Moving A Planet

[Tech Support]

So... plausible, but it would take a while.

Now thats an understatement

[PakledHostage]

Now thats an understatement

Perhaps equally implausible, but what about the idea of bumping Oort cloud objects into Earth flyby trajectories? If carefully engineered, the gravitational interactions between the Earth and the comets could slowly adjust the Earth's orbit inward or outward over a period of millions of years.

This is the principle behind the Nice model of solar system formation that posits that Uranus and Neptune may have swapped positions due to millions of years of interactions with trans-neptunian objects.

[NERVAfan]

Perhaps equally implausible, but what about the idea of bumping Oort cloud objects into Earth flyby trajectories? If carefully engineered, the gravitational interactions between the Earth and the comets could slowly adjust the Earth's orbit inward or outward over a period of millions of years.

If you had a solar-system-wide civilization and unlimited time and resources, it could work. Extract deuterium from the comets, use that for fuel for a high-specific-impulse fusion engine to move them into trajectories that get an Earth gravity assist, which will change the Earth's momentum. It's just that when we do spacecraft gravity assists the spacecraft mass is small enough that the momentum lost by the planet is completely insignificant. But if you used millions or billions of comets, you might get a noticeable effect.

(Or use a smaller number of larger icy bodies like the outer planet moons and KBOs.)

[-Velocity-]

For decades, we've known that the best way to move an entire planet would probably be to send asteroids on flybys. To move the planet inward, you have the flyby give the asteroid a gravitational assist from the planet. This robs a tiny bit of the planet's orbital energy and gives it to the asteroid, boosting the asteroid's orbit and moving the planet inward slightly. To move the planet outward, you have the flyby apply a gravitational brake to the asteroid, which transfers orbital energy from the asteroid to the planet, boosting the planet's orbit by a tiny bit.

The problem is, it takes millions- maybe billions- of asteroid flybys. So it's a project that takes thousands or millions of years. Obviously, since the asteroids are not destroyed, you can reuse them, so the idea of moving a planet with asteroid flybys is at least theoretically possible with the materials we have on hand here in the solar system.

That said, if your goal is to cool down a planet, there are A LOT cheaper ways to do it. You simply fly a large star shade directly between the planet and the star in the L1 Lagrangian point, which blocks some fraction of the star's light. There have even been papers about doing this with Earth in the near future, should climate change get bad enough. Besides the engineering challenges, supposedly there are some issues with changing the amount of sunlight that various latitudes receive, which could lead to a different set of climatological problems.. After doing a quick calculation myself a few months back though, I'm not entirely convinced that these supposed problems are valid, but it was close...

That said, if it really were a problem, you could conceivably build a truly massive, partly transparent starshade (probably with carbon nanotube ropes for rigging and tension applied by a small amount of spinning). Such a star shade would be incredibly big, but not theoretically impossible, and it would block an equal amount of sunlight at all latitudes.

Anyway, I don't think we'll ever move planets, it will always be cheaper to use star shades to cool down too-hot planets, and greenhouse gases to warm up too-cool planets.

Edited October 5, 2014 by |Velocity|

[tomf]

If you can do proton-proton fusion then the energy available in the oceans is almost exectly the same as the energy to completly disassemble the earth, far more than is required to just move it a bit. I propose fusion powered railguns.

[shynung]

For decades, we've known that the best way to move an entire planet would probably be to send asteroids on flybys. To move the planet inward, you have the flyby give the asteroid a gravitational assist from the planet. This robs a tiny bit of the planet's orbital energy and gives it to the asteroid, boosting the asteroid's orbit and moving the planet inward slightly. To move the planet outward, you have the flyby apply a gravitational brake to the asteroid, which transfers orbital energy from the asteroid to the planet, boosting the planet's orbit by a tiny bit.

I think a direct impact is more efficient than asteroid flybys. It'll shake the planet a bit, but hey, the original goal was simply 'move the planet'.

[ZetaX]

No, a fly-by can create a greater change of momentum than an impact.

Additionally, you do not want to make earth bigger, that would just be more mass to work with.

[Red Iron Crown]

No, a fly-by can create a greater change of momentum than an impact.

Additionally, you do not want to make earth bigger, that would just be more mass to work with.

I don't see how a fly-by could change momentum more. The asteroid leaves the flyby with some momentum remaining, a collision imparts all of the asteroid's momentum to the Earth.

[ZetaX]

The fly-by ideally leaves the planet's SOI with the momentum (relative to the planet) opposite to the one it entered with. That's twice the change in momentum.

[Red Iron Crown]

The fly-by ideally leaves the planet's SOI with the momentum (relative to the planet) opposite to the one it entered with. That's twice the change in momentum.

I was neglecting the directional component, thanks for clarifying.

[shynung]

The fly-by ideally leaves the planet's SOI with the momentum (relative to the planet) opposite to the one it entered with. That's twice the change in momentum.

Huh. Didn't think of that one. Thanks.

[-Velocity-]

The fly-by ideally leaves the planet's SOI with the momentum (relative to the planet) opposite to the one it entered with. That's twice the change in momentum.

In addition to what ZetaX said, do you really want to impact a habitable planet with asteroids?!?!

And even further- asteroids are limited, you'll run out if you don't use them, and after the planetary encounter, it won't take too terribly much energy to put it back into the proper orbit again. It will certainly take a lot less energy than going and snatching a new one from the asteroid belt! Furthermore, you'll probably want to eventually mine the asteroids, not blast them to the bottom of a big gravity well that you will later have to lift them out from!

Many, many reasons to not collide the asteroids with the planet you're trying to move. The only reason TO collide them with the planet you're trying to move is if you're also trying to terraform the planet at the same time, in which case, you'll likely be colliding comets packed with volatiles, not asteroids.

Edited October 7, 2014 by |Velocity|