Is it possible to move planets to get them nearer to Earth?

Question

Could we move rocky planets such as Mercury, Venus, and Mars to be closer to Earth's orbit?

I know it sounds like a silly question and it's impossible right now, but if there is a small possibility, we could reduce the travel cost significantly and potentially terraform these 3 rocky planets. If we assume this technology is available, or better said, if you could place the planets wherever you want in the solar system, is there a combination of orbits where each planets were near or sufficiently close to each other without interfering orbits or interfering with each other in a functional way? I'm just hypothesizing and I really want an open discussion with people who know.

Answer 1

we could reduce the travel cost significantly

No, we couldn't. The travel cost between planets is negligible. The main travel cost is the cost leaving the gravity well, which is entirely independent of the distance between planets (unless you're getting close to the Roche limit, which has its own problems). So if you're paying the cost to leave the gravity well, you're not much worse off by paying the travel cost as well.

This is why we talk about mining the asteroid belt rather than Mars or Venus. The distance is greater but the expense is lower because we do not need to get out of a planetary gravity well. Also, we don't care about the travel time for mined goods so long as the throughput is sufficient.

There is a certain time cost that could be reduced. But the actual fuel cost of moving from Earth orbit to Mars or Venus orbit is not so great now. The main cost is getting stuff into Earth orbit. And the time isn't so bad. Even a Hohmann transfer can get you between Earth and Mars in about 344 days. Add more power and you can cut that to 130 or so. Venus is even closer, with only 183 days for the Hohmann transfer. But you won't gain as much with the powered assist.

Moving planets has the same problem. Yes, it makes things like the Hohmann transfer easier, but you can't accelerate as much during the journey (accelerating takes time). So cutting the distance in half doesn't cut the time in half, even when just looking at the orbital transfers. Again, the gravity well maneuvers won't be affected at all.

It would be possible to move planets. It would probably take a long time, as fast methods of adding propulsion would tend to break the planet that you're moving. But planets are simply big. If you're willing to wait, it seems possible.

A greater problem is that closer orbits may not be stable. To make this work, you might have to move Jupiter and move the Earth, Mars, and Venus into a conformation with it (e.g. orbit or the Sun-Jupiter trojan points). Because most stable multi-body systems require one to be much bigger than the others. You can get a two body stable orbit (around the much more massive Sun) as a binary planet. But trying to put the Earth, Mars, and Venus into a single conformation with only the Sun to balance may not be stable.

Also, I presume you are doing all this with an inhabited Earth. So whatever you do, don't miss. A small mistake could have tragic results, since it will be hard to fix later.

Answer 2

I'm going to assume you want to move these planets more for the sake of terraforming and colonization, than for mining resource. Generally, it will always be easier to move stuff you get from a planet, than to move the entire planet.

Now, how to move a planet is the question -- the simple answer is gravity. NASA has been examining gravity tugs for use in deflecting asteroids from collision orbits with the Earth. It takes a long time, but if you detect the potential collision a few decades out (as we can, with large enough rocks), you can send a solar sail or ion drive craft that will use its own mass to very slowly tow the asteroid by their mutual gravitation.

The same trick can, in theory, be done with planets (see Larry Niven's World out of Time). For something a little less far fetched than Niven's method, consider mounting very large mass drivers on a body like Ceres, Pallas, or Vesta. Maneuver the body into a position where its drive can more than offset its "weight" due to the gravity of the planet you want to move, and keep it there until the body to be moved has accelerated enough for each maneuver, then move the asteroid away.

You'll need multiple maneuvers to, for instance, move Mars into a Trojan orbit relative to Earth -- but if you have a source of reaction mass and unlimited energy (and if you don't, you wouldn't be considering moving planets for living space), and don't mind taking decades to get the job done (unavoidable, really), it's very much possible. In fact, you could even move an inhabited water world like Earth this way, though you'd have to watch the tidal effects closely.

Answer 3

Not feasible with any technology we can see right now.

To change the orbit of a planet you have to impart a lot of energy. The term astronomical number was coined for things like this. Generating the energy will be impractical, and if it were practical then the waste heat would become the next problem. It would wreck the surface of the planet.

If it was practical to mount a drive on a planet, drive technology would be so good that the distance is no problem any more.

Answer 4

It is possible. Though the term possible here means 'possible within our understanding of the laws of physics and will not violate fundamentals like conservation of mass' and not 'possible for the human race to do'. Let's talk orbital dynamics: [Side note: I'm just going to be using Sun instead of Sol, because this is mainly explaining the abstract concepts.]

All orbits are elliptical in nature (with one of the foci of the ellipse being the object in question, i.e. the Sun ), though most planet's orbits are fairly close to being circular that the change of distance between them and the Sun is somewhat insignificant. (In other words, Earth never leaves the Goldilocks zone.) There are six useful vectors when it comes to discussing moving orbits. The two most useful to us are prograde and retrograde. To get them, draw a tangent to the point where the planet is on the orbital plane. Prograde is forward, retrograde is backwards. (The other four vectors are to the sun / away from the sun, and north/south relative to the orbital plane.)

In other words, if you drew a cube around the planet, the directions we're interested in are the faces that correspond to the orbital path, not the vertical faces, nor the faces that point to the Sun.

See, orbital dynamics are a bit weird to jump to from plain old Earth kinematics. The reason being that exerting force to the Sun isn't a good way to make the orbit closer to the Sun. Reason being that if you tally up the vectors the planet already has, it already possesses a vector towards the Sun. It has to, otherwise it wouldn't be able to get to the far side of the Sun. (Would it work eventually? No, because you'd decay the orbit to the point were it wasn't stable.)

The way to reduce an orbit so that the object would have a closer orbit to the Sun would be to apply a retrograde vector. In other words, force applied opposite the direction the planet is going. This would directly reduce the orbit on the other side of the orbit. Remember, orbits are ellipses, not circles, so it's possible to have a somewhat non-standard orbit. So, if you took Mars as an example, you apply a retrograde vector at a point in Mars's orbit. (Preferably the apoapsis.) That would reduce the point in orbit 180 degrees away to Earth's orbit, assuming you applied enough force (and it'd be a lot). Then, half a Martian year later, when Mars reaches the 180 point, you apply force again, and reduce the orbit on the other side so now both sides are Earth-like.

As for applying the vectors - well, honestly, that's really hard to do without destroying the planet. I suppose as long as we're okay discussing these levels of energy, we can just Dyson sphere the planet, even though that's not typically what you'd use a Dyson sphere for, and apply the force to the Dyson sphere. Or possibly you apply a heat ray to specific portions of the atmosphere to turn it to plasma, replacing the atmosphere using comets as need be. But that seems like it could be another question entirely, I'm just spitballing here.

You may run into some more problems with orbits, given that Mars and Earth share a similar, but not identical, orbit plane. But that's what the other four vectors are for. A fine-tuning problem, if you will.

Answer 5

I'm not an expert of orbital mechanics (so I can't provide any maths about it), but it could somehow be feasible (obviously, not at any possible technology level in short, medium, long and very long term).

The idea would be to deviate the path of some big asteroids with a suitable drive (be it a light sail or any onobtainium-like device), so that it would fly by the planet we want to bring closer to Earth. The effect of a properly planned flyby would be to make the planet lose kinetic energy (for Mars, like in the slingshot effect) or gain energy (Venus and Mercury). Since the planet is a lot of magnitude orders more massive than an asteroid, it should approach at high speed in order to have an even minimum effect on the orbit.

But launching this way some hundreds thousands asteroids through the centuries, I think it could be possible to change the orbit of the planets as wanted, if you don't want to radically alterate the orbit, but only slightly reduce/enlarge its radius