 km/s in its orbit around the sun (in a solar-centric reference frame), so you "only" need to impart a 12 km/s delta-V to reach solar escape velocity. Only 11,999.996 km/s to go. $\endgroup$
    – user
    Jul 31, 2015 at 13:33
  • 4
    $\begingroup$ +1 for the reminder that transmuting 10% of the planet into energy is neither renewable nor green. $\endgroup$
    – Sidney
    Jul 31, 2015 at 14:34
  • $\begingroup$ @MichaelKjörling You only need 12m/s if you're OK with going in the same direction as your rotation. However, if you want 42km/s velocity away from the sun, you need [almost] all 42. $\endgroup$
    – iAdjunct
    Jul 31, 2015 at 16:33
  • 1
    $\begingroup$ @iAdjunct True, but assuming you are happy going in the orbital plane of the solar system, there should be a point at which a parabolic trajectory will get you to where you want to go. (Rocket science is actually pretty easy.) Oh, and I assume you meant 12 km/s and not 12 m/s. $\endgroup$
    – user
    Jul 31, 2015 at 16:49

This is most certainly possible and technically feasible, if we slightly change our first assumption and ignore cost. Particularly if the race is highly cooperative and possesses great ingenuity!

I will address each of the concerns raised in the first two links mentioned above and of course the concerns raised here.

Lets start with building a massive, distributed nuclear fission thruster system; actually, lets not. Trying to move massive objects over great distances with propellant based system is a pipe dream. Instead, let's use our massive, distributed nuclear reactor to sustain our energy needs for millions of years. We will need it.

Since we still need to move the planet, what can we use? Well, we can use one of those shiny RF resonant cavity thrusters, or any kind of quantum vacuum plasma thruster.

sciencealert com/independent-scientists-confirm-that-the-impossible-em-drive-produces-thrust

On multiple parts of the planet, with the main thrusters being positioned within diametrically opposed bore holes (we've got to slow down too!). Only the opposed bore holes need to reach the core. The other thruster bore holes can settle for scaled down geothermal and nuclear power. We can use the smaller thrusters for thrust vectoring.

Using @iAdjunct 's 50 Gg of Uranium and the geothermal heat we have available will provide us plenty of electrical power to sustain our operations for at least a few millennia.

We will need to use @Thucydides suggestion of patience and interplanetary momentum transferring to get started. But once we've started moving, we will rely only on our thrusting mechanisms.

Since it will take some time to actually get moving (and for our engineers to invent these exotic thrusters), let's work on getting to the core. And while we're at it, let's consider some of the other challenges/opportunities we will face:

  1. Losing the atmosphere

We can start our work by bottling up the entire atmosphere. This will reduce the massive amount of pressure that would be on our thrusters & our underground systems. This partially removes the pressure problem raised in the second link above. Once we start moving, we will lose this atmosphere either way, so this is absolutely necessary for our long term success. And it will take some time.

  1. Surviving the conditional extremes

We need something to survive the extreme temperatures coming from below the thrusters, within the underground system (via the planet's mantle, core, etc) and from without and above (i.e the cold gas of a nebula, the heat from a near by star).

We need something to survive the extreme pressure as we build structures deeper into the planet, and as we travel closer or further away from massive objects.

Can we address all of these problems and the concerns raised by @D. Elliot Lamb and some of others in the second link? Turns out we can! We have our unobtainium: Aerogel composites!

Sufficient research could yield a host of aerogels with the necessary properties for most of our engineering needs:

Today's aerogel is already capable of maintaining functionality under extreme temperature differentials.

Aerogel's porosity gives it the mechanical properties necessary to bear high loads. Creating a composite with the right formula could allow our engineers to create a sort of planetary spring/sponge. This will be particularly useful for absorbing the forces created during acceleration and from tidal forces from other large planets.

As a bonus we can also store our atmosphere within the aerogel, killing two birds with one stone. Closer to the bore hole walls, the aerogel structures can absorb some of the steam produced from continuous wall cooling, using the aerogel as a thermoelectric catalyst to produce hydrogen and oxygen necessary for our chemistry and survival respectively (producing ammonia for food, breathing).

And while we're at it, let's grow things micro-organisms our aerogel pores :)
Eventually our scientists will figure out how to evolve a collection of organisms which can regrow what will become our symbiotic planetary host.

2a. The serious issue of building large structures

Even after removing the atmosphere, we still must support the pressure of our entire system as we move closer to the planet's outer core. Using Earth as an example, that's a 2,890 km (1,800 mi) overhead we have to deal with.

The composition of the mantle is locally a solid, but essentially a fluid over time. And its temperature (using Earth as an example) can range from between 500 to 900 °C (932 to 1,652 °F) at the upper boundary with the crust; to over 4,000 °C (7,230 °F) at the boundary with the core. [shameless copy pasted from Wiki]

Can we provide an anchoring solution strong enough to withstand the heat and pressure of geological time?

I think so, in the present moment we have (predicted) Hafnium Carbide superalloys with of melting points of about 7,460 degrees Fahrenheit:


And Hafnium superalloys are already known to be excellent an neutron moderators, to shield us from the neutrons produced from nuclear reactions in the planet.

Scientists are already pursuing Hafnium Carbide aerogels!


Having strong, porous materials along with an abundance of atmospheric gases can allow us to making floating platforms which withstand the large pressures from above. Technology like this also already exists:

aps org/units/dfd/meetings/upload/Weinbaum_DFD03.pdf

In conclusion, by simultaneously constructing a planetary spring/storage system, a geothermal/nuclear energy system, and all the other planetary scale systems you'd need included, you could satisfy all of the requirements of creating a traveling planet which could sustain its own needs and survive the internal and external forces of traveling intergalactic space :)

  • $\begingroup$ The "em drive" described in New Scientist is not a real thing and cannot work. If you want to postulate some new physics for your story to violate concervation of momentum, you can do better with bosons; e.g. a photon rocket can work without reaction mass because photons can be made from energy. $\endgroup$
    – JDługosz
    Aug 15, 2015 at 7:17
  • $\begingroup$ arstechnica.com/science/2015/08/… $\endgroup$
    – JDługosz
    Aug 15, 2015 at 7:31

Moving planets can be done, but this requires a lot of patience and playing a long game of interplanetary billiards.

When a spacecraft like New Horizon "slingshots" around a gian planet like Jupiter, there is a momentum transfer; the spacecraft gains energy, while the gas giant loses a corresponding amount of energy. Given the mass differentials, you will have a very hard time measuring the reduction in Jupiter's orbital velocity.

However, early in the evolution of the solar system, billions of small bodies, asteroids and comets filled the protoplanetary disk. As the gas giant planets passed through this mass of bodies, some were accelerated away into deep space by the gravitational interaction, slowing the forming gas giant down and moving its orbit closer to the sun. It is equally possible for more of the protoplanetary material to be decelerated, speeding up the growing gas giant and causing it's orbit to move outwards from the sun.

So if you want to move a planet today, you would have to start in the Oort cloud or Kuiper belt and start sending bodies on carefully calculated orbits to trade momentum with the planet you want to move. There are some advantages; since the momentum exchange is done through gravitational interaction spread over a long period of time (it would take millions of asteroids passing the Earth to change its orbit), the physical effects on the planet would be minimized, and you have plenty of time to adjust the biosphere through genetic engineering and so on the match the new solar constant.

The disadvantage is since the objects have to fly through the entire solar system, orbital calculations would be insanely difficult, to account for the perturbations of the other planets as the objects passed, and also to ensure these bodies didn't hit vital space infrastructure as they moved past the planet who's orbit you are changing. You also have to account for the bodies after they make their momentum exchange passes, are you going to reuse them or do they settle down in a new, highly eccentric orbit around the sun?

So moving a planet of any size is possible, given enough time and resources. The amount of effort will be so huge that there would have to be a very compelling reason to do so.

  • $\begingroup$ The fuel needed to launch those bodies towards the planet would be impossibly prohibitive. $\endgroup$ Aug 1, 2015 at 17:29
  • $\begingroup$ The energy would be massive, but solar or magnetic sails could be used, or the Deuterium in the bodies fused to provide energy, or other forms of external energy beaming used to do the work. With enough patience, a smaller amount of energy could be used, if you were willing to take tens of thousands to millions of years to complete the project. $\endgroup$
    – Thucydides
    Aug 1, 2015 at 22:53
  • $\begingroup$ This has the advantage of being how the planets were moved. See this recent SETI Seminar on the history of the Kupier Belt and the Solar System. $\endgroup$
    – JDługosz
    Aug 15, 2015 at 7:40

My physics are weak. But wouldn't some kind of magnetic field be required to keep the planet's structure intact as the gravitational forces on it shift (zipping by other solar systems, gas giants, passing by black holes etc)? On a long enough journey who knows how many different and unexpected gravitational influences might test the planet's structure?

Running a continuous energy field through the planet's structure would be a huge fuel cost. Unless some kind of mechanical reinforcement could substitute. But installing steel meshes inside the planetary structure sounds like one hell of a project.

  • $\begingroup$ Magnetic field to mitigate tidal effects? I don't think so. $\endgroup$
    – JDługosz
    Aug 15, 2015 at 7:12

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